Fairford synergy, User Manual

Page: 187 / 233

Synergy Series Soft Start User Manual
MAN-SGY-017. Version 01. 27/06/2016
187
8.3.2 The Induction Motor (continued)
The acceleration of a motor-load system is caused by the difference between the developed torque (motor)
and the absorbed torque (load), and is shown by the shaded area in the next figure:
Figure 8.3.3: Torque/Speed Curve – Accelerating Torque
Obviously, the larger the difference, the faster the acceleration and the quicker full speed is reached - and,
coincidentally, the greater the stresses experienced by the supply and drive systems during the acceleration
process. An “ideal” start would accelerate the load with just sufficient force to reach full speed smoothly in a
reasonable time, and with minimum stress to the supply and drive mechanisms.
Broadly speaking, the motor speed/torque characteristic is controlled by the rotor resistance - a motor with
high rotor resistance can generate it’s peak torque (pull-out torque) at standstill giving the high break-away
torque characteristic, which reduces steadily as the speed increases and becoming zero at synchronous
speed. At the other end of the scale, a motor with a very low rotor resistance will produce a low starting
torque but will generate its peak torque closer to the synchronous speed. Consequently this type of motor
runs at full power with higher operating efficiency and low slip speed. It is possible to combine the twin
requirements of high starting torque and efficient full-speed operation within a single motor by techniques
such as double-cage or deep bar design, and this, usually, is the motor characteristic chosen for lifting and
hoisting applications:
(see over)
8. Applications (continued)