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Questions Regarding Selection Of Single Phase Generators


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#1 AB2005

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Posted 07 April 2012 - 07:23 AM

Hi Friends,

I have some questions regarding selection of generators,
A small weaving machine has a single phase 1HP motor and don’t require a high starting torque. A 125% (almost 1KW) rating generator is sufficient for this motor?
If we run above motor through a soft starter, would the rating of generator reduce from 125% to 100%?

Second case
A cutting machine has a 1.5kw 1phase motor which runs the machine by rotating a fly wheel and then a clutch. Normally, flywheel accelerates to full speed within 8sec. A doubled capacity generator would be sufficient for that application?
If we run above motor through a soft starter, would the rating of generator reduce from 200% to 100%?

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

#2 marke

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Posted 07 April 2012 - 09:30 PM

Hello AB2005
First case. The motor needs to be started initially. The normal starting current for an induction motor can be 600 - 800%.
If you apply a soft starter, you can reduce the start current provided that there is still sufficient torque to accelerate the motor to full speed.
As you reduce the start current, you reduce the start torque by the square of the current reduction, so typically, you will need 350 - 400% current to start an easy load. I would suggest that the generator needs to be able to supply an overload of several KVA in order to start this motor.
Self excited alternators typically have very little overload capacity so I would expect that you would need to use a generator of perhaps 3KVA capacity.

Second case, much the same as the first. I would expect that even with a soft starter, the generator would need to be in the order of 4.5KVA or higher.

Best regards,
Mark.

#3 AB2005

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Posted 08 April 2012 - 06:28 AM

Hello AB2005
First case.
Self excited alternators typically have very little overload capacity so I would expect that you would need to use a generator of perhaps 3KVA capacity.


Do I need 3KVA generator in both cases if i start the motor direct or thorough soft starter? There is no any advantage by using soft starter?

I saw a place where a single phase 1kva generator was starting a 1HP pump motor. When motor was started, generator RPM decreased almost 30% and then accelrated to full speed and they were doing that practice for a long time without any trouble.

I am sorry Mark but by seeing such applications people don't accept our argument to install a higher capacity generator as you suggested. Can you put some light that what would be the bad effects/results experienced by installing generator which capacity is very near to the required input power of load motor?
"Don't assume any thing, always check/ask and clear yourself".

#4 marke

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Posted 09 April 2012 - 06:40 AM

In order to answer categorically, we would need to know precise details of the motor, load an possible generators.
My earlier cooments apply, but that does not mean that some smaller combinations may work with a drop infrequency and voltage. - A case of try it and see.




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