Reducing PMDC motor noise ?

[Eric R Snow]

I have a CNC mill in the shop that radiates profusely in the AM band.
The servo motors are brushed PMDC motors. I am wondering if I can use
capacitors across the motor leads at the motor to help attenuate the
noise. The motors are powered by modern servo amps and the power
supply output is 100 volts at 15 amps max. The CNC control is a
retrofit done by me and purchased from Ajax. Ajax has terrible
customer service and has not answered my questions about reducing
electrical noise. The motors are all properly grounded and the motor
cables are enclosed in braided copper conduit. If caps can be used at
the motors what kind at what capacity should I use? Better yet, how
can I calculate what capacitors to use?
Thank You,
Eric R Snow

 

[Phil Allison]

"Eric R Snow"

I have a CNC mill in the shop that radiates profusely in the AM band.
The servo motors are brushed PMDC motors. I am wondering if I can use
capacitors across the motor leads at the motor to help attenuate the
noise. The motors are powered by modern servo amps and the power
supply output is 100 volts at 15 amps max. The CNC control is a
retrofit done by me and purchased from Ajax. Ajax has terrible
customer service and has not answered my questions about reducing
electrical noise. The motors are all properly grounded and the motor
cables are enclosed in braided copper conduit. If caps can be used at
the motors what kind at what capacity should I use? Better yet, how
can I calculate what capacitors to use?

** This would be the WRONG newsgroup - wouldn't it ?

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

 

[default]

I have a CNC mill in the shop that radiates profusely in the AM band.
The servo motors are brushed PMDC motors. I am wondering if I can use
capacitors across the motor leads at the motor to help attenuate the
noise. The motors are powered by modern servo amps and the power
supply output is 100 volts at 15 amps max. The CNC control is a
retrofit done by me and purchased from Ajax. Ajax has terrible
customer service and has not answered my questions about reducing
electrical noise. The motors are all properly grounded and the motor
cables are enclosed in braided copper conduit. If caps can be used at
the motors what kind at what capacity should I use? Better yet, how
can I calculate what capacitors to use?
Thank You,
Eric R Snow

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?

 

[James Beck]

I have a CNC mill in the shop that radiates profusely in the AM band.
The servo motors are brushed PMDC motors. I am wondering if I can use
capacitors across the motor leads at the motor to help attenuate the
noise. The motors are powered by modern servo amps and the power
supply output is 100 volts at 15 amps max. The CNC control is a
retrofit done by me and purchased from Ajax. Ajax has terrible
customer service and has not answered my questions about reducing
electrical noise. The motors are all properly grounded and the motor
cables are enclosed in braided copper conduit. If caps can be used at
the motors what kind at what capacity should I use? Better yet, how
can I calculate what capacitors to use?
Thank You,
Eric R Snow

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
0.1uF ceramics. You might want to go with more capacitiance and at
least double the voltage or better, especially if they do any type of
braking or reversing.

Jim

 

[Eric R Snow]

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?

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

 

[Eric R Snow]

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
0.1uF ceramics. You might want to go with more capacitiance and at
least double the voltage or better, especially if they do any type of
braking or reversing.

Jim

Thank You Jim for the reply.
Eric

 

[Jamie]

I have a CNC mill in the shop that radiates profusely in the AM band.
The servo motors are brushed PMDC motors. I am wondering if I can use
capacitors across the motor leads at the motor to help attenuate the
noise. The motors are powered by modern servo amps and the power
supply output is 100 volts at 15 amps max. The CNC control is a
retrofit done by me and purchased from Ajax. Ajax has terrible
customer service and has not answered my questions about reducing
electrical noise. The motors are all properly grounded and the motor
cables are enclosed in braided copper conduit. If caps can be used at
the motors what kind at what capacity should I use? Better yet, how
can I calculate what capacitors to use?
Thank You,
Eric R Snow

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..

 

[Eric R Snow]

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

 

[Jamie]

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.