# Question on the right PWM frequency for Permanent Magnet DC Motors

I know that there are some posts in this site about this question. I don't know if there's a way to find out the "ideal" PWM frequency for motor control. Usually they recommend to be safe in a range from 2 to 25 kHz. For example, in this post: Is there an ideal PWM frequency for DC brush motors? they recommend to compute the motor time constant in order to find the proper frequency.

Now, the main question is this. I took the following test fragment from a thesis.

Is this approach correct or reliable?

This is the motor I'm controlling:

The board that controls this motor measures speed and current. Does this data help in finding what is best for this motor?

• 2kHz and above will be just fine. This is pretty big motor and has plenty of inductance. Commented Oct 14, 2018 at 19:30
• Thank you. If it's a big motor, does it mean it has plenty of inductance? Plenty of inductance means the motor has low cutoff frequency?
– user115094
Commented Oct 14, 2018 at 19:36

Now, the main question is this. I took the following test fragment from a thesis. Is this approach correct or reliable?

The quoted passage is about field-control of a wound-field DC motor with a commutator. It has no relevance to armature control of a permanent magnet DC motor.

This is the motor I'm controlling: The board that controls this motor measures speed and current. Does this data help in finding what is best for this motor?

The motor nameplate provides the rated: armature voltage and current and the rated horsepower and RPM. You could see if the manufacturer has published the motor time constant and use the procedure given in the other question. You could also look for the motor manufacturer's recommendations about controllers.

• A fast question about that motor. It has 3 wires: Red, Black and Green. The nameplates says: Motor must be grounded. What does the green cable mean?
– user115094
Commented Oct 14, 2018 at 20:37
• Green should always indicate ground for equipment sold for the USA market. However it would be unusual for a motor to be sold with a ground wire attached. The ground wire of the supply cable is usually attached directly to the motor frame.
– user80875
Commented Oct 14, 2018 at 20:54
• Ok. I just asked because I'm using only red and black wires. The green one is floating.
– user115094
Commented Oct 14, 2018 at 20:58