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Components Sizing For Closed Transition Star Delta Start


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#1 jessie_zac08

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Posted 27 January 2008 - 01:58 PM

Hello guys,
How are you!!?
One querry i have here about the the sizing of the components in the Closed Transition Star Delta Start.

What is the or how to size the 3 resistors and 4 contactors should be, if i will apply it in the screw compressor star delta start. The compressor is 110HP with 360A star/1074A delta starting current, 218A max oper currents and 85.2kW power inputs (power inputs are based from certain ambient condition we selected).

Thanks and Regards,
JZ

#2 marke

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Posted 28 January 2008 - 06:46 AM

Hi JZ

Welcome to the forum,

The main contactor and the delta contactor should be AC3 rated for 1/Rt3 of the rated motor current provided that the main contactor and the delta contactor are both connected to the incoming supply. If the input to the delta contactor is connected to the output of the main contactor, the main contactor would need to be AC3 rated at the motor rating.
The star contactor should be AC3 rated at one third of the motor rating.

The auxilliarly delta contactor should be rated at 1/rt3 of the motor rating (same as the delta contactor).
The resistors should be sized such that the current through the resistors when in series with the delta configured windings, should be similar to the current through the resistors when connected in star. The resistors must be capable of dissipating the power when the resistors are connected in star (until the star contactor opens) and the dissipation in delta until the delta contactor closes.

For more information, see http://www.lmphotoni.../star_delta.htm

Best regards,

#3 jessie_zac08

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Posted 28 January 2008 - 07:08 AM

QUOTE (marke @ Jan 28 2008, 09:46 AM) <{POST_SNAPBACK}>
Hi JZ

Welcome to the forum,

The main contactor and the delta contactor should be AC3 rated for 1/Rt3 of the rated motor current provided that the main contactor and the delta contactor are both connected to the incoming supply. If the input to the delta contactor is connected to the output of the main contactor, the main contactor would need to be AC3 rated at the motor rating.
The star contactor should be AC3 rated at one third of the motor rating.

The auxilliarly delta contactor should be rated at 1/rt3 of the motor rating (same as the delta contactor).
The resistors should be sized such that the current through the resistors when in series with the delta configured windings, should be similar to the current through the resistors when connected in star. The resistors must be capable of dissipating the power when the resistors are connected in star (until the star contactor opens) and the dissipation in delta until the delta contactor closes.

For more information, see http://www.lmphotoni.../star_delta.htm

Best regards,



Thanks a lot for a valuable reply sir.

Follow up question please also related to this kind of starter.

Would it be necessary that i have a timer to installed to make an enough delay prior to the closing of the delta contactor (KM2 - as shown from the link you given), I mean there would be an enough delay before closing the KM2 after the auxilliary delta contactor (KM4) closes.

Because i'am thinking if i can only install one timer just for the delay of energizing the coil of KM4 after the KM3 and KM1 energized, then the coil of KM1 will be deenergized thru the auxiliary contacts driven by the KM4, then the KM2 will be energized by the auxilliary contacts driven by the KM1.


#4 marke

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Posted 28 January 2008 - 08:14 AM

Hi JZ

Refer to the sequence on the web page.
You will need a sequencer or a series of timers to make this work.

The first stage is with the main contactor closed and the star contactor. This stage should last for long enough for the motor to get up to full speed.

Next, the auxiliary delta contactor is closed,

Next the star contactor is opened

Next delta contactor is closed.

With star delta starters, it is important that in addition to the delay before the star contactor is opened, there is an additional delay before the delta contactor is closed. Star delta timers provide a dual time sequence with a long time before the star contactor opens and a short delay before the delta contactor closes. You can not rely on the auxiliary contacts alone to provide this delay

Best regards,




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