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Soft Starter With Pfc Capacitor


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#1 Ronald

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Posted 07 November 2006 - 09:26 PM

We have installed a pfc capacitor in our ac motors with soft starter, and was connected after the soft starter, during the running phase, we engage the capacitor by turning on our controller, we observed that speed is slowing down at instant then back to normal speed. Afterwards, we knew that the soft starter was already short circuited. How does it happened?

#2 jraef

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Posted 08 November 2006 - 12:08 AM

I'm not sure I understand the terms you are using or your sequence of operation. Are the capacitors controlled by a separate contactor? When you say 'short circuited" do you mena bypassed with a contactor? Please describe exactly what happens in order, starting with a start command to the soft starter.
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#3 marke

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Posted 08 November 2006 - 10:03 AM

Hello Ronald

You must never, under any circustances, connect the power factor corretion on the output of a soft starter. You will probably cause the SCRs to fail by doing so.
Al;ways use a separate contactor and connect to the input side of the soft starter once the starter has ramped to full voltage.

Best regards,

#4 Ronald

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Posted 08 November 2006 - 09:58 PM

Thanks for the reply. If that so, i must use on-delay timer so that when the starter ramped to full voltage, our pfc capacitor with contactor will then energize. Is it 100% safe if we will connect it in the input side of the starter?

#5 jraef

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Posted 08 November 2006 - 10:05 PM

QUOTE(marke @ Nov 8 2006, 02:03 AM) View Post

Hello Ronald

You must never, under any circustances, connect the power factor corretion on the output of a soft starter. You will probably cause the SCRs to fail by doing so.
Al;ways use a separate contactor and connect to the input side of the soft starter once the starter has ramped to full voltage.

Best regards,


Mark,
This makes me wonder though. If he did have a separate contactor for the Caps, AND he had a Bypass Contactor and only turned on the Capacitor contactor AFTER the bypass is pulled in, would there still be damage? In other words, even if there was still a high dV/dt from energizing the caps, the SCRs are already gated fully on anyway and the bypass would shunt any transient around them. Conversely, the harmonics from firing the SCRs would already be gone by the time the caps are energized, so no excess resonance and heating of the caps either.
"He's not dead, he's just pinin' for the fjords!"

#6 Ronald

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Posted 10 November 2006 - 09:17 PM

After a series of power quality test using our HIOKI instrument, we found out that there is a high percentage of THD in the output side of the soft starter and low percentage in the input side of the soft starter. We also noticed that soft starter is still engaging eventhough the motor reaches its 90% full load. Thus we conclude to remove the PFC capacitors and never attempt to install it.
Thanks for your fruitfull suggestions and discussions.
Regards,




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