Motor Brush Noise 1

[farmer2]

I want to compare 24 volt DC motor brush noise between several motors in order to determine which brush material or capacitor addition gives the lowest EMC emissions?

What instrument could be used for this instead of having to send each motor to an EMC test lab? Once the motor with the lowest emissions are determined that motor would be sent to a lab.

 

[waross]

Will you be comparing volume, frequency spectrum or something more esoteric?

Bill
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"Why not the best?"
Jimmy Carter

 

[edison123]

capacitor addition ? EMC emissions ?

 

[Clyde38]

AM Radio?

 

[farmer2]

The emc test lab measures emissions in units of dB micro volts per meter. The emissions on a motor, tested at an emc lab, were slightly over the max. in the 40-45 mhz frequency range. A .047 micro farad capacitor was added from the field to ground and had no effect on emc emissions.

The motor mfg will try different things to reduce these emissions and needs a way to compare motors at their facility for the best solution without having to send each one to the lab.

It is a 24 volt DC, series wound motor. Also, need ideas on how to reduce the emissions which are coming from the brushes.

 

[itsmoked]

Find a radio receiver of that frequency range and drape an antenna across the motor in a carefully repeatable position. Listen to no-motor power. Turn on the motor and hear the noise it contributed to the receiver's output. It will be very recognizable. Tune the radio to maximum focus on that noise. Set the volume level low to prevent psychotic events. Mess with the antenna placement until you find an easy location. Easiest would be X inches to the side of the case so you only need to pick up and set down the motor.

Once that's set they can screw with brushes and caps and inductors to their hearts content. A large reduction in the noise volume will mean a clear reduction in EMI, worthy of sending back to the test house.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

To elaborate on Keith's thinking. It is very difficult to judge anything by aural or visual means. Attach a volt-meter to the speaker of that radio. Make sure you haven't tuned to a broadcasting station. Then you can read noise intensity from your volt-meter.

There are also USB scopes with respectable FFT analysis at quite affordable prices. Some of them will easily measure 50 MHz (not millihertz, please) signals.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[sreid]

The way to kill emissions from a brush type DC motor is to use inductors (installed right at the motor). Ferrite beads can be very effective and inexpensive. Brush noise tends to be high frequency (all the little brush arcs).

 

[farmer2]

Followup: The motors passed tests by reducing the metal content of brushes in most of the motors while a couple required capacitors between the armature and ground. Two different cap sizes were used. A .047 microfarad and 1 microfarad.

Thanks to all for your help.

 

[itsmoked]

Hey, thanks a bunch for the feedback!

Keith Cress
kcress -

http://www.flaminsystems.com