View Full Version : Anyone tried motor-running caps + line reactor to lower VFD noise?

noah katz
11-21-2007, 08:51 PM
I just got a Magnetek GPD 403 1 HP drive to run a Palmgren 3/4 HP 3-ph grinder, 2.7 FLA.

The Magnetek must have a PWM freq well down into the audible range, as it's really noisy.

I looked through the manual for a freq setting but didn't see it.

At certain freq setting the motor just sings.

Besides being a bit distracting, I'm concerned it might be prone to insulation breakdown.

I put a 4 A 6.5 mH line reactor between the drive and the motor and it didn't make a bit of difference that I could tell.

Could I put 3 motor-running caps across each pair of motor leads to absorb the high freq?

What voltage rating and max mfd rating (the higher the better the noise suppression) should I use?


11-21-2007, 09:23 PM
If you aren't careful you will create a L-C tank circuit. and if you hit resonate frequency motor hum will be the least of your problems

noah katz
11-22-2007, 01:20 AM
Hmm, I thought the circuit configuration made it a 2nd-order lowpass filter...

11-22-2007, 07:20 AM
I am not familiar with that model but have worked with 505s which are now obsolete.Google Yaskawa for in depth technical manuals on Magnetek drives.The device you speak of between the motor and drive is a load reactor. I'm not sure if line and load reactors are designed the same. One option is to change the carrier frequency to reduce noise if that parameter is available on your drive model. Excellent phone support is also available from Yaskawa.

11-22-2007, 08:22 AM
Look at it this way, if caps on the output of a drive reduced the carrier noise one of the drive manufacturers would be offering it.

11-22-2007, 09:05 AM
You have to consider that the drive is not generating 3 phase, it's throwing out a series of pulses that when connected to the motor it is configured for will drive it in a way which closely approximates 3 phase.
If you start shoving stuff in the line between motor & drive things could get interesting, scary or smokey,

11-22-2007, 09:40 AM
Have you tried swapping motor leads?I don't know exactly why,but I had a Marathon motor that screamed from 42-65htz,until I swapped leads,just swapped T1 and T2 and problem solved.

11-22-2007, 09:51 AM
What Wierd said. Mine was a Baldor, swapped T1 and T2, and the screaming jet had taken off.

11-22-2007, 09:57 AM

About three years ago I installed a inverter into a mcc panel.
Fit all well, ohmed out the motor leads and there was continuity and free from ground.

Hooked up the inverter and it smoked. Capacitor-resistor networks in the motor pecker head. TOOK out the power transistors in a heartbeat. They put them in as spike surge protection for plcs all over the plant.

OOPS. Removed and replaced inverter and it is still running today.

If the motor makes too much noise? turn the stereo up.

Forrest Addy
11-22-2007, 12:35 PM
Most every FVD manual I've ever paged through specifically cautioned against caps in the load side of the VFD. It relies on the inductive kick from the motor to tailor its response to changing load conditions. Cap reactance will screw that all up and maybe interact with the motor inductance. It may cause a resonance possibly containing large circulating currents and overvoltage and smoke the putput transistors.

The GPD 405 is older technology using a lower carrier frequency. What you hear is magneto constrictive noise. Iron changes volume very slightly fas the magnetic field varied. If the varying field is square wave and in the audible range is results in the motor emitting a noise. Unless you can select a higher or less irritation carrier freq, there's not much you can do about the motor squeal except build a sound proof enclosure anound it.

Adding. Swap any two line leads in a three phase motor and you reverse it.

noah katz
11-22-2007, 12:58 PM
"You have to consider that the drive is not generating 3 phase, it's throwing out a series of pulses that when connected to the motor it is configured for will drive it in a way which closely approximates 3 phase."

The line reactor is an inductor which is 1st-order lowpass (high cut) filter, and the caps would make it 2nd-order with a steeper rolloff of the high freq carrier, making it a better approximation to a sine wave.

That's the theory, but I'll heed Forrest's warning.

Now that I think about it, lower carrier freq might decrease chance of insulation failure because of lower voltage spike rate.

And I'll try the lead swap, too, thanks.

11-22-2007, 07:03 PM
You are very correct, lower carrier frequency does not stress the motor insulation as badly. Especially good on non inverter duty motors. I wish i photographed the motor that was killed by a VFD.