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Topic Title: Power factor rating on UPS
Topic Summary: Understanding power factor rating of a UPS
Created On: 13 August 2013 11:29 PM
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 13 August 2013 11:29 PM
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Joined: 13 August 2013

I ordered a UPS with an output rating of 6kVA/6kW and a quoted power factor of 1.0. What has actually turned up is a model rated at 10kVA/6kW with a stated power factor of 0.6. Since I don't have an electrical engineering degree I'm trying to figure out what the difference between these two really is.

As I understand it a low pf is bad, but that it's a function of the load, not the source, so I'm not entirely sure what it means to quote a pf output rating since the UPS has no control over the pf of the applied load. After reading some articles on power factor I have concluded that all the 0.6 pf rating means is that at maximum load the load can have a pf as low as 0.6 for the 10kVA/6kW UPS, and at maximum load on the 6kVA/6kW UPS the pf must be 1.0.

So although a low pf is bad, a lower pf on a UPS with the same kW rating is actually better because is can handle a load with the same real power consumption but with a lower pf. And so UPSs that market a high pf rating as good are only good if you size based on kVA only and have a load with a pf at least as high as the rating on the UPS.

Am I understanding this correctly, specifically that the power factor rating isn't actually important and that the only important bit is whether the kVA and kW ratings are high enough for the intended load?

 15 August 2013 08:12 AM
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You don't need an engineering degree to understand power factor - just some AC theory.

A ups with rating 6kVA/6kW means for resistive loads (pf=1) you have 6kW maximum. For reactive loads you would have to derate to suit.
A ups with rating 10kVA/6kW generally means derating is not required. And if you work it out a theoretical load of 6kW @ pf0.6 would require a UPS rated at 10kVA.
In practice this is assumed pf0.6 lagging not leading as a leading pf0.6 would be very 'capacitive'.

So in AC theory it is the load that determines power factor.

Many years ago I had a project that had PSU's with typical pf0.7 lagging but worse case pf0.49.
This would mean your 6kW UPS would have to be able to support 12.3kVA.
 15 August 2013 09:41 PM
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This is a small UPS probably built to a competitive price so don't expect too much. Most UPS units are used to Power a load that consists of IT equipment.

Typically, with modern IT equipment, you find it imposes a leading power factor (capacitive) load on the UPS, 0.9 leading is typical.

There are many technical reasons for this, you can find out why if you want to! But I only suggest you do this if you like really boring technical stuff.

So your UPS must handle the load power factor, you may have bought one that is designed for a lagging power factor, and if you have, it might get a bit warm with a leading power factor.

What you need to know is whether your load is within the "capability" of the UPS, the following link gives a good explanation.

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