Using a 1:2 CT dry transformer on generator

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  #1  
Old 01-13-09, 06:45 PM
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Using a 1:2 CT dry transformer on generator

Hello,

I have a small generator, one of those Honda EU2000i inverter types with 120V only output. Fuel economy is important, and this unit can run a long looong time with the eco-throttle engaged.

I have two loads I'd like to power with this generator to keep my house warm during outages. A boiler with a Beckett oil burner and two small circulators. They draw about 5.5A on 120V. My furnace is one of those gradual start blowers, that draws about 1.4A on 240V. Both of these are measured values while running.

The starting current is likely double+, but still within the capacity of the genset. The 240v furnace is a problem though.

I was considering wiring a 1:2 CT dry transformer at the generator to convert the 120 to 120/240. They're available for not too much $, and easily rated for 2-5kva.

Has anyone tried something like this before?
Problems? Recommendations?

(I will state that I have a transfer switch, an L14-30 inlet, and I'm not playing with suicide cords.)

-Steve
 
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  #2  
Old 01-14-09, 05:15 AM
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The best place to ask whether your loads are too large for your generator is the manufacturer. That being said, I think what you are suggesting is possible depending on the generator size. Did you get the starting current of the blower from actual measurements or is that a guess. The blower motor nameplate should have the locked rotor amps (LRA) listed. It could be more than two times the running amps. And starting current is taken into account when sizing a generator, not just running amps.

Also remember that 1.4 Amps at 240 V is 2.8 Amps at 120 V. You probably don't need any more than a 1 kVA transformer, but oversizing it would not be a problem if that is what you want to do.
1.4 Amps * 240 V = 336 VA
 
  #3  
Old 01-14-09, 07:33 AM
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Originally Posted by scpeters View Post
I was considering wiring a 1:2 CT dry transformer at the generator to convert the 120 to 120/240. They're available for not too much $, and easily rated for 2-5kva.

Has anyone tried something like this before?
Problems? Recommendations?
That is a common way to get a 120/240V system out of a 120V only system. Depending on other factors in your setup, it may be better to place it near the generator (need to run 3 wires plus ground from there) or near the transfer switch (need to run heavier wires from the generator). An autotransformer may, or may not, be an option for you, also depending on how things are set up (particularly the grounding).
 
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Old 01-14-09, 08:12 AM
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Originally Posted by scpeters View Post
I was considering wiring a 1:2 CT dry transformer at the generator to convert the 120 to 120/240. They're available for not too much $, and easily rated for 2-5kva.
Yes, it's okay to use the transformer, just make sure to get everything grounded as appropriate. The optimal transformer size is about 80% of the load. A transformer that is too large for the load has a poor power factor which reduces efficiency wasting generator capacity and fuel.
 
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Old 01-14-09, 10:05 AM
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While the transformer will certainly work I'm intrigued by the boiler requiring 240 volts for operation. Is this a European boiler? I am unaware of any US made residential boiler that would require 240 volts.

Do the circulator pumps draw 5.5 amperes each or together? If each, then they are probably the old style pump and motor connected with a mechanical coupling. The newer "canned" (or cartridge) pumps use quite a bit less power.

If I were to use a transformer in this particular case I would size it for the boiler load and install it at the boiler and then reconnect the branch circuit for the boiler as a 120 volt circuit.

What I have for my own system is a Yamaha 3 kW inverter generator (120 volt only) and a transfer switch that feeds a sub panel with my critical loads. I wired the transfer switch to parallel the two "hot" leads to my sub panel when in the generator position. Of course this means I cannot have any multi wire branch circuits or 240 volt loads in that sub panel.
 
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Old 01-14-09, 11:03 AM
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Originally Posted by Skapare View Post
Depending on other factors in your setup, it may be better to place it near the generator (need to run 3 wires plus ground from there) or near the transfer switch (need to run heavier wires from the generator).
I was considering placing the transformer at the generator, so I could easily swap in a larger generator (borrowed from neighbor) with 120/240 if needed to run more than just the heat. I can then use my existing inlet and transfer hardware. The hesitation behind putting it at the furnace is that it would need to operate 24x7x365 unnecessarily.

Originally Posted by furd View Post
While the transformer will certainly work I'm intrigued by the boiler requiring 240 volts for operation. Is this a European boiler? I am unaware of any US made residential boiler that would require 240 volts.
Do the circulator pumps draw 5.5 amperes each or together? If each, then they are probably the old style pump and motor connected with a mechanical coupling. The newer "canned" (or cartridge) pumps use quite a bit less power.
The furnace is 240V (Goodman AHU). Boiler is 120V. (Crown Freeport, Beckett burner, 2 Taco 007 circulators). The ~5.5A is for burner plus both circulators. It's a little bit of an unusual setup - the boiler is the second stage for a heat pump. When using the generator the heat pump compressor clearly won't be operating. (11.5A operating/27 LRA @ 240V)

All amperage measurements were made by me on the branch circuits while operating. I don't own a peak reading ammeter, but I have watched the reading jump about 150-175% when the motors start/stop. Of course, I can't accurately measure the starting current without the right equipment.
 
  #7  
Old 01-14-09, 11:16 AM
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If I understand correctly you have a heat pump as your primary source of heat. When the heat pump cannot supply all the heat then a boiler supplies heat via a coil in the ductwork. Does the boiler and coil add to or replace the heat pump operation?

Do you also have electric strip heaters installed in the air handler? Are these connected and if so, do they ever come into operation? If no on the electric heaters then you might be able to replace the 240 volt blower motor with a 120 volt model and change the air handler power requirements to 120 volts.
 
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Old 01-14-09, 11:53 AM
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I was planning on looking for an isolation transformer rated ~1.5kva, which is close to the running power of the generator.

The 120V from the generator would step up to 120/240V, which I would then connect through my transfer switch inlet, which is a standard L14-30. All power would go through the transformer, not just the 240 needed for the furnace. This would save me a lot of hassle since both legs have branch circuits in the generator panel.

Most of the transformers I've seen are 240/480 primary and 120/240 secondary. Would it work to connect 120V from the generator across what is considered the "240V secondary." The "240/480 primary" would then have 120/240V across it.

My concern is the rating for the transformer winding is based on double the voltage (half the current) that I'll be using. Will this require a kva rating that is double what I'll need?

A 120-120/240 autotransformer might work, but I think I'd be more comfortable with a two-winding transformer.
 
  #9  
Old 01-14-09, 12:19 PM
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Originally Posted by furd View Post
If I understand correctly you have a heat pump as your primary source of heat. When the heat pump cannot supply all the heat then a boiler supplies heat via a coil in the ductwork. Does the boiler and coil add to or replace the heat pump operation?

Do you also have electric strip heaters installed in the air handler? Are these connected and if so, do they ever come into operation? If no on the electric heaters then you might be able to replace the 240 volt blower motor with a 120 volt model and change the air handler power requirements to 120 volts.
GOOD QUESTIONS!

The boiler adds to the heat pump operation. The boiler is in warm weather shutdown most of the year, and only operates during the deep depths of winter.

The AHU has 5kw (maybe 10kw) strip heaters that do operate during the defrost cycle, but they are on a non-generator circuit. The compressor is similarly wired to the non-generator panel. The 24V control transformer, control circuit, and AHU blower are the only items on the 240V circuit from the generator panel.

So - yes, the AHU blower is on it's own circuit alone, and could perhaps be replaced. The challenge is that the AHU blower is a variable speed control model that slowly ramps up the operating speed over 30 seconds or so, and is very nice. I haven't seen that style motor elsewhere, and it is sort of deeply intertwined with the furnace control board. Perhaps it could be done though? I've always wondered if the variable speed motor is in fact a 3ph motor with a very small VFD attached.
 
  #10  
Old 01-14-09, 01:07 PM
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This is an interesting problem. If I understand correctly you want to be able to fire the boiler and run the blower in the air handler for at least some heat during a power outage. The problem is that the air handler blower and controls require 240 volts.

Your blower has the GE ECM motor or an equivalent from a different manufacturer. You definitely do not want to replace it since it has a price of around $500. I don't know how much of the motor control is integral to the motor and how much is in the control board. Suffice to say it is not a matter of simply swapping a few wires.

Certainly what you originally proposed is the easiest and also allows for any 240/120 generator to use the installed inlet box and transfer panel. The problem will be in finding a transformer and then paying the freight. Most small transformers, units under about 5 kVA are designed as step-down transformers and don't work all that well when connected backwards. I don't recall just what the problems are, maybe just higher losses within the transformer and maybe poorer regulation from no-load to full-load. I just looked at the Grainger catalog (not necessarily the best place to buy transformers) and saw a 2 kVA transformer for $370 and a 3 kVA for $514. These were uncased units and would require an enclosure.

One thing to keep in mind is that the furnace control board is rather susceptible to failure from low voltage, or so I was told by a furnace repairman. He told me that was the primary cause of control board failures when running from a generator and that needs to be kept in mind if you are going to use a transformer wired "backwards" from its design.

I'm sorry I don't have a better answer.
 
  #11  
Old 01-14-09, 10:46 PM
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There is one thing about if you run the transfomer in reverse manner aka backward format.,

It will have slightly poorer voltage totrance so if you have 120/240 X 120 run in reverse just give you a example 125 volts primary side then output will go 125/250 volts but not very effecentally.

The other option you can is get 120 X 240 volts transfomer that is pretty common to do that However just becarefull with grounding.

There one way you can do is have 120X240 volts transfomer perment connected so that way when you switch over the generator power you only have to switch over the 120 v circuit instead of 240 volts due the transfomer will take care of it.

If you go that route becarefull with breaker selection espcally on 120 side due you will draw double of current compared what you ran on 240 volts.

If you have more question just holler one of us will help ya.

{ a gotcha is need to know the air handler model number the reason why I ask due one of us can able look up the model number and depcher to see if can able get the control side and heater grid on seperated [I know most heater grids useally be on seprated breaker but not always the case ]}

Merci,Marc
 
  #12  
Old 01-15-09, 07:18 PM
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I've found a MUCH better solution.

The ECM 2.3 motor nameplate states that it is a 120/240V motor. Searching for the service manual for this motor doesn't turn up much from GE Industrial, but a number of vendors who use the GE motors have service manuals and helpful diagrams. I found one at Climate Master, one at Thermo Pride, and a few other places.

Turns out that the 5-pin power connector is usually populated with two wires plus ground. If that is all that is connected, the motor runs on 240V, with both wires hot plus the chassis ground.

If you jumper the two unused pins using a pair of AMP Contacts "AMP 350537-1" and a short section of wire, then the ECM will operate on 120V. You feed to 120V in the original pair of wires, however the diagram shows the wire closest to the ground as the 120V neutral, and the wire on the end of the connector is the 120V hot.

BE CAREFUL - IF YOU APPLY 240V WHILE THE JUMPER IS IN PLACE, YOU WILL DESTROY THE MOTOR.

Haven't done it yet. Might wait until the weather warms up. We can't be without heat when it's 10F outside.
 
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Old 01-15-09, 08:29 PM
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Of course this could be done manually but if you wanted to normally use 240v you could devise a relay circuit to disconnect from 240v, jumper the terminals and apply the 120v. Sorry can't tell you details but if my idea is good someone here could supply details. The important thing would be it was a break before you make relay. I can imagine a relay with a 240v coil that would move to its second position (120v )when 240v supply was interrupted.
 
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Old 01-16-09, 06:10 AM
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I forgot to mention another small detail, should someone else want to try this based on the thread later...

The control transformer for the thermostat and furnace board is a 240V/24V step down. Since the control circuit will still need 24V, this will need to be replaced with a 120V/24V of the same power rating. Perhaps I will find one with a 120/240 primary in case switching back and forth becomes necessary.
 
  #15  
Old 08-09-11, 07:27 PM
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Just have to butt in here. If you size the transformer for 80% of the load the transformer will overheat and fail.the load should be no more than 80% of the rating of the transformer, ideally less. The better the quality of the transformer (and the more money of course) the closer to the rating you can load it. The power factor is related to the size of the core, and can be corrected by the correct addition of capacitance if this is an issue . One of the reasons many motors have start/run caps is to help correct for the pf of the motor inductance. Take it from an EE who does use isolation transformers.
 
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Old 08-10-11, 05:05 PM
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Good info, but the OP is over two years old.
 
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