View Full Version : servo help

10-16-2003, 12:57 AM
I just looked again at one of my new servo motors and noticed something I had not noticed before. On part of the label it reads:

1/3 hp PWM It is a brushed dc Reliance motor that will make 980 oz/in at 10 amps. Terminal voltage is 120v with the motor turning 15 rpm for every 10 volts applied. My question is based on an assumption I have had for some time. I though PWM meant pulse width modulation? Does this have any bearing on the step/dir system I had planned to use?


Mariss Freimanis
10-16-2003, 02:40 AM
Leaving the PWM thing aside for the moment, there is something wrong with your numbers. These numbers should mesh and yours don't. Heres why:

1) Kt = 1.351 Kv

2) Watts = RPM * in-oz / 1351

3) There are 746 Watts per Horsepower

A Kv of 15 RPM / 10V would mean the motor would have a no-load RPM of 180 at 120VDC.

From (1) it would make its Kt = 20.2 in-oz / Amp. At 10A its torque would be 202 in-oz.

From (2), its power would be 202 * 180 /1351 Watts or 27W

From (3), 1/3 HP = 249 Watts. 27W is not even close.

Please check your numbers. One of them is very badly off; the above is a reality check.



10-16-2003, 08:28 AM
Mariss thank you for your response. I am including here the total info that I have from the seller. These are new motors but were made in 95.

Back emf constant, ke=68V/kRPM
Torque Constant, kt=5.75 lb-in/amp
Rated Peak Torque: 1080 oz/in

The 1/3hp PWM is all that is on the motor. The 15 rpm I got from the seller in an email and the 980 oz/in should have been 920.

With the numbers above can you help, and at the same time explain the PWM thing for me.