Motor Power Factor Correction

[fornhamspark]

Could any of you clever people help me with a small query,
to correct the power factor of the motor, i know how to work out the mf by vertical component calculations etc, but i ahave been told that the speed has some relationship to this.

My question is can anybody tell me the relationship speed has and possibly how to add this variable in an equation for power factor correction??

 

[Skogsgurra]

Speed has nothing with PF compensation to do.

That's the easy answer.

But if you have different frequencies to get different speeds, then speed has a lot of influence.

BUT! You never compensate a variable frequency drive motor. Never!

The only case where speed could have som influence is if you move between different regions with different mains frequencies (you can if you live in Japan where 50 and 60 Hz grids are quite close to each other). But for normal use, speed has absolutely no influence on power factor correction.

 

[electricpete]

The only very remote tie-in I can think of is if you were trying to estimate the required correction before you had actual motor data... slower speed motors tend to have lower power factors and require more capacitance for correction.

However, once you have the nameplate data, that power factor and full-load amps (and voltage of course) are all you need to determine correction and speed is no longer relevant.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[fornhamspark]

Thats what i thought, but when we talk to ABB capacitor division they always want to know the speed of the motor.

 

[cbarn24050]

Hi, the slip speed has a very big influence on the pf.

 

[Skogsgurra]

I think that you mean efficiency?

 

[cbarn24050]

Hi skogsgurra, are you suggesting that slip speed does not affect the pf? I can't really believe you are.

 

[Skogsgurra]

Yes, I did.

I mis-read your post since there had been a discussion about speed and PF correction earlier in the thread. I was pre-conditioned to read "speed" and did not see the "slip" - it slipped my eyes...

But we have to sort things out. I am talking about the PF on the name-plate. The PF that you use when you calculate your compensation capacitors.

Of course, the delivered power increases as torque goes up and magnetising current stays essentially constant. So the PF is very low at low load and gets better as load increases. So you could turn things upside down and say that PF is slip dependent, yes. But I would still say that it is load dependent, since the load is the determining factor.

 

[mc5w]

The total magnetizing current of a 4 pole induction motor is approximately twice as much at full load as at no load. The no load magnetizing current is what developes the counter voltage of the motor and is mostly voltage dependent and slightly dedendent on speed. The other half at full load is essentially needed to push power across the air gap so as to develope torque.

Slower speed motors have a harder time developing a given amount of counter voltage for a given amount of magnetizing current. A high reactance rotor also will lower power factor. A high reactance rotor is an alternative to a high resistance rotor for starting purposes.

At zero speed (locked rotor) both contributions to magnetizing current are very high - rotor not turning impairs countervoltage production and increases the need for torque.

Mike Cole, mc5w at earthlink dot net