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?

  1. 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

Reply to
valemike
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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

Reply to
valemike

last time i did that it was around 170 volts?

Reply to
Jamie

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