Types of electric motor

It is not the intention of this Chapter to be a comprehensive guide to the various types of electric motor. Guide to European Electric Motors, Drives and Controls gives a detailed descrip­tion of the whole electric motor market and the variants avail­able. Performance characteristics, design features and acces­sories such as starters are all described.

However a brief resumй of the most popular types used with fans is included for completeness.

Motors for alternating current fall into two main groups:

• induction motors

• all other types

From the point of view of characteristics, induction motors are similar to direct current (DC) shunt wound motors and are said to possess shunt characteristics. They are inherently constant speed machines, which run at just a little lower than synchro­nous speed for the supply frequency and the number of poles on the field of the machine. The difference between the actual running speed and synchronous speed is known as the “slip”. A further rather important point about induction motors is that al­though poly-phase machines will start without assistance, sin­gle-phase induction motors are inherently non-self-starting. This is the reason for the many different types of single-phase motor.

The relationship between poles and speeds of alternating cur­rent motors is given in Table 13.2.

Frequency

40 cycles

50 cycles

60 cycles

No. of Poles

Speed — r. p.m.

Speed — r. p.m.

Speed — r. p.m.

Synchro­

Nous

Nominal

Approx.

Synchro­

Nous

Nominal

Approx.

Synchro­

Nous

Nominal

Approx.

2

2400

2240

3000

2800

3600

3350

4

1200

1120

1500

1400

1800

1670

6

800

720

1000

900

1200

1080

8

600

560

750

700

900

830

10

480

455

600

570

720

685

12

400

375

500

470

600

565

14

343

320

430

400

514

480

16

300

290

375

360

450

430

Table 13.2 Relationship between poles and speeds of alternating current motors

Apart from synchronous motors (which run exactly at synchro­nous speed) and induction motors, all other types of AC ma­chines may be said to possess series characteristics and are not limited to speeds dependent on the supply frequency.

However, the majority of AC fan drives are performed by induc­tion motors, as they are more reliable and generally require less attention than other types of AC machines. Invariably they are also less expensive. Any speed tolerances quoted in this sec­tion for induction motors assume exact maintenance of supply frequencies, and since supply systems are often heavily loaded an additional tolerance of plus or minus 4% may easily arise from this cause.

3-phase motors

Squirrel cage induction motors

These consist of a stator wound normally for 3-phase supply and with a rotor of squirrel cage construction, (see Figure 13.1). They are essentially a constant speed drive, but motors spe­cially designed for fan drives may be arranged to give speed regulation of up to about 50% of normal speed by means of volt­age reduction. Pole-changing motors are available giving two speeds in the ratio of 2 to 1 by re-connection of the stator wind­ings. Alternatively, multiple-wound stators provide two or occa­sionally more speeds in any ratio.

This type may be purchased in sizes up to quite large powers. Low kilowatt machines, up to about 4 kW may generally be

Types of electric motor

Types of electric motor

Figure 13.1 3-phase AC squirrel-cage induction motor

Started direct-on-line. For greater powers the following two main methods are used for starting:

1. The voltage is reduced by means of a resistance or auto-transformer (usually wound in open delta for econ­omy). The machine is generally started on light load, as the starting torque is reduced when the voltage is reduced.

2. Star-delta starting is used quite often on moderate power. This is achieved by arranging that the motor has the end connections of each winding brought out to six terminals. The machine is designed to run normally with its winding connected in delta, that is, with each winding connected to the full supply voltage. During the starting period, how­ever, the windings are connected in star by means of a special switch, which in effect reduces the voltage across each winding to about 57% of the supply voltage and con­sequently reduces the starting current drawn from the mains to one-third of that for direct starting. When the ma­chine is running close to full speed the switch is operated and the machine is delta-connected for running, thus putt­ing full voltage on each of the windings. There is no radio interference from this type of machine.

Important note: Induction motors may also be used as vari­able speed machines by altering the frequency of the AC sup­ply. This is best achieved by the use of an inverter, a method which has now received universal acceptance. The method is discussed more fully in Chapter 5.

Typical characteristics of squirrel-cage induction motors:

KW range 0.25 to 100

Starting torque 150% to 250% of full load torque

Starting current 6 to 8 times full load current

Power factor 0.8 to 0.9

Speed tolerance ± 5% for small sizes and low speeds

± 2% for larger sizes

Wound-rotor induction motors

Types of electric motor Types of electric motor

Figure 13.3 3-phase AC synchronous induction motor

подпись: 
figure 13.3 3-phase ac synchronous induction motor

Figure 13.2 3-phase AC wound-rotor induction motor

подпись: 
figure 13.2 3-phase ac wound-rotor induction motor
These machines are different from the squirrel cage induction motor in that the rotor is wound, and the end of the windings brought out to slip rings. (See Figure 13.2.) They are inherently speed regulating machines, this being achieved by adding re­sistance to the rotor circuit via the slip rings. They make excel­lent fan drives, particularly when volume regulation is required, the range of speeds obtainable being virtually from standstill to
full speed of the machine. However, in order to keep the speed regulator to economical proportions, it is usual to regulate from full speed down to about 50% of full speed. They are available in any size, though machines of larger powers are more com­mon because of the comparatively high expense of the lower power machine compared with other types of AC motor of similar horsepower.

In order to limit the current on starting, the machines are usually arranged to start at the lowest speed position of the speed regu­lator and interlocks are normally fitted to ensure that this oc­curs. Starting currents may be kept down to 1.5 times full load current. There is generally no radio interference from these machines, but some may be experienced if the slip rings and collectors are allowed to get into poor condition.

Typical characteristics of wound-rotor induction motors:

KW range 5 to 1000 and over

Starting torque * 150% to 300% of full load torque

Starting current * 1.5 to 3 times full load current

Power factor 0.7 to 0.9 according to degree of

Speed regulation

Speed tolerance ± 2% at full speed

*at lowest speed

Synchronous induction motors

3 to 2000

150% of full load torque 1.5 times full load current 0.8 to 1.0

подпись: 3 to 2000
150% of full load torque 1.5 times full load current 0.8 to 1.0
Synchronous motors are rarely used for fan drives, except where power factor correction is necessary for a large continu­ous-running fan installation. The leading power factor current drawn by the synchronous motor compensates for the low power factor of other installed electrical equipment. Synchro­nous motors usually have field supplied by AC, while the rotor is supplied by DC generated by an excitor mounted on the same shaft, (see Figure 13.3).

They are inherently non-self-starting and must be run up to speed on light load either by means of an auxiliary motor or, as is more common by means of a squirrel cage or other windings constructed in the pole faces of the rotor. In the latter case the machines are started up under light load as induction motors, after which the rotor DC supply is switched on and the ma­chines have sufficient torque to pull themselves into synchro­nous speed. The windings in the pole faces of the rotorthen act as damping windings to prevent hunting with load fluctuations.

Synchronous motors are also made in very small sizes with per­manent magnetic rotors, and these are becoming popular for fan applications.

The DC excitor emits continuous radio interference and provi­sion for suppression should always be installed.

Typical characteristics of synchronous induction motors

KW range 15 to 100 and over

Starting torque 50% to 150% of full load torque

Starting current 2 to 5 times full load current

Power factor 1.0 to almost anything leading

Polyphase AC commutator motors

It is probable that the majority of polyphase commutator motors are built for specific purposes rather than for general industrial drives. A well-known type of commutator motor, which has been used as a fan drive where speed regulation with minimum loss is required, is the Schrдge motor. It comprises a rotor with a primary winding, connected to the supply by slip rings, and a low voltage commutator winding in the same slots. The sec­ondary windings on the stator (one for each phase) are fed from the commutator by means of brushes whose positions may be varied simultaneously, giving speed variation above and below synchronous speed. It has two main advantages. At a given brush setting it possesses shunt characteristics, i. e. speed var­ies very little with torque variation. Also, losses due to speed regulation are low.

Provision should be made for suppression of radio interfer­ence.

Typical characteristics of the Schrдge motor:

KW range Starting torque*

Starting current’

Power factor Speed tolerance ± 5%

* When started at lowest speed

Types of electric motor Types of electric motor Types of electric motor

Figure 13.5 Single phase AC (or AC/DC) series motor

подпись: 
figure 13.5 single phase ac (or ac/dc) series motor
Types of electric motor

Figure 13.4 Single-phase AC motor

подпись: figure 13.4 single-phase ac motorSingle-phase AC motors

KW range Starting torque Starting current Power factor Speed tolerance

подпись: kw range starting torque starting current power factor speed toleranceThese machines have a single field winding and a wound rotor with short-circuited brushes. (See Figure 13.4.) The speed and direction of rotation are dependent on the position of the brush axis. They are sometimes used for fan drives and are available in low power sizes. Low-power machines may be started direct on to the supply, whilst higher-powered machines are arranged to have the voltage reduced on starting by means of auto-trans­former, series choke, or series resistance.

In some machines starting and speed regulation are obtained by moving the position of the brushes. The starting torque is quite high. The machines emit continuous radio interference, which should be suppressed.

Typical characteristics of AC range motors:

KW range Starting torque Starting current Power factor Speed tolerance

подпись: kw range starting torque starting current power factor speed tolerance0.33 to 7.5

300% to 400% of full load torque

3 to 4 times full load current 0.7 to 0.8

Below 0.33 h. p. per 1000 r. p.m ± 20% above 0.33 h. p. per 1000 r. p.m ± 15%

Types of electric motor

In fractional kW sizes these machines are invariably known as universal motors, as they may be run on both alternating or di­rect current. Their speed torque characteristics are generally similar to those of DC series motors, but the same machine will run at a higher speed on DC than on the same voltage AC (see Figure 13.5).

They are sometimes used for fan drives where speeds in ex­cess of maximum AC synchronous speeds are required, and for AC/ DC supplies where it is not essential to have the same speed on both supplies. Alternatively they run on a different voltage on either supply. Speed regulation on fan loads may be obtained by means of a series resistance.

At speeds below about 5000 r. p.m. commutation is generally poor on AC For this reason these machines are usually made only in fractional power sizes and high speeds. They are invari-

подпись: in fractional kw sizes these machines are invariably known as universal motors, as they may be run on both alternating or di-rect current. their speed torque characteristics are generally similar to those of dc series motors, but the same machine will run at a higher speed on dc than on the same voltage ac (see figure 13.5).
they are sometimes used for fan drives where speeds in excess of maximum ac synchronous speeds are required, and for ac/ dc supplies where it is not essential to have the same speed on both supplies. alternatively they run on a different voltage on either supply. speed regulation on fan loads may be obtained by means of a series resistance.
at speeds below about 5000 r.p.m. commutation is generally poor on ac for this reason these machines are usually made only in fractional power sizes and high speeds. they are invari-
AC series motors ably short-time time rated. Starting is usually direct on line, and the starting torque is high.

Continuous radio interference is emitted and suppression de­vices should therefore be fitted.

Typical characteristics of AC series motors:

0.01 to 0.4

300% to 500% of full load torque 5 to 9 times full load current 0.5 to 0.7

Below 0.25 kW per 1000 r. p.m. ± 20%

Above 0.25 kW per 1000 r. p.m. ± 15%

13.4.3.1 Single-phase AC capacitor-start, capacitor-run motors

These motors have a stator with two windings, the phase of one of them being practically 90° (electrical) different from the

Types of electric motor

Phase of the other. This is achieved by the insertion of a capaci­tor (condenser) permanently in series with one of the windings. The rotor is of squirrel cage construction, (see Figure 13.6). The performance of these machines can be quite high, ap­proaching that of a true 2-phase motor. The power factor is high and the motor forms an excellent fan drive.

A limited range of speed variation on fan loads only may be ob­tained with a specially designed machine of this type. By regu­lating the voltage to the stator by means of an auto-transformer or series choke, speed reductions of about 50% of nominal speed may be achieved. Two speeds may be obtained by means of double winding or pole changing. The machine is nor­mally made in fractional and low power sizes, although ma­chines up to 7.5 kW have been produced.

KW range Starting torque Starting current Power factor

подпись: kw range starting torque starting current power factorReversal is quite easily obtained by reversing the connections of one of the stator windings. In low power sizes the machine is usually started direct on to the supply. A compromise must be made by the designer in the choice of capacitor to permit both starting and running of the machine on a single capacitor, which gives a lower starting torque than is ideally obtainable. Higher power machines are usually fitted with an extra capacitor, which is used during the starting period only, giving additional starting torque. When the machine is up to running speed this capacitor is switched out and the machine runs on the remaining capaci­tor, which has been chosen for optimum performance at run­ning speed. The machine with two capacitors is not suitable for speed regulation. Capacitors must be extremely reliable and are usually of a high quality paper insulated type. In the case of high power machines it may also be necessary to reduce the voltage on starting by means of an auto-transformer, series choke, or series resistance. There is no radio interference from this type of machine.

Typical characteristics of capacitor-start, capacitor-run motors:

KW range 0.33 to 7.5

Starting torque 200% to 300% of full load torque (some special permanent capacitor types for fan drives have only 75%)

Starting current 1.5 to 2.5 times full load current

Power factor 0.95

Speed tolerance ± 5% for small sizes and low speeds ±

Types of electric motor

Starting

Capacitor

подпись: starting
capacitor

Starting

Winding

подпись: starting
winding

Starting switch

подпись: starting switch
 
Types of electric motor Types of electric motor2% for larger sizes

Single-phase AC capacitor-start, induction-run motors

These are generally similar to capacitor-run motors, but the ca­pacitor and additional winding are used only for starting, after which they are cut out at speed by means of a relay or switch, usually a centrifugal type mounted on the motor shaft, (see Fig­ure 13.7). They then run, as single-phase induction motors.

The capacitor is usually a short-time-rated electrolytic type. The motor is normally a constant speed machine. Reversal may be achieved by reversing the connections of the starting winding. The starting torque is quite high with correspondingly high starting current. These motors are less suitable for fan drives than the capacitor-start, capacitor-run type. They cannot be regulated, since speed reduction would cause the re-con­nection of the starting condenser and rapid burn-out of the ma­chine. They have an inferior efficiency and power factor, while the high starting torque provided is unnecessary for fan drives. No continuous radio interference is emitted, but clicking will be heard when the centrifugal switch operates.

Typical characteristics of capacitor-start, induction-run motors:

0.1 to 1

200% to 300% of full load torque

3 to 5 times full load current 0.65 to 0.75

Speed tolerance ± 5% for small sizes and low speeds ± 2% for larger sizes

Single-phase AC split phase motors

In the case of the two types of motor, just described, a capacitor is employed to achieve electrical angular displacement be­tween the magnetic fields of the two windings, producing ap­proximately two-phase conditions. In the split phase machine there are again two windings, but the displacement is achieved either by inserting resistance in series with the starting winding, or by so constructing the starting winding to give a higher ratio of resistance to reactance than the main winding, (see Figure

13.8).

Either method creates a displacement of phases between the fields of each winding sufficient to start the machine. When the motors have attained normal speed, the starting winding is cut out by a switch which may be operated manually, by a relay con­
trolled by the main winding current, or more commonly by a centrifugal switch mounted on the shaft. The motors then run as single-phase induction motors. These machines have the same disadvantage for fan drives as the capacitor-start, induction-run type.

Two speeds may be obtained by either double winding or pole changing. Reversal is possible by reversing the connections the starting winding. They are made only in fractional sizes and are suitable for low powerfan drives. They are started direct on supply. There will be no continuous radio interference, but clicks will be heard when the centrifugal switches operates.

Typical characteristics of split phase induction motors:

0.

KW range Starting torque Starting current Power factor

подпись: kw range starting torque starting current power factor03 to 0.25

100% to 200% of full load torque

4 to 6 times full load current

0. 5 to 0.7

Speed tolerance ± 5% for small sizes and low speeds

+ 2% for larger sizes

Single-phase shaded pole motors

These are the simplest form of self-starting, single-phase in­duction motors. They have a squirrel cage rotor and the field is so constructed as to have an offset short-circuited coil produc­ing a magnetic field displaced electrically from the main field, (see Figure 13.9). Compared with other types of single-phase motor the performance is poor and power factor very low, but this is counter balanced by cheapness and robustness. As losses are normally quite high it is generally impossible to damage the machine by overload.

Types of electric motor

Power factor 0.4 to 0.6

Speed tolerance ± 5% for small sizes and low speeds

Single-phase repulsion-start induction motors

These machines have a single field winding and are similar to the repulsion motor in that they have a wound rotor and com­mutator, (see Figure 13.10). They are started as a repulsion motor, that is, the brushes are short circuited. When running speed has been attained a centrifugal switch operates a short-circuiting ring making contact with all of the commutator segments. The machines then run as single-phase induction motors. They may be reversed at rest by altering the brush position.

Types of electric motor

Shorting ring

Types of electric motor

Figure 13.10 Single phase AC repulsion-start induction motor

SHAPE \* MERGEFORMAT Types of electric motor

KW range Starting torque Starting current Power factor Speed tolerance

подпись: kw range starting torque starting current power factor speed tolerance

Types of electric motor

Figure 13.9 Single phase AC shaded pole motor

The speed may be regulated on fan loads only from full speed to 50% of full speed by voltage reduction. The machines are essentially non-reversing. Their starting torque is very low. They are a very popular drive for small fans requiring powers not exceeding 1/50 horsepower and may be started direct on the supply. There is no radio interference from these motors.

Typical characteristics of shaded pole induction motors:

KW range 0002 to 0.15

Starting torque 50% to 150% of full load torque

Starting current 10.5 to 2 times full load current

Repulsion-start, induction-run motors are not very suitable for fan drives, as they are essentially constant speed machines, and the high starting torque is not required. However, they are sometimes the only available motors in the larger sizes for use on single-phase supplies. They emit continuous radio interfer­ence during the starting period, but none when running at speed as induction motors.

Typical characteristics of repulsion-start induction motors:

0. 2 to 3.5

300% to 500% of full load torque

4 to 6 times fun load current

0. 7 to 0.8

± 5% for small sizes and low speeds ± 2% for larger sizes

Current (DC) motors

Series wound motors

These motors are eminently suitable for use as direct fan drives as the speed of the motor will adjust itself until the motor output balances the fan load, (see Figure 13.11). They are quite sim­ple to speed regulate, but where the full speed power exceeds 1 kW, the regulators tend to be rather bulky and the electrical losses in the regulator rather high when the fan is being regu-

Types of electric motor

Figure 13.11 DC series wound motor

Starter and Speod regulator

подпись: starter and speod regulator Types of electric motor Types of electric motorLated. Series motors should not be used on indirect fan drives because if the load is disconnected, for example through belt failure, the speed will rise to a dangerous level. Reversal may be obtained by reversing the connections of the armature.

The starting torque of these motors is high. When the machine is connected directly to the supply the starting current is of the order of 5 to 8 times full load current. With large motors this may be higher than the permissible current allowed by the authori­ties; in that case a controller is used whose function is to limit the normally high starting current. As the same current passes through the field and armature, a series resistance will serve to reduce the rating of the motors on starting and so reduce the current consumed. This resistance is made variable so that it can be gradually reduced as the machines gather speed. Con­trol is generally by hand, but automatic controllers are pro­duced. The starting current with a controller is usually limited to

1.5 times full load current.

These machines emit continuous radio interference and provi­sion should always be made for suppression. A tolerance of plus or minus 10% on speed is normally to be expected from se­ries wound fan motors, rising to plus or minus 20% for the fractional powered versions.

Shunt wound motors

Shunt wound motors are essentially for constant speed, al­though speed regulation is possible by adjusting the strength of the field. In this case the frame would be larger than would be necessary with a constant speed machine of the same power. These motors are suitable for a constant speed drive of any horsepower and may be reversed, if suitably designed, by re­versing the connections to the armature. The starting torque of these motors is not as high as that of a series wound motor.

A starter is usually necessary to avoid instability during the starting period, (see Figure 13.12). This starter is arranged to limit the starting current to about 1.5 times full load current and to ensure starting on full field if the motor is of the shunt field regulating type. The starting resistance in this case is in series with the armature only while the field receives full supply volt­age. Starting is usually carried out manually, although auto­matic starters are available.

Types of electric motor

Figure 13.12 DC shunt motor

Radio interference is continuous and provision should always be made for suppression. Normal tolerances on speed to be expected in the manufacture of these machines are as follows:

Below 2 kW per 1000 r. p.m. plus or minus 10%

Between 2 & 7.5 kW per 1000 r. p.m. plus or minus 7.5%

Over 7.5 kW per 1000 r. p.m. plus or minus 5%

DC compound wound motors

Compound wound motors may be designed to exhibit charac­teristics ranging from those of the series machine to those of the shunt machine. When used for fan drives the best type is probably one, which, whilst exhibiting characteristics similar to those of a series machine, is sufficiently compounded to pre­vent dangerously high speeds on light load. Although suitable for power, they are normally used where drives of 1.5kW or above are required.

Speed regulator

Types of electric motor

Speed regulation is usually achieved by reducing the strength of the shunt field, (see Figure 13.13). As in the case of shunt motors, the frame for a regulating machine would be larger than for that of a constant speed machine of the same power. If suit­ably designed the machine may be reversed by reversing the connections to the armature.

Figure 13.16 Cross-sectional view of “inside-out” motor fitted to forward bladed impeller

Courtesy of PM°DM Precision Motors Deutsche Minebea GmbH

подпись: 
figure 13.16 cross-sectional view of “inside-out” motor fitted to forward bladed impeller
courtesy of pm°dm precision motors deutsche minebea gmbh
The method used to start a compound wound DC motor is to use a variable resistance in series with the armature and series field. The shunt field is given the full supply voltage and exer­cises a retarding influence on both speed and current. Starting gear is generally designed to limit the starting current to about 11/2 times the full load current.

Radio interference is continuous and provision should always be made for suppression. Normal tolerances on speed to be expected in the manufacture of these machines are as follows:

Below 2kW per 1000 r. p.m. plus or minus 10%

Between 2 & 7.5 kW per 1000 r. p.m. plus or minus 7.5%

Over 7.5kW per 1000 r. p.m. plus or minus 5%

“Inside-out” motors

Single-phase as well as 3-phase induction motors can be built as conventional inner rotor motors or as “inside-out” external rotor motors. For fan application, an external rotor motor, in which the cowl-shaped rotor revolves around the inner stator wound with copper wire, is especially advantageous. The short length of the winding head enables space-saving design and reduced copper losses. In addition, such motors are very com­pact because of the bearing system (sintered sleeve bearings or precision ball bearings) integrated into the stator’s interior. The motor installed inside an impeller results in a fan unit re­quiring minimum space. The unique integration of the motor curved

And the impeller permits precise balancing which guarantees low loads to the bearing system. The motor is positioned di­rectly in the air stream, so the very efficient cooling extends life­time expectancy.

Figure 13.14 shows the space saving possible for an axial flow fan, whilst Figures 13.15 and 13.16 show this motor variant ap­plied to a small forward curved bladed centrifugal fan.

Posted in Fans Ventilation A Practical Guide


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