Phase convertor sizing

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frogneck77

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Shelton,CT
Hello All,
I need to size a phase convertor for a 3phase AC condenser. Ive looked at a couple of web sites and they all want to know the HP of the unit. The specs that I have for the condenser include the total unit amps,MCA,MOCP,compressor amps,condenser fan data,etc.I cant find anything for compressor hp, or anything showing total unit hp. Should I size the unit according to equivalent hp based on the total unit amps? I know I should wait until tomorrow and call the convertor manufactures but I was hoping to get the proposal out tonight.
Any info would be appreciated
Bill
 
VFD drives will also work as phase converters so that may help in sizing. All the phase converters I have wired have been for personal use and all were sized by HP.
 
One horsepower per ton.

How many ton is the condensing unit? Add in also any condensing fans. Many/most are 1/3 or 1/2 HP each.

Wouldn't it be easier to just get the right condensing unit? Unless it's something special, you'll have as much in the phase converter as you did in the condensing unit.
 
Bob Kraemer said:
Also don't forget by using a phase converter you reduce the efficiency of the motor by around 20% or so I think.
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Depends on the type, but that is untrue for most modern rotary's. You may need to oversize the phase converter a bit, but the output of the connected load will remain essentially the same.
 
mdshunk said:
Depends on the type, but that is untrue for most modern rotary's. You may need to oversize the phase converter a bit, but the output of the connected load will remain essentially the same.
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Oops Thanks Marc,
I forgot to add that, that is true with static style converters and not rotary phase converters.
 
The unit is a 7 ton condenser. The building only has single phase power. According to the HVAC contractor the largest single phase unit he can get is 5 tons. He suggested to the GC that it might be more cost effective to go with a 3 phase 7 ton unit than two single phase units.The total unit amps at 208-230 3 phase is 27.6. The condenser fans are (2) 1/4 motors, 1.5 FLA each. the compressor data only says 25.6A, nothing about HP
 
BTW- I dont know much about compressors, I normally ask the HVAC guy for the specs, and follow the 10A per ton rule w single phase. Where would it say on the spec sheet the tonnage?
 
frogneck77 said:
BTW- I dont know much about compressors, I normally ask the HVAC guy for the specs, and follow the 10A per ton rule w single phase. Where would it say on the spec sheet the tonnage?
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Tons is a unit that is exactly equal to 12,000 BTU's per ton. The spec sheet normally says BTU's and you just do the math for tons. The unit's model number will always have the BTU's coded in it. For instance, if you see a 36 or a 48 in the model number, that would be a 3 ton and 4 ton unit, respectively. Look for the multiple of 12 in the model number. Look for the BTU's on the spec sheet and divide by 12,000 to get tonnage.

For your 7 ton unit plus the condensing fans plus control power, you should get a 10hp rotary phase converter.

You may also desire a control arrangement that would permit the phase converter to only run when necessary. That being separate "control power" to the condensing unit, and a relay that would be first in line on a call for cooling that would fire up the phase converter, wait a short time delay, then fire up the condensing unit.
 
what the a/c contractor is saying is great for him but if he was paying the power bill he'd go with two single phase units! this installation is gonna be a real pain for the customer and i think is gonna be cheaper to change the a/c's than buying and installing a phase converter. take the a/c spec sheet and send it to the converter manufacturer because for this type load it's gonna be oversized - big time!
 
Marc-
Thank you very much for your input. The unit is 90,000 BTU's-so a little over 7 tons. Thanks for the control aspect. Ive only installed 1 phase convertor for a air pack filling compressor(on demand). I didnt think of the fact that the phase converter in this application would run constantly.
Thanks again,
Bill
 
Charlie tuna said:
this installation is gonna be a real pain for the customer and i think is gonna be cheaper to change the a/c's than buying and installing a phase converter.
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I agree whole-heartedly.

This is a ridiculous arrangement for comfort cooling.

For some sort of special process refrigeration that only comes in 3-phase, that's a different story. The cost of the installed phase converter will be more that the cost difference between a 7 ton 3-phase unit and two 3 or 4-ton single-phase condensing units. Factor into that then that you'd have the operating costs associated with the phase converter.

I disagree with Charlie that a rotary phase converter would need to be way oversized to start a refrigeration compressor. Maybe back in the day, but in these times, the next standard horsepower size up will do just fine as long as the phase converter isn't being started coincidently with the connected load.
 
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frogneck77 said:
The unit is 90,000 BTU's-so a little over 7 tons.
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I was a little puzzled when you said 7-ton, because that's not a standard size. 90K BTU is 7-1/2 tons, which is a standard size.

1, 1-1/2, 2, 2-1/2, 3, 4, 5, 7-1/2, 10, 15, 20, 50.

Some manufacturers throw some oddballs in between there too.
 
The phase convertor will be @ $1,000 dollars plus labor, controls. What would the price difference be for (2) 4 ton units Hvac and electrical cost? What additional wattage does the phase converter consume-versus two ac units?Ugh-so many variables
 
frogneck77 said:
The phase convertor will be @ $1,000 dollars plus labor, controls. What would the price difference be for (2) 4 ton units Hvac and electrical cost? What additional wattage does the phase converter consume-versus two ac units?Ugh-so many variables
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By the time you get that 1,000 dollar phase converter installed, the bill will be 3,000. You'll also have a 10 horsepower or more phase converter running (lightly loaded a lot of the time, mind you) 24/7 for the whole cooling season. Plus, if this is important a/c, you have a duty to recommend that your customer keep a spare phase converter at hand since this isn't something you normally just pop into your local supply house and pick off the shelf.

These are games HVAC guys play to win the work. They'll sell them the cheap 7-1/2 ton 3-phase unit, and tell them that the electrician will just have to put this little gadget on.

FYI.... 7-1/2 ton single phase units are available at slightly more expense than the 3-phase units. What brand, or even better, brand and model is spec'd? I'm just curious to see if it comes in single phase. I've got a 10-ton single phase Lennox to wire up some time before the cooling season hits at a pre-school.
 
im thinking the a/c unit to be a scroll compressor which which will drive the power converter crazy and if the converter isn't sized large enough to handle the design high starting current of the scroll compressor it will damage the compressor and/or it's controls. i'm thinking the voltage drop at start will effect the a/c's performance. and as mark said, the a/c contractor just sweet talked the gc into believing "it's no biggy"!!!
 
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