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VFD sizing for a range of motor sizes

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qstorm

Electrical
Nov 17, 2002
16
PH
Can a VFD rated for a 184 kW motor successfully operate a motor with a much smaller rating, say 55 kW? I'm working on a project which would require VFD control for the sewage pump motors. Initially, 55 kW pumps would be installed but these would be scheduled for replacement with 184 kW after about 11 years operation when facility requirements would be higher. I intend to design for the utlimate capacity rather than for the initial size since replacing the VFD later would also be very expensive.
 
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Yes. I would say that it can very well be done.

Especially if it is a scalar drive - and a pump drive usually is.

The only thing that I can think of is: Can the thermal protection be set for the smaller motor? That is normally not a problem.

If you think that a vector drive is necessary (can't see why) then you may need to check to make sure that the R:s and X:s (resistances and impedances) of the smaller motor are civered by the parameters in the drive. Again, with a 1:4 ratio between motor and inverter, that should not be a problem at all.

There might be a physical problem with the cabling. A cable for a 184 kW motor may be a problem to connect to a 55 kW motor. But that is solvable, just put a junction box close to the small motor and connect with a smaller area from there. Remember to fuse accordingly!

Gunnar Englund
 
I for one would not waste the money on a larger drive if you don't need it for 11 years! An 11 year old drive right now is not worth repairing and is an anacronism compared to new drives. VFD technology will continue to evolve, who's to say what the predominant technology will be in 11 years. Capacitors will not likely last that long anyway, 5 - 7 years is the average, but by then you may as well take advantage of whatever the latest technology is when you need it. In addition, the way prices have been going on VFDs, the larger drive will most likely cost you a lot less in the future.

Eng-Tips: Help for your job, not for your homework Read faq731-376 [pirate]
 
In 11 years, we may all be using matrix drives with no capacitors to fail in 5 - 7 years!!

We may also be faced with requirements for low harmonic drives which wil require active rectifiers or similar.

Best regards

Mark Empson
 
Wiser than a poodle?
Is that a compliment or what...? You Swedes have expressions that are almost as daft as those used by the English!



----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!
 
Yes. A poodle is a very wise animal. I think that it all started with Martin Luther that said something about a poodle. Not sure though.

Gunnar Englund
 
Gunnar,
If that hadn't come from you, I don't think I would have interpreted it as a complment!

Mark,
Matrix drives, good point. The technology is expensive now, but basic PWM drives were comparitively expensive 11 years ago and look at them now. That is likely the wave of the future that far out.

Eng-Tips: Help for your job, not for your homework Read faq731-376 [pirate]
 
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