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Anonymous Poster

How to Reduce Motor Load Current with a Capacitor Bank

07/03/2009 11:09 AM

Power Factor correction by capacitor bank installation will reduce the current consumption of a particular motor. I have a motor with the followings specs:

225kW 300HP 2965rpm

FLC 358Amp @ 415V 50Hz PF 0.94

During pump performance test, my motor was running at 420Amps ( short time ) but we still unable to get the required pressure for the pump. Hence, we have to reduce the motor amps further. The motor has been tested to run at 450 amps more than 10 years ago during a similar test.

1. Can we install capacitor bank to improve the PF further hence lower the running amps during the performance test?

2. If possible, what is the value of the capacitor? and how much further can we reduce the amps reading ?

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Guru
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#1

Re: How to Reduce Motor Load Current with a Capacitor Bank

07/03/2009 11:31 AM

Is the voltage you are applying to the motor 415V 50Hz? If so, the motor has a problem or the load is to much. Applying capacitors will not lower the amperage the motor requires. Capacitors will lower the amperage on the conductors feeding the motor.

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Guru

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#3
In reply to #1

Re: How to Reduce Motor Load Current with a Capacitor Bank

07/03/2009 11:55 AM

Supporting wareagle ….

The capacitor bank is used not to improve the P.F of motor itself; it is used to improve the P.F of the system. Even you add the capacitor bank at the motor terminal; it will reduce the current through the motor cable but not the current (of course the magnitude of the current) drawn by the motor. What is the S.F (Service Factor) of the motor? If it is less than 1.15, 420A can damage the motor, however, I would suggest you contact the motor manufacturer and find out how long the motor can sustain for 15%-20% overload and take the decision.

-MS

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Anonymous Poster
#4
In reply to #3

Re: How to Reduce Motor Load Current with a Capacitor Bank

07/03/2009 12:55 PM

Obviously, 10 years.

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Guru
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#5
In reply to #4

Re: How to Reduce Motor Load Current with a Capacitor Bank

07/03/2009 4:25 PM

If its 10 years, then why worry?

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Commentator

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#2

Re: How to Reduce Motor Load Current with a Capacitor Bank

07/03/2009 11:37 AM

Are you saying that the motor/pump produces too much pressure?

If you are saying this, reducing the motor current will not solve your problem. As a matter of fact, it sounds like your motor is working perfectly since you state its current (amps) usage for the past 10 years has been quite stable. If you reduce the motor current, you reduce its torque, which may cause other problems for you.

What you need is a Variable Frequency Drive but since the motor is at least 10 years old, it would only be advisable to use a VFD after the motor has been tested for faults - insulation resistance, voltage stress faults etc etc. Also, it is highly recommended that the motor have at least a "Class F" insulation before even considering connecting a VFD to it. Further, if the motor is "unenclosed" (has ventilation slots) & is 10 years old, it is highly advisable NOT to connect a VFD to this motor unless it has been fully tested.

Power Factor Correction will not reduce the current used by the motor.

If you want formulas, let that be your next question.

I'm curious...where are you & what is your job?

Caviat: this only applies to induction motors.

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Guru
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#6

Re: How to Reduce Motor Load Current with a Capacitor Bank

07/03/2009 9:16 PM

I'm not very knowledgeable about PF but, from what I know, it won't reduce the current consumption (as others have already said).

From experience, however,your motor seems seriously overloaded. If the motor's current consumption is over the FLA, and you're still not getting enough pressure, that tells me that your motor and pump are not matched. It would be best to obtain the required hp of the pump and buy the appropriate motor.

regards,

Vulcan

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Guru

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#7

Re: How to Reduce Motor Load Current with a Capacitor Bank

07/05/2009 6:39 AM

If you increase the power factor the efficient part of the consumed Amps will be lower so if you manage the reach the pf=1 your motor/capacitor_bank system will consume about 395 Amps.

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Guru
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#8

Re: How to Reduce Motor Load Current with a Capacitor Bank

07/06/2009 3:43 AM

<....still unable to get the required pressure....>

There could be loads of factors other than the motor. Try looking at these as well:

  • Has the system been added to so as to create loads more parallel paths for the flow other than the original design ones over the last 10 years?
  • Is the pipework clean inside, and not partially blocking the flow anywhere, particularly on the inlet?
  • Is there a partially-open valve where there shouldn't be one?
  • Is there a non-return valve on parallel pump sets that is letting a small reverse flow through, due to 10-year corrosion, or some foreign object wedging the port partly open?
  • Is there a blocked strainer somewhere that is choking the inlet flow to the pump?
  • Is the inlet connection of the pump fully flooded at all times while the pump is running?
  • Is the 10-year-old pump impeller in good condition, with no wear or damage? What about the volute?
  • Is the correct size impeller still fitted or has a get-it-going-again maintenance spare of the wrong size been fitted instead?
  • Using the manufacturer's published performance curve for the pump, if the pressure is too low, is the flow too high? Whereabouts on the curve is the pump operating? Is this realistic?
  • Is there a leak in the pipework somewhere that isn't currently being detected?
  • Are their weirs on the system that would limit the delivery pressure of the pump regardless of what was tried to increase it?
  • Is there a downstream valve that could be throttled to provide the required pressure, albeit at a reduction in flow?
  • etc.
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Guru
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#9

Re: How to Reduce Motor Load Current with a Capacitor Bank

05/28/2012 1:03 PM

Dear Friend,

1.You have not mentioned how much capacitor is connected to the system. 2. PF 0.94 is it on the name plate of the Motor or you want to have a PF of 0.94. 3.You have not mentioned what was the pressure developed by the pump and what is the full pressure.

Theoritically you should have 168.6 KVAr if PF is 0.8 and nearest capacitor would be 175KVAr. But 2965 RPM Motor will have a better PF on its own. For FLC of 358 amps, PF 0.94 and 415Volts the Power is 241.88 KW. and if PF is taken as 0.8, the power is 205.85 KW. and both values are inconsistent to the referred full load.

Therefore the problem will be on the pump side also, by way of re-circulation with in the pump, affecting the discharge, which is indicated by the pressure on delivery side.

The issue is to be deeply studied on mechanical aspect also.

PL. post the full data so that we can understand and suggest some remedy.

Thanks,

DHAYANANDHAN.S

Hence the problem

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Guru
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#10

Re: How to Reduce Motor Load Current with a Capacitor Bank

05/29/2012 11:12 AM

Depending on where you take your measurement, power factor improvement on a motor will appear to reduce the current. But it does NOT reduce the energy consumed by the motor to any degree worth mentioning. It is a common falacy used by many scammers to sell PFCCs as "energy savers", they are not. What determines the energy consumed by a motor is the LOAD on the motor. If your motor current is above the Full Load Amp rating on the nameplate, you are doing damage to the motor, there is no magic device that will prevent that. If you managed to reduce the current once upon a time with capacitors, that was a false benefit, you are only reducing the APPARENT current in that motor, you have NOT changed the loading of the motor, it is still over loaded.

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