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Discussion Starter · #1 ·

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?

50hp
3 phase 480v
60hz
 

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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
 

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Certified Organic A-Hole
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I hooked up a VFD for my senior project in tech, ran a 3 phase 208 motor, motor whined until a certain speed was reached when the centrifugal switch kicks out.
 

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Discussion Starter · #8 ·
Lone Crapshooter said:
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
Do you mean that the sine wave is distorted?
 

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IBEW L.U. 1852
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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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Yes there is wave distortion. Do a wiki for PWM drive and there is a general explanation of a PWM drive. LC
 
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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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