VFD causing disturbance for Stand-by generator

[vetlover]

Gentlemen,
I have an issue with a Yaskawa VFD (480v) and a new stand-by generator. When the VFD is starting the blower motors on HVAC units, the generator is sending wide voltage swings and even causing generator engine speed disturbance. I am thinking the generator may be undersized (250kw) but wanted to see if anybody new the ratio of gen to VFD kva required. I know a generator needs to be at least twice the size of a UPS, is a VFD comparable or is there something else to look at? Thanks for your input.

 

[waross]

What size is the motor? What are the settings of the VFD?
Does the genset have a Permanent Magnet Generator option?
If you are ramping the start load too fast on a capacity challenged generator the prime mover will slow down. When the frequency drops, the Under Frequency Roll Off feature of the Automatic Voltage Regulator will drop the voltage almost proportionately.
Motors are not quite like UPSs. When a UPS is drawing full load and charging dead batteries simultaneously the input may be double the rated output.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[vetlover]

Bill,
Thanks for your response. There is not a PMG> There are (2) 10HP motors. I do not presently have the settings, but understand your ramp concern. I can say that the voltage is not proportional, it is acutally fluctating both up and down during the speed build up. I think I see where you are going, are you thinking that the PMG will help the regulator stay independent of the main generator? This is our baseline analog voltage regulator also, nothing special.

 

[dpc]

There have been issues with harmonics created by VFDs causing a lot of problems for older analog voltage regulators. Some additional filtering and shielding generally cured this problem.

I would think a new 250 kW generator would not be having this problem, but you never know.

Does the wiring from the VFD to the motors pass close to the generators? This motor feeder is a powerful source of RFI.




"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

 

[ghostbuster7]

We have had good success with true drive isolation transformers in containing these types of V.F.D.elctrical disturbances.

 

[itsmoked]

You might consider an input EMI filter for the VFD. Often the drive's manual will recommend specific ones.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Does changing the ramp setting on the VFD have any effect on the generator response? Can you put a scope on the power and sense terminals of the AVR to see if you are getting serious voltage dips or waveform distortion?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[BSEE1980]

Does the VFD have incoming AC filter caps? Unloaded VFD's have a very poor (leading) PF that can supply a lot of VARS to the genset, causing exactly what you are seeing. I agree with the other posts that it is probably an interaction between the AVR and the system.

You might have to start some regular AC motors to give you some reactive VARS before you start your VFD, or delay the AC filter caps in the VFD until you have some load.

Or i have also seen resistors added to load the genset up some before starting the VFD's, but not so sure they really helped that much.

Or just bypass the VFD's when on the genset and start the motors across the line. A 250KW genset will start 2 10HP motors with no problems, especially a low inertia load like blowers.
Good luck!
Steve