Other Automatic Controls

Modern heating and cooling systems contain electrical control cir­cuits that are interconnected and interlocked with the various sys­tem components by a series of switches and relays. Most of these components, particularly the heating and cooling equipment (fur­naces, boilers, compressors, condensers, and so on) and most sys­tem controls have been described in other chapters of the book. This chapter is reserved for a description of the fan and limit con­trols; the various electrical control circuit switches and relays; transformers; and a number of different control devices used in cooling systems.

Fan Controls

A number of different devices are available for controlling the oper­ation of fans in heating and/or cooling installations. Most of these devices function as fan safety controls; a few of them serve as fan primary controllers. The following are fan controls described in this chapter:

Fan control

Air switch

Fan relays

Fan center

Fan manager

Fan timer switch

Fan safety cutoff switch

Caution

Always disconnect the power supply before installing, servicing, or repairing any of the electrically operated devices described in this chapter. Failure to do so may result in damage to equipment and/or electric shock.

Fan Control

A fan control is a device used to turn the system fan on and off in response to air temperature changes in the furnace plenum. This fan controller is frequently combined with a limit controller in one unit (see Combination Fan and Limit Control in this chapter).

In the operation of the furnace, the burner or burner assembly starts first and heats the air, which rises through the heat exchanger to the furnace plenum. The fan control is located in the plenum and is present for a specific cut-in temperature. When the temperature of the rising air reaches the cut-in temperature setting on the fan controller, the fan is automatically turned on and warm air is moved through the distribution ducts. After a period of time, the room thermostat will no longer call for heat and will shut off the burner. The air in the plenum then begins to cool. When the air temperature drops below the cut-in temperature of the fan con­troller, the fan is automatically shut off.

In most forced warm-air heating systems, the fan control is usu­ally a line voltage device wired in the hot lead (L1) of the power supply to the fan motor (see Figure 6-1). If a step-down transformer is used to provide a low-voltage control circuit for the room ther­mostat, the fan motor and fan controller will connect at the line side of the transformer (see Figure 6-2).

FAN

CONTROLLER

Other Automatic Controls

Figure 6-1 Fan controller wired in the hot lead.

(Courtesy Honeywell Tradeline Controls)

The following procedure may be used for setting a fan control:

1. Allow the burner or burner assembly to operate for a normal running period.

2. Lower the thermostat setting so that the burner(s) will not operate during the fan control setting procedure.

3. Place a thermometer in the furnace plenum or bonnet or in one of the warm-air ducts near the furnace.

TRANSFORMER

Other Automatic Controls

Figure 6-2 Typical warm-air fan control circuit.

(Courtesy Honeywell Tradeline Controls)

4. Set the fan adjustment lever and the fan differential adjust­ment (when used) to their lowest or coldest position so that the fan will run continuously.

5. Watch the thermometer until the temperature drops to about 5F (3C) above the temperature normally maintained in the rooms being heated.

6. As soon as the temperature on the thermometer has reached the appropriate level (see step 5), slowly move the fan temper­ature adjustment lever up to a point where it will stop the fan.

When setting a fan according to the aforementioned procedure, the speed of the fan must be set so that the average temperature rise through the furnace is about 90F (50C).

In some installations, a drafty condition may result from the location or types of warm-air outlets. This condition can be corrected by the following adjustments to the procedure described previously:

1. Place the thermometer in front of the return air grille in the room or space that is most difficult to heat.

2. When the air leaving the room begins to feel cool, slowly move the fan adjustment up until the fan just stops.

Air Switch

An air switch is a device designed to control a two-speed fan in response to air temperature changes in the furnace plenum (see Figure 6-3). When the temperature in the plenum rises to the set­point, the air switch will change fan operation from low to high. In other words, the air will be removed from the plenum at a higher rate of speed in order to lower the temperature of the air in the fur­nace. Because this device is frequently used in conjunction with a fan controller or a combination fan and limit controller, it is some­times referred to as an upper fan control.

Other Automatic Controls

TO GAS-VALVE CONTROL CIRCUIT

Typical wiring connections for L6068 uses to control a 2-speed fan motor. R-W makes. R-B breaks on temperature rise to set point.

Figure 6-3 Model L6068A air switch and wiring diagram.

(Courtesy Honeywell Tradeline Controls)

Air switches are generally available with fixed temperature set­tings (e. g., 125F, 135F, 165F, or 200F) or with an adjustable tem­perature range (125F to 165F or 160F to 200F). T emperature setting adjustments can be made as shown in Figure 6-4.

Other Automatic Controls

Figure 6-4 Adjustment and connection points on a model L6068 air

Switch. (Courtesy Honeywell Tradeline Controls)

An air switch can also be used to shut off the burner and turn on the fan when the temperature between the filter and heat exchanger rises to the setpoint. In this manner, it functions as a limit control (see Secondary High-Limit Switch in this chapter).

Fan Relays

A fan relay is a primary controller designed to provide 24-volt circuit control of line voltage fan motors and auxiliary circuits in

Heating and/or cooling systems (see Figures 6-5 and 6-6). It also provides manual fan operation at any time by using the manual fan switch of the thermostat base.

Other Automatic ControlsFigure 6-5 Model R85I fan relay (contactor) provides 24-volt control of single — or two-speed fan motors up to xh hp.

(Courtesy Honeywell Tradeline Controls)

Fan relays are available in a variety of different models based on the switching element contact position. The following are some examples:

• Single-pole, single-throw (SPST) switching element—both contacts normally open.

• Single-pole, double-throw (SPDT) switching element—one contact normally open and one normally closed.

Double-pole, single-throw (DPST) switching element—one normally open main contact and one normally open auxiliary pole.

Fan relays are often used with multiple-speed fans to provide low-speed fan operation during the heating cycle and high-speed operation during the cooling cycle.

A 24-volt room thermostat is generally used to switch the fan relay controlling the indoor fan (120- or 240-volt AC power). In cooling systems, a second fan relay must be added if switching con­trol of the condenser fan motor is desired.

Other Automatic Controls

Figure 6-6 Fan relay used for low-voltage control of line voltage fan motors and auxiliary circuits.

(Courtesy Honeywell Tradeline Controls)

The operation of a fan relay can be checked by applying power to the coil and listening for the click of the contacts closing or by testing for electrical continuity. If the fan relay does not operate, check the voltage to the coil.

Sometimes a fan will operate at low speed but not at high speed. Check the fan relay first. If it is not defective and is receiving proper voltage, the failure of the fan to operate at high speed may be caused by loose wiring or dirty contacts. The method used for cleaning relay contacts is described elsewhere in this chapter (see Cleaning Contactors).

A defective fan relay is also the occasional cause of compressor short cycling; however, this operating problem is more commonly traced to dirty air filters and other air movement restrictions on the low side of the compressor. These possible causes should be checked first.

Fan Center

A fan center is a primary controller designed to provide automatic low-voltage control of line voltage fan motors and auxiliary circuits in heating, cooling, and heating/cooling circuits. A typical wiring hookup with a two-speed fan motor, electronic air cleaner, and humidifier is illustrated in Figure 6-7.

ISOLATION RELAY

Other Automatic Controls

CONTRACTOR OR COMPRESSOR CONTROL PANEL

Other Automatic Controls

In addition to providing the same switching functions as a fan relay (see previous section), a fan center includes an integral low-voltage transformer and a terminal board for low-voltage system wiring. In air-conditioning systems, a thermal delay relay is often added to a fan center to prevent short cycling of the compressor motor.

Fan Manager

A fan manager is used in compressor-operated air-conditioning sys­tems and heat-pump systems to enable the blower to continue run­ning for a short time after the compressor has shut off (see Figure 6-8). This short delay increases the cooling efficiency of the system by allowing the blower enough time to force the residual cooled air into the living spaces. Heat-pump systems can be wired so that the blower shutoff delay also occurs in the heating cycle.

Fan Timer Switch

A fan timer switch is a device that provides timed fan operation for forced warm-air furnaces and unit heaters when they are wired in parallel with a furnace or heater controller. The switch operation is

Other Automatic Controls

Other Automatic Controls

VALVE RELAY CONTACTOR

/ Power supply provide disconnect means overload protection as required.

Figure 6-8 (continued)

Independent of furnace plenum temperature changes. This factor ensures fan operation and eliminates unnecessary recycling at the beginning and end of burner operation. The use of a fan switch is particularly recommended for horizontal and downflow furnaces.

The Honeywell S876 fan timer switch shown in Figure 6-9 con­tains a heater-actuated SPST bimetal snap-action switch that turns the fan on after the burner starts and off after the burner stops. Typical wiring connections for a timer switch are shown in Figures 6-10 and 6-11.

Fan Safety Cutoff Switch

A fan safety cutoff switch can be installed in any heating, ventilating, or air-conditioning system to control fan motor operation (see Figure 6-12). These are manually reset mercury switches that automatically break the fan motor circuit on temperature rise to the setpoint. The setpoint is established by adjusting the temperature-setting screw on the unit. This switch is designed to lock out to prevent the return of fan operation until manually reset.

Other Automatic Controls

TRANSFORMER

Other Automatic Controls

O

подпись: oProvide disconnect mean and overload protection as required.

Other Automatic Controls

Figure 6-11 Two-speed fan control circuit with low fan speed controlled by a fan timer switch. (Courtesy Honeywell Tradeline Controls)

Limit Controls

Limit controls are also used to prevent the buildup of excessive and dangerous high temperatures in the furnace plenums. They accom­plish this task by shutting off the burner when the maximum tem­perature setting on the control is reached and by turning it on again when the air temperature returns to normal.

The following limit controls are described in this chapter:

Limit control

• Secondary high-limit switch

Limit Control

A limit control is a device designed to provide high-limit protection for a forced warm-air furnace (see Figure 6-13). It controls the oper­ation of a burner or burner assembly in response to air temperature changes in the furnace plenum. If the air temperature in the plenum becomes excessively high, the limit controller shuts off the burner or burner assembly until the air temperature returns to normal.

Other Automatic Controls

HELICAL

ELEMENT

подпись: helical
element
Other Automatic ControlsTEMPERATURE

3-S

подпись: 3-s

MERCURY

SWITCH

подпись: mercury
switch

TEMPERATURE SETTING SCREW

подпись: temperature setting screw

RESET

LEVER

TERMINAL

BLOCK

подпись: reset
lever
terminal
block

CONDUIT OUTLET

подпись: conduit outlet Other Automatic ControlsSCALE

Figure 6-12 Fan safety shutoff switch. (Courtesy Honeywell Tradeline Controls)

Other Automatic Controls

Limit controls are available in models suitable for use in low — voltage, line voltage, and self-energizing (millivolt) systems. Typical wiring hookups for these different systems are shown in Figures 6-14 and 6-15. These limit controls have so-called universal contacts in the limit switch, which makes them suitable for all voltages from millivolt to line voltage.

TRANSFORMER

1 ^ Provide disconnect means and overload protection as required.

*

THERMOSTAT

 

POWER

SUPPLY

O

 

Other Automatic Controls

Figure 6-14 Limit control in a low-voltage circuit.

(Courtesy Honeywell Tradeline Controls)

 

Other Automatic Controls

LINE VOLTAGE LIMIT CONTROL

L1

(HOT) “ L2

Other Automatic Controls

.. PRIMARY "J SAFETY CONTROL

 

4-o o

 

POWER

SUPPLY

O

 

©

подпись: ©Provide disconnect means and overload protection as required.

Figure 6-15 Limit control in a line voltage circuit.

(Courtesy Honeywell Tradeline Controls)

A limit control contains a snap-action switch operated by either a fluid-filled or bimetallic sensing element.

Fluid-filled sensing elements are connected to the control by a length of capillary tube, which is available in lengths up to 72 inches. The tube is filled with a temperature-sensitive liquid. A tem­perature change causes the liquid to expand against a diaphragm that operates a snap-action switch (see Figure 6-16).

Bimetal sensing elements are available in helical, flat-blade, and spiral types (see Figures 6-17, 6-18, and 6-19). The bimetal sensing element is connected directly to the switch operator.

Other Automatic Controls

Figure 6-16 Fluid-filled sensing bulb, diaphragm, and snap mechanism.

(Courtesy Robertshaw Controls Co.)

Other Automatic Controls

Figure 6-17 Helical bimetal sensing element.

The temperature setting of the limit control should be high enough not to interfere with the normal operation of the furnace, but low enough to shut off the burner or burner assembly before air temperatures in the furnace plenum reach the danger point. After the temperature in the furnace plenum has cooled and dropped below the setting on the limit control, the limit switch closes and starts the burner or burner assembly again.

Secondary High-Limit Switch

Figure 6-18 Flat-blade bimetal sensing element.

подпись: 
figure 6-18 flat-blade bimetal sensing element.
The same type of air switch used to provide two-speed control of fan motors can also serve as a secondary high-limit switch (or upper-limit con­trol) on downflow or horizontal warm­air furnaces (see Figure 6-20).

Downflow and horizontal furnaces are sometimes subject to a reverse air circulation condition that can result in a dangerous buildup of temperatures. This condition is usually caused by fan failure or clogged filters. The secondary high-limit switch is a safety device used as a backup system for the regular high­limit controller (see Figure 6-3). It is par­ticularly important to have a secondary high-limit switch on a furnace if there is a possibility that the location of the reg­ular high-limit controller may cause it to fail to detect a fan malfunction.

The secondary high-limit switch is located between the filter and the fur­nace fan (see Figures 6-21 and 6-22). When the air temperature exceeds a certain setting, the switch opens and shuts off the burner or burner assembly and turns on the fan. Secondary high-limit switches are either automatic or manually

Other Automatic Controls

Other Automatic Controls

Other Automatic ControlsReset types. The automatic type is a single-pole, double-throw (SPDT) switch that turns on the burner when the limit cuts out. The manually reset switch must be reset before the burner will operate again.

AIR

FLOW

Other Automatic Controls

Other Automatic Controls

WARM AIR

Other Automatic Controls

OUTLET

Figure 6-21 Approximate location of a secondary high-limit control on a downflow warm-air furnace. (Courtesy HoneywellTradeline Controls)

FAN CONTROL HIGH — LIMIT CONTROL

Sl; secondary

^ HIGH — LIMIT CONTROL

When the air temperature between the filter and the heat exchanger reaches the setting (i. e., setpoint) in the air switch, one internal switch closes (R to W) and another opens (R to B). This operation shuts off the burner and starts the fan.

Combination Fan and Limit Control

A combination fan and limit control combines the functions of a fan controller and a limit controller in a single unit. One sensing element (either bimetal or fluid-filled) is used for both controls (see Figures 6-23 and 6-24).

ROTATES ON TEMPERATURE RISE

Other Automatic Controls

Other Automatic Controls

HIGH — LIMIT INDICATOR

подпись: high -limit indicator Other Automatic Controls

HIGH — LIMIT TERMINAL SCREWS

подпись: high -limit terminal screws

JUMPER-SEE

‘WIRING’

подпись: jumper-see
'wiring'

MOUNTING

HOLE

подпись: mounting
hole
Figure 6-24 Model L40I7 combination fan and limit control.

(Courtesy Honeywell Tradeline Controls)

Combination controllers are wired in much the same way as the individual controls. Examples of some typical wiring hookups are shown in Figures 6-25, 6-26, 6-27, and 6-28. These combined con­trols can be used in line voltage, low-voltage, or self-energizing mil­livolt systems.

The combination fan and limit controller should be located where it will provide the best possible operating characteristics.

Limit switch terminals are on the left side of the control, fan switch terminals on the right. This arrangement is true of combina­tion fan and limit controls as well as single-purpose types (see Figures 6-29, 6-30, 6-31, and 6-32).

On the fan and limit controls shown in Figures 6-29, 6-30, 6-31, and 6-32, temperature settings can be changed by moving the temperature-setting pointers. Temperature settings are interlocked to prevent the limit off from being set as low as the fan on pointer. Sometimes the limit off setting will be factory-locked to a specific setting. If this is the case, do not attempt to adjust this setting. A safety interlock prevents the fan on pointer from being set as high as the limit off pointer.

Other Automatic Controls

2 1 To controlled low-voltage equipment.

Figure 6-25 Combination fan and limit control wiring diagram with limit controller in the low-voltage circuit. (Courtesy Honeywell Tradeline Controls)

Other Automatic Controls

2) To controlled low-voltage equipment.

Figure 6-26 Combination fan and limit control wiring diagram with limit controller in the line voltage circuit. (Courtesy Honeywell Tradeline Controls)

Some combination fan and limit controls are equipped with a manual summer fan switch to provide continuous fan operation for summer ventilation. To operate the blower during summer weather without the burner in operation, move the switch lever on the manual summer fan switch from auto to on position. This will provide continuous fan operation until the lever is moved to the auto position.

Other Automatic Controls

Figure 6-27 Combination fan and limit control in warm-air heating control circuit with low-voltage limit. (Courtesy Honeywell Tradeline Controls)

Other Automatic Controls

COMBINATION

CONTROLLER

POWER

SUPPLY

©

Provide disconnect means and overload protection as required.

О

Other Automatic Controls

Figure 6-29 Fan control with adjustable fan differential and summer fan switch.

(Courtesy Robertshaw Controls Co.)

As shown in Figure 6-33, the metal tabs on the temperature dial of the Robertshaw combination fan and limit controller can be bent back to serve as stops for the fan and limit setting pointers. The dial is held to prevent rotation, and the pointer is pressed in and rotates to the proper temperature setting. The first tab above the pointer is bent back 90°toward the sensing element.

A certain amount of caution must be exercised when installing a combination fan and limit control. These controls can be either located in the furnace plenum or mounted directly on a panel, but the sensing element must be located in the path of free-flowing air. Never mount the control near the cool-air intake, and keep the

Other Automatic Controls

Figure 6-30 Limit control without summer

Fan switch. (Courtesy Robertshaw Controls Co.)

Sensing element away from any hot metal surfaces. Furthermore, make sure you mount the control where it is accessible for making temperature adjustments.

Switching Relays

A switching relay is a device used to increase system switching capabilities, to isolate electrical circuits, and to provide electrical interlocks in a heating and cooling system. These devices are espe­cially useful in systems where the heating and cooling equipment have separate supplies.

A typical switching relay contains an integral transformer and a magnetic relay with contacts designed to make or break an electri­cal circuit. These contacts will be either normally open or normally closed, depending on the design of the relay and its purpose in the heating and/or cooling system.

Other Automatic Controls

Figure 6-31 Combination fan and limit control with nonadjustable fan differential and summer

Fan switch. (Courtesy Robertshaw Controls Co.)

The 24-volt switching relay illustrated by the wiring diagram in Figure 6-34 is designed to control 115-volt and 24-volt or millivolt circuits. It incorporates a 20 VA 115 V/24 V transformer and a 24 V/60 Hz 0.2-ampere magnetic relay with two normally open con­tacts. One set of the relay contacts is line voltage rated for the switching of a circulator or other device. The other set of contacts is used for the switching of a self-energized (millivolt) or 24-volt cir­cuit. A terminal board is located on top of the relay cover with screw terminals for connecting a thermostat (terminals T1 and T2) and a gas valve or oil burner control (terminals X1 and X2).

The switching relay illustrated in Figure 6-34 is shown as used in a gas-fired, forced hot-water heating system. In operation, a

Other Automatic Controls

Figure 6-32 Combination fan and limit control with adjustable fan differential and summer

Switch. (Courtesy Robertshaw Controls Co.)

Thermostat or some other switching accessory (e. g., an aquastat or zone valve) connected to the T1 and T2 terminals starts the circula­tor and boiler simultaneously by energizing and closing the two — pole, normally open relay. When the relay is activated by the thermostat, it closes a circuit from the integral transformer to a magnetic switch, which causes high voltage to be fed to the circula­tor to start the system pump. At the same time, the second pole of the relay switches 24-volt power to the gas valve or oil burner con­trol, thereby starting the boiler.

A switching relay can also be used as a pilot duty relay to power a contactor and control a crankcase heater for the compressor motor. This type of switching relay has normally closed contacts

Other Automatic Controls

Figure 6-33 Temperature dial metal tabs bent to form a stop.

(Courtesy Robertshaw Controls Co.)

That complete an electrical circuit to the heater until the thermostat calls for cooling. When this occurs, the relay switches to break the heater circuit and power the compressor motor circuit.

In installations where a cooling system has been added to a self — energizing (millivolt) heating system, a switching relay may be used to isolate the cooling and heating power supplies (see Figure 6-35). When the room thermostat calls for heat, an isolating relay is used to switch the heating equipment directly.

Heavy-duty switching relays are used for control of high-current loads such as cooling compressors or electric heating where sudden high-current demands are not unusual. The relay is wired to break both sides of the circuit with DPST switching (see Figure 6-36).

Impedance Relays

An impedance relay (see Figure 6-37) is used to provide lockout and remote reset in refrigeration, air-conditioning, and other

I

I

B L

W

Я-

———- —-

! r-< 1

‘-vmr—» !

L

I

N

TR

R-^OWWV

N

SWITCHING

RELAY

JV

HI-LIMIT

AQUASTAT

 

T2

 

X1

 

X2

 

THERMOSTAT

 

Other Automatic Controls

TH

 

GAS VALVE 24 V

 

Other Automatic Controls

R

 

CIRCULATOR

Other Automatic Controls

 

H

 

115 V 60 HZ

 

G

 

Other Automatic Controls

Figure 6-34 Switching relay used with 24-volt power supply.

(Courtesy Hydrotherm, Inc.)

Systems. A typical wiring diagram for a low-voltage impedance relay is shown in Figure 6-38. A low-voltage relay requires the use of a transformer with an open circuit secondary of between 24 and 27 volts AC.

As shown in Figure 6-38, one pair of contacts is normally open and the other pair is normally closed. During normal operation, the normally closed contacts of the pressure controls (i. e., the low — pressure and high-pressure cutout switches) and the motor overloads short out the impedance relay coil so that the compressor contactor pulls in. If one of the pressure controls or overloads opens, the impedance relay coil is energized in series with the contactor coil and most of the available voltage is used by the high impedance of the relay coil. Because insufficient voltage remains to operate the contactor coil, the contactor drops out and compressor operation stops.

As the impedance relay pulls in, its normally closed contacts open to keep the contactor out, even though the pressure control or

Other Automatic Controls

Figure 6-35 Switching relay used to isolate cooling and heating power

Supply. (Courtesy Honeywell Tradeline Controls)

Overload (automatic reset) remakes. The system can be reset by breaking the contactor circuit to allow the impedance relay to drop out. In most systems, this is accomplished by moving the thermo­stat subbase switch to off and back to cool again.

Heating Relays/Time-Delay Relays

In air-conditioning installations, a time-delay relay is often installed in the control circuit to provide protection for the compressor and contactor. This device is activated or deactivated by the room ther­mostat. In operation, it causes a time delay between turning down the thermostat setting and the start of the compressor unit of approximately 20 to 45 seconds (depending on the manufacturer). On the shutoff cycle at the thermostat, the same delay occurs. This control prevents rapid short cycles from occurring.

A time-delay relay is essentially a switching relay that contains a small heater wound around a bimetal element. The relay heater is

Other Automatic Controls

Other Automatic Controls

IMPENDANCE RELAY

PRESSURE, OVERLOAD CONTROLS (MAY BE ONE OR SEVERAL)

CONTRACTOR

подпись: contractorHH

Hf-

подпись: hf-POWER

THERMOSTAT

подпись: thermostatSUPPLY

©

©

подпись: ©(XI)—— 0_r

0

0

подпись: 0
0
CHECK LITE

Provide disconnect means and overload protection as required.

2 j Transformer for low-voltage wiring or R8231 relays. R4231 are used in line-voltage systems (omit transformer).

Figure 6-38 Impedance relay in a low-voltage circuit.

(Courtesy Honeywell Tradeline Controls)

Energized through the cooling contacts of the thermostat and heats the bimetal element. After a time delay of approximately 20 to 45 seconds, the heated bimetal element bends to provide the switching force. In other words, it closes a set of snap-action contacts, thereby completing the circuit through the starter or contactor coil required to start the compressor. The relay may be wired to provide a delay in breaking the circuit after the thermostat is satisfied.

The time-delay relay shown in Figure 6-39 is used with a two — wire low-voltage thermostat (and remote-mounted thermostat) to provide a time delay between stages for electric heaters in furnace ducts. Each relay can control up to 6000 watts at 240 volts AC. One relay is required per stage or time increment. This particular model provides a delay of approximately 75 seconds between the on cycles of consecutive stages. The sequencing of heating loads is permitted by auxiliary contacts.

Because of the dual nature of their function, time-delay relays are also referred to as time-delay switches, thermal switching relays, ther­mal time-delay relays, thermal relays, and heating relays. Other exam­ples of time-delay/heating relays are shown in Figures 6-40 and 6-41.

Potential Relay

In air-conditioning installations, the potential (start) relay serves as a switch to disconnect the starting capacitors when the compressor motor has overcome the initial starting torque.

•is

11 TT

M

Nj®

 

Figure 6-39 Thermal switching relay used for control of electric furnaces or electric duct heaters.

(Courtesy Honeywell Tradeline Controls)

 

Figure 6-40 Electric heating relay used with a two-wire thermostat for control of electric boilers, duct heaters, fan coils, or electric furnaces.

(Courtesy Honeywell Tradeline Controls)

 

Other Automatic Controls Other Automatic Controls Other Automatic Controls

An important fact to remember is that each potential relay is specifically designed for the compressor to which it is attached. Should this relay fail or become erratic in its operation, no attempt should be made to repair it. The relay must be replaced with an identical component.

Other Automatic Controls

Figure 6-41 Time-delay relay used in electric baseboard heating.

(Courtesy Singer Controls Co. of America)

Pressure Switches

A pressure switch is a safety device designed to stop or start heating or air-conditioning equipment in response to gas — or air-pressure changes. These switches are used in either positive-pressure or differential-pressure systems.

Gas-pressure switches used in the control of gas-fired furnaces and boilers are described in the section Pressure Switches in Chapter 5 (Gas and Oil Controls)’. Pressure switches used as refrigerant con­trollers in a cooling system are described in this chapter (see Low — Pressure Cutout Switch and High-Pressure Cutout Switch).

The National Electrical Code requires that a duct heater be inter­locked with the system fan so that the heater cannot be energized unless the fan is also energized. This can be accomplished by using either a fan interlock relay or a built-in air-pressure switch.

An air-pressure switch designed to provide fan interlock control consists of an internal diaphragm that is actuated by positive air pressure. The switch sensor is mounted so that it extends into the air stream (see Figure 6-42). Movement of the diaphragm closes an electrical switch, which permits the duct heater to turn on. When there is no airflow or low airflow, the switch will open and turn off the duct heater.

Other Automatic Controls

Figure 6-42 Air-pressure switch. (Courtesy Vulcan Radiator Co.)

Sail Switches

A sail switch consists of a steel or polyester film sail mounted on a switching device (see Figure 6-43). The combined unit is mounted so that the sail is located in an air duct. When the air velocity increases, the switch makes an electrical circuit. Figure 6-44 illus­trates the location of the sail switch in a gas control circuit.

Sail switches are used in forced warm-air heating systems, in air — conditioning systems, and with gas-fired unit heaters. Some sail switches are designed to provide on-off control of electronic air cleaners, odor-control systems, humidifiers, and other equipment that is energized when the fan is operating. In these applications,

Other Automatic Controls

Figure 6-43 Sail switches. (Courtesy Honeywell Tradeline Controls)

Other Automatic Controls

Figure 6-44 Location of the sail switch in a gas control circuit.

The sail switch completes a power circuit to auxiliary equipment, which can be wired independently of the blower motor. Sail switches are also used in electric heating systems to provide mini­mum airflow.

Other Switches and Relays

Other switches and relays used in heating and/or cooling systems include the following:

• Balancing relays Manual switches

• Auxiliary switches

Other Automatic ControlsA balancing relay (see Figure 6-45) is used with an electric motor that does not have an integral balancing relay. The relay is mounted separately from the motor so that vibrations will not affect it.

Other Automatic Controls

MOUNTING

LUGS

BALANCING

RELAY

TERMINAL

STRIP

COVER SCREWS (2)

Figure 6-45 Balancing relay. (Courtesy Honeywell Tradeline Controls)

A manual switch is used to manually perform one or more oper­ations in a heating and/or cooling installation. These switches are generally two-position types (on-off), although multiple-position switches are also available. An example of the latter would be the heat-off-cool switch on a heating and cooling thermostat. Examples of some special switching hookups in which manual switches are used are shown in Figures 6-46, 6-47, and 6-48. In each of these examples, a relay could have been used instead of the manual switch.

THERMOSTAT #1 THERMOSTAT #2

Other Automatic Controls

Figure 6-46 Manual switch (DPDT) used to transfer control of an electric motor from one thermostat to another.

(Courtesy Honeywell Tradeline Controls)

An auxiliary switch (see Figure 6-49) is used in conjunction with an electric motor to provide control of auxiliary equipment. This control functions as a direct extension of motor operation. The aux­iliary switch may be an integral part of the motor or an external unit fitted to the motor and adjusted to open or close at the desired point in the motor stroke (see Figure 6-50). As shown in Figure 6-51, the internal auxiliary switches are in a single-pole, double-throw (SPDT) configuration.

Sequence Controllers

A sequence controller (also referred to as a sequencer or step con­troller) is a device used to operate two or more electric switches in

Other Automatic Controls

Optional low limit not used in cool position.

Figure 6-47 Manual switch (DPDT) used for reversing control so that the same thermostat can be used for both heating and cooling.

(Courtesy Honeywell Tradeline Controls)

Predetermined sequence. This function is accomplished by means of a proportional electric or pneumatic operator.

Sequence controllers are most commonly used to provide sequenced starting of a number of electric heating elements or com­pressor motors. This prevents the massive drawing on the current that simultaneous starting would cause.

Sequence controllers are manufactured in a number of different sizes and capacities. The Honeywell S984 step controller, shown in Figure 6-52, provides up to 10 adjustable switches. It can also be used to control at least two other step controllers when greater switching capacity is required.

Each switch in the Honeywell step controller is operated by a cam mounted on the main shaft (see Figure 6-53). Adjustments of the step controller can be made by setting each of the switches to make or break a circuit at the correct time or angle in the stroke. The procedure for setting the switches in the Honeywell step con­troller is as follows:

1. Loosen the setscrew so that the cam assembly will turn freely on the motor shaft.

2. Run the step controller motor until the desired switch make point is reached. This will be determined by the time or degrees of camshaft rotation.

3. Turn the cam until the switch just makes and tighten the setscrew.

4. Run the step controller back to the desired switch break point.

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Figure 6-48 Manual switch (three-pole, double-throw) used to transfer thermostat control to one or the other of two motors.

(Courtesy Honeywell Tradeline Controls)

5. Loosen the lockscrew and turn the differential cam so that the switch will break at this point.

The instructions for adjusting the Honeywell step controller rec­ommended setting the make points of all the switches first and then setting all the break points. This procedure avoids unnecessary cycling of stages.

The sequence controller, shown in Figure 6-54, provides time­delay switching of up to eight electric heater banks and a fan or pump. When the thermostat calls for heat, it starts the low-voltage

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Figure 6-49 Electric motor with an integral auxiliary switch.

(Courtesy Honeywell Tradeline Controls)

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AUXILIARY

SWITCH

Figure 6-51 Internal auxiliary switches in a single-pole,

Double-throw (SPDT) configuration. (Courtesy Honeywell Tradeline Controls)

COMMON
TERMINAL

NORMALLY OPEN

TERMINAL

NORMALLY CLOSED TERMINAL

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RECYCLE

RELAY

 

BALANCING

(MOUSETRAP)

RELAY

 

DRIVE

MOTOR

 

SNAP SWITCHES (NUMBER DEPENDS ON MODEL)

 

LOW-VOLTAGE

TERMINAL

BOARD

 

SWITCHING CAMS

 

POWER-SUPPLY

TERMINAL

BOARD

 

FEED BACK POTENTIOMETER

 

Figure 6-52 Model S984 step controller. (Courtesy Honeywell Tradeline Controls)

 

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Sequence motor, which operates the rotating cams. The first cam locks in the motor circuit so that if the thermostat should break immediately, the motor will continue running, rotating all cams back into the starting position.

The second cam starts the fan. The remaining cams switch on the heater banks with a specific time delay between the energizing of each bank. The motor will stop running after 180°rotation as long as the thermostat calls for heat. The fan and all heater banks will

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Continue to be energized. When the thermostat is satisfied, the motor starts again and switches off the heater banks at the prede­termined time interval. The fan is deenergized, and the motor stops when the cams are back in the starting position.

Contactors

A contactor is essentially a switching relay device that functions as a primary control in a cooling system. Its operation is based on either magnetic or mercury-to-mercury contact action.

A magnetic contactor (see Figure 6-55) is similar in design and operating principle to a relay but is larger in size. In a mercury con­tactor (see Figure 6-56), the contacts are made and broken between two pools of mercury separated by a ceramic insulator. Mercury-to — mercury contact action results in a quieter operation than magnetic contactors can provide, but a mercury contactor has the disadvan­tage of being position sensitive. It must be mounted in an upright position.

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Figure 6-55 Magnetic contactors. (Courtesy HoneywellTradeline Controls)

A contactor is used for applications requiring heavy current, high voltage, or a large number of poles—in other words, applica­tions where the capacity of a relay would be inadequate.

In a cooling system, an electric-driven compressor motor may be cycled by a thermostat (either a low-voltage or line voltage type) or a low-pressure control. Because these controllers are usually unable to handle the high current drawn by the compressor motor,

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Figure 6-56 Mercury contactors. (Courtesy HoneywellTradeline Controls)

A contactor is installed between the thermostat or pressure control and the motor where it functions as a primary control. This contac­tor is the electrical contact switch to which the main electric power is supplied. When activated by the room thermostat, the contactor causes the compressor and condenser blower motor in an air — conditioning system to begin operating. Contactors are used to control all but the smallest compressor motors.

The size of the contactor selected for use in a cooling system will depend on such variables as the size and type of compressor motor and the auxiliary loads.

Both single-phase and three-phase motors are used in compres­sors. As shown in Table 6-1, the motor current (expressed in amperes) will vary according to the size of the compressor. Contactors are rated in amperes and should be selected to match the rating of the compressor motor. The number of pole contacts is also an important consideration in selecting a contactor. A one — or two-pole contactor is required for a single-phase compressor motor, and a three-pole contactor for a three-phase motor. Auxiliary poles may be used for interlock switching, fan loads, or crankcase heaters (see Figure 6-57).

Other Automatic Controls 277 Table 6-1 Compressor Size and Motor Current Ratings

Single-Phase Motor Three-Phase Motor

Compressor Size Motor Current Compressor Size Motor Current (tons) (amperes) (tons) (amperes)

2 TOC o "1-5" h z 18 3 18

3 2530 4 2530

4 3040 5 3040

5 3550 71/2 3550

(Courtesy Honeywell Tradeline Controls)

TO

THREE PHASE /TTN

POWER SUPPLY v!/

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1 Add disconnect means and overload protection as required. I Includes line-or low-voltage power supply and thermostat.

Figure 6-57 Typical hookup of a three-pole contactor.

(Courtesy Honeywell Tradeline Controls)

Troubleshooting Contactors

Always check the fuse box (or circuit breakers) first to make certain that the fuses are good and the switch is in the on position. If this is not the source of the problem, then set the thermostat and base to cooling position and check for continuity across contacts. If the cir­cuit is open, repair or replace the thermostat.

If the system fails to start and the contactor is open and buzzing, check the voltage at the contactor coil (see Figure 6-58). Normal voltage will be within plus or minus 10 percent of the rated coil volt­age. Subnormal voltage may be caused by an undersized transformer or low voltage at the supply side of the transformer. Thermostat wiring that is too long can also cause a subnormal voltage reading. A normal voltage reading under these circumstances (that is, an open, buzzing contactor) is generally caused by a tight or fouled contactor armature. The armature should be cleaned or the contactor replaced.

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(Courtesy Honeywell Tradeline Controls)

If the system compressor will not start and the contactor is open but not buzzing, the contactor coil may not be powered. A voltage and continuity check must be run to locate the problem. This is a more complicated procedure than the one described for an open, buzzing contactor.

The voltage to the contactor coil should be checked first (see Figure 6-58). If the voltage is normal (within plus or minus 10 percent of the rated voltage), the contactor should be replaced. If a zero voltage reading is obtained, check the voltage at the transformer secondary (see Figure 6-59). If a zero voltage reading is obtained, check the line voltage side of the transformer (see Figure 6-60). A normal supply voltage reading here indicates that the transformer is defective and should be replaced. A zero voltage reading, on the other hand, indi­cates a problem with the power supply. Check the fuses, circuit break­ers, line disconnect switch, and the power at the service entrance.

If the voltage checked at the transformer secondary (as shown in Figure 6-59) is normal, the control circuit to the contactor coil is open. With a continuity checker, jump across the terminals of the following controls:

• Low-pressure switch

• High-pressure switch

Other Automatic Controls

(Courtesy Honeywell Tradeline Controls)

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Figure 6-60 Checking voltage at the line side of the transformer.

(Courtesy Honeywell Tradeline Controls)

• Room thermostat

• Heating/cooling interlock switch Time-delay switch

Lockout relay

If the system starts with a particular control out of the circuit, the control is probably defective and needs to be replaced. If none of these controls is defective, check the circuits for broken wires or loose connections. An open internal thermostat or overload relay switch in the compressor is another possible cause.

If the compressor hums but will not start, and the contactor remains closed, the problem may be in the motor starting circuit.

Check the motor circuit wiring for loose or broken wires. If there is no problem with the wiring, check the starting capacitors and motor starter. A defective motor is also a possibility. If the motor starting circuit is not defective, check for abnormal system pressures. Another possible cause is a tight, stuck, or burned-out compressor.

If the contactor is closed and the compressor motor neither starts nor hums, check the continuity of the overload switch and the open compressor motor windings. Check also for broken or loose wiring. If none of these is the cause, the contacts in the contactor are probably burned. Replace the contacts.

Cleaning Contactors

Sometimes a contactor will fail to operate because a layer of dust and lint has accumulated on the electrical contacts. This dust and lint can be removed by placing a file card between the contacts, closing the contacts against the card, and sliding the card back and forth. This will usually clean the contacts. Do not use abrasive material to clean the contacts because this will scratch and possibly ruin the surface.

Replacing Contactors

A contactor contains a stationary contact that operates in conjunc­tion with a contact bar (see Figure 6-61). The contacts should be replaced if they show evidence of uneven wear.

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CONTACT

BAR

SWITCH

MOLDING

PRESSURE CONNECTOR FOR LINE-VOLTAGE WIRING

Always compare the ratings of the replacement contactor with the old one. They should at least be equal in rating. To be on the safe side, it is better to overrate a contactor and replace the old one with a contactor of slightly higher rating.

When replacing a contactor, make certain the terminal connec­tions of the new one fit the installation. The mounting holes and dimensions should also be compatible.

Some manufacturers provide replacement coils for their contac­tors. Never replace a contactor coil until you have located and cor­rected the cause of the original coil failure. If you fail to do this, the replacement coil will probably burn out, too.

Always disconnect the power supply before attempting to remove a contactor. Be sure to tag the wiring connections to the contact terminals as soon as you have removed the contactor. This will minimize the possibility of confusion when the replacement is installed.

Motor Starter

When an electric-driven compressor motor stalls or is overloaded, it draws current many times its full load rating. If the condition lasts any length of time, the motor windings overheat and a fire may start in the insulation. This will result in very expensive damage to the motor. One method of guarding against the occurrence of an overload condition is by installing a motor starter in the control cir­cuit (see Figure 6-62).

A motor starter consists of a contactor plus one or more over­load relays. Each overload relay consists of a bimetal contact in series with the motor contact coil and a heater in series with the compressor motor. The overload relay (or relays) disconnects the motor from the power supply when the motor temperature and/or the current drawn by the motor become excessive.

Overload Relay Heater

As shown in Figure 6-62, an overload relay consists of a bimetal contact in series with the motor coil, and a heater in series with the compressor motor. An overload relay heater is a small electric heat­ing device designed to work in conjunction with the bimetal con­tact. When the compressor motor becomes overloaded or stalled, the heavy continuous current through the heater causes the bimetal contact to bend until it opens the motor-starter coil circuit. This action stops the flow of current through the motor starter and results, in turn, in the opening of the load contacts to stop the flow of current to the compressor motor.

TRANSFORMER

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Figure 6-62 Motor starter in the control circuit.

(Courtesy Honeywell Tradeline Controls)

An overload relay heater must be accurately sized for the instal­lation. Generally the manufacturer of the cooling equipment will provide instructions for sizing overload relay heaters.

Inherent Protector

An inherent protector is another safety device used to protect an electric-driven compressor motor from overload damage. It accom­plishes this purpose by disconnecting the motor from the power supply when the motor temperature or current becomes excessive. Its function is similar to that of the overload relay.

An inherent protector is essentially a thermostat operated by the snap action of a bimetal disc. As shown in Figure 6-63, it consists of a heater, thermostatic disc, and contacts.

Other Automatic Controls

TO LINE

INHERENT PROTECTOR

CONTACTS

MOTOR WINDING (SINGLE PHASE)

THERMOSTATIC

DISC

TO LINE

подпись: to line

INHERENT PROTECTOR

подпись: inherent protector

CONTACTS

подпись: contacts

MOTOR WINDING (SINGLE PHASE)

подпись: motor winding (single phase)

1 THERMOSTATIC DISC

подпись: 1 thermostatic disc Other Automatic ControlsCLOSED POSITION OPEN POSITION

Figure 6-63 Inherent protector. (Coun. esy Honeywell Tradeline Controls)

In operation, the motor current flows through both the motor winding and the heater. When an overload condition occurs, the critical temperature level is reached in the motor winding at exactly the same time that the protector reaches its tripping point. When the temperature rises to the rating of the bimetal disc, the disc snaps open, reversing its curvature, and cuts off the flow of current to the motor. When the temperature drops, the action is reversed.

Inherent protectors are available for all sizes of single-phase compressor motors. For three-phase motors, they are available for motor sizes up to 1V2 hp.

Pilot Duty Motor Protector

Some manufacturers will install a pilot duty motor protector (or pilot duty thermostat) on the compressor motor to protect the motor from overcurrent damage. This device is a small temperature — sensitive thermostat mounted inside the compressor on the motor windings. If the motor windings become overheated, the pilot duty thermostat breaks the circuit to the contactor relay and shuts off the compressor motor.

The pilot duty thermostat is usually wired in the 24-volt control circuit. This can be accomplished by interrupting the transformer secondary of the control circuit. A wiring diagram of a pilot duty thermostat connected in a 24-volt circuit is shown in Figure 6-64. Pilot duty thermostat contacts can also interrupt 120-volt and 240- volt circuits.

IMPENDANCE

RELAY

CONTACTS

TRANSFORMER

R

3 (4;

2….

‘ °VERLOAD FAN RELAY JCONTAC> HPCO

TO FAN CONTROL

 

RB100, WA212 OR WB212

 

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TO THERMOSTAT SUBBASE

 

POWER (-’2’! SUPPLY^

 

INTERNAL

THERMOSTAT

 

I 1 Heinemann overload must be manual rest type if not used in rest circuit as shown.

 

Other Automatic Controls Other Automatic Controls

Provide disconnecting means and overload protection as required.

Figure 6-64 Pilot duty thermostat. (Courtesy Honeywell Tradeline Controls)

Capacitors

A capacitor is a device that provides the phase shift in the running and starting windings of an electric motor in order to increase the torque and efficiency of the motor-compressor assembly.

Capacitors are used in the following single-phase induction motors:

• Capacitor-start motors

• Permanent-split capacitor motors

• Capacitor-start, capacitor-run motors

A capacitor-start motor (see Figure 6-65) is used to power fans, blowers, and centrifugal pumps where constant-speed drive is

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Necessary. This motor develops high starting torque on the frictional horsepower ratings and moderate starting torque in the lower ratings.

In operation, an auxiliary winding is connected in series with a capacitor in the motor circuit. When the motor approaches running speed, a centrifugal switch cuts the capacitor and auxiliary winding out of the circuit.

A capacitor-start, capacitor-run motor (see Figure 6-66) also develops high starting torque. This is accomplished by employing a starting capacitor and a running capacitor. The starting capacitor gives good starting ability but is suited for short-time operation only. The starting capacitor is cut out of the circuit during the run­ning period. The running capacitor provides high efficiency at run­ning speed. Capacitor-start, capacitor-run motors are used to power compressors, reciprocating pumps, and similar types of equipment.

On some capacitor-start, capacitor-run motors, the starting capacitor may be cut out of the circuit with a centrifugal switch (see Figure 6-66). An alternative method of cutting out the starting capacitor is by using a back-emf relay (see Figure 6-67). Back-emf relays are usually used on hermetic (sealed) compressors where it is impractical to install a centrifugal switch. The coil of the back-emf relay is connected in parallel with the auxiliary winding. When the

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Figure 6-66 Capacitor-start, capacitor-run motor.

(Courtesy Honeywell Tradeline Controls)

Other Automatic Controls

RUNNING CAPACITOR

ROTOR STARTING

BACK EMF RELAY CONTACT (STARTING POSITION)

BACK EMF RELAY COIL

LINE

MAIN WINDING.

‘ ‘AUXILIARY WINDING

Motor approaches running speed, the relay pulls in, opening its contacts and taking the capacitor out of the circuit.

Troubleshooting Capacitors

A defective capacitor may be the cause of the following operating problems:

• Condenser fan will not run.

Condenser fan will run, but compressor will not start. Compressor hums but will not start.

A defective running (or run) capacitor is often the direct cause of the compressor motor cycling on overload. A defective capacitor can be removed and replaced in the field, but first it should be tested to make certain that the capacitor is actually the source of the problem. Before testing the capacitor, inspect it for leaking or swelling. Corrosion around the terminals and bulging caps are caused by leaking electrolytics. These symptoms are an indication of a defective capacitor that can be removed and replaced. There is obviously no need to test it.

Warning

Always completely discharge a capacitor before testing it. This must be done to avoid serious shock injury as well as to protect the testing equipment from potential damage.

Discharge the capacitor by connecting it to a 15,000-ohm and 2- watt resistor, and then disconnect the wires. Check for a ground in a motor-run capacitor by connecting an ohmmeter or test neon lamp in series with the capacitor and the metal part of the case for each capacitor. A ground is indicated if a continuity of circuit exists. Both terminals should be tested to ground, and this must be done with the capacitor disconnected from the compressor.

Replacing Capacitors

When putting a running (or run) capacitor back into a unit, check the capacitor terminals for a marked or identified terminal. The marked terminal may be indicated by a dab of solder, a paint mark, or a stamping on the case. This terminal must always be connected with the wire leading directly back to the contactor. If this capacitor should become defective by a ground, a fuse in the power circuit will blow. If the capacitor is not properly connected, a defect by ground will cause a flow of power through the start-and-run wind­ings before reaching a fuse and cause compressor damage.

High-Pressure Cutout Switch

A high-pressure cutout switch is a pressure-actuated refrigerant controller connected to the pilot (low) voltage circuit of the main contactor in a refrigeration system (see Figure 6-68). These switches are installed in all refrigerating units over 1 hp to provide protec­tion against dangerously high head pressures. These excessive and unsafe pressures develop as a result of a number of different abnor­mal operating conditions, including (1) air in the refrigerant lines, (2) excessive refrigerant charge, (3) dirty condenser, (4) faulty or inoperative condenser fan, and (5) insufficient water in a water — cooled condenser.

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Figure 6-68 High-pressure cutout switch.

(Courtesy Honeywell Tradeline Controls)

As shown in Figure 6-69, the high-pressure cutout switch is con­nected to the so-called high side of the system. When the head pres­sure exceeds the control setting, the high-pressure cutout switch opens the compressor power circuit and prevents excessive head pressures from building in the condenser by shutting off the com­pressor motor.

Other Automatic Controls

If automatic reset is used, the compressor motor will start again when the pressure returns to normal. More specifically, the contacts of the high-pressure cutout switch close automatically and reestab­lish the compressor power circuit when the head pressure drops the amount of control differential.

Automatic reset allows the equipment to cycle off the high — pressure control and continue to provide some degree of cooling while the high-pressure condition is being corrected. However, if the high-pressure condition is not corrected and the equipment con­tinues to cycle over an extended period of time, damage may be caused to the motor or compressor. For obvious reasons, this can­not occur when a manual reset high-pressure cutout switch is used.

Low-Pressure Cutout Switch

A low-pressure cutout switch is similar in design to a high-pressure switch (see previous section) except that it provides low-pressure cutout protection.

As shown in Figure 6-70, this switch is connected to the so — called low side of the compressor and is designed to open the com­pressor power circuit if the low-side pressure drops below a desired level. These excessively low pressures are usually caused by dirty filters, evaporator fan failure, a stuck damper, damper motor fail­ure, or some other interruption in the air supply.

HIGH-PRESSURE

Other Automatic Controls

Figure 6-70 Low-pressure cutout switch. (Courtesy Honeywell Tradeline Controls)

The cutout action of the switch prevents the temperatures in the evaporator from falling below the temperature at which frost would form on the coils. It also prevents the feeding of an excessive

Amount of liquid to the compressor. Low-pressure cutout switches may be designed to provide either manual or automatic reset.

Transformers

A transformer is an inductive stationary device designed to transfer electrical energy from one circuit to another. Each transformer con­tains a primary and secondary winding. A changing voltage applied to one of the windings induces a current to flow in the other wind­ing. In this manner, electrical energy is transferred from one circuit to another. Usually the changing voltage is applied to the primary winding and a current is induced in the secondary. The electrical energy may be transferred at the same voltage (a coupling trans­former), at a higher voltage (a step-up transformer), or at a lower voltage (a step-down transformer).

The transformers used in heating and cooling systems are step — down transformers. They are designed to reduce (step down) the higher line voltage power to the 24 to 30 volts required by low — voltage control circuits. Some models are designed to power ther­mostats, gas valves, and relays in HVAC 24-volt systems. Others are designed primarily for powering air-conditioning systems, although they can be used in other applications if they do not exceed the listed ratings. In most installations, the room thermostat is operated by a low-voltage (24-volt AC) circuit. Some gas valves are also operated by a low-voltage circuit.

All wiring connections to transformers must be done in accor­dance with the recommendations of the National Electrical Code. A single thin copper wire is used in a low-voltage circuit. It is com­monly (but not always) identified by its red and black insulation. High-voltage circuits use a larger-diameter wire that is commonly covered with white or black insulation.

Note

If you have any doubts about the voltage of the circuit, check it with a voltmeter before beginning any work. Keep in mind that a small number of transformer models are connected to high-voltage circuits, not the customary low-voltage ones.

Interconnected transformer secondaries are not permitted by the National Electrical Code. One method of avoiding the need for interconnecting transformers is by using a single transformer rated to carry both the heating and cooling load. Using a thermostat and subbase combination with isolated heating and cooling circuits is also an acceptable method. Still another successful method utilizes an isolating relay to isolate the heating power supply from the cool­ing power supply.

Sizing Transformers

Transformers are not 100 percent efficient. There will generally be some loss of energy between the primary and secondary coils. In any event, the secondary coil must have enough remaining energy to drive the load connected to it.

When the equipment in a heating and/or cooling system is not adequately powered, check the transformer primary and secondary voltages. If these readings are within plus or minus 10 percent of the rated voltage and there is no problem with the wiring, the trans­former may not be large enough for the system. A transformer too small for the system can be a very serious matter because it will sup­ply abnormally low voltage to the control circuit. As a result, con­tactors or motor starters will not operate properly, and eventually the compressor may suffer damage.

When replacing a transformer, always select one that is the same size or larger than the one being replaced. For new installations, follow the equipment manufacturer’s recommendations.

The capabilities of a transformer are described by its electrical rating. This information will include the primary voltage and fre­quency, the open-circuit secondary voltage, and the load rating in volt-amperes (VA).

The Class 2 transformers used in low-voltage control circuits have a maximum load rating of 100 VA and a maximum open — circuit secondary voltage of 30 volts. The secondary current must also be limited. This can be accomplished by using an energy-limiting transformer or by adding a 3.2-ampere (or less) fuse in the secondary. In the latter case, the maximum load rating of a typical 4-volt Class 2 transformer is 77 VA (24 volts X 3.2 amperes = 77 VA).

Installing Transformers

Always closely follow the transformer manufacturer’s installation instructions, because they will vary depending on the model and the specific application. The following guidelines apply to most trans­formers:

Caution

Never attempt to remove or install a transformer unless you are a qualified electrician or have the required training and experi­ence. Improper removal or installation can result in damaged equipment and/or serious electric shock.

Note

The following guidelines cannot replace the specific step-by-step instructions in the transformer manufacturer’s installation instructions.

1. Disconnect the power supply to prevent equipment damage or electric shock before attempting to remove or install a transformer.

2. Separate and tape each exposed, unused lead wire. Note: All wiring must comply with local electrical codes and ordinances.

3. Do NOT short the transformer secondary terminals or you may burn out the overload protection.

4. Check the specification section of the transformer for lead wire color-coding (see Figure 6-71).

5. Connect the primary lead wires to the line voltage power supply.

6. Connect the transformer secondary to the 24-VAC control system. If the transformer model has a primary or secondary conduit spud, connect the wires first and then screw the con­duit onto the spud.

Other Automatic Controls

Other Automatic Controls

Other Automatic Controls

7. Check the secondary voltage before connecting the trans­former to the power supply.

8. Turn on the power supply and operate the system for one or two complete cycles.

Control Panels

A control panel combines many of the heating and cooling controls into a single, unified preassembled package. As a result, field wiring and control troubleshooting are greatly simplified.

A complete line of standard control panels is available for a vari­ety of different functions. Depending on the requirements of the installation, it is possible to obtain a variety of different combina­tions of the following control components in a panel:

1. Transformer.

2. Line voltage and low-voltage wiring terminals.

3. High- or low-pressure cutout.

4. Two-stage cooling time-delay circuit.

5. Compressor contactor or motor starter.

6. Auxiliary relays.

The internal view of the Honeywell heating-cooling panel that is illustrated in Figure 6-72 shows the typical arrangement of these control components.

Other Automatic Controls

QUICK-CONNECTS

Figure 6-72 Heating-cooling control. (Courtesy Honeywell Tradeline Controls)

Posted in Audel HVAC Fundamentals Volume 2 Heating System Components, Gas and Oil Burners, and Automatic Controls


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