RE: Stepper Motor Specs

If you end unable to obtain the specifications for any stepper motors, you may be able to derive the specifications under testing. As you increase the energy into (voltage and current) a stepper motor, it will give you torque and heat. If you apply too much electrical energy the motor will burn up. At some high temperature the magnets may loose some of their strength. The temperature rise in the motor is not only related to the energy you apply. The type of driver used has a great deal to do with the efficiency of the torque vrs. the heat generated. A chopper drive (constant current) is one type of driver, for example.

The measurement of torque sounds a little difficult, but it is not. If you wind some cloth fishing line around a 1" diameter spool attached to your motor, you can hold more and more weight until you identify the holding torque. Next you can back off the weight and try stepping foward without "loosing it". If you need to identify running torque, you may do so with a little more trouble, but it can be done by expanding the crude fishing line method for a little distance such that you can reach the speed before you hit the spool. It takes time but it works.

So how do you know how much heat is within what a motor may tolerate? To me that is a little like fishing. After running the motor for 1/2 hour touch it and see if it feels too hot. If it feels too hot, it is too hot. They will run a little warm without trouble.

So what does the driver have to do with it? The driver induces electrical energy into the windings which have resistance and inductance as well as some dynamic properties (such as reflected impedance from the mechanical load). At 1000 RPM's a heavy load requires more energy than no load at all. A heavy load will appear electrically different to a driver than no load at all. Different kinds of drivers respond differently to the static and dynamic properties of the stepper motor. The kind of driver you are using will therefore make a great difference in what you observe as the motor performance. The officail specifications for a motor are often done with a chopper driver because the performance is better than with a voltage driver.

The funney thing about a stepper motor is that the electrical energy is easy enough to get into them. It is another matter to get the electrical energy back out so that the coils do not hold the motor in the last position while you want it to advance to the next. A chopper drive has an advantage in this manner and therefore the speed is usually better. I designed a driver that improves this even more as well as takes care of the resonance (buzzing) problems. I sold quite a few of them several years ago. I did a lot of torque curves for that project, so I will add one more note. If whatever driver you wish to use seems to increase the fundemental and harmonic resonance issues with a motor, you may observe lower torques at these speeds such that your curve gets pretty roller coaster like.

David Howland



-----Original Message-----
From: PTENGIN@... [SMTP:PTENGIN@...]
Sent: Thursday, September 09, 1999 9:06 PM
To: CAD_CAM_EDM_DRO@onelist.com
Subject: Re: [CAD_CAM_EDM_DRO] Stepper Motor Specs

From: PTENGIN@...

In a message dated 09/09/1999 5:34:16 PM Hawaiian Standard Time,
johncrad@... writes:

<< Any pointers would be gratefully accepted. >>
John,
Before anybody can give out advise, I think they would need to know a
little bit about your intended use. Do you intend to gear down? Ball screws?
Rack and pinion etc. A slow metal cutting machine versus a fast router. My
first impression of your motors are the voltage is too high. This means the
inductance of the coil may be high, and thus they will not run very fast. Not
good for anything but a very small and slow application. The motors I look
for have a very low voltage rating and a high current rating. Peter

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