Operation within tolerance of ±10 percent of the rated voltage.

• Operation from a sine wave of voltage source (not to exceed 10 percent deviation factor).

• Operation within a tolerance of ±5 percent of rated frequency.

• Operation within a voltage unbalance of 1 percent or less.

Operation at other than usual service conditions may result in the consumption of additional energy.


When the line voltages applied to a polyphase induction motor are not equal, unbalanced currents in the stator windings result. A small percentage of voltage unbalance results in a much larger percentage current unbalance. Consequently, the temperature rise of the motor operating at a particular load and percentage voltage unbalance will be greater than for the motor operating under the same conditions with balanced voltages.

Voltages should be evenly balanced as closely as they can be read on a voltmeter. If the voltages are unbalanced, the rated horsepower of polyphase squirrel-cage induction motors should be multiplied by the factor shown in Figure 11.1 to reduce the possibility of damage to the motor. Operation of the motor with more than a 5-percent voltage unbalance is not recommended.

When the derating curve of Figure 11.1 is applied for operation on balanced voltages, the selection and setting of the overload device should take into account the combination of the derating factor applied to the motor and the increase in current resulting from the unbalanced voltages. This is a complex problem involving the variation in motor current as a function of load and voltage unbalance in the addition to the characteristics of the overload device relative to

IMAXIMUM or 1 AVERAGE. In the absence of specific information it is recommended that overload devices be selected and/or adjusted at the minimum value that does not result in tripping for the derating factor and voltage unbalance that applies. When the unbalanced voltages are unanticipated, it is recommended that the overload devices be selected so as to be responsive to

Figure 11.1 Polyphase squirrel-cage induction motors derating factor due to unbalanced voltage.


The effect of unbalanced voltages on polyphase induction motors is equivalent to the introduction of a "negative-sequence voltage" having a rotation opposite to that occurring the balanced voltages. This negative-sequence voltage produces an air gap flux rotating against the rotation of the rotor, tending to produce high currents. A small negative-sequence voltage may produce current in the windings considerably in excess of those present under balanced voltage conditions.

11.4.1 Unbalanced Defined (MG 1 20.56.2)

The voltage unbalance in percent may be defined as follows:


Voltage = 100 x Unbalance

Maximum voltage deviation from average voltage average voltage

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