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Connecting A Capacitor As A "snubber" In Ups.


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#1 AB2005

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Posted 27 May 2012 - 03:37 AM

Dear Mark,
Our people widly installed square wave UPS due to power break down problems. Many people have a problem of malfunctioning of fans dimmers which run on square wave UPS. I checked a fan running on square wave UPS by osciloscope and found that during swicthing on and off of transistor, fan produced high voltage spikes as "inductie kick back". I connected a 2.5uf capacitor with fan and found that high voltage spikes disapeared. Please see the below pictures. Now i want to ask that as a capacitor also creates a load on supply component, connecting a capacitor with fan in above mentioned condition put extra load on UPS? (2.5uf = 33VA almost)

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

#2 marke

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Posted 27 May 2012 - 08:42 AM

Hello AB2500

I would be very carefull connecting a capacitor across the load. If there is a triac in series providing a variable voltage to the fan, then the switch ON current will be very high and will cause the triac to fail. You need to put a resistor in series as a minimum, and I would aim to reduce the amplitude, (lengthen the pulse), not to eliminate it.

Best regards,
Mark.

#3 AB2005

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Posted 30 May 2012 - 09:41 AM

Dear Marke,

I have understood that the capacitor has a minimum resistance during few micro seconds in start which behaves like a " Short Circuit". Lets there is no any Triac in series with fan and we connect a capacitor 2.5uf, would capacitor put extra load to the utility or on to UPS? I understand that if the power factor is legging then capacitor tries to improve it but if PF is almost ok and we connect capacitor, would it put extra load on utility or on UPS?
"Don't assume any thing, always check/ask and clear yourself".

#4 marke

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Posted 30 May 2012 - 09:25 PM

Hello AB2005

If the UPS is direct coupled on the output, then the use of a capacitor without a series resistor will put a lot of extra stress on the switching devices as there will be a high voltage across the devices as the capacitor charges up resulting in high switching losses.
If the output of the UPS is transformer coupled, the leakage reactance of the transformer will reduce the effect of the capacitor charging.

In regard to the power factor correction, this is only relevent with a sinusoidal inverter. A squarewave inverter has lots of harmonics and at the harmonic frequencies, the load is capacitive if it is corrected to unity at line frequency. This will increase the losses.

Best regards,
Mark.




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