Reducing PMDC motor noise ?

"Eric R Snow"

Is the AM band noise related to PWM drive of the motors?

If so, adding caps across the motors will make it worse.

Is the AM interference affecting AM radios outside of your building ?

Or just the portable one the operator has sitting next to the machine ?

....... Phil

You don't say what control scheme they use for controlling motor speed and direction and that may give a clue. Modern servo amps may imply pulse width modulated drivers.

Describe the noise. Static noise may be brush noise and buzzing may be caused by the driving circuit. Is the noise consistent with motor speed? Does it change with loading? ETC....

A small ceramic disc cap across the motor leads, at the motor end, may help. Something like a point one microfarad 300 VDC unit, noise spikes will have higher peaks than the supply - if it is indeed brush noise. Short leads as possible from brush to brush.

The shield should be grounded at one end only (as a general rule - but safety may require it grounded at both cabinet and motor).

Inductors in series with the motor leads - something like a toroid with several turns of wire sufficient for the current.

A commercial filter may be a good idea mounted as close to the motor as possible and grounded to the motor frame.

Was the noise always present or just start up?

Warn, loose motor brushes will cause noise due to excessive arcing or a commutator that is worn will also do it. Gives a static sound.

I had a situation where we used brushed motors for adjusting the dies on a tablet press - they were Bodine gear motors (not PM) and set the pressure of the dies - continually on the fly. No amount of filtering would cure a bad set of brushes on those motors - we gave up trying and just changed the brushes frequently.

Noise can radiate in several ways - ground loops are another thing to look out for. If you have several machines all connected together mechanically but served from different power supplies they should have (go to) one common ground not a series of individual grounds - and that may involve safety so you'd need someone capable of resolving that problem.

Why is it a concern? Is the AM radio necessary? Can it be moved to solve the problem?

In my RC planes that still use brushed motors, I put a capacitor from each brush to the motor can and one across the brushes. I usually use

Jim

Thanks for the speedy reply, The noise is I believe both brush and driver noise. When the machine is idling it is noisy. When moving there seems to be two types of noise and both rise and fall with RPM. I am almost positive the amps are PWM types. To listen to an AM radio station I must instead listen on-line and broadcast from the computer in the FM band with a small xmitter I built from Ramsey. So changing stations is a hassle. Cheers, Eric

Thank You Jim for the reply. Eric

That is normally not an issue in shops since you should be working and not listening to the radio:) unless you are disturbing some one out side your premises? That would be understandable of course.

The brushes could be generating the noise, there are a few ways of removing it but, I really don't think that is the mojor problem.

One of the problems could be the drive it self on the service generating the noise due to it's nature in operation. The most common ways I can think of off the top of my head is an isolation xformer or a reactor in the L1,L2,L3 lines to the drive.

The motor it self should be symmetrically sealed with it's metal case and metal seal tight leads from the drive cabinet, to the pecker head.

The armature leads should be in it's own metal race way or flex seal. Twisting them also helps to prevent noise from coming back through the leads since this, effective R.F.

Good luck..

Gretings Jamie, I've been told by the boss that I'm not working hard enough but since I am the boss I just ignore the old coot. The drives, AKA servo amps, are fed DC from rectified and filtered 3 phase, the filtering being one large cap. This power supply scheme is the one provided by the maker of the drives. Would an oscilloscope be useful to track down the source? Could one lead be used as an antenna that could be waved around the various machine components to help spot noise emanations? Thanks, Eric

Using a scope with direct connections may not yield much because you most likely will see this noise all over the inside of the cabinet.

Make a small loop of wire about 1 inch radius (2 inch across) with a good amount of turns. Use some very thin wall wire like teflon, enamel (bell wire) ect. Connect to scope across the leads. Set the scope in the millivolt range and use the loop as a sniffer.

If you don't have an isolation xformer or reactors, the R.F. generated from the drive firing circuits mainly on the output will still radiate over the wires to the main. The lead wires from the supply to the drive inside will generate a nice radiation of noise. From this point i'm assuming you may have a PWM type drive that is using the motor's inductance as part of the equation. This type of system can be very noisy.

what you want to do is concentrate on using common mode chokes to cancel out self generate noise and maybe an xformer. a 3 phase reactor is a good starting point and maybe through in a bucket filter. In your case, you almost have one. Placing a burden (load) resisters on the PS helps in this matter.

To be honest, I would mount a 1:1 xformer on the outside of the enclosure where the drive is housed (metal I hope). Have the secondary only leads going in side the cabinet. At the entry point inside, I would have a 3 phase reactor to help things along. Also, using rope constructed conductors reduces the skin effects greatly that carries this signal, along with swing chokes on the output of the drive..

P.S. Try putting loops in the wire as an experiment to see if it reduces the problem. I find putting a single loop at L1, L2, L3 serves as a service check point for current measurements and, it also acts like a choke.