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· Electric Al
4,813 Posts
View attachment 32002
When I turn the vfd to 68% the motor makes this loud humming noise. Sounds like it's vibrating. If I go past 68% on the vfd. The noise stops and the motor sounds great. Any ideas?

3 phase 480v

Just curious ! What does it drive ?

Does it act like that when there is no load ?

· Premium Member
2,048 Posts
The voltage going into the motor is nonsineusotial in it's shape.That causes the windings to vibrate a odd frequencies. What you are hearing is normally heard on motors that are powered by a drive. LC

· IBEW L.U. 1852
5,250 Posts
Meh....no worries there. Every VFD driven motor I've ever worked on has make some wierd humming sounds in the lower speed ranges right up to about 75% speed or so.
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· Registered
5,774 Posts
You probably hit a resonant frequency. Hopefully it will be a non-issue with the blade on.
Bingo. Most machines have at least one if not multiple resonant frequencies. It means that there is some sort of mechanical speed at which vibrations in different parts interact with each other and amplify to where they are noticable, maybe even dangerous. Most of the time these are lost in the flood of other vibrations and you can't tell they are there. When you run an uncoupled motor, nothing else is vibrating so you can notice it.

Most good VFDs have a feature called "critical frequency lockout" or "skip frequencies", something sounding like that. What that does is allow you to jump over or stay below a resonant frequency if and when you find one (or more). But as 8V71 said, don't react to anything until you are running the real load, it will all change again because the mechanical issues change.
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