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Using Under Rating Inverter In An Emergency Case


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#1 AB2005

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Posted 09 October 2009 - 10:11 AM

Dear Friends;

I have been facing a problem in 11KW Vacon brand Inverter (for 10kw motor) in our Corrugator machine. An internal fault has been displaying after 2-5 hours. The arrangement of new inverter needs some days and if the inverter would be permanently in fault, we would be stuck.

I want to know, is it possible to install an available 5.5kw inverter for that 10kw motor within inverter’s maximum output current limit? I mean, we would run the machine slow so that inverter current may not increase. The application is the rotating slitting knifes which shear the paper in linear direction and doesn’t need high starting torque. Does anybody have such experience for using the under rating inverter?

"Don't assume any thing, always check/ask and clear yourself".

#2 yuri

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Posted 09 October 2009 - 05:48 PM

QUOTE (AB2005 @ Oct 9 2009, 10:11 AM) <{POST_SNAPBACK}>
Dear Friends;

I have been facing a problem in 11KW Vacon brand Inverter (for 10kw motor) in our Corrugator machine. An internal fault has been displaying after 2-5 hours. The arrangement of new inverter needs some days and if the inverter would be permanently in fault, we would be stuck.

I want to know, is it possible to install an available 5.5kw inverter for that 10kw motor within inverter's maximum output current limit? I mean, we would run the machine slow so that inverter current may not increase. The application is the rotating slitting knifes which shear the paper in linear direction and doesn't need high starting torque. Does anybody have such experience for using the under rating inverter?


Hello.
Among many many (unknown to me yet) cites I found "CR4" to be a very informative cite with many clever engineers too - I would post there also if I had the same question (under the "electrical engineering" division).
Meanwhile, I believe, the productivity of the machine forced to work a half or maybe one third of its capacity will be very insufficient.

#3 VST

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Posted 10 October 2009 - 02:45 AM

Please find out how much current your 10KW motor is drawing in loaded condition.If it is belowthe 10amp.(below 50%) you can use 5.5Kw drive for your application.

#4 marke

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Posted 10 October 2009 - 05:49 AM

Hello AB2005

Provided that you keep the current within the inverters rating, you will not have a problem. The one thing to be careful of when undersizing the inverter, is that the inverter must be able to supply current higher than the magnetizing current.

On small motors, the magnetizing current can be as much as 60% of the rated current of the motors and on large 2 pole motors, the magnetizing current can be as low as 20% of the rated current of the motor.

Best regards,
Mark.

#5 AB2005

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Posted 11 October 2009 - 02:50 AM

QUOTE (marke @ Oct 10 2009, 11:49 AM) <{POST_SNAPBACK}>
on large 2 pole motors, the magnetizing current can be as low as 20% of the rated current of the motor.

Best regards,
Mark.


Thanks Mark;
But I think if we increase the number of poles in large motors, the magnetizing current would also be increase as close as in small motors.

What do you say about using an inverter say 10kw for a 5kw motor? I think no any issue only that we need to enter correct overload current of motor. This is a general question only.
"Don't assume any thing, always check/ask and clear yourself".

#6 yuri

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Posted 11 October 2009 - 08:57 AM

QUOTE (AB2005 @ Oct 11 2009, 03:50 AM) <{POST_SNAPBACK}>
Thanks Mark;
if we increase the number of poles in large motors, the magnetizing current would also be increase as close as in small motors.



It does not apply to your case - you need to reduce the power consumption of the oversized for its VFD motor, but if you increase the number of poles you will increase the power and make the motor even more oversized.

The answer to the "general question" - an absolute "yes".

Sorry for interfering, however.

#7 marke

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Posted 11 October 2009 - 05:59 PM

Hello AB2005

Yes, as you increase the number of poles, the magnetizing current does tend to increase, but not as high as 60% for large motors.
Provided that the rating of the inverter is higher than the magnetizing current, it should work.

Best regards,
Mark




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