Stepper Motor Torque

[sammybrock]

The application for a stepper motor includes the expected torque output of approximately 142 in-ozs. What we have measure is approximately 1/2 of expected torque output at the output shaft. Expected torque output of the gearbox (100:1 planitary gearbox, 85% efficient) should be (142 x 100)/16) x .85 = 754.4 in/lbs of torque. Motor torque output is in accordance with the manufacturers torque/speed curves.

We have tested the system for harmful electrical noise and isolated all possibilities.

Are there any other considerations that should be included such as resonance, gear box losses, etc.

 

[jtsamuels]

Torque is a function of the current waveshape that is being conducted by each field winding. For example,... if the waveshape is not a square wave, the the amount of torque you will get from the motor is a percentage of the stated output. Also, if you are you using a chopper scheme to prevent the motor from overheating, then you must be sure that the point at which the controller begins chopping the drive signal is at a point where you still get full rated torque. The single most useful tool for diagnosing stepper drive problems is a Clamp-On Current Probe attached to a decent quality digital scope. This will not only show you current waveshape and chopped levels, but also allow you to check each winding and make a composite image of what the stepper is being driven by. Good luck.

 

[Allphase]

http://www.micromo.com/library/docs/notes&tutorials/Torque-Senitivity.pdf

 

[xyzz]

Sammybrock,
you say "Motor torque output is in accordance with the manufacturers torque/speed curves."
How you came to this conclusion?

Also, a gearbox efficiency is usually rated in some range of input power.
Are you in this range?