Stepper motor tester torque measurement

[Richard CSL]

Hi all, does anybody know if there is such a device that can measure the strength of a stepper motor.

 

reason for asking is I have several used steppers, I recondition many of them ie: strip clean lubricate the bearings, polish the laminate core with a dremel. but when reassembled many of them feel so weak (without power connected) it is as if the core armature magnet goes weak.

 

I just wondered if there was any method of measuring the torque easily. either with power connected or without.

[greenalien]

It's not unknown for stepper motors to lose their magnetism over time - this is more likely if their bearings have tightened up in use, usually due to dirt ingress, in which case the magnet is unable to respond to the moving magnetic field around it, which has a demagnetising effect.

With power applied, it should be very difficult to move the spindle by hand - if there isn't a lot of resistance, the motor is knackered.

These days, stepper motors are pretty cheap, it's not really cost effective to spend time attempting to repair them.

[David Duffy]

I have heard that steppers are magnetised after they are assembled. I have also heard that as soon as you take them apart, the magnetic field is damaged.

 

I experienced a similar situation a couple of years back. A stripped & re-assembled motor was always weak compared to a new one. Even weaker than just before it was pulled apart.

[KevinE]

The problem is that a stepper motor's performance is inextricably linked to its drive circuit and load. Different manufacturers use different drive circuits and methods, so although there may be a motor manufacturer's proper way to test the performance, there's no saying that just because its still 'in spec' that it will work reliably in a particular fixture or at a particular temperature. Getting specs may be a little hit and miss given the huge number of chinese OEMs making the majority of steppers seen in our field of work. As well as proprietory drive waveforms, modern fixtures not only use PWM but Vector-control algorithms, for example upping the current setpoint during acceleration and deceleration, reducing current during idle periods, microsteps (half-current steps in between main steps) and so on and so forth.

 

As an example, I had to recently change 2 strobe-shutter motors in some moving heads. They felt very torquey when operated in the palm of the hand and cycling the shutter channel up/down, they 'felt' right with power on and off, they 'sounded' right, and they worked ok when tried on the large colour wheel....but they just wouldnt strobe, instead preferring to just buzz without moving. In went 2 new motors and everything was fine.

 

To be fair, most motors are relatively cheap compared to the cost of a replacement fixture, I only salvage or repair motors if its something simple like a seized bearing or the ferrite shellac has broken up and jammed the stator gap.

 

Pattern motors can be obtained for very little money (a tenner) from the US if you buy enough of them, they come in all popular shaft and winding formats.