# Current Variation In Line Side And Load Side Of Soft Starters

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### #1 amb2000

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Posted 01 March 2014 - 04:46 PM

Dear Sirs,

please look into my problem and help me. We have one MCC with Main Incomer MCCB and 3 Soft Starters  in Outgoing side  with Individual Outgoing Breakers. There is one Capacitor bank also controlled by dedicated Power Factor Regulator.

​At the Motor starting (Any one out of the three)  incomer Current in the digital Meter and Outgoing Current in the Digital merter Installed in the soft starter circuits are Same at 0.75 power factor Incomer and Outgoing Currents are  84A- L1, L2,L3. But after Power Factor correction PF - 1, Incomer Current is falling to 64A- L1, L2,L3  but the Outgoing Soft starter current is still  84A- L1, L2,L3.

​Can anyone help me please, we have checked the CT and measured the current with separate Clamp Meter also, again the scenario is the same.

Abhilash,  abhi.mureeckal@gmail.com

### #2 marke

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Posted 01 March 2014 - 08:39 PM

hello amb2000

The current drawn by a motor comprises a resistive component and an inductive component.

The resistive component is in phase with the voltage and the inductive component is 90 degrees behind the voltage.

The current measured is the vector sum of both the resistive and inductive currents.

Power factor correction uses capacitors to draw capacitive current which leads the voltage.

The capacitive current is 180 degrees out of phase with the inductive current and can be used to cancel the inductive current.

With power factor correction installed, the current into the motor is the resistive current and the inductive current. There is no change resulting from the addition of the pf correction.

The current drawn from the supply is the current into the motor plus the capacitive current.

If the capacitive current is equal to the inductive current, the inductive current is cancelled from the supply, so the net current is reduced.

This is the whole role of pf correction.

Best regards

Mark

### #3 AB2005

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Posted 02 March 2014 - 08:08 AM

Hi amb2000;

Can you please share the drawing of your system for better understanding?

"Don't assume any thing, always check/ask and clear yourself".

### #4 amb2000

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Posted 02 March 2014 - 10:13 AM

Can you please provide your mail ID, I will sent it thru it or kindly advise how to share here

Abhilash

### #5 amb2000

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Posted 03 March 2014 - 04:25 AM

hello amb2000

The current drawn by a motor comprises a resistive component and an inductive component.

The resistive component is in phase with the voltage and the inductive component is 90 degrees behind the voltage.

The current measured is the vector sum of both the resistive and inductive currents.

Power factor correction uses capacitors to draw capacitive current which leads the voltage.

The capacitive current is 180 degrees out of phase with the inductive current and can be used to cancel the inductive current.

With power factor correction installed, the current into the motor is the resistive current and the inductive current. There is no change resulting from the addition of the pf correction.

The current drawn from the supply is the current into the motor plus the capacitive current.

If the capacitive current is equal to the inductive current, the inductive current is cancelled from the supply, so the net current is reduced.

This is the whole role of pf correction.

Best regards

Mark

Hello Mark,

Thanks for your kind support, Just need to check with you few things,

1) Is there any possibility of making both the currents the same. How we can reduce the current in the Outgoing side of Soft Starter as we cannot connect the capacitors in the Outgoing side.

2) Is this the same scenario in all the circuits where ever SoftStarters and Drives are coming along with Capacitor banks.

3) Can I advise my customer, our system is working perfectly and there is no problem to worry in this regard.

Abhilash

### #6 AB2005

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Posted 03 March 2014 - 04:37 AM

Very easy. Just prepare the drawing, get its image and attach with your post.

"Don't assume any thing, always check/ask and clear yourself".

### #7 marke

marke

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Posted 03 March 2014 - 04:56 AM

Hello Abhilash

1) If you measure the current at the actual input and output of the soft starter, you will find that they are equal. The current is only reduced on the line side of the PFC connection.
2) Any system that has power factor correction will (should) show a reduced current on the line side of the power factor correction connection point.

In the case of VFDs, you do not apply power factor correction capacitors because the displacement power factor is already better than 0.95  The true power factor will typically be much less because the harmonic power factor is less than 0.95

3) Definitely!!

Best regards,

Mark

### #8 amb2000

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Posted 03 March 2014 - 06:53 AM

Dear Mark,

Kindly note for above Point 1 , I would like to share the single line diagram for more clarity.  Please go through this and advise, Soft Starter Line side and the Capacitor bank connection points are the same here. Then why there is difference in current in both Ammeters.

Abhilash

### #9 AB2005

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Posted 08 March 2014 - 07:51 AM

Hello amb2000

There is no any diagram attached with your post.

"Don't assume any thing, always check/ask and clear yourself".

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