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Floating DC motor hums when stalled

Can someone explain this observation:

I am running a brushed DC motor with an H-bridge.
Voltage of motor bus: 140VDC (120vac * 1.414 = 140VDC)
PWM frequency: 20khz
5V for digital part, 15V for the motor driver; 15V Gnd of motor driver
module connects to motor return at one point.
The 5V circuitry is isolated from the 15V/motor circuitry via
optoisolators.
The 15V/140V driver/motor circuitry is FLOATING.

I notice that there is a nasty hum on some motors when i encounter a
stall. I tried hooking up the ground plug of a floating oscilloscope to
the motor return, and all the humming magically disappeared!
But if I put a .1uF capacitor from the motor return (which is also 15V
Ground) to EarthGround, then the humming disappears. WHY??

Also, I noticed that, with the .1uF capacitor alone, and I probe the
voltage across one leg of the motor:
In one direction, I will notice a clean rectangular duty cycle from 0
to 140. But when the motor is turning the other way, instead of being
at ideally 0V, I will notice some intermittent 1MHz spikes of
significant magnitude when the PWM is off. If i put a resistor in
parallel with the .1uF capacitor, then the lower i go with the resistor
value seems to minimize these noise spikes.

So what I wonder about, in summary is:
1. Why does merely putting a scope's ground probe on the motor driver's
Vss pin cause all humming to disappear?
2. Without the scope's ground probe, the .1uF capacitor from the Vss
pin to earth ground also the humming to disappear. What effect does
this have?

3. The lower the resistance between motor Vss to earth ground, then the
less noisy voltage spikes across the motor. What is the "standard"
configuration: A floating motor? Or a motor whose return is directly
connected to earth ground (0 ohms). I thought that a floating motor is
safer so that one does not get electric shock, vs. one that is hooked
up to earth ground. Doesn't my galvanic isolation with optocouplers
then get rendered useless when the digital ground and the motor ground
are each connected thru a resistor to earth ground?

-Mike
 
oops. i meant 100VAC. I actually have a 100VAC tap from the primary.
And I have a 50VAC tap. I use a relay two switch between which taps i
want to use, depending on the motor rating. So i have 141VDC and 71VDC
 
J

Jamie

Jan 1, 1970
0
Can someone explain this observation:

I am running a brushed DC motor with an H-bridge.
Voltage of motor bus: 140VDC (120vac * 1.414 = 140VDC)
last time i did that it was around 170 volts?
 
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