AIUI, an AC induction motor under the control of a variable frequency drive (VFD) has its speed controlled by varying the frequency of the current supplied by the VFD. OK, so my expectation is that when I hear whining noises from the motor, I should expect to hear a very low frequency when the motor is just starting from a dead stop, varying smoothly to a high frequency when the motor is spinning fast.
However, I sometimes hear very different noises from various motors, including subway trains and industrial blowers. On one occasion, the sounds I heard were analogous to the shifting of an automatic transmission: rising pitch for a second or two followed by a jump to lower frequency, at which point the pitch rose for another second or two before repeating that pattern a couple more times. No apparent interruption/change of acceleration when the pitch changes. My uninformed speculation is that phases are being incrementally added or subtracted to the current delivered to the field windings, and that this somehow accounts for the sudden pitch changes. If that’s not it, I’m curious to know what’s really happening.
More recently I heard a blower spinning up that had a different pattern; the frequency of the whining was constant for several seconds as the blower spun up from a dead stop, and then the frequency began rising steadily as the blower continued to accelerate. Can’t quite make sense of what’s happening here.
The output from a VFD is kinda funky. What you are hearing is the varying switching frequencies used to generate an approximate sine wave. I think you are picturing a much simpler switching than what they actually do.
They’ll vary the switching frequencies in order to get a more sine-wave type output, though they rely on capacitance in the line and in the system in general to filter it to an actual sine wave (or sine wave-ish).
So in that plot you linked to, the VFD has two positive voltage levels, two negative voltage levels, and a zero voltage level, and it dithers from one level to another, relying on the inductance of the motor’s field winding (and whatever capacitance the VFD has onboard) to ultimately create a field current that’s roughly sinusoidal - and what I’m hearing is the high-frequency “fuzz” on that sine wave?
Am I now understanding correctly? If so, then the situation I recently saw with the industrial blower makes more sense: the VFD can stick with a low switching frequency until the rotor RPM is sufficiently high that it (the VFD) has to increase the switching frequency so that it can continue to approximate a decent sine wave at the higher field RPM.
But the other situation I heard, i.e. the one that sounded like an automatic transmission, still is puzzling. Why would the switching frequency be repeatedly ramping up and stepping down as motor RPM steadily increases? (FWIW this was on a subway train in Japan)
ISTR rail in Japan uses DC. the cars could have electronically commutated motors which could vary the commutation frequency and number of poles based on motor speed.
Have operated many pumps and ventilation systems with VFDs. I can not remember any one of the drives or motors making noise if the bearings were in good shape. The fans and pumps did make noise. But a motor with a drive on it made the same noise as a motor connected acrossed the line.
Perhaps they vary the switching frequency for some reason, such as optimizing the relative stresses on winding insulation, drive capacitors, or core eddy current losses. Given that there is a microprocessor program controlling the switching, it would take very little motivation to execute such a thing.
