Motor Starting

[bob]

Hi,

 

I am commissioning a 90 k W 400/600 V 50 Hz motor DOL starting. The motor circuit breaker tripped instatantaneously when the motor is started. I measured an inrush current of 1400 A by the Fluke 435 analyser. The magnetic setting on the motor breaker is 2400 Amps. I suspect the motor breaker is faulty. The rated current of the motor is 160 Amps and if I assume a LRC of 900 % i.e 1440 Amps, then the motor breaker should have been able to start the motor. I have also connected the motor in star and it drew 90 Amps per phase and I assume the motor is OK.

Any comment ?

 

Bob

[marke]

Hello Bob

 

Yes, you are correct with your currents, except that when you DOL start, there is an initial infush curent which can last for a cycle or two which can be more than two times the LRC of the motor. If the magnetic trip is fast enough, it could be operating on the inrush current rather than the LRC.

The inrush current is the initial current that establishes the magnetic field which then limits the current to the LRC.

Perhaps the Fluke is not fast enough to pick it up. - could be a settings thing.

 

Best regards,

Mark

[bob]

Hi Marke,

 

Thanks for your usual prompt and expert reply. I have replaced the motor breaker with a similar one and the motor startS well.

 

Regards.

 

Bob

[animna]

Hello Bob

 

Yes, you are correct with your currents, except that when you DOL start, there is an initial infush curent which can last for a cycle or two which can be more than two times the LRC of the motor. If the magnetic trip is fast enough, it could be operating on the inrush current rather than the LRC.

The inrush current is the initial current that establishes the magnetic field which then limits the current to the LRC.

Perhaps the Fluke is not fast enough to pick it up. - could be a settings thing.

 

Best regards,

Mark

 

 

 

Hi again Mr. Mark,

 

I have read several replies of yours to our co-members in this forum; And I'm quite impressed with the way you handle questions.

Regarding the "inrush current which is greater than the LRC" can you kindly give me reference for it. I would like to know more....=)

 

Thanks in advance.

 

Best Regards,

Animna

[marke]

Hello Anima

 

Thank you for your comments.

 

When you first apply full voltage to an induction motor, there is a current surge called the inrush current. This is a very short burst of current and is the current required to establish the flux in the iron.Once the iron is fluxed, the current drops down to the locked rotor current where the current is determined by the effective rotor impedance, leakage reactance and stator impedance.

 

When you apply power to a transformer, you also get an inrush current which is the same sort of current. - the current required to establish the flux in the iron.

 

Best regards,

Mark.

[Jeff Millar]

Hello Bob

 

Yes, you are correct with your currents, except that when you DOL start, there is an initial infush curent which can last for a cycle or two which can be more than two times the LRC of the motor. If the magnetic trip is fast enough, it could be operating on the inrush current rather than the LRC.

The inrush current is the initial current that establishes the magnetic field which then limits the current to the LRC.

Perhaps the Fluke is not fast enough to pick it up. - could be a settings thing.

 

Best regards,

Mark

[Jeff Millar]

Hello Marke,

 

I appreciate your answer also on that subject.

 

Can you help clarify the following. I have experience with NEMA Motors, and not

much experience with IEC applications.

 

Recently came across " Absorbed load " and on review became confused, perhaps you

can solve the confusion.

 

load factor = absorbed load / motor rating static load.

 

Thanks

 

Jeff

 

 

 

 

[marke]

Hi Jeff

 

That is not a term that I use, but I expect that the "absorbed load" is the shaft power as opposed to the input power.

IEC motors are rated in maximum shaft power. Many people confuse this with the electrical input power and this is higher as it includes the motor losses.

A 10KW motor has a shaft power of 10KW and in input power at rated load of 11.5KW

 

Have a good day,

Best regards,

Mark.

[animna]

Hello Anima

 

Thank you for your comments.

 

When you first apply full voltage to an induction motor, there is a current surge called the inrush current. This is a very short burst of current and is the current required to establish the flux in the iron.Once the iron is fluxed, the current drops down to the locked rotor current where the current is determined by the effective rotor impedance, leakage reactance and stator impedance.

 

When you apply power to a transformer, you also get an inrush current which is the same sort of current. - the current required to establish the flux in the iron.

 

Best regards,

Mark.

 

Dear Mr. Mark,

 

Thanks for your expertise.

You have respect.

 

 

Best Regards,

 

animna

[bob]

Dear Mark,

 

I understand that the LRC is a function of the characteristics of the motor. However, the inrush current is directly dependent on the mode of starting and applied voltage. Am I right ?

 

Best regards.

 

Bob