Brushed Motor - Current Ringing 2

[CaptainCarp]

I am not experienced with driving motors and would very much appreciate some guidance in a problem that I am having.

I am trying to drive a 12VDC, brushed motor, with a h-bridge driver / external MOSFETs combination.
I have no specs on the motor but I did measure ~ 1uH of inductance and 0.3 Ohms of resistance across it's leads.
It draws ~ 6A when not loaded.

There is a pronounced "ringing" of the current waveform (see attachment, trace #4) when I try to PWM it.
I tried some other "smaller" motors that we have laying around (~ 2A) and they seemed to work just fine.

If I apply a "load" to the motor shaft, the overall current rises, but the ringing appears to be the same in amplitude and frequency.
The voltages present at the board's motor A and B leads are fairly "clean" (traces 1 and 2).
The voltages at the motor, seperated from the board by about 3 feet of 12ga. wire, are "ringing" pretty badly.
When I placed a 0.1uF capacitor across the motor leads (at the motor), the "ringing" amplitude increased.

Any ideas as to what characteristic of the motor is the cause of this current "ringing"?
Is it too much capacitance inside of the motor (and if it is, how do I compensate for it)?
Are there brushed motors that are optimized for constant DC and not meant to be PWM driven?

Any help would be appreciated.
Thank you,

 

[mikekilroy]

1 good attachment deserves another

IMHO I suggest your motor inductance is about 2000 times too LOW to allow a PWM @ 5khz to control current into this motor.

see Lmin equation at link. IF you want to to just wing it, try adding a 1mh inductor in series with this motor and you may find current more too your liking (and control)

 

[CaptainCarp]

Hi Mikekilroy,

Thank you very much for the information.

I double checked my notes and the motor inductance is actually ~ 100uH, so I'm actually only about 20x too low

I took the motor apart and found what I am pretty sure is a 470nF capacitor connected across the leads.
When I removed the cap, the current "ringing" pretty much disappeared and all I had left was the current ripple.

I would guess that the cap is there to reduce brush noise, but it is a rather large value and I am still wondering if DC brushed motors are design optimized for the type of drive being used and that this particular motor manufacturer used this cap value because the motor was only meant to be supplied with a constant voltage source and not PWM.

 

[rhatcher]

I am guessing that the 'capacitor' that you found is probably for radio noise suppression.