Power Factor Correction Capacitors

[duck-boy]

We have 2-15HP and 2-7.5HP pump motors coming off the same MCC. Having issues with the 2 new 7.5HP motors failing on overtemp. Manufacture is saying we need Power Factor Correction Capacitor Bank installed to correct problem. I have my own beliefs as to why the overtemp BUT for arguement sake lets just say we do as manufacture suggests. OK so we do and here is what we see. Motor pulls about 6A per phase @ 480VAC before adding 3KVAR bank. We add bank to secondary side of breaker feeding starter (per manufactures installation instructions because of periodic bumping of motor which we do sometimes) and now we see about 3A per phase at breaker, about 3A per phase going to bank, and about 5.8A per phase going out to motor. So results are that we have dropped Amperage going to motor by about .2A but the interesting thing is at the breaker feeding the section where we have reduced by about 50%. Lets say we now have no motor fails on overtemp once again for argument sake. (time will tell) Ok so here are my questions.
#1. Do these readings seem right?
#2. Since the bank is wired in parallel to the secondary side of the breaker feeding the motor section which is always turned, do i really need a second bank for the second motor if only one will run at a time (since the bank is technically in parallel with the whole MCC at any given time)
#3. Only one 7.5HP will run at a time but there might be a time when both 15HP and 7.5HP are running. How will this affect the banks results? Thank you in advance....

 

[kwired]

7.5 Hp motor 480 volt pulling 6 amps does not sound like is overloaded. Likely is rated for at least 8 amps. Does it get sufficient airflow to cool it? Frequent starts or frequent reversing could also be a problem that requires derating the motor that is used.

 

[duck-boy]

Actually its rated for 10.7 Amp and I agree it is not overloaded. Like I said I belive the problem is not what manufacture says but im more curious now on what I see with banks installed since this is my first time dealing with them.

 

[kwired]

First thing - the capacitors do not change anything in the motor itself with the exception of maybe some improvement in voltage drop, which would result in less current being drawn.

The motor still operates at same power factor, the effects of the correction only apply to branch circuit at points upstream from the point where the capacitor is connected. The reactive power that is being corrected is flowing between the motor and capacitor instead of between the motor and the power source.

Is this a motor with a constant load?
Was your current measurement taken during period when maximum load is encountered, if not a constant load?
Again does motor have sufficient cooling? If you have failure due to over temp but do not have overload this sounds like something that needs checked.

I have to assume we are talking about a motor connected directly to the line and not to a VFD. If connected to a VFD that opens up several other possibilities and external power factor correction should not even be involved with a VFD.

 

[duck-boy]

Yes motor is at constant load and no VFD. I agree with you the cooling is the issue. Sounds like the .2 A drop is about what is to be expected.

 

[Jraef]

kwired is right of course, that capacitor is doing next to nothing in terms off helping that motor if it is overheating, that was a red herring to begin with. They were just trying to deflect from the fact that they have no clue.

And by the way, that is NOT how to connect the PFC capacitors, they need to be DOWN STREAM of the motor starter, otherwise they are on-line all the time and when that motor is not running, you are going to be over correcting, which is just as bad but for everything else that is running at the same time. If they told you to hook them up that way, it's further proof that they are nincompoops.

Real possibilities, in addition to what kwired already said:
1) Have you checked you voltage balance? Unbalanced voltage causes what is called "negative sequence currents" to rotate in the rotor of the motor, which causes negative torque that opposes the torque that the normal power is providing. this makes the rotor heat up faster for the SAME amount of stator current. In other words it is entire possible, and in fact probable, that you can cause the motor to over heat without ever exceeding the FLA rating of the motor, so the Overload Relay will never trip out. That is why they make Phase Imbalance Relays. If you have more than about a 2% imbalance between phases, you may have a problem. Most motor mfrs recommend never operating a motor with greater than 5% imbalance, and a 10% imbalance is a kiss of death.

Measure it with the motor RUNNING, not when it is off. Also, voltage balance can change throughout the day, especially if there is a lot of residential loading near by. That's because residential loads are all single phase and although the utility tries to keep them balanced as much as possible, they have no control over who is home and who is not. I have seen a swing of >10% imbalance in one area where there were a lot of housing developments going in and a few remaining farmers with well pumps that kept burning up. We installed phase monitor relays and they would all trip off at the same time of day throughout the week, when people came home from work and turned their AC's on when it was hot.

2) Bad bearings in either the motor or the pumps. Bearing heat in the pump can transfer to the motor shaft and up into the motor housing.

 

[duck-boy]

Thanks for the info jraef & kwired. As far as the wiring of the capacitor bank, that info is from Eaton (the manufacture of the capacitor bank) but I do understand what you are saying. It says "IMPORTANT! , if motor will be bumped, plugged, reversed, or frequent start and stopped to wire it upstream of motor starter. I can always change this but that is why I questioned its location as there is usually one of 4the motors running at any given time. I will look into voltage difference between phases when motor is running also and consider the relays you stated. Its a long story so I wont get into it but they gave us the wrong motor and now im just going through motions to please my bosses to prove motor rep. wrong. Thank you for all the additional info.

 

[kwired]

Thanks for the info jraef & kwired. As far as the wiring of the capacitor bank, that info is from Eaton (the manufacture of the capacitor bank) but I do understand what you are saying. It says "IMPORTANT! , if motor will be bumped, plugged, reversed, or frequent start and stopped to wire it upstream of motor starter. I can always change this but that is why I questioned its location as there is usually one of 4the motors running at any given time. I will look into voltage difference between phases when motor is running also and consider the relays you stated. Its a long story so I wont get into it but they gave us the wrong motor and now im just going through motions to please my bosses to prove motor rep. wrong. Thank you for all the additional info.

If it is frequently started and stopped you could switch the capacitor with its own contactor initiated by starting the associated motor and use an off delay timer so that it does shut off if the motor is not started within a prescribed time. Then it is not in the circuit for very long when not needed.

 

[Jraef]

... As far as the wiring of the capacitor bank, that info is from Eaton (the manufacture of the capacitor bank) but I do understand what you are saying. ...

Eaton (ex Sprague Capacitors) provides very generalized information on how it should be done, they don't know, or care, how it affects your system overall. They just want you to buy capacitors so they show you the cheapest way to implement them, otherwise they might expose the flaw in their entire product concept in that unless you are being penalized for poor power factor, capacitors are mostly a waste of money.