Convert single phase to three phase

Status
Not open for further replies.
Minuteman

Minuteman

Senior Member
Location
The Sooner Nation
I have been asked to install wiring on 2 Roof Top A/C units replacements for a church. The original 5 ton units are single ? 60 amps and the new ones are 3 ? 50 amps. The question is, would it be cheaper to swap out the 3 ? compressors or to install inverters.

To be honest, I have never worked on an inverter before. Would that be a "Buck-Boost"? Is that something I can pick up the supply house or do I have to rig it up?
 
Last edited:
There used to be several manufacturers of rotary converters that converted single phase to three phase. I believe Ronk was one of them. Haven't searched the internet so I don't know exactly what to search for. I think their method was to have a motor driven generator for conversion.
Hope this helps.
 
Well...IMO there's two ways to do it.
One, as you suggested is to use a AC Variable frequency drive to convert the incoming single phase to 3 phase.
The other is to install 3 phase power for the compressors.
How many HP are you talking?
Is there 3 phase power near?
I've used inverters in industrial applications (way back) for variable speed drive applications. They're OK. I'm just not sure about starting and stopping a compressor with one, although you can adjust the acceleration and torque.
Maybe someone with some recent experience and knowledge of inverter drives will jump in.
They are package units and it's pretty much a matter of placing them and hooking them up. Do a search for "Variable Frequency Drives", there are several manufacturers.
A 15 HP drive will probably cost you around $800-1000.
If it were me and there was any way possible, I'd either install 3 phase power, or swap the units for single phase.
just my 2 cents.
steve
 
I've installed a couple of Phase Adders from Eylander Electric and they work well. http://www.angelfire.com/planet/eylander/index.htm You should check with them to make sure it will work in your application. One that I installed was for a dry clean machine with several motors and one small freq drive, and the drive wouldn't work with the coltage changes when the other motors started and stopped.
 
For an a/c unit, the multiple starts and stops per day will probably create a problem for a drive, unless specifically rated for such an application.

You essentially need an ac-dc-ac converter, with single phase in, 3 phase out.

Contact Ametek, (formerly Solidstate Controls). They build a lot of equipment for the power generation industry and are top quality. They also build custom application equipment.

http://www.solidstatecontrolsinc.com/index.html
 
I have just finished a redesign to add AC to a church space using equipment donated by a member. Holy Cow what trouble it was. It is typically better to design and build it right the first time than have a potentially dangerous situation with improper equipment. Either find available 3-phase power or swap out equipment to have it operate on the available power. As soon as you add a 3-phase converter to the facility, the owner now has a piece of equipment they don't know what it does and what maintenence is required.
 
This seems pretty risky to me. It makes me wonder since these are changing from single phase to three phase how much do they weigh compared to the existing units? Can the roof support them? Also have you verified the voltage on the units is what is out there?
 
After looking at the spec's. on Variable frequency drives I find that the cut-off for 1 phase in/3 phase out is about 3HP.
As I said, it's been a long time since I've had any experience with this application and I'm sure that the technology has changed a lot.
Is there any way to change single phase to three phase without using a roto-converter? I'm speaking of solid state without any moving parts, 5 HP and up.
Just curious.
steve
 
There are static converters, but a lot of them are poorly designed and won't give you the same performance as a rotary converter. Some even generate a phase to start a 3-phase motor and then drop to a single phase run condition. Obviously, this is not desirable and potentially even dangerous.

There are rotary phase converters that can start and run up to 75 HP motor, so the size of your application shouldn't be an issue. Plus, a properly sized rotary can run multiple motors.
 
A VFD will convert single phase to 3-phase output. It is very common to have a 240 V single phase input to operate a three phase motor using a VFD. Care must be used in selecting the correct HP, actually amp, rating of the VFD. With single phase input only half of the input diodes will be connected. so for a 5 hp 208v motor FLA rating of 16 Amps, a VFD rated at minimum of 32 amps will be required. The ouput waveform of the VFD doesn't make it a good choise for something other than a 3-phase motor, but you will be able turn on and control the fan. I don't recommend it for compressors however.

That's my way of fitting a 3-phase peg in a single phase hole.
 
Status
Not open for further replies.
Top