Rob's Critical Power September Newsletter
September 2021
Hello Reader,
This month we are going to discuss one of my favorite engineering topics, Power Factor. As Power Factor is a complex subject, I've included a PDF of a section from my book to help explain at farther.
As always, your feedback is welcome and encouraged.
Thanks again for reading.
Rob
Power Factor
Last month a customer called because they want to double their current 10kVA UPS system size. Their current system is loaded to over 90% and they need to add more IT equipment.
As we reviewed their electrical infrastructure, we realized to double the system from 10kVA to 20kVA would be an expensive undertaking. The UPS is on the third floor and the electrical feed starts in the basement electrical room. All new conduct and wire would need to be installed between the electrical room and the third floor. There was also a concern the current generator cannot support the increase in size.
We did determined a 15kVA system could be installed with very minimal change. However, the customer was concerned they were only getting 50% more power.
When we took a deeper look, their current UPS System is rated at a 0.8 Power Factor (PF), meaning it is 10kVA, but it’s only 8kW. Most of today’s IT loads use power closer to a 1.0 PF, meaning kVA and kW are equal.
The new UPS system has an output rating PF of 1.0, making it a 15kVA/15kW unit. This new system would essentially double their usable power.
But what is this Power Factor magic that allows us to do this?
Engineers define it is as the ratio between real power (watts) and apparent power (volt-amps), expressed as a number between -1.0 and 1.0. We often use the power triangle to visualize it.
The best analogy I’ve seen to explain power factor is a glass of beer. In the glass, there is the beer, what you drink, and the foam, what you can’t drink. However, you need the entire glass to hold both the beer and the foam.
The beer is the real power, the part you need, and the foam is reactive power, the part you can’t use but are paying for. The combination of the beer and foam is the apparent power, the total power supplied to the load.
Modern IT loads use all real power and very little reactive power. Today’s larger UPS systems are rated at a PF of 1.0, allowing them to support a load's real power without derating.
Some smaller UPS systems are still rated with lower output power factors, and caution should be used when sizing them.
Power factor is a complex subject and I cannot do it justice in a brief newsletter. So I have included a section from my book that can be download below.
UPS-Handbook-Power Factor.pdf
It is also one of my favorite engineering topics, so be sure to contact me if you want to discuss it more.
You can learn more about UPS System and batteries in my book, "UPS Handbook, A Layman's Guide to Uninterruptible Power Supply Systems."
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