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HP ratings on variable speed AC and DC motors

HP ratings on variable speed AC and DC motors

HP ratings on variable speed AC and DC motors

(OP)
I've seen a piece of equipment that claimed a 6 HP PMDC motor yet operates from a 120VAC, 20 amp line. Even with unity power factor and 100% efficiency, 3.2 HP would be the max. Is there any way the claim could be valid? Perhaps torque output equivalent to a 6 HP brush DC motor? Is their a standard way to rate motors designed for variable speed drives yet independent of the drive?

RE: HP ratings on variable speed AC and DC motors

Hi, check your decimal point.  6hp is large for a PMDC motor.

RE: HP ratings on variable speed AC and DC motors

powerbob

I believe your right about the maximum power rating of the 120Vac 20amp circuit. For a DC motor the equation for torque is

T = Ia * field flux + K1

Where Ia = armature current
      Field Flux = constant for PM motor
      K1  = constant

HP = (T * n) / 746

Where n = speed
      746 = constant for converting KW to HP

It is possible that the 6 HP rating is actually not a continuous rating but a momentary rating.  With PM, high performance motors, the manufacturers will allow as much as 4 or 5 times the normal current, which equals torque, for short periods of time ( a few seconds). A typical standard DC motor is rated for 150% current for 1 minute. You might check the ratings on the motor.

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