Page 110 - Industrial Power Engineering and Applications Handbook

Starting and control of slip-ring induction motors 5/91
5.2.5 Temperature rise limits = 0.553 R
As in NEMNICS-2-2 13. the exposed conductor resistance
750 '2
unit:, can attain a temperature rise up to 375°C. whereas and for T,,, (120%) R2, = >3 x 435 x 1.2 (1 etc
when they are enclosed in a housing, the temperature
rise must be restricted to 350°C. Accordingly, the Determine slip S, to give the same value of R,, as above.
temperature rise of the air in the vicinity of the housing, Then for this slip, determine one step on the torque curve.
when measui-ed at a gap of 1 inch from the ericloaure, Calculate for this slip S,, R,,, etc. as shown in Table 5.2.
should not exceed 175°C. Since the torque at synchronous speed is zero and the end
portion of a torque curve is almost a straight line, the same
results are obtained if a straight line is drawn from starting
Corollar;i, point 'a' to zero torque 'A'. Where it meets the T,,, line
Very high temperature-rise permissible limits of resistance determines one step. Here again, the torque goes up to T,,,
unit:, render them unsuitable for installations which are by another resistance value and this can also be joined to A.
Continuation of such a procedure gives a point where it meets
tire-prone. such as pulp and paper industries, chemical the motor curve. To reduce the number of steps, slight variations
industries, refineries, textile mills, etc. For specific in T,,, and T,,, can be made as shown. Note that complete
applications and surroundings, however, resistance design resistance cannot be removed at step f otherwise the torque
can be altered (derated) to restrict the temperature rise to will jump to about 212%, which may not be desirable. Thus,
within desirable limits. the total number of steps amount to seven (resistance
Moreover, high-temperature variation may result in segments six), which is reasonable.
We must also check whether the starting time of the motor
large variations in the resistance of the grid and may with this profile of starting torque would be safe for the motor
vary the performance of a variable-speed drive if care is to pick up to the rated speed. Considering the same data as
not exercised in selecting a proper alloy of the resistance. for Example 7.1, GD: = 1866 kgm'
An alloy such as aluminium-chrome steel should be
preferred when it is required to perform a speed control 450 x 974 o,5
or a speed variation. as discussed above. and T, = 980
= 223.62 mkg
5.3 Hypothetical procedure to 1866 x 980
calculate the rotor resistance :. Accelerating time t, = 375 x 223.62
= 21.8 seconds
The tollowing is il nioi-e appropriate method to dctcrrriine This seems to be high and must be checked with the
the number of steps and resistance of each step of the thermal withstand time of the motor (Section 3.5).
resistance grid, making use of only rotor data and desired The upper limit of the starting torque chosen at 180%
limits of T,,:,, and T,,,, as ascertained from the available appears to be too low and can be raised to, say, 210%. This
load curve. The concept used in arriving at the number will also help in reducing the number of steps. Number of
of steps is based on the fact that the rotor current varies steps at 4-6 are preferable to economize on the cost of
in direct proportion of torque (equation (I .I)). For more switchgears, and yet provide a reasonably smooth (free from
clarity we will discuss this method using a practical overshoots) starting torque.
Considering T,,,
= 210%
exarriple. The procedure is generally the same as that
adopted in Example 5.3.
Example 5.4
Consider a conveyor system, requiring an average torque of = 65%
100% during pick-up. The motor data are as follows: and fs = 16.77 sec
kW= 450 which is quite reasonable. A modified accelerating torque
N, = 980 r.p.m. diagram is drawn in Figure 5.7(b), providing a smooth
V, = 6.6 kV acceleration. Now we can cut off the last resistance at e,
I, = 50 A giving a jump in T,,, of even less than 210%.
sse2 = 750 V To obtain the starting characteristics according to Figure
I,, = 435 A 5.7(b), it is essential to calculate the total rotor resistance,
Tpa = 250% R,, and resistances between each step along the lines of
R2 = 0.02 R (star connected) Table 5.2.
Solution
For conveyors, the torque should not be very high, to
economize on belt size and cost. Otherwise a higher safety 5.3.1 Calculation of time between
factor must be considered for the belts, resulting in a wider or each step
thicker belt at an extra cost. Therefore considering a torque
demand of T,,, as 180% and T,,, as 120%, for a conveyor
torque of 1 OO%, providing an average starting torque of 150% To make the whole starting sequence automatic in a
and an accelerating torque of 50%, (Figure 5.7(a)):
contactor type automatic starter unit it is essential to
know the time the motor will take to accelerate from one
For T,,, (1 80%) R2, = 750 12 slip to another between each step. It is required to select
,3 x 435 x 1.8