Rockwell Automation 150 MNL/SMC DIALOG+/APP-PROD GUIDE - Manual (Page 155)

8-2 Reduced Voltage Starting
Reduced Voltage The most widely used method of electromechanical reduced voltage
starting is the autotransformer. Wye-Delta (Y- D), also referred to as
Star-Delta, is the next most popular method
Figure 8.2 Bulletin 570 Autotransformer
All forms of reduced voltage starting affect the motor current and
torque characteristics. When a reduced voltage is applied to a motor
at rest, the current drawn by the motor is reduced. In addition, the
torque produced by the motor is a factor of approximately the square
of the percentage of voltage applied.
For example, if 50% voltage is applied to the motor, a starting torque
of approximately 25% of the normal starting torque would b
produced. In the previous full voltage example, the NEMA Design B
motor had a starting torque of 180% of full load torque. With only
50% voltage applied, this would equate to approximately 45% of full
load torque. Table 5.A shows the typical relationship of voltage,
current, and torque for a NEMA Design B motor.