Voltage notch depth in MT circuit

Hello to every one and happy new year,

during a dynamic test on a medium tension AC motor, I've found a repetitive notch depth.

I have read that could be linked to a non linear load, but the questions are:

- in MT circuit is there a % limit referenced to an IEEE standards?

- the motor is drive a feedwater pump inside a combined cycle power plant. Which non linear load could be the origin of signal distortion?

- in attachment the image of the signal. What do you think about the notch depth level?




Photos (1)
Original Post

There are 6 equally-spaced notches per cycle. So this is associated with commutation of the 6 electronic switches in a three phase full bridge converter (example rectifier bridge or SCR bridge).    It is either an electronic converter upstream of this motor (dedicated to the motor or maybe to a larger bus that includes the motor) or an electronic converter powered from the same bus as this motor.

Either way it does not indicate any problem of the motor itself.  Also it generally does not indicate a condition dangerous for the motor   Motor inductance acts like a low-pass filter to high-frequency voltage notches such that the resulting THD of the current is much less than that of the voltage... so you're not going to get high current THD or associated heating effects.  If anything there might be a concern for the rate of change of voltage (high rate of change of voltage can stress the motor turn insulation).  

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