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Derating of Induction Motors Due to Waveform Distortion

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Derating of Induction Motors Due to Waveform Distortion

Electrical induction motors are designed on the basis of balanced three-phase sinusoidal input voltage. Nonsinusoidal voltage has detri­mental effects on induction motor performance, and derating of the machine is required. IEEE Standard 519 suggests that no derating of the motor would be necessary for a harmonic content of up to S"lo. The derating of induction motors due to harmonic distortion is discussed in detail.

Based on the initial results, it is decided to calculate the derating due to second harmonics (case a) only. To determine the derating, a computer program reduces the output power of the machine until the temperature due to the abnormal condition is less than or equal to the normal operating temperature corresponding to class B insulation.

Here. the first term represents the loss due to harmonics, and the second term will give the rotor copper loss due to the fundamental. In order to maintain constant output torque, the last term of (22) will vary an amount given by (23), which is proportional to the resultant torque produced by the harmonic currents.

The thermal resistances R ;, R j, and R 1 are determined for a 5-hp and for a 100-hp machine. The values found for the 5-hp machine are used as representatives of machines ranging from O to 25 hp, and those found for the 100-hp machine for machines ranging from 25 to 1000 hp. Insulation class B is assumed for all cases.

Slot leakage inductance reduction due to harmonic distor­tion is considered equal to 10% for all waveforms and rotor slot leakage inductance is taken to be 30 % of total rotor leakage for all machines. In all the computations, an ANSI M-22 steel and 1.0-T maximum air-gap magnetic flux density is used.

When supplied with waveform a, machines of both types of enclosure and for all ratings would require derating at 5 % harmonic distortion. Fig. 7 also shows the derating due to 8 % second harmonic.

While, in the cases of harmonic waveforms b and c, 5 % limitations on harmonic content is acceptable, results for case a are significant. We find that a restriction for the second harmonic should be included on the harmonic distortion limits established by IEEE Standard 519 [ 1], and derating in some cases should be considered for less than 5 % harmonic distortion.

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Derating of Induction Motors Due to Waveform Distortion