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Power Factor Correction


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Seasonal Greetings.

1. Which one of the following alternatives is preferrable location for installing the power factor correction capacitors for squirrel cage AC 3 phase 415 V induction motors ?

i)In the main control panel with separate Circuit Breakers for capacitor

ii)In the starter on the supply side (common circuit breaker for Motor and Capacitor)

iii)In the starter on the motor load/side (common circuit breaker for Motor and Capacitor)


2. In our organisation, in few installations we have capacitors connected in main panel as in alternative (i) above, some times the operator forgets to switch on the capacitors when the motors are started and in running. This results in low power factor problem and consequent penalities. Hence I think that the capacitors can be connected in the starter near the motor load centre as in alternative (iii) above. Is it correct and safe? Will such method result in any additional problem for differnt type of starters such as Auto Transformer starter, Soft Starters, Star Delta Starter?


Kindly help me

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First, have a look at http://www.LMPhtonics.com/pwrfact.htm and see the difference between static correction and bulk correction.

The addition of power factor correction correctly used, will reduce the current drawn from the supply. This will reduce any maximum demand KVA penalty that may be applied to the power bill. It will not alter the power drawn by the motors.

If you have significant cable losses between you main switchboard and the motors, then adding the power factor correction at the motor will reduce the current in those cables and reduce the voltage drop and the KW losses in the cable.


If you use bulk correction, you should use a power factor controller to control the capacitors. This will keep the power factor to a required level. If you manually switch the capacitors, you need to switch them as the load changes and you will never get it right.

Too much capacitance is as bad as too little, or if there are resonance problems, it can be very damaging.


The choice comes down to economics.

If you have the capacitors at the main supply or switchboard, I would recommend that you add a controller to control the capacitors.


Best regards,

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