Fan type capacitor in Desi UPS

Hello there guys!

Coming straight to rhe point, i wanted to know any pros and cons of using a 4uf fan type capacitor at the output of a desi ups. Well, i want to install one because i think it will help if this ups is being used at inductive loads like sewing machine motors 100-150 watts.

Does this effect backup time of a desi ups? If yes, how?

Do we need to remove the in built capacitor from the ups kit (card) if we install fan type capacitor?

What happen if we use both?

What happen if we use fan type capacitor only and remove the built in one?

Does number of FETs effect backup time of a desi ups? If yes, how much time effect if we use one additional on each side? And so on..

Any logical answer will be highly appreciated!!

[quote=", post:, topic:"]

Does this effect backup time of a desi ups? If yes, how?

[/quote]

Yes, it will become a permanent load on the UPS. This is because when a capacitor is connected to AC voltage, it becomes like a resistor (there is a property of a capacitor known as Capacitive reactance which is equivalent to resistance in a DC circuit). For example a 4uF capacitor connected to a UPS (220VAC 50Hz) will behave as if a 795 Ohm resistor has been connected. This is approximately 0.3A load (equivalent to 66VA ).

[quote=", post:, topic:"]

Do we need to remove the in built capacitor from the ups kit (card) if we install fan type capacitor?

[/quote]

The internal capacitor on the kit is of very small value of about 0.15uF which is equivalent to a 21K Ohm resistor (2.3VA load) thus it has negligible effect as compared to a 4uF capacitor. The internal capacitor is not meant for power factor correction. It is there acting as a snubber which minimizes the spikes produced by switching transistors. These spikes are actually created on the transistor's side of transformer but they pass through to 220V side as well.

[quote=", post:, topic:"]

What happen if we use both?

What happen if we use fan type capacitor only and remove the built in one?

[/quote]

The answers to first two questions should give you an idea.

[quote=", post:, topic:"]

Does number of FETs effect backup time of a desi ups? If yes, how much time effect if we use one additional on each side? And so on..

[/quote]

Number of MOSFETs does have some effect. If you parallel many MOSFETS, the switches losses are reduced. This will in turn increase the conversion efficiency and increase the backup time.

Unfortunately there are so many variables in computing the effect of adding one additional MOSFET that it would be much better to practically measure it rather than doing the maths. Instead of adding MOSFETs, it is more beneficial to use MOSFETs with Low On-State Resistance. They have low losses and they stay cool.