EMI suppression of PMDC motors
There are 2 ways to control EMI. One is restrict it from happening, the other one is restrict it from spreading.
a. Restrict EMI from happening
EMI of PMDC motor comes mainly from commutating sparks. Restricting sparks will suppress motor’s electromagnetic interference.
Add varistor between commutator bars. This is a very effective method.
Change the position of brushes. Change connection between winding coils and commutator bars. Make the motor commutate in advance. Such method sacrifices the efficiency of winding coils.
Increase commutating bars to reduce bar to bar voltage. Reduce commutating voltage.
Change the composition of the brushes. Increase the resistance of commutating circuit.
b. Prevent EMI from spreading
The above mentioned methods reduce electromagnet interference. But we can’t totally eliminate EMI. It is just a matter of more or less. We can also suppress EMI by preventing it from spreading.
Add a capacitor between terminals to filter conducted (voltage) emission from 150KHz to 30MHz. Examples are: Add an X-capacitor (0.47µF—2.2µF) to HVDC motor. Add an electrolytic capacitor (10µF—100µF) to LVDC motor that rotates in one direction. For LVDC motor that runs in both directions. add a non-polarized metal film capacitor or a monolithic capacitor with capacitance greater than 0.47µF.
 Add a choke to each terminal. Inductance can be a few µH to 100 µH. It suppresses conducted emission (power) at 30MHz to 300MHz and radiated emission at 30MHz to 1GHz effectively.
Add a capacitor with small capacitance between the terminals and motor housing. Examples are: Add Y222 capacitor to HVDC motors. Add 103, 104 capacitors to LVDC motors. They suppress conducted emission (power) and radiated emission at 300MHz and below.
Below chart is an EMI suppression scheme for a LVDC motor (uses both battery and grid electricity as power supply).