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Join Date: Jun 2011
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Why PF improvement capacitors are in Delta connected? y not in Star?

12/20/2011 11:47 AM

Why PF improvement capacitors are in Delta connected? y not in Star?.

In Star-Delta starter, at which point it is better to connect capacitor for improving p.f.

@ delta contactor? or @ main contactor? or @ line side?....... pls guide me.........

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#1

Re: Why PF improvement capacitors are in Delta connected? y not in Star?

12/20/2011 11:58 AM
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#2

Re: Why PF improvement capacitors are in Delta connected? y not in Star?

12/21/2011 12:29 PM

hello.......lyn..... i know about pf. understand my question...ok. dont answer as like that........

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#3

Re: Why PF improvement capacitors are in Delta connected? y not in Star?

12/22/2011 7:23 AM

Capacitor banks in PFC Units are connected in Delta because sums of stored KVARs are required in parallel across a three phase network to improve the power angle on the load. The summations of KVARs are done in electrical stepping sequences via the PFC Unit controller depending on the number of capacitor banks in the PFC Unit.

Where to connect capacitor to Star-Delta Starter?

Reasoning is used here; please find out more before attempting.

Star-Delta Stater is soft starter for motors with high torque. To connect capacitor at the main contactor or the line side will be exposing the capacitor to instantaneous in-rush current magnitude and the rise in current magnitude switch from star to delta, which may in the long term have reducing effects on the life of the capacitor. Therefore, connect the capacitor to the delta contactor. So that the capacitor is working on the delta contactor when the motor in is full running. Where its power factor will be better improved at running load. The capacitor may still be connected to the line side, but as stated, its life may be reduced when exposed to in-rush current and the change in current magnitude from star-delta.

Another suggestion is that, it may be connected to the line side, via a capacitor contactor, protection fuses, and a timer. So that it may be switched in seconds after the motor runs in delta mode. Motor contactors will not withstand the arching energy release from capacitors, where a capacitor contactor must be used.

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#4

Re: Why PF improvement capacitors are in Delta connected? y not in Star?

12/29/2011 6:33 AM

Draw the equivalent circuit of a delta connected capacitor. You will find that for the same cell capacitance (C), delta connection will give 1.5C between any two lines, whereas if they are star connected, you would only get 0.5C between any two lines. Which is cost effective?

Having said that, please note that this delta connection of capacitors is practised only in low voltage. All HV capacitors are invariably Star connected. Because, if HV capacitors too are Delta connected (for the total capacitance advantage, as in the case of LV Capacitors), the insulation has to be done for line voltage. Whereas if you connect it in star, you need only the phase voltage insultaion, which would greatly reduce the cost.

In, LV there is no big cost difference between phase voltage (240V) insulation and Line Voltage (415V) insulation; so, you get cost-effective by going for delta connection so that you get a bigger line capacitance with a smaller cell capacitance.

Whereas in HV, there is considerable cost difference between phase voltage insulation and line voltage insultaion and this cost advantage far outweiighs any advantage that one gets from a bigger line capacitance, had it been connected in Delta.

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