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Measuring PWM Driven Motor Terminal Voltage

Question asked by coolbrian on Jul 3, 2013
Hello all,

My customer is probing the voltage between the two motor terminals.  The scope ground is connected to the negative lead (MTR-) and the probe tip is connected to (MTR+).  The scope (old Agilent 54622A) displays the waveform just fine.  However, it introduces lots of noise into the system.  Without the scope ground on MTR-, the system functions just fine with the usual motor noise.

The motor controller is a low-side PWM driven type, where MTR- switches from the system ground to ~330V.
The motor controller is floating and not connected to the earth.  The scope itself is not connected to the earth either.

Even though connecting the scope ground to a switching node seems bad, I cannot explain how the noise is introduced to the system.  Could anyone show some light on how the extra noise is introduced when the scope ground is connected to MTR-.

Thanks in advance.