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PM motor with additional external inductor.

PM motor with additional external inductor.

PM motor with additional external inductor.

(OP)
We have a specification of PM synchronous motor driven by VFD. The motor spec requires external inductance about 20 microHenry. Our understanding was that this inductance is required to reduce dv/dt of PWM voltage pulses, thus reducing a stress applied to motor insulation, reducing accelerated insulation aging effect, and optimize a service life. However, the motor MFG states that this additional inductance is required to reduce rotor overheating. Is this true? Unfortunately, the motor MFG (outside of USA) would not provide further explanation.

RE: PM motor with additional external inductor.

In principle, added inductance will reduce the current ripple in the motor phases that is created by the VFD's PWM output. And reducing current ripple does reduce resistive losses.

But, with typical motor phase inductance measured in millihenries, adding 20 microhenries will not make much difference in this regard. And in most cases, the added losses from the ripple are minor compared to the losses from constant current flow.

But the added inductance will "soften" the PWM edges, as you surmised, which does have the benefits you stated. In addition, it can significantly reduce the EMI produced. In many cases, it can be required to meet EMC regulations.

Curt Wilson
Omron Delta Tau

RE: PM motor with additional external inductor.

(OP)
Thank you, CSWILSON. You made a good point. This specific motor has values of Ld/Lq around 30 microhenry (and not milliHenry's range). Probably, they require another 20 microHenry to minimize thermal effect of current ripples. We just never saw before such justification of additional inductance.

RE: PM motor with additional external inductor.

(OP)
430 kW.

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