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Variable speed drives


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#1 leeks

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Posted 26 November 2003 - 04:13 PM

can someone tell me, If I running a 0.75kw or 0.55kw induction motor at 10Hz with 415V by using an invertor without independence cooling fan, will it burn the motor? what is the normal recommanded lowest frequency can operate for the induction motor?

#2 marke

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Posted 26 November 2003 - 06:11 PM

Hello leeks

Operating a motor at less than it's rated speed will reduce the cooling of the motor due to the reduced fan speed. If you operate the motor at rated torque and reduced speed, then you will inclrease the heating of the motor and this will shorten the motors life. At a very low speed, such as 10 Hz, there will be no appreciable cooling from the cooling fan, so you will be relying entirely on the convection cooling of the motor. Provided that the losses in the motor are low enough, and the load is light enough, you will not cause the motor to fail. The maximum shaft load is dependent on the motor losses and cooling and this varies from motor to motor. To get a definative answer, you would need to contact the motor manufacturers, however I would question why you would want to operate a motor at 10Hz. The effective maximum power of the motor is only 20% of its full speed power at this speed. (Assuming a 50Hz motor!!)
It is best to select the motor (no of poles) and coupling ratio such that the motor operates close to it's rated speed.
If you can supply more details, perhaps someone can comment further.
Best regards,

#3 Carl

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Posted 22 July 2005 - 05:45 PM

Hello

There are two options, that I know of.
1. you can force cool the motor by attaching a muffin fan that blows into the motor.

2. run the motor through a gearbox.

some people reckon that motors should run cool I've seen motors pumping hot cooking oil to hot to touch. An old engineer said to me once if you spit at the motor and it don't spit back then its fine.


hope this helps

#4 jraef

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Posted 22 July 2005 - 08:16 PM

In every VFD demo case that I have used, if we left the drive running at 10Hz and under for any length of time, it would seriously overheat the motor to the point of letting out some magic smoke! These demo units had no load on them whatsoever. At some point, even convection cooling becomes ineffective to overcome the additional heating caused by the harmonics coming from the drive.
"He's not dead, he's just pinin' for the fjords!"

#5 marke

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Posted 22 July 2005 - 10:31 PM

Its very hard to put that smoke back in again isn't it!!!

#6 moltra

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Posted 04 August 2005 - 04:07 PM

Running at that low a speed is very bad for the motor. install a powered fan to remove the heat generated

Moltra

#7 jraef

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Posted 04 August 2005 - 04:46 PM

QUOTE
Originally posted by marke
Its very hard to put that smoke back in again isn't it!!!


I know, I have tried over and over (as long as the end user is not looking over my shoulder), and so far it has never worked.

Once I give up, I start looking for someone else to blame!
"He's not dead, he's just pinin' for the fjords!"




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