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Contactor for DOL Starter with PF Capacitor

01/02/2017 1:04 AM

The PF improvement capacitor connected parallel to motor.
1. In case Of DOL starting the contactor selection for the Motor should be "Capacitor switching contactor" or normal "AC3" contactor?
2. How is the inrush current? Does the PF capacitor decrease or increase the inrush current? Normally if the capacitor is switched ON, the capacitor itself draws a inrush current. In this case will there be inrush in Motors as well as in capacitors?

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Guru

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#1

Re: Contactor for DOL Starter with PF Capacitor

01/02/2017 1:37 AM

I used normal AC3.

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Subramanyam
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#2

Re: Contactor for DOL Starter with PF Capacitor

01/02/2017 3:33 AM

Tell us what you think, after reading this.

Inrush current - Wikipedia

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Guru
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#3

Re: Contactor for DOL Starter with PF Capacitor

01/02/2017 7:16 PM

No difference in the contactor selection. The capacitor switching contacts are for when capacitors are the ONLY load on them.

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#4
In reply to #3

Re: Contactor for DOL Starter with PF Capacitor

01/03/2017 2:49 AM

JRaef,
Thanks, Could you kindly explain why the capacitor switching contacts are for when capacitors are the ONLY load on them?

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Guru
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#5
In reply to #4

Re: Contactor for DOL Starter with PF Capacitor

01/03/2017 5:35 AM

Because they are the only things that are in circuit downstream of the contactor.

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#6
In reply to #4

Re: Contactor for DOL Starter with PF Capacitor

01/03/2017 9:14 AM

A contactor designed for switching capacitor banks has pre-charge resistors built in. Switching a motor via these contactors would blow the resistors to bits.

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#7
In reply to #6

Re: Contactor for DOL Starter with PF Capacitor

01/03/2017 9:51 AM

Or, without resistors, vacuum contact dielectric to interrupt high peak currents (typically high voltage applications).

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#8

Re: Contactor for DOL Starter with PF Capacitor

01/03/2017 10:01 AM

Capacitors sized for 95% net power factor correction are a small component of the total load when starting an induction motor, the extra inrush current is not a significant factor in the design of the starting contactor.

Capacitors can be used as part of a reduced voltage starting scheme, to reduce the demand on the upstream power system. The net effect over time is a reduction in current from the power system.

https://www.powellind.com/sites/downloads/ProductAssets/01.4TB.071%20Starting%20Methods%20for%20Large%20Medium%20Voltage%20Motors.pdf

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