120/240V Single Phase Generator for Backup of 208Y120V 1P Load

[hdp]

I have a customer that needs to provide standby power for lyophilizers (freeze drying equipment). They are rather expensive, multiple compressor loads. These loads are 208V single phase, fed from a 208Y120V service. To minimize expense, the customer wants to use a 120/240V residential single phase generator to provide standby power. I'm concerned about retransfer to utility power after restoration and have recommended a delayed transition transfer switch. Am I being overly concerned about the difference in phase angles and voltages between the two sources?

Thanks for any opinion/guidance.

Project is in the U.S.

 

[wayne440]

I would first make doubly sure that 120/240V will not damage the "rather expensive, multiple compressor loads".

My generic answer to your question- if the compressors are small a switch with "in phase" monitoring might do the job as well as any, but it probably can't hurt anything to use a delayed transiton. If they are larger, use delayed transiton and make sure they don't all start at once. Also, a 240V rated switch might not have enough adjustment range to accept 208V power as normal.

More information will yield better responses- i.e. a multiple compressor load could be 2 units or 5 units, and 1/2hp or 25hp, what do you have? Are all the units fed from the same phase and subpanel? What are the potential losses from a failure-hundreds of dollars, thousands of dollars or ?

In my (limited) experience with standby power, "to minimise expense" is often difficult reasoning to justify in the long term; even more so when a failure will result in substantial losses.

 

[hdp]

Hello Wayne,

Thanks for your reply. These units have multiple 1hp compressors. They probably run $20k US each and are laboratory grade devices. I can cost the equipment; however, since this is a research facility, I am not calibrated to the potential cost of "lost" genomic research..it could be hundreds of thousands, or potentially more. My initial approach was to use a small natural gas fired commerial grade 3 phase generator. My client wants to go the Lowes.

 

[hdp]

P.S. Muliple compressors = 2 each. Number of units = 2 each. Not a substantial load; I'm concerned with potential ill effects with transfer from the generator to the utility source, given the difference in phase angles and voltages. Perhaps it's not an issue with single phase motors. This application is admittedly outside of my realm of experience.

 

[wayne440]

I personally would not "go to Lowes" for this application. There are several reasons, the first being that I am familiar with the brand and quality of the sets they usually sell. Using a decent delayed transition switch with one of those generaters would be similar to wearing $600 shoes with a $150 suit. With a $20K piece of equipment, a call to the manufacturer would drive much of my decision between delayed transition or in phase monitored transfer.

I think a 120/208V set makes sense for this, if nothing else because I see problems later on with the 240V set feeding a 208V service, and it is almost universally customary to back up utility service with like voltage standby. The NEC and or electric inspector may have something to say there as well. Besides that, your name will come up when someone "asks who in the #$%& did this", and some later installed 208V device may fail when it gets 240V instead.

If you have the luxury of being able to do so, this may be a job best done your way or passed on.

 

[waross]

If the customer wants to risk hundreds of thousands of dollars of research on a Lowe's gen-set, Adjust the generator voltage to 208 Volts and use a delayed staggered restart. You can use the same starting arrangement for both generator starting and grid starting.
Put another way, you will probably need an automatic staggered start arrangement to get the units up and running on the gen-set. Use a few seconds delay and then use the same controls to restart on the grid.

If you have the luxury of being able to do so, this may be a job best done your way or passed on.

I'll second that.

I'd suggest a minimum of a 10 KW set( 10 KW at 240 Volts). That will allow you to start 2 compressors together if you have to. It will also recognizes the drop in gen set rating that will result from dropping the voltage setting. There is also a little extra for fans. If you have electric defrosting you better tell us.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[hdp]

Thanks much for the replies.

According to the manufacturer's data sheet, the equipment proposed is rated 208/240 VAC, single phase. The "no heat" option has two compressors, one -1/2 hp and one 1.0 hp. The "heat" option uses two - 1.0 hp compressors.

I have repeatedly expressed my objections to this solution. However, the client says that "..we did it at another facility". At that facility, all the loads are 120V, refrigerators, security system, phone system, small line-interactive UPS, on a 14kw Lowes gennie.

This is a agricultural research facility. Those who have spent much time around power systems on farms have probably seen just about everything.

 

[wayne440]

...the client says that "..we did it at another facility". At that facility, all the loads are 120V, refrigerators, security system, phone system, small line-interactive UPS, on a 14kw Lowes gennie.

Your client is comparing apples to oranges. If you also have 120V loads, and a single phase set, either your 120V loads get 104V, or your 208v loads get 240V. While such a set might be made to "work" in the proposed application, I would not put my name on it. Customers often insist on compromises in the name of economy, then forget whose idea it was when something goes wrong. This reads like such a scenario in the making. Good luck.

 

[waross]

If all the compressors are rated 208/240 Volts them use the 120:240 Volt genny. Still allow enough time for the motors to stop before restarting.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[hdp]

Thanks for your insight, Bill and Wayne440. I appreciate both your interest, and your time.

Dave

 

[ccjersey]

I would if at all possible use a single generator to backup the whole facility, matching whatever the utility voltage/phase supply is. If they really want something just to backup those machines, and to have it automatic, I would guess that the cost of installing an automatic transfer switch in a subpanel location within the facility would exceed the installation of a modest sized whole facility backup system with automatic transfer switch at the building's service entry.

The other part of this question is how it will be fueled and maintained so it will function when needed. I doubt Lowes offers service contracts to keep the consumer grade generators they're selling going long term. I expect they don't really want to hear about one once it goes out the door, though it might be a missed opportunity for them.

Going to the local "big box" store and buying a generator sounds like something you would do (along with every other desparate homeowner) after there has been a large power outage. The results might be satisfactory in light of the present situation, but rarely are satisfactory long term.

 

[itsmoked]

You client is being.. a moron.

There that's off all our chests.

Use the Home Despot generator. Use it in 120/240 mode as designed. Leave the voltage setting at 240.
Install 32V buck transformers to buck from the 240 to 208. The compressors will be fine and he'll still save over a 120/208 generator.

The system will work and you'd have nothing besmirching your reputation down the road.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[TheBlacksmith]

Kind of seems like having a BMW and buying no-name tires from the local garage, but he's the client.