# 3 Phase to single phase actual materials?

#### Mark Steele

Joined Dec 11, 2020
13
Hi I'm looking for someone to help me figure out what materials I would actually need to convert a 3 phase 15kw generator to single phase 120/208. I can't just use two legs at 10kw, it isn't enough power. It's my understanding that it can be done by converting to DC then back to AC. I need to know what size rectifiers etc. that I would need to do that. Would anybody like to help me?

#### Papabravo

Joined Feb 24, 2006
16,523
It is going to take a good deal more than rectifiers to accomplish this task. What voltage is the 15kw 3 phase generator producing? The rectifiers you need will be more than capable of handling the current and able to withstand more than the maximum reverse voltage without breaking down. I'm guessing you won't find those at a price you would consider either attractive or reasonable. In fact I'd encourage you to consider the cost of a single phase generator as the costs of this project mount.

#### crutschow

Joined Mar 14, 2008
27,477
What are you going to power with the generator?

#### Mark Steele

Joined Dec 11, 2020
13
It is going to take a good deal more than rectifiers to accomplish this task. What voltage is the 15kw 3 phase generator producing? The rectifiers you need will be more than capable of handling the current and able to withstand more than the maximum reverse voltage without breaking down. I'm guessing you won't find those at a price you would consider either attractive or reasonable. In fact I'd encourage you to consider the cost of a single phase generator as the costs of this project mount.
Thank you for replying! I know it's going to take more than rectifiers. That's what the "etc." was for. I have an Onan generator. This is all the info on the tag on the generator.
Model and Spec No. 15JC-4R/5066P

SERIAL NO. – 27C944732

STBY. KW - 15

CONT. KW – 15

A.C. VOLTS – 120/208

A.C. AMPS – 52.5

CY. – 60

EXCITER - 06SX1N3B

GEN. DATA – 15JC/179

#### Mark Steele

Joined Dec 11, 2020
13
It is going to take a good deal more than rectifiers to accomplish this task. What voltage is the 15kw 3 phase generator producing? The rectifiers you need will be more than capable of handling the current and able to withstand more than the maximum reverse voltage without breaking down. I'm guessing you won't find those at a price you would consider either attractive or reasonable. In fact I'd encourage you to consider the cost of a single phase generator as the costs of this project mount.
What are you going to power with the generator?
I want to use it to power my house during an electrical outage. I have a 6 ton geothermal heat pump, 208/230 with a 50 amp breaker, so if my calculations are correct, two legs of the generator (10kw) come just short of what I need to run the heat pump. Thanks!

#### Just Another Sparky

Joined Dec 8, 2019
213
Honestly your best bet is going to be in either swapping the alternator head or finding a new genset. A 15kW inverter is going to far exceed the cost of a used 15kW generator set after accounting for the resale value of the machine you've already got.

Alternatively, get the nameplate values of the heat pump itself and the alternator on the genset - the alternator, not the genset's nameplate. Sometimes I've come across gensets fitted with alternators rated slightly higher than the genset nameplate. For instance, I've got a "5kW" Katolight fitted with a 240V, 24A Pincor alternator - 5700W. Likewise, your heat pump probably doesn't consume 50 amps at full load... North American electrical code incorporates 20% headroom for circuits supplying continuous loads. That 50 amp breaker might not ever actually see more than 40 amps of steady-state load.

Another "off-the-books" consideration is that your alternator is not going to be seeing full load anyways if you are only using two of three phases. Slightly overloading those two (I.E. 5-10%) while leaving the third entirely unloaded is likely not going to create any sort of thermal overload as long as the windings are able to conduct the additional losses into the stator core efficiently. Heck, it might even have a 1.15 service factor right on the nameplate.

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#### Papabravo

Joined Feb 24, 2006
16,523
Thank you for replying! I know it's going to take more than rectifiers. That's what the "etc." was for. I have an Onan generator. This is all the info on the tag on the generator.
Model and Spec No. 15JC-4R/5066P

SERIAL NO. – 27C944732

STBY. KW - 15

CONT. KW – 15

A.C. VOLTS – 120/208

A.C. AMPS – 52.5

CY. – 60

EXCITER - 06SX1N3B

GEN. DATA – 15JC/179
I would start with rectifiers rated at 75 Amperes and 600 volts PIV

#### Mark Steele

Joined Dec 11, 2020
13
Honestly your best bet is going to be in either swapping the alternator head or finding a new genset. A 15kW inverter is going to far exceed the cost of a used 15kW generator set.

Alternatively, get the nameplate values of the heat pump itself and the alternator on the genset - the alternator, not the genset's nameplate. Sometimes I've come across gensets fitted with alternators rated slightly higher than the genset nameplate. For instance, I've got a "5kW" Katolight fitted with a 240V, 24A Pincor alternator - 5700W. Likewise, your heat pump probably doesn't consume 50 amps at full load... North American electrical code incorporates 20% headroom for circuits supplying continuous loads. That 50 amp breaker might not ever actually see more than 40 amps of steady-state load.

Another "off-the-books" consideration is that your alternator is not going to be seeing full load anyways if you are only using two of three phases. Slightly overloading those two (I.E. 5-10%) while leaving the third entirely unloaded is likely not going to create any sort of thermal overload as long as the windings are able to conduct the additional losses into the stator core efficiently. Heck, it might even have a 1.15 service factor right on the nameplate.
Thanks I am keeping my eye out for a different generator, or head, but this is the first I've heard that the project would cost more than it's worth. Someone else told me that it would cost less than $100.00. But now I'm hearing different. Do you think I could save money via ebay or some other way? What is it that would cost so much the inverter? #### Just Another Sparky Joined Dec 8, 2019 213 A heat sink for 15kW worth of IGBTs is probably about a$30-50 billet of aluminum. IGBTs can be had for $10-20 each shipped. Capacitors for that kind of a DC bus maybe$100-200. Microprocessors are fortunately cheap. The time and labor investment required to assemble and program it all into a working inverter? Very steep. Alternatively, a used 3 phase motor drive may or may not play nice with a single phase compressor motor and ancillary equipment. Sort of a janky proposition but you never know.

As I mentioned before though, all of this might not even be strictly neccesary. In terms of raw horsepower you should definitely be able to run that sort of a heat pump. On a theoretical level, it's just a matter of ensuring you're not going to be passing excessive current through the alternator windings and generating excessive heat there. Check the heat pump's nameplate - I'll bet if that branch circuit was sized correctly you might just be able to squeak by with what you've got.

If all else fails, 4.8kW electric space heaters are about $60-70 each. One across each phase would put you at 11268 watts/38448 BTU/hr. of heat after adjusting for 208 volts. Turning 15kw three phase into single phase is terribly complicated. Turning 15kw three phase into heat is dead simple. Last edited: Thread Starter #### Mark Steele Joined Dec 11, 2020 13 A heat sink for 15kW worth of IGBTs is probably about a$30-50 billet of aluminum. IGBTs can be had for $10-20 each shipped. Capacitors for that kind of a DC bus maybe$100-200. Microprocessors are fortunately cheap. The time and labor investment required to assemble and program it all into a working inverter? Very steep. Alternatively, a used 3 phase motor drive may or may not play nice with a single phase compressor motor and ancillary equipment. Sort of a janky proposition but you never know.

As I mentioned before though, all of this might not even be strictly neccesary. In terms of raw horsepower you should definitely be able to run that sort of a heat pump. On a theoretical level, it's just a matter of ensuring you're not going to be passing excessive current through the alternator windings and generating excessive heat there. Check the heat pump's nameplate - I'll bet if that branch circuit was sized correctly you might just be able to squeak by with what you've got.

#### Ian0

Joined Aug 7, 2020
2,722
The maximum you can get out of it on single phase simply by rewiring it would be 65% of full load.
If you need more than 10kVA and don't want to spend much money on it the best way would be to see if you can rearrange the load to use more than one phase.
Single phase alternators are becoming rarer, as the world likes to standardise on the "12-wire" design, with two 115V windings on each phase, which can be rearranged into all sorts of voltages, but no more than 65% of full load if used single phase.

Don't forget that a 15kW generator will give you 30kW of heat, and your geothermal is only giving your 21kW. (correct me if I'm wrong with that conversion). If you cut the geothermal power to 8.5kW, then you would only require 6kW electrical, which would give 12kW of heat from the generator, and you'd be warm enough.

Just checking - is it geothermal or is it a ground-source heat pump? Ground-source heat pumps are usually about 300% "efficient", so I would expect 30kW out from 10kW in.

Another thing worth checking - most heat pumps have built in inverters, so that they can change the motor speed. The first thing that happens is that the AC is rectified to DC to run the compressor via a variable-frequency drive.

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#### Mark Steele

Joined Dec 11, 2020
13
Hi Thanks so much for commenting! My heat pump is a 6 ton ground source. The tag on it says 15kw 60 amps. I don't want to underpower it and burn the thing out. Also I have a well pump, and the usual house appliances. When you mentioned heat form the generator, are you talking about hooking the generators cooling duct into the house heat? I don't think there's a way to cut the geothermal power down. It's just a compressor, blower, and water pumps. The compressor is wired right to the house 220. What do you think?

#### Just Another Sparky

Joined Dec 8, 2019
213
Hold up - we just jumped from a 240V, 50A branch circuit to a 15kVA heat pump - i.e. an 80 amp branch circuit. Which is it? If your pump truly is 15kVA then it had better not be on a 50A branch circuit with #8 conductors.

#### Ian0

Joined Aug 7, 2020
2,722
Let me sort out the weird units first.
A ton of air conditioning is the power required to melt a ton of ice per day. 334kJ/kg x 1000kg divided by 86400 seconds = 3.86kW. (I don't like British Thermal Units either)
6 tons = 23kW. I would have expected an electrical input to the compressor of no more than 7kW, it's a poor heat pump if it's COP is <3.
How much of its 15kW is compressor, and how much is blower and water pumps? The various power consumptions will be written on the various parts. It may possible, with a bit of rewiring, to put the compressor on one phase and the rest on the others.
If the engine will run it, and you can use the heat from the engine as well, you'll only need run it about a third of the time. It will probably end up cheaper in diesel than buying electricity from the utility company.
Maybe you could run the water from the ground source around the engine then into the heat exchanger!

#### Just Another Sparky

Joined Dec 8, 2019
213
Dividing 15kVA of itemized single phase load onto a 15kVA 3 phase generator set is going to be sketchy at best - One of the phases is bound to end up overloaded. At this point it's sounding like the electric space heater option is going to be the way to go. It sounds like this is for a cost-effective emergency heating and power solution rather than an energy-efficient permanent HVAC installation.

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#### Mark Steele

Joined Dec 11, 2020
13
Hold up - we just jumped from a 240V, 50A branch circuit to a 15kVA heat pump - i.e. an 80 amp branch circuit. Which is it? If your pump truly is 15kVA then it had better not be on a 50A branch circuit with #8 conductors.
Sorry I was reading the backup heat module. Here's a photo of the tag on the heater. Thanks!

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#### Mark Steele

Joined Dec 11, 2020
13
Dividing 15kVA of itemized single phase load onto a 15kVA 3 phase generator set is going to be sketchy at best - One of the phases is bound to end up overloaded. At this point it's sounding like the electric space heater option is going to be the way to go. It sounds like this is for a cost-effective emergency heating and power solution rather than an energy-efficient permanent HVAC installation.
It's to keep the wife happy in the face of a power outage.

#### Just Another Sparky

Joined Dec 8, 2019
213
So 15kVA/208/Sqrt(3)=41.63A
The nameplate on your heat pump calls for 31.7A at 208/230V, minimum circuit ampacity 40.

That'll chooch. Connect A, B and neutral with #8s via a 40A interlocked circuit breaker and bob's your uncle. Should have enough power to spare for basic neccesities and conveniences too. Lights, refrigerator, etc. Just don't expect a clothes dryer, range or electric water heater to play nice at the same time.

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