DESIGN CHARACTERISTICS
NEMA standards state that motors should operate properly at rated load with a variation of up to 10 percent in the voltage. However, this does not indicate that the motor will operate at its rated performance. For example, a motor may not be able to accelerate a driven load if the voltage is reduced by 10 percent because the torque-speed curve will change. The major effects of voltage variation on motor operation are as follows:
Reduced voltage
1. Increased temperature rises
2. Reduction in starting torque
3. Reduction in maximum torque
4. Decreased starting current
5. Increased acceleration time
Increased voltage
1. Increased starting and maximum torques
2. Higher inrush current
3. Decreased power factor
Any of these conditions can shorten the effective service life of a motor.
A motor designed for 60-Hz operation can operate at 50 Hz on selected voltages at 80 or 85 percent of the 60-Hz rated horsepower. Table 8.2 illustrates the voltages required for this operation.
