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Higher Current At Lower Speed


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#1 chaterpilar

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Posted 14 August 2007 - 06:35 AM



We have a Fan ( 375 rpm) run by a 3hp 1720 rpm motor ( speed reduction by pulleys-V-belts).

Now i have installed a 7.5 hp VSd and i have coupled the motorshaft direct to the Fan shaft, ( no pulleys).

I started the motor at 13 hz ( equivalent to 375 rpm) using VSd.

Now, the current ( 4.5 amps) on the input side of VSd is matching with current the motor used to draw using conventional pulleys method, however the cuurnt on the output of VSD is higher ( 7.5 amps).

The VSD manufacturer say that this is because it is running at very low Hz and we have to use a 8 pole motor so VSd can run at 30 hz.

Why is the current increasing, since the work done is the same.

Chaterpilar


#2 marke

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Posted 14 August 2007 - 08:26 AM

Hello Chaterpilar

You have fallen into the classic mistake!!

When you use a mechanical speed changer, the power rating of the motor (and system) does not change. As the shaft speed reduces, the shaft torque increases, provided that the motor speed remains constant.

If you reduct the speed of the motor using a VSD, the flux in the iron remains constant. Therefore, the torque output of the motor shaft remains constant. As power equals speed times torque, the effective power rating of the motor reduces with speed.

Therefore, if the motor was operating at full load when you used a mechanical speed changer, the motor will be overloaded to almost 500%, and I would expect the fan to be running slow due to the motor slipping. The motor will overheat and fail if you operate it this way!
The other issue to consider, is that the cooling of the motor will be severely compromised at this low speed. If the motor was to operate near rated torque at less than 30Hz, you should apply forced cooling to the motor.

Best regards,

#3 chaterpilar

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Posted 15 August 2007 - 03:36 AM

Thanks for your response Marke, but if i use a motor with 8 poles or 12 poles then the VSd would be running higher than 30 hz and also the cooling would increase.

Actually in this application, the Fan cools the motor because the air (which the Fan is pushing ) moves over the motor.

Chaterpilar

#4 marke

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Posted 18 August 2007 - 01:32 AM

Hello chaterpilar

If you use a lower speed motor, it will produce more torque output because the speed is reduced by the number of poles.
An eight pole motor will produce 4 times the torque of a two pole motor aof the same size when running on the normal supply (rated speed).
Replacing your existing motor will increase the torque output for any given shaft speed.

Best regards,




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