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Transformer For Motor


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#1 gul

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Posted 30 June 2009 - 08:57 AM

I WANT TO KNOW THE EXACT RATING OF THREE HASE TRANSFORMER FOT THREE PHASE 30 HP INDUCTION MOTOR ,
HOW TO SELECT THE REQUIRED TRANSFORMER RATING PLEASE GUIDE
THANKS

#2 yuri

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Posted 02 July 2009 - 04:36 PM

Hi.
Why? Look for a transformer with the needed primary and secondary voltages and the appropriate power and thats it.

#3 marke

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Posted 04 July 2009 - 05:05 AM

Hi gul

Welcome to the forum.

Please do not type in capital letters only as it makes it harder to read, and is considered to be shouting. Many people will not answer capitalised posts.

I presume that you are after a transformer to alter the voltage applied to an induction motor, for example a 3.3kv primary to a 400V secondary.

There are two major considerations. The VA rating must be greater than the VA rating of the motor, and the transformer must be sized such that the voltage during start does not drop too low.

Transformers have an internal impedance which is commonly between 4% and 7%. This is the voltage drop on the output of the transformer at it's rated output VA.
When you start a motor, it will draw a high current until it reaches full speed. If the motor is DOL started, it will typically draw 6 - 8 times the rated current of the motor.
If the transformer has an impedance of 5% and is rated at twice the VA rating of the motor, then when the motor is fully loaded, the voltage drop will be in the order of 2.5% and under DOL start conditions, the voltage drop will be in the order of 7 x 2.5% = 17.5%

The maximum voltage drop during start is determined from the minimum voltage that can be applied to other equipment operating off the transformer secondary. Contactor coil voltages have a limited useful operating range.

Best regards,
Mark.





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