What kind of frequency does a DC brush motor generate? There are a few explanations online as to what causes EMI in DC brush motors (commutator sparking for example) and how to reduce it, but no mention of frequencies at all.

I know this is a poor question, but there I simply cant find an answer to it. I know it varies on the motor and other factors, but I am simply looking for an order of magnitude (kHz, MHz, GHz...).

  • \$\begingroup\$ It would depend on the rise and fall time of the current in the brushes make or break, which is dependent on the RPM as well as a host of other, essentially, unmeasurable mechanical parameters specific to the motor. It's like asking what is the typical audible noise spectrum of an engine. My understanding is it tends not to reach the GHz range though. \$\endgroup\$ – DKNguyen Oct 16 at 17:51
  • \$\begingroup\$ I could provide the datasheet of the motor if that helps. But a estimation like yours is what I'm looking for, obviously the more precise the better. \$\endgroup\$ – Ryan Oct 16 at 17:56
  • \$\begingroup\$ I think it is in the MHz range though, because what isn't? I think there was a small window in history when battery and motor technology made electric power viable for radio controlled models but before brushless motors and 2.4GHz radios were used. You should ask those old-timers. Those will be very small motors however. The pilots used the 72MHz band and drivers used the 54MHz band. \$\endgroup\$ – DKNguyen Oct 16 at 17:58
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    \$\begingroup\$ I am an old-timer who has been playing with radio controlled models since the '70's. EMI was a big problem with 27MHz AM receivers, not so bad at 40-72MHz with FM but still sometimes hard to eliminate (caused a few of my planes to crash!). At 2.4GHz it's not a problem anymore, due to lower EMI levels and the receiver's ability to reject interference. \$\endgroup\$ – Bruce Abbott Oct 16 at 21:08
  • \$\begingroup\$ @BruceAbbott This is interference from brushed motors right? Not engines? (wait...that might be a dumb question since glow engines don't have spark plugs). \$\endgroup\$ – DKNguyen Oct 16 at 22:56

The fundamental frequency is determined by the commutator switching speed (which varies with motor rpm), but the rf produced by arcing can go up to GHz frequencies. Here's an example:-

enter image description here

The blue line is before adding ferrite cores to the motor leads, the green line after.

At lower radio frequencies (below 100MHz or so) small capacitors connected from each motor terminal to the case (possibly combined with small inductors in series with the leads) are usually sufficient to reduce EMI to an acceptable level. At GHz frequencies the wiring itself blocks it, but various internal parts of the motor can act as antennas. If the motor has large cooling slots or a plastic end bell these frequencies can easily radiate.

At audio frequencies EMI is mostly conducted through the wires as a result of the pulsing current. This is mitigated by twisting and/or shielding the motor wires and keeping sensitive circuits away from them, putting large capacitors across the supply rails, avoiding ground loops etc. In severe cases the motor might need to be on a separate power supply.


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