Motor Power Factor 1

[microaid]

Does using a larger motor for the same load cause it to run at a lower power factor and result in a lower power consumption. If so what are the disadvantages of doing this which deter it from being a common practice.

 

[jbartos]

Suggestion: Visit

http://www.turi.org/PDF/Chap 3 - Motors.pdf

for a motor performance as a function of load

 

[smms]

If you design a large motor for a lower rating the power factor will increase (lower induction). But the efficiency will probably decrease (higher friction losses for a larger motor, ...) so the effect will be negative.

 

[electricpete]

In general a larger motor at same load will draw a lower power factor.

The core will be larger and more reactive magnetizing current required.

The real power output is determined by the load and will not change.

There are some losses within the motor which affect efficiency. With larger than required motor, portions of those losses will go up (core losses), and portions of those losses may go down (I^2*R losses... depending upon winding). Maybe someone else can comment which direction efficiency will go but I'm not sure.

Of course a big factor is initial cost. Larger than required motor will cost more. Usually motor is sized some margin above expected load to allow for errors in calculation and possible future change in loading.

 

[Iwe007]

The larger the motor the more expensive it is! You might need to consider if weight and space are issues at the end user's application. I'm sure if you look up to the sky you can think of a few more!

 

[buzzp]

A underloaded motor (compared with nameplate) is almost always less efficient than one working at nameplate ratings.

 

[yongyc]

Hi, I like to join in by asking about power factor and of 3 phase induction motors rated 600V as follows:-
1) pf tends to go down from the same manufacturer for the same horsepower rating as the numbers of poles increase, that's to say, from 2,4,6, 8 ...so on, the pf drops, can some one explain pls,
2) is it true that from the same manufacturer/manufacturing technique & design and same pole numbers, for a given horsepower rating say 200 hp, the pf is lower for a submersible motor (IP 68) direct coupled to submersible sewerage pump compared to the same motor rating of TEFC type from the same manufacturer,
Lastly, appreciate if you all out there can advise how to improve the pf of submerged motor to that of TEFC motor form the same manufacturer, and what's the IP rating possible with TEFC motors.

Cheers
Jyaces1

 

[rlpuck]

First know that a low power factor is not a good thing, you always want a higher power. Power factor is simply a comparision ratio of real to reactive power. Secondly you need to understand that a larger motor will require a higer starting current, starting current is dependent on lock rotor amps of the motor and not depend on the load. Third a larger motor will need larger cables run to it, and a larger starter, and a larger breaker frame and etc. So there is more to it then just specifing the motor size. You will always find that proper sizing of the motor to the load is your best choice, some oversizing of the motor is needed to compensate for flucuations in load condition but this is usually taken care of in specifing a higher service factor for the motor. Determine what the bhp the load requires then size the motor accordingly allowing some overage.