hp rating of Dc motor 1

[jarvis]

I've got some variable speed dc motors from treadmills that I'd like to use in some projects. I've been told that the hp ratings on treadmill motors was not the same as ratings on regular motors. Is this true? If so why? What is the conversion factor if one exists? Thanks for your help.

Ben Englund

 

[jarvis]

I just looked at some new treadmills. Some seem to have two ratings on them like 2.5tHP and 1.5cHP on one machine. I assume that the cHP would be common horse power and tHP would be treadmill horsepower. So would I take 40% or so off the treadmill rating for the actual HP rating? Would it have anything to do with the duty cycle the motor is run at. I suppose it probably varies slightly from manufacturer to manufacturer? Any thoughts?

Ben Englund

 

[rbulsara]

Could it be intermittent (non-continuous) use HP and continuous HP?

 

[rmastro]

The ratings are continuous and intermittent. Just like many consumer products, the treadmill manufacturers have fallen to run-away ratings pressure. The continuous rating of the motor is the manufacturers rating at speed, current and voltage, under the thermal limit. However, since the treadmill only outputs high power when the foot hits it, they can claim a high rating for that instant (and if you are not to heavy and slow, then you don't trip the breaker, which happens because you cannot get 2.5hp for too long from a 1phase 120v 15 amp circuit). For much of the time the treadmill is running unloaded (in between your steps). If it is producing 0.25hp for 50% of the time and 2.5 hp for 50% of the time, then the rms value is about 1.8hp.

Here is what I would do.
Measure the resistance.
Take a power drill and back drive the motor, measure the voltage generated at speed (probably 500 rpm for a cordless drill) then convert this to what it would be at 1000 rpm. The torque constant can be calculated from the voltage constant. Take the amount of volts generated at 1000 rpm and multiply it by .085, the result is the torque constant in lb-in/amp.

With the rated current from the name plate and the above info you know almost everything about this motor.

Do not exceed an rms current of more than the continuous rating. And I would limit the peak current to about 2X the continuous (so the copper won't burn up). Keep in mind that if you are at 2X (that is 4X heating losses) you would not be able to do for long and still have the rms current equal the continuous value. Put enough voltage to overcome the would be generated voltage at the speed you want to run (stay within 2x the rated nameplate speed for mechanical reasons).


good luck, hope this helps.

 

[jarvis]

Thanks guys

Rmastro,
Thanks for your information, I'll definately use it. I would be using the variable speed drivers originally for the motors. I was just wondering about the horsepower numbers. I guess I should have thought about the load on the motor. It makes sense to me now. Thanks again

Ben Englund