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Lightly loaded motors causing low power factor?
2

Lightly loaded motors causing low power factor?

Lightly loaded motors causing low power factor?

(OP)
Our plant has some 350Hp, 480V fan motors that are started across the line. According to the nameplate they have a FLA of 385A, but according to the meter data they generally only pull about 200A.

Seems like I've read before that this contributes to low power factor. True? and Why.

Also, one of our objectives is to change out the main 480V fuse that is feeding these lightly loaded motors - to reduce the arc flash potential.

This has a NEMA Code Letter G...can you use the typical running Amps in the calculation when figuring the starting current?
 

RE: Lightly loaded motors causing low power factor?

Starting current is a property of the motor. Starting time is influenced by the load and therefore the duration of the starting current has a load component, but peak is just motor.  

RE: Lightly loaded motors causing low power factor?

Quote:

Our plant has some 350Hp, 480V fan motors that are started across the line. According to the nameplate they have a FLA of 385A, but according to the meter data they generally only pull about 200A.

Seems like I've read before that this contributes to low power factor. True? and Why.
It is true that your motors appear to be operating at low power load, and their power factor will be lower at low load than high load.

Quote:

Why.
It can be seen from the equivalent circuit.

First a simple model – neglect magnetizing reactance.  Then we can see we have a magnetizing branch which draws constant vars and a load branch which draws kw roughly in proportion to load.  As load increases the ratio kw/sqrt(var^2+kw^2) = pf increases.

A slightly more accurate model adds back the contribution of magnetizing branch.   Then we the constant block of magnetizing vars which does not vary with load, plus leakage vars which increase proportionately to load.   Since real power is also proportional to load, the "power factor" excluding magnetizing component would be constant with respect to load (ratio of 2 things proportional to load is constant). So when we add back in the magnetizing branch we again conclude that lower load = lower power factor and higher load = higher power factor.

Quote:

This has a NEMA Code Letter G...can you use the typical running Amps in the calculation when figuring the starting current?
I agree with David, running current has nothing to do with starting current.  FLA has an indirect relation to starting current. Knowing code letter and horsepoweryou can calculate the starting kva, and knowing the voltage you can convert that to starting current.
 

=====================================
(2B)+(2B)'  ?

RE: Lightly loaded motors causing low power factor?

Ouch. A correction in bold:
First a simple model – neglect magnetizing reactance.
should have been
First a simple model – neglect leakage reactance.

=====================================
(2B)+(2B)'  ?

RE: Lightly loaded motors causing low power factor?

Here is how to calculate starting current as mentioned above:

460VAC 350HP motor
KVA Code G motor => kva/hp = 5.6-6.3  per NEMA MG1
Multiply by 350 gives
1960-2205 starting kva
I = VA/(sqrt3*VLL) = 1000*KVA / (sqrt3*VLL) gives
2460 – 2768A starting current at 460vac
 

=====================================
(2B)+(2B)'  ?

RE: Lightly loaded motors causing low power factor?

2
Side note:
How is it that you think changing the fuses is going to reduce the arc flash potential? Fuses in a motor circuit by the way are there to provide short circuit protection. That does not change based upon the running amps if that was what you were thinking.

In this pdf, there is a chart on page 2 that shows typically how PF drops with load.
http://www1.eere.energy.gov/industry/bestpractices/pdfs/10097517.pdf
 


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RE: Lightly loaded motors causing low power factor?

Switch motors to the correct size is the best solution.  Use the current draw and the motor's curves to discover the actual HP needed.

An alternative would be to install a VFD which could correct the power factor and allow fan speed changes.  The VFD could possibly be down sized since you don't need the full motor rating.

Or add capacitors to correct the power factor.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Lightly loaded motors causing low power factor?

Power factor is the ratio of real power to apparent power.
Apparent power is comprised of real power and reactive power.
Reactive power doesn't change much over the range of normal motor loading.
Real power does change with motor loading. Hence the ratio or power factor changes with the load.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Lightly loaded motors causing low power factor?

(OP)
Thanks for the good answers.

jraef,

I'm not talking about the fuses on the individual motors.

I'm talking about the upstream main fuse on the transformer, this also protects the MCC feeder as well.

This large transformer feeds little more than these two big motors, so we're thinking of changing these fuses out.



 

RE: Lightly loaded motors causing low power factor?

Transformer fusing needs to accommodate transformer inrush, which is independent of the load and power factor.

Also unless you have already performed an arc flash analysis, reducing fuse on the primary would not have as much impact on the secondary side arc flash as you might think. I would rely more on the secondary side main's settings to address arc flash on the secondary side.

Rafiq Bulsara
http://www.srengineersct.com

RE: Lightly loaded motors causing low power factor?

(OP)
rbulsara,

We haven't done the arc flash study yet. The main fuse I'm talking about is on the secondary of the transformer. There is no main breaker. It is a bolted pressure switch.

The inrush on the transformer...what is it 12X the FLA?  only the primary sees this inrush right?
 

RE: Lightly loaded motors causing low power factor?

Yes.

Look up NEC 430 for sizing a feeder overcurrent protection for multiple motors.

Rafiq Bulsara
http://www.srengineersct.com

RE: Lightly loaded motors causing low power factor?

"The inrush on the transformer...what is it 12X the FLA?  only the primary sees this inrush right?

Correct.  

Unless you energize the transformer from the secondary.  Then inrush will probably be higher than 12X.

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