VFD & Servo Noise

[jmoore@jhm]

Is it possible that VFDs can cause noise to a degree of intermitent fuse blowing on Servo drives fed from the same immediate power source?

We have 4 VFDs each powering a 1 1/2 hp motor, then connected to the same power source in the same nema inclosure are three AB Ultra3000 Servo drives which each power a small 1 1/2 hp servo motor.

None of the drives, VFD or Servo are overloaded, yet we have expereinced intermitent fuse blowing on the input power to the Servo drives. The drive have the exact 5amp fuse as recommended by AB and the system operates at 480 vac 3ph.

If the VFDs are suspect, what is the solution. This problem seems to be isolated to this system, since no other surounding equipment is experiencing problems.

 

[rsdoran]

Do the VFD's have line reactors? VFD's can create harmonics on the incoming power source that may cause problems.

 

[jmoore@jhm]

No we have the VFDs connected directly to the main input power just as the servos.
It is now become a quesiton as to if we need line reactors to eliminate the potential for the VFDs to back feed to the input power and send a spike to the input of the servos on the same line.

I was told today that may be the cause, I am looking for additonal input on the subject, this is a major system installation and this condition has exsisted for several weeks and I now need to find the true "root cause".

 

[rsdoran]

Checkout this thread: http://www.plctalk.net/qanda/showthread.php?s=&threadid=4701

http://www.mtecorp.com/load.html

Use of an oscilloscope can see the harmonics on the line side.

 

[DickDV]

jmoore, you have to consider that "VFD noise" can be one or more of three different types. There is harmonic noise projected back into the power supply network. There is radiated noise (RFI and EMI) from the high frequency pulses on the motor leads. And, there is the worst of the three---common mode noise which comes from the motor lead pulses capacitively coupling into the ground system and then searching for a path back to the drive DC bus.

The easiest one to deal with and certainly the best understood is the input harmonics. At your horsepowers, an input reactor makes an inexpensive filter unless you have low input voltage problems. Check first to see whether the drive comes with some input reactance standard. Some do.

Whether any of these noise types is your problem is hard to determine. However, while not being a servo expert, if noise is coupled into the speed regulator on the servo amplifier, it may be making the motor do thru sudden (high current) accel/decel cycles. This may be very fast noise and not readily visible by the drive display or ordinary metering. It seems reasonable that such short-term high current transients could eventually take the input fuses out.

If the harmonics aren't the problem, try shielding the motor leads and/or separating them as much as is possible from any servo leads, especially the speed or torque references.

If the problem is common mode noise, check back here. That subject is complex and just simply ugly. Let's hope the problem goes away before we get to considering that.