Stepper Motor Testing

[Steve Cavill]

Hi guys,

 

After looking at the spares shelf today, I have come across around 30 used stepper motors, none of which have been labelled to signify whether they have a fault or not.

 

Is there an easy way of testing these? Such as a 12v battery charger @ 6amps? Or would this cause some serious problems?

 

I really dont fancy wiring them all up 1 by 1 into a fixture just to find that theyre naff and I've got 29 more to try before I'm finished..

 

Any infor would be great.

 

Cheers,

Steve

[Jivemaster]

They have to be driven by a suitable controller which will drive pulses into the windings with the correct timing of the pulses. Applying DC may well harm them.

[Steve Cavill]

So, I would be better off purchasing something like this:

 

http://www.milinst.com/DMX/dmxtext.htm#1_550

 

Then modifying it to have push terminals on, and setting to DMX Address 1, and proceeding to test with a controller.

[DavidLee]

So, I would be better off purchasing something like this:

 

http://www.milinst.com/DMX/dmxtext.htm#1_550

 

Then modifying it to have push terminals on, and setting to DMX Address 1, and proceeding to test with a controller.

There are much cheaper stepper motor drivers that may be suitable, available for about £20. You may be able to use one of these to build a tester.

 

eg Velleman and Magenta

 

David

[KevinE]

They require sequenced pulses in the right order and polarity applied to the (usually four or six) leads. A stepper as used in lighting are usually 4-phase and are either driven with a microstepping ic linked to a bipolar bridge (4-wire) , or a unipolar driver (6-wire with centre taps), giving artificial half-current steps inserted by the control IC to get a smoother rotation. Fixtures drive the phase current in sequence and regulate the current by PWM (making them whistle quietly) which modulates according to the speed and inertia algorithms written into the control board.

 

Different motors need different voltages and currents to drive them properly. Motors have various resistance windings between models, the drive circuits are designed for the specific motor to be used...refer to the correct data sheet. Identical-looking motors will not necessarily have the same electrical spec.

 

One of the faults with steppers in lighting is that the magnetism fades after years, I think it's related to how hot the fixture operates. A duff motor may work on one fixture (say as a colourwheel operator) but fail in torque if used as a mirror operator...maybe skipping one step every few minutes causing the thing to lose position and possibly crash into a hard stop. I've built simple stepper circuits driving motors but really the conditions on the bench render such experiments little more than a diversion as far as I can see.

 

So I dont think there's a cheap and easy way to do this apart from trying it over time by substituting the correct motor into the correct fixture. The Milford unit may well drive a duff motor perfectly!

 

The magnetism can be gauged with ALOT of experience by twiddling the shaft between the thumb and forefinger and 'feeling' the reluctance of the stator-rotor.

 

The windings rarely burn out as they're very low resistance to start with and few drive circuits have the power..preferring to blow themselves first. Steppers naturally feel 'gritty' but a different type of feel belies dirt, rust or metal filings clinging to the rotor...again..experience!!

 

 

 

<_<

[AndyJones]

I have now worked out why you have a motor as your display picture <_<

[Brian]

TBH you're better off just measuring the winding resistances to make sure none of them have gone o/c and then giving the shaft a spin by hand to feel for worn bearings. There's not much can go wrong with a stepper. If you've got one you know is broken then open it up and get a feel for what's inside.

[jodyflorian]

I've been hoping to find a way of testing various motors in trackspots, intellabeams and golden scans. If one were to have a stepper motor controller for each of the motor types, would it not be possible to test them by testing the force/torque at which they start skipping steps? Perhaps this could also be tested at different speeds and motor positions?

[KevinE]

I really wouldn't bother. Just keep one motor of each on the shelf and test by substitution. It really is the only way to be certain.

 

As I said before, the torque itself is modulated by the pcbs so that, for example, if a rapid movement is needed, the current to the motor can be increased to cope with the movement, and then reduced when the motor is stationary (holding current) to prevent the motor overheating. I cant see how you'd simulate this without condemning a few ageing but still good motors...an expensive hobby! Sometimes manufacturers drive a small motor very hard to acheive a certain motion, (maybe above its design spec) for short periods depending on the vectors in the micro. Motors get increasingly hard to drive as they go faster because of the reactive nature of their windings, (they effectively increase their impedance at higher speeds); this is also compensated for by the drive current modulation. And then you've got acceleration and deceleration ramps to think of (motors can skip steps if the speed-time curve is too sharp). I think you need to speak to an intelligent-light design engineer!

[jodyflorian]

Ok I see what you mean, I think!

 

So maybe there's a half way house? Use your idea about testing by substitution.

 

You said in an earlier post that motors' magnets tend to lose strength which is the main problem, so we want to be able to test that? An old motor could be compared with the performance of a new motor - just by applying the same test to each, with a low current, and see what torque it produces?

 

I may well be missing the point here, but it is something I need to find an answer to, so don't hurt me!! <_<

 

Edit: Forgot to say thanks for your reply! You gave a lot of very good to-the-point info - thanks :** laughs out loud **:

[Pashonlie]

Hi guys I hope you dont mind me tagging onto the end of this posting, Ive just picked up some VRX Gladiators and the Colour wheel motors and the shutter motors have seized or got really stiff at the end of the rotation. Ive striped and lubricated a couple to get them to work but others are just dead, What causes them to seize is it just age or the fact that a certain pattern in a programme does not use the far end of the colour selection, or does not use the shutter feature?

 

Is it a common problem with the VRX units and where can I obtain steppor motors at a reasonable price.

 

Regards

Paul

[jodyflorian]

No problem (I hope because I did the same!) ... If the colour wheel goes back to its previous colours correctly then I would have thought it would be the electronics, although it would be very strange for the programme to malfunction. If you're programming the sequences yourself, double check them - if you're sure they're correct then maybe it's a setting on the VRX or your desk that is not calling up the colours at the end - maybe you have a master setting that's messing up the full range you think you're sending to the units. Failing that, I guess it must be a stepper motor problem, I can only imagine that there must be something getting in the way?!

 

Edit: I know I'm just mumbling common sense stuff but easy to forget solutions... try taking the motor out of the unit, but leave it connected to the board and see what it does, maybe without being connected to anything physically it'll behave differently. Failing that swap it with another motor that is the same type?

[DMXBandit]

Pauls post reminded me of a stepper problem I once had.

I bought some second hand Martin Destroyers a couple of years ago, and one of them had a strange problem where the stepper motor would loose position and eventually stop all together. If you left it and then turned it on again it would work fine for a while and then start going wrong again. When I ran it on the bench with a test sequence with constantly changing gobos you could see it slowing down, but turning the lamp off would mean that the motor would start to speed up again after a few minuets, so it was obvious that the heat from the lamp was causing this. Turning the motor by hand was also much harder when it was hot, eventually becoming impossible to move.

 

It wasn't a big problem, I just swapped the motor out and it was fine, but I've never managed to work out why the heat from the lamp would cause one motor to completely seize up while others in identical effects would be fine. Is it fairly common for steppers to be effected by heat?

[KevinE]

The motor rotors have a dust-core which is usually sealed in between the magnets via some sort of resin or varnish. This can flake away with heat, and let the iron dust float about a bit (most of it will stay). If this gets in between the very narrow gap between rotor and stator pole pieces, it will jam the motor, or make it feel very gritty. A single speck is enough to make it intermittent. With care you can re-varnish the core, rub the excess away and give it a very good clean with doubled-over gaffer tape (its a real faff) and away you go. But it's not one if you're on a tight schedule.

 

I suppose a motor seizing when its warm may be because the rotor is getting hot and binding against the stator poles. Another problem with cheaper motors is that the stator poles can actually rust and swell! This'll also give trouble.

 

The bearings can of course fail (they're usually minature ball races which can jam) and you can obviously change them..but, again, look at the replacement costs before opening up and spending the afternoon messing about.

 

It's well known that heating a magnet slowly and cooling it slowly will cause the magnetism to weaken..maybe this happens with the steppers; its common in hot-running lights such as the earlier Roboscans and certain disco-grade scanners and effects. And it seems more common with those motors mounted closest to the lamp...eg the dimmer shutter.

[DMXBandit]

Ah-ha. Well the lamp in a Destroyer is pretty much mounted to the colour/gobo wheel. Thanks Kevin.