HP ratings on variable speed AC and DC motors

[powerbob]

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?

 

[cbarn24050]

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

 

[electrageek]

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.