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26th September 2011, 03:23 PM #31Cba
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Ray, thank you for the link to the ABB technical note. As a side note some 40 years ago I was an apprentice at ABB in Switzerland (back then is was still called BBC, before merging with the Swedish Asea to become ABB). For about a year I was also testing motor drives for BBC, DC drives back then in the 70's.
I found this simple picture to visualize how a modern VFD creates a nearly sinusoidal motor current by modulating a PWM carrier consisting of rectangular pulses: https://www.ohiosemitronics.com/pdf/...ng-VFDs(F).pdf
I would think small HP motors do normally not need reactors or line filters between VFD and motor to achieve this sinusoidal current shape. This is so, because small motors (say under 5HP) would already have a high impedance for high frequency transients (unlike large motors, small motors are made with lots of windings of thin diameter wire). But as you say, if the line between VFD and motor is some 50m long, things change, and this is why filters and reactors are optionally available from the VFD makers.
The insulation of an ordinary new 3 phase motor is designed to withstand the voltage when hooked up in Y (415V). But if using a single phase VFD, we hook up this motor in delta (240V) which certainly stresses the insulation less to begin with.
VFD's are nowdays very widespread, and I believe any newer motor on the market is nowdays manufactured with VFD use in mind. Even older motors will cope very well. The problems are more with large motors (10's of HP). Or with older motors that already had a history of overheating that previously weakened the insulation. Moisture and /or dirt inside the motor will also weaken the insulation over time. Maybe a good idea to carefully clean and dry an old motor before hooking up to a VFD.
Chris
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