EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: drummerdimitri on September 22, 2017, 01:31:08 pm
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So I want to buy a UPS for my mining rig that currently consumes 630 W of power according to my power meter. It is showing me a power factor of 0.97.
Does that mean a UPS with a rating of 800 VA will be enough to keep it runing a couple of minutes before shutting down or does the 0.6 power factor still means that a 800 VA UPS will have a Wattage or around 480W since it affects the UPS itself and not the load attached to it?
What I am asking is if the UPS' wattage will change depending on the load that it is attached to and how that is correlated to the power fator shown on my power meter?
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Although the real power (Watts) is what gets consumed, the reactive portion still has to be switched and conducted by the UPS.
These create internal losses.
This constraint is not limited to an UPS. Transformers are rated the exact same way. VAs, not Watts
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Read the UPS data sheet carefully. They often promote the VA rating 'cause it's a bigger number.
They should also specify a rating in Watts.
If they don't specify a rating in Watts, you'll have to estimate by multiplying their rated VA by their rated PF.
Also you need some margin. I guess at least 20% (others may suggest more) to account for 'optimistic' data sheets.
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20% is the rule of thumb and is sufficient if the equipment is decent quality. I would actually suggest getting a used rackmount UPS and replacing the batteries since it would be a cheap way to get a high end UPS.
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Please help me understand the concept first.
I bought a UPS with a VA rating of 1500 and a Wattage of 900 so does it mean that with a power factor of 1 it could deliver 1500W of power to my computer but with a PF of 0.6 it will only be able to provide the rated output of 900W?
I would like to understand this first...
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The double rating with VA and Watts is that way, that both numbers can be limiting. For the 1500 VA, 900 W example they claim to deliver up to 1500 VA if the real power is below 900 W. So for a load with PF of 1 they can deliver up to 900 W. They can still deliver up to 900 W even if the power factor gets less good, down to about 0.6. With a power factor that is even worse (e.g. 0.5) the VA rating would be limiting - so no more than 750 W at a PF of 0.5 as this would reach the 1500 VA rating.
The power factor is used to describe two possible effects of the load. One are phase shifts (e.g. from an inductive load like an AC induction motor. The other are nonlinear loads like many LED lamps or old / poor DC power supplies - these cause a non sinusoidal current flow. For something like an transformer the PF is the correct number to describe the losses. However it is not clear that the electronics in the UPS also reacts similar to both types of PF, so the VA rating might not be valid for all cases. At least there is an additional limit on the peak current, even if the VA rating is not exceeded. So it is a good idea not rely on the full VA rating of the UPS.
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The double rating with VA and Watts is that way, that both numbers can be limiting. For the 1500 VA, 900 W example they claim to deliver up to 1500 VA if the real power is below 900 W. So for a load with PF of 1 they can deliver up to 900 W. They can still deliver up to 900 W even if the power factor gets less good, down to about 0.6. With a power factor that is even worse (e.g. 0.5) the VA rating would be limiting - so no more than 750 W at a PF of 0.5 as this would reach the 1500 VA rating.
The power factor is used to describe two possible effects of the load. One are phase shifts (e.g. from an inductive load like an AC induction motor. The other are nonlinear loads like many LED lamps or old / poor DC power supplies - these cause a non sinusoidal current flow. For something like an transformer the PF is the correct number to describe the losses. However it is not clear that the electronics in the UPS also reacts similar to both types of PF, so the VA rating might not be valid for all cases. At least there is an additional limit on the peak current, even if the VA rating is not exceeded. So it is a good idea not rely on the full VA rating of the UPS.
Just the explanation I was looking for thanks a lot!
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UPS, when powered from battery, has 2 power converter stages:
1. 24V battery->high voltage DC converter
2. High voltage DC to AC converter
The first one limits maximum real power output (W), while the second one limits total apparent power output (VA).
There are many different ways UPS systems are implemented, and they behave differently.
Just to name one alternate design: low voltage H bridge driving a low-frequency transformer; gets you 230 V AC from battery voltage with one-and-a-half stages, so to speak.
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If the UPS is going to be used only for your mining equipment and assuming that the mining equipment has one or more electric motors then your load is most likely inductive. I would think if the PF became controlling then you might look into adding capacitive load to the equipment and attempt to bring the final PF closer to unity. Then the total watt limit might allow you to use a smaller UPS. But since cost of a smaller UPS might not be worth the added cost of the capacitors.
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I guess "mining rig" most likely refers today to the electronic power-wasting-machinery prospecting virtual coin.