Page 84 - Industrial Power Engineering and Applications Handbook

Duties of induction motors 3/65
(One cycle) on its speed-torque curve, the motor will not drop its speed
substantially and cause high slip losses.
Note For such duties, the starting heat is kept as low as
possible by suitable rotor design to eliminate the effect of
frequent starts and stops. The margin for starting heat and
braking heat should be taken into account if these are
considerable. The manufacturer is a better guide for
suggestions here.

3.9 Shock loading and use of a
flywheel

The application of a sudden load on the motor for short
duration, in the process of performing a certain load
duty, is termed 'shock loading'. This must be taken into
account when selecting the size of a motor. Electric
mp. temp hammers, piston pumps, rolling mills, cane crushers and
(e = O) cane levellers, sheet punching, notching, bending and
Time + 0 cutting operations on a power press, a brake press or a
shearing machine are a few examples of shock loading.
*e, - Corresponding to no-load losses They all exert a sudden load. although for a very short
0- Actual heating curve duration, during each load cycle, and may damage the
@ - Thermal curve for 7.5 kW motor motor as well as the machine. Such machines. therefore,
Figure 3.17 the motor rat,ng (CMR) for a short- experience a sharp rise and fall in load. Figure 3.18 depicts
time duty wch a load cycle, having excessive load P2 for d short

T
t

p3

No load
i losses

= Corresponding
to no load
losses
Time - r

P, - Light loading
P2 - Severe to very severe loading
Figure 3.18 A typical shock loading duty