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Motor Starting Current Based On It's Rated Current Or Working Current?


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#1 schow

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Posted 10 September 2014 - 08:11 AM

Dear all,

 

Good day!

 

I have a 30kW motor rated 58A @ 400VAC / 3-phase used for hydraulic pump.

 

I am using a soft starter to start this motor and the starting current is approx 100A.

 

The motor is being started before the load engaged, where it is running at 16A only.

 

My question is since the running current before load engaged is 16A, why my starting current is as high as 100A, which is almost 6.25 times of the running current.

 

My understanding is that with soft starter, normally the starting current would be 3 to 3.5 times of the running current, right?

 

Or we should worked out the starting current based on the rated current of motor, which is 58A and NOT the running current of 16A???

 

So when we size which kW soft starter to be used, should we size based on the motor's rated current on the name plate OR the actual running current?

 

Please kindly advice, cheers!

 

 



#2 marke

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Posted 10 September 2014 - 09:40 AM

Hello Schow.

 

When the motor is running under high slip conditions, the impedance of the motor is a function of motor slip (speed) and is independant of the motor shaft load.

 

The torque produced by the motor is reduced by the square of the voltage reducetion, and as ohms law applies, it is reduced by the square of the current reduction.

 

Under vull voltage start conditions, the start current equals the locked rotor current and slowly drops as the motor accelerates towards full speed and only drops dramatically as the motor reaches full speed. The actual current speed curve, is a function of the rotor design of the motor.  see motor Start Characterisitcs

 

So, the motor shaft load does not alter the full voltage start current.
The minimum start current is the current that is required to develop enough torque to accelerate the load to full speed and is related to the full voltage start current.

Typically, on a lightly loaded motor, the start current can be in the order of 300% Full Load current, but the actual current is dependent on the motor characteristics and the load characteristics, but independent of the "run" current.

 

Best regards,

Mark.



#3 schow

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Posted 10 September 2014 - 09:53 AM

Hi Marke,

 

Thanks for your detailed explainations! You are always my "GURU" in this field!!!

 

People always say that the starting current of a soft starter would be around 3 times of the FLC, so if I understand your post above correctly, this "FLC" is actually referring to the motor's name plate rating and NOT the actual "RUN" current, right?

 

Thus, am I right to say that for a motor with rated current of 30 amps for example, even it has no load attached to it, the starting current with a soft starter would be approx 90 amps (eventhough the free load running current is only 10 amps)?

 

Thanks a lot!!!



#4 marke

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Posted 10 September 2014 - 06:50 PM

Hello Schow

 

Yes, the start current is always related to the rated full load current of the motor.

The actual start current is dependent on the rotor design and the shaft load.For very lightly loaded motors, the start current may br in the order of 250 - 300% FLC, and for higher start torque loads, it can be 450 - 500%.

 

The modern high efficiency motors are usually very inefficient under high slip conditions so require a much higher start current for a given load.

 

Best regards,

Mark






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