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How fussy are VFD'S using "dirty" three phase power

Brian Campbell

Plastic
Joined
Dec 14, 2021
I have a 7.5kw lathe motor that needs to be soft started as the local power utility has a limit of 4kw direct online starting for three phase motors as this can cause voltage drops which affect neighbors.



The three-phase power I have is generated using a rotary phase converter. I stagger start my rpc so have enough capacity for the lathe motor. I will try to balance the output with caps but don’t expect to get the voltage balance perfect. All the machines I need three phase power for are old so do not need perfectly balanced power.



I have a SEW MOVIDRIVE vfd I am planning on using, its capable of starting a 7.5kw motor.



The concern I have is that with all the fancy electronics in the vfd will this be thrown out of order using “dirty” three phase power from a rpc?



Thanks, Brian
 

Karl_T

Cast Iron
Joined
Jun 13, 2008
Location
Dassel,MN,USA
I am doing exactly this on my 5hp manual mill, no issues at all. Need the generated three phase for the power feeds
 

mksj

Cast Iron
Joined
Aug 10, 2010
Location
Tucson, AZ
You could also oversize the VFD, run it off of single phase in a derated mode. You would want to use it with a DC choke to decrease the THD and fusing requirements. There can be some losses in running a VFD off of an RPC, I typically go to the next size up VFD size then I need, and then adjust the motor parameters. There can be some sizing issues relative to the RPC.
 

JST

Diamond
Joined
Jun 16, 2001
Location
St Louis
An RPC is not really "dirty" power. It is slightly unbalanced, maybe, but it's not particularly obnoxious in any other way. No spikes or much of anything other than maybe some harmonic content.
 

BT Fabrication

Stainless
Joined
Nov 3, 2019
in my machine there is a reactive transformer that balances out the spikes a little bit. sure you want as clean as possible with in 5%
 

JST

Diamond
Joined
Jun 16, 2001
Location
St Louis
It should be perfectly fine.

Remember, the VFD itself makes the dirtiest power you can think of.... all pulses and spikes. If THAT does not bother it, a little extra harmonic content won't be an issue.

Very high harmonics, far worse than any idler motor makes, might be a problem for the bus capacitors, due to "ripple current". Otherwise, not likely.
 

jstolaruk

Cast Iron
Joined
Apr 2, 2005
Location
SE Mich.
Having been bit a few times with neighboring machines throwing noise back on the buss and messing with the sensitive electronics, I routinely design in a line reactor in front of VFDs and Servos. They are inexpensive and save time if I didn't design it in and found out when commissioning it was needed.
 

JST

Diamond
Joined
Jun 16, 2001
Location
St Louis
Having been bit a few times with neighboring machines throwing noise back on the buss and messing with the sensitive electronics, I routinely design in a line reactor in front of VFDs and Servos. They are inexpensive and save time if I didn't design it in and found out when commissioning it was needed.
That's actual "dirty" power.

OP was only asking about RPC output going to a VFD. No spikes, nothing but some low harmonics. Nothing I'd call actual "dirty power".
 








 
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