Looking for info on Torque of AC vs DC Motors

testsubject

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Join Date
Feb 2004
Location
Chicago, Il
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I realize this is not about PLCs, but there have been a lot of questions answered on here about motors so I thought I would place this here.

What is the difference in torque between an AC Motor and a DC Motor? I just got off the phone with an AC Drive tech (I am thinking of going from DC Drive/Motors to AC Drive/Motors) and he informed me that the torque of a 1 HP, 1800rpm, 90VDC motor (the motor shows 36 in/lb) was different than the torque of a 1 HP, 1800rpm, 230VAC motor (3 lb. ft.) I do not see how this is possible. I mean torque is torque and if the HPs are the same should not the torques be the same?

Looking at the above examples, they look the same. (36in.lb=3 lb.ft)

I thought that I would be able to directly swap out the one motor for the other....

Thanks in advance,

Bob
 
Bob

The two values are the same. To convert in-lbs to ft-lbs, divide by 12 (12 in = 1 ft).

The full load torque of a DC and AC motor will be the same for the same HP and speed. Torque (lb-ft) = HP x 5252 / RPM.
 
That's what I thought. He was trying to tell me that I would need a bigger motor to get the same torque. I did mention that I sometimes run this motor at 30% rated rpm. I do realize that torque will drop but it should still be the same as the other motor at 30%.
 
DC motors will produce their rated torque down to zero RPM. AC drives, without encoder feedback, are not so good at low RPM. You should be ok at 30% speed. Watch for motor over heating if running loaded at low speed for a long time.
 
Torque curves for different motors are different - period (
and even if you look at AC style motors only, they are not
the same). Difference between AC and DC is not to be ignored
you have to select proper motor for the application.
AC Motors are not good for low rpms primarily because they
overheat due insuficient airflow at low rpms. If you put
separate motor that runs all the time at full speed
and provides sufficient cooling for the low rpm motor,
you can go slower. Depending on duty cycle and size and
construction of the motor you might want to keep it above
certain speed.
 
As you have correctly found, an AC motor nameplated 1hp 1800rpm will develop 3ft-lbs of torque as will a DC motor nameplated similarly. However, since nameplate data is a snapshot of motor performance only at full speed and full load, it is wise to look a little further when varying speed.

First, depending on how the motors are cooled, they may overheat at slower speeds. If either type of motor is cooled by a shaft fan as in ODP or TEFC enclosures, slow speed heating can be a problem when delivering heavy loads. The speed of 30% was mentioned. I would expect that either type of motor would behave thermally at full load at or above this speed.

Second, since nameplate data is a continuous rating, it generally tells you little about short term overtorque capabilities for either type of motor. An AC induction motor wound to NEMA Design B (the most common type) will be capable of an absolute maximum short term torque output of about 220% of continuous. Above that, the motor breaks down magnetically and will actually produce less torque as its speed sags further and further to stall. It is important to realize however, that the AC drive powering the motor will most likely not be able to provide the current required for this high overload level (it can be if sized for it). Normal AC drive sizing will give you either 110% short term overload (normal or variable torque rated) or 150% overload (heavy duty or constant torque rated). While a shunt wound or permanent magnet DC motor has somewhat larger short term overload capabilities, the DC drive is likely to be rated similar to the AC drives (110% or 150%) and so is the limiting factor as in AC.

The third consideration is torque development at very low speeds. The DC motor will have no problem developing its rated torque from stall right up to full speed (this is without regard to heating so it must be short term). The AC motor may or may not have the same capability depending entirely on the AC drive technology. An ordinary (cheap) Volts per Hertz drive will not develop much slow speed torque but a good sensorless or flux vector drive will duplicate the DC performance.

So, the bottom line is that you need to know within reasonable precision just what torque your machine requires and then make sure the motor-drive package can produce it. It can be either DC or AC as I have shown. Good luck!
 

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