Starting the fan and motor

NiL

N,

Equ 13.1

Where:

Pi

Pico

Ni

N100

Power at any instant power at full speed speed at any instant full speed

F

For vee belt-driven fans there will be additional small power losses in the bearings and belt (varying directly as the speed), but for the following analysis, these are ignored.

It is usual for electric motor manufacturers to produce torque speed curves. It is therefore necessary to calculate the torque required by the fan.

I*-

Figure 13.14 Comparison of space required for an axial flow fan fitted with an “inside-out” and conventional motor respectively

It will be seen that this is a square relationship. We may there­fore draw a curve of torque versus speed. This starts at the ori­gin, for when N, = 0 then T = 0. N100 and T100 will be the full speed and corresponding torque taken by the fan under the stated conditions of gas air density and point of operation (damper closure etc). In fact, with a fan impeller mounted on a shaft running in bearings, there will be a small amount of torque at the instant of starting. This is due to the “stiction” in the bear­ings and is known as the “break away torque”. It is only of any significance with sleeve bearings and again will be ignored in the present analysis.

Figure 13.15 View of forward curved centrifugal fan fitted with “inside-out" motor Courtesy of PM°DM Precision Motors Deutsche Minebea GmbH

% Full-load speed Figure 13.17 Torque available for acceleration

Average torque available for acceleration (average of all ordi­nates taken over very small Increments of speed).

In the examples which follow “f is approximated for some of the most popular types of motor and starter. However, there is no substitute for a detailed analysis when actual fan and motor torque/speed curves are drawn to scale on the same base. This will enable “f to be accurately assessed.

The time allowablefor starting is dependent on a number of fac­tors. Acceleration produces additional stresses in the fan im­peller and shaft but these are not usually of significance. More important are the effects of higher motor winding temperatures, suitability of starter overload relays, and the ability of power lines to accept the additional current. Usually a time of around 18 seconds is therefore recommended, but this may not be achieved with very large units. The whole installation must then be discussed between fan, motor, and starter manufacturers to achieve the best solution.

To assist in the calculation of these times, it is necessary to have accurate values of the inertia of both motors and fans. However, typical values are given in Tables 13.2,13.3 and 13.4, which may be used for initial calculations at the project stage. They should be replaced by actual values, once the fan and motor manufacturers have been selected.

In most cases the power absorbed by the fan will be within a small percentage of the motor installed power. Assuming them to be equal, at this stage of the analysis, we may then plot curves for the motor and fan. The various types of motor and starter may now be considered and factors “f” determined to give approximate run up times:

Direct-on-line (DOL) induction motor

This method of starting is usually employed up to about 7.5 kW, and for motors of this size the torque/speed characteristic is generally as shown in Figure. 13.15. As may be seen the avail­able torque varies from 200% to 0% of the motor full-load

% Full-load speed

Equ 13.10

Equ 13.11

100

Equ 13.12

40 60

% Full-load speed

20

80

Slip ring stator rotor

Figure 13.21 Induction motor characteristics, auto-transformer starting

Slip-ring motors/stator-rotor starting

This is one of the most satisfactory methods of fan starting since by inserting resistance in the rotor circuit, the torque char­acteristic is arranged such that maximum is available when re­quired. Figure 13.22 shows a higher torque is available than in most other cases. The correction factor may be as low as 0.4 although 0.5 is a reasonable figure to use.

Equ 13.13

In all cases it is good practice to limit the value of t to about 18 seconds. The value of Pf to insert in the formula is that relating to the conditions of start up.

It is important to note that these approximate formulae make the assumption that the fan absorbed power and the motor rat­ing are almost equal and certainly within 10% of each other. If a larger motor is installed then this will reduce the starting time. Strictly speaking a new correction factor should be assessed. However, an indication of the starting time, likely to result, may be obtained by the use of the graph in Figure 13.23. Simply by multiplying the time calculated by the use of equations 13.10 to 13.13 by the factor kT, the reduced time may be calculated.

% Full-load speed Figure 13.22 Slip-ring motor characteristics, stator-rotor starting

% Full-load speed

Figure 13.23 Indication of reduction in starting time

Correct voltage selection is also important, and care should be taken to ensure that the motor is rated at the line voltage.

For example, a motor wound for 440 volts connected to a 380

^38qA2

Volt supply will develop only I I x100, i. e. 75% of normal

Torque, but more important, in star connection, the torque avail­able for starting the fan may be as low as 20% of the direct on-line value.

Summary:

From the above remarks it can be seen that a general formula may be derived to calculate the run-up time of any AC motor, i. e.:

Equ 13.9

.-.kT =0.61

•• Used =18×0.61 = 11 seconds