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View Full Version : Uninterruptible power supplies and VA vs. Kw



Micheal Donnellan
12-29-2007, 1:06 AM
What size UPS would you recommend for a 60W laser that says its uses <1KW. UPS's are sold with VA marked on them, this I am told is based on power factor? and a 1000VA is not 1000W. So anyone know what size would be needed? and how to correctly convert it to wattage rating.

Luke Phillips
12-29-2007, 9:57 AM
Michael - UPS have both maximum Watt ratings and maximum VA ratings. Neither the Watt nor the VA rating of a UPS may be exceeded. It is a de-facto standard in the industry that the Watt rating is approximately 60% of the VA rating for small UPS systems, this being the typical power factor of common personal computer loads.

In some cases, UPS manufacturers only publish the VA rating of the UPS. For small UPS designed for computer loads, which have only a VA rating, it is appropriate to assume that the Watt rating of the UPS is 60% of the published VA rating. For larger UPS systems, it is becoming common to focus on the Watt rating of the UPS, and to have equal Watt and VA ratings for the UPS, because the Watt and VA ratings of the typical loads are equal.

Not sure if you're using 230V or 120V for your laser but would recommend a 1.5kW rated UPS.

Micheal Donnellan
12-29-2007, 12:53 PM
Thanks for the reply I have something to work with now. As for 60%, from working out the stated values on some UPS's I have seen from 56% to 85%. 85% being more common, if 60% is standard, sounds like market hype as they were small machines.

Richard Rumancik
12-29-2007, 1:20 PM
I agree what Luke is saying; 1500VA would probably be prudent as you want some margin. However you might consider plugging your PC/monitor into the same system unless you have a separate one for them. This would raise the requirements a bit and you might have to go up to say 2200VA. Leave some margin (say 15-20%) so you are not running close to the limits.

I would not plan to actually run the machine during and outage; just use it as a surge suppressor.

Here's a paper that might help

www.ptsdcs.com/whitepapers/12.pdf (http://www.ptsdcs.com/whitepapers/12.pdf)

You might consider a rack mount UPS like those used for Network/Server applications. For example APS Smart UPS units. As an example, one unit rating is:

APC Smart-UPS, 1980 Watts / 2200 VA,Input 120V / Output 120V, Interface Port DB-9 RS-232, SmartSlot, USB, Rack Height 2 U

You can see that the Watt rating is set at 90% of the VA rating. Most switching power supplies in newer lasers will probably be running with a power factor of 95% or more.

This kind of supply will set you back maybe $1000. If your budget does not accommodate this, one option is to buy a used one cheap with the idea that you might have to put in $200 worth of batteries. The telecom services, Internet companies, utilities, universities etc may have used ones for sale. I bought a "new" old-stock unit (a few years old but never installed) which was bought as a spare unit for a data center. Then they went to newer technology and sold the old ones.

Skip Weiser
12-30-2007, 11:40 AM
Would this unit be worth looking at or is it too small for a laser with a 1kw power supply like Micheals, and mine? I'm more interested in the voltage regulation aspect. I'm sure my voltage fluctuates frequently but power outages are rare.

Ultra / ULT33046 / 2000 VA / 1200 Watt / AVR Backup System / UPS w/ Digital Display (http://www.tigerdirect.com/applications/searchtools/item-Details.asp?EdpNo=1815470&sku=ULT33046&CMP=EMC-TIGEREMAIL&SRCCODE=WEM1535MH)

Thanks.

Richard Rumancik
12-30-2007, 1:13 PM
Skip, I am not an expert on this but here are my thoughts. The unit you are looking at is intended to be a simple backup supply. It has only 2100 joules of surge suppression. I would try to have 3000 or 3500 joules. (This could be done by adding a high-quality surge suppressor in front of this box, but it might cost another $50.)

You say the main reason for getting it in your case is to regulate voltage, but I don't think these entry-level units are set up to do that. They tend to "pass through" whatever is coming down the line, minus the spikes. But they don't try to regulate the voltage up or down. In other words, if you have a sag to 109 volts or it goes up to 125 volts on the input, I would expect the lower end units would just pass this through to the laser. I can't say for sure on this particular unit, as it does not show the complete specs.

What you are probably looking for is a unit with "Buck" and "Boost" capability. Also called AVR for Automatic Voltage Regulation. It will drop the voltage if it gets too hight and boost it if it gets too low. My rackmount unit has this. It adjusts the output continuously if the line voltage fluctuates.

Look at the specs for the Ultra unit:


Power Protection Type: UPS Battery Backup
Input Voltage: 110V-120V
Recommended Electrical Service: 120V
Watts: 1200 Watt
Voltage: On Battery 120V 5%
Volt Amps: 2000 VA
Output Frequency Regulation: 50/60Hz 10% (auto sensing)
Output Voltage Regulation: On-Battery Output Voltage Regulation Nominal 2%
Surge Energy Capacity: 2100 Joules

Notice the "on-battery" qualifier in two places. To me this means that while it is running on the battery it will regulate the output. Otherwise I expect it will not try to regulate at all.

This unit is certainly better than a regular surge suppressor/power bar but I don't think it will do what you are hoping for. You will probably have to get into the $500-$1000 range to get buck and boost capability (AVR).

However, I can't tell you that you absolutely need to have AVR. You would need to decide. In my case I am more concerned with disturbances, spikes, and brownouts. The reason I am less concerned with AVR is that the switching supply in my laser is quite tolerant to voltage input. If you see that the power supply is marked with something like: Input ~100V-250V then it means that it will operate acceptably anywhere within this range. (Most modern supplies are self-switching between 120 and 240V inputs; older PCs and lasers might have a high-low switch.) So having AVR is better but not necessarily mandatory.

My own preference would be to go to more of a server/utility/hospital type backup unit which is more "industrial" than consumer grade. You might be lucky if you enquire with IT departments at businesses, schools, etc. But its better to have something installed than to wait.

Skip Weiser
12-30-2007, 9:03 PM
Thank you for the info, Richard !!!

Skip

Jerry Allen
01-02-2008, 12:43 PM
Most of the newer UPSs have AVR, but for $100-$200 you cannot expect one to handle the load requirements of your laser. Even if it could, you cannot expect it to run for long on the battery and would be useless if a long job was running. I use them on my computers but not on my laser.
My laser is rated at ~1000W. I use a line conditioner (magnetic tranformer type) which is rated for 1200W and two surge suppressors in series. It cost about $120. Most surge suppressors use MOV's to suppress spikes. They deteriorate over time. Adding them in series works and adds protection.
Even though computer power supplies can use a wide range of voltages and some can auto switch, they still get problems with dirty power and can have their life shortened. The power supply in the laser is no exception and is not cheap. I have read the the power supply for the beam itself is subject to line fluctuation and the beam will be more consistent by using line conditioning. I can't prove this is true, but I have had my laser using a line conditioner since day one for three years without a hiccup. However, lightening is some nasty stuff and can induce voltages in wires just by induction, not actually striking the device. Therefore, I do not depend on any protection device when lightening is close and just shut the laser and computers down until it's over.