Page 149 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

MAINTENANCE OF MOTORS

8.4 CHAPTER EIGHT

TABLE 8.1 AC Motor Nameplate Voltages for Corresponding
Distribution System Voltages (60 Hz)

Three-phase motors Single-phase motors
System voltage Motor voltage System voltage Motor voltage
208 200 120 115
240 230 240 230
480 460
600 575
2,400 2,300
4,160 4,000
4,800 4,600
6,900 6,600
13,800 13,200
Source: ANSI C84.1-1970, American National Standards Institute, Inc., 1430
Broadway, New York, NY 10018.

The supply voltage must be known in order to select a motor for an application. Table
8.1 shows the required voltage which should exceed the nameplate voltage by a small
amount. Ideal power supplies provide constant voltage, frequency, and phasing. In reality,
voltages vary from 10 percent above to 10 percent below the nominal values. The fre-
quency is usually controlled within a tight tolerance, but it, too, can vary. Phasing, which
is normally balanced when the voltage in each of the three phases is equal, can be unbal-
anced by a few percentage points.

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.

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