Generator Question - Step up Transformer

[chipdog4]

My brother has this generator, 7500 watt 120 volt 35 amp circuit breaker.

He’s been using it fine for 120 volt but I had questioned about getting a step up transformer.

My understanding is it would double the voltage but half the amp output. Half of 35 wouldn’t be worth it.

But I’m seeing 62.5 amps listed in the specs here. 30 amps at 240 volt would be worth the trouble of getting a step up transformer.

Am I misunderstanding the specs here?

https://www.electricgeneratorsdirect.com/Cummins-Onan-A064K832/p15745.html

 

[Max]

Yes. You are assuming that the 240V generator is using a transformer to get 240V - and it isn’t. Instead it is using different windings on the generator.

There is no free lunch with a transformer, so the power you put in is the power you get out, minus some generally minimal losses in the transformer. Power = Voltage x Current, so Pin equals Pout (ignoring losses) and if you double Voltage on one side you get half the current.

 

[dave*99]

That generator has a 2 pole breaker and I assume it has 2 outputs of 120V @ 35A each.

It’s an RV generator and most have only 120V loads.

 

[mm08822]

From the spec sheet it is a 2 pole 35a cb. Odd that it doesn't spec the gen as 120/240vac @ 35A. However, it all has to do with how the windings are configured/placed within the stator.

• It could be 1 winding @ 70a 120vac capacity, split into 2 35a ckts. (Possibly, but I would find this odd.)
• It could be 2 separate windings @ 35a 120vac each in phase with each other so there is no chance of additive voltage configuration. (Possibly, but I would find this odd.)
• More common, would be one winding, center-tapped and broken out as 120/240vac @ 35a.

Not sure if this "anomaly" is geared toward the TT-30 camping use. Are there 2 of these recepts on it? (Still a 2pole would be odd.)

 

[mm08822]

Measure across L1 - L2. Is it 0 volts or 240 vac?

Even if you do read 240vac, this , to me, is so odd, that it warrants further research.

 

[dcg9381]

Not sure if this "anomaly" is geared toward the TT-30 camping use.

Geared toward RV use (mobile) yes. That's why it's oddball. Not TT-30 though, it's setup for 2 x ~35A but windings are in phase and most RVs like this use 14-50R (not at the gen, but at the converter). RVs really just pull between the hots and neutral.. Not both hots. This generator won't supply 240v, but it pushes more power than you get from a TT-30 on two breakers that are in phase. Typical generator for 5th wheels and some diesel pushers with no more than 2 AC units.

Most RVs don't use 240V power.

 

[chipdog4]

From the spec sheet it is a 2 pole 35a cb. Odd that it doesn't spec the gen as 120/240vac @ 35A. However, it all has to do with how the windings are configured/placed within the stator.

• It could be 1 winding @ 70a 120vac capacity, split into 2 35a ckts. (Possibly, but I would find this odd.)
• It could be 2 separate windings @ 35a 120vac each in phase with each other so there is no chance of additive voltage configuration. (Possibly, but I would find this odd.)
• More common, would be one winding, center-tapped and broken out as 120/240vac @ 35a.

Not sure if this "anomaly" is geared toward the TT-30 camping use. Are there 2 of these recepts on it? (Still a 2pole would be odd.)

There aren’t any receptacles on it, it’s meant to be hardwired to an RV.

 

[mm08822]

There aren’t any receptacles on it, it’s meant to be hardwired to an RV.

Ok. I don't follow the RV world. Home standby gens, yes.

The q in post #5 is the next step.

 

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[dave*99]

 

[dcg9381]

30 amps at 240 volt would be worth the trouble of getting a step up transformer.

What's the use case for needing 240V? Are you going to use this for home backup?
That's a great generator, one of the higher end deals for RV/mobile.

It still could be used for home backup if you're willing to live without 240V loads... So it depends on the house.

I keep a dedicated (portable) 240V inverter generator around. They cost a fraction of what that Onan does... But they're not diesel.

 

[mm08822]

That's what I was worried about......in phase or perfectly 180 degrees out of phase........ anything in between is trouble for the gen and/or loads. Otherwise, it would be marketed as 120/240.

 

[dave*99]

That's what I was worried about......in phase or perfectly 180 degrees out of phase........ anything in between is trouble for the gen and/or loads. Otherwise, it would be marketed as 120/240.

Yes, and I suspect you are aware it is possible to phase lock 2 inverters 180 out of phase. Some of the small inverter generators have this feature. But it appears the RV folks have discussed this specific generator and 240V. I didn't see anyone mention that possibility.

 

[theoldwizard1]

My brother has this generator, 7500 watt 120 volt 35 amp circuit breaker.

I thought this was very unusual until ...

There aren’t any receptacles on it, it’s meant to be hardwired to an RV.

What he really needs is 240VAC split phase output ! 240VAC is required to run well pumps, stove, water heater, etc. 17A is probably enough to run a pump, but not a hot water heater or stove. Of course this does depend on what loads are on the 120VAC circuits.

In THEORY, you can use a 240/120 transformer wired backwards to make 240VAC (without a neutral), which should be fine for a pump. For safety, the transformer should be at least 2KVA rated. Not cheap (unless you can find a used one on eBay).

Qualified electricians please comment !

 

[wyliesdiesels]

My brother has this generator, 7500 watt 120 volt 35 amp circuit breaker.

He’s been using it fine for 120 volt but I had questioned about getting a step up transformer.

My understanding is it would double the voltage but half the amp output. Half of 35 wouldn’t be worth it.

But I’m seeing 62.5 amps listed in the specs here. 30 amps at 240 volt would be worth the trouble of getting a step up transformer.

Am I misunderstanding the specs here?

https://www.electricgeneratorsdirect.com/Cummins-Onan-A064K832/p15745.html

correct. so you would have about 17a @ 240v

 

[wyliesdiesels]

Yes. You are assuming that the 240V generator is using a transformer to get 240V - and it isn’t. Instead it is using different windings on the generator.

There is no free lunch with a transformer, so the power you put in is the power you get out, minus some generally minimal losses in the transformer. Power = Voltage x Current, so Pin equals Pout (ignoring losses) and if you double Voltage on one side you get half the current.

huh? he was asking about using a step up transformer to get 240v off a 120v generator. nothing about a 240v generator. the gen in question isnt outputting 240v so i dont know what your reply has to do with

 

[mm08822]

Yes, and I suspect you are aware it is possible to phase lock 2 inverters 180 out of phase. Some of the small inverter generators have this feature. But it appears the RV folks have discussed this specific generator and 240V. I didn't see anyone mention that possibility.

Yes, I am, but that is not the OP's gen the question is posed to.

The gen is offered in a 120/240v option, per website OP provided a link to.

Not sure why anyone would want a straight 120v unit. The 120/240 unit gives more flexibility.

 

[Max]

huh? he was asking about using a step up transformer to get 240v off a 120v generator. nothing about a 240v generator. the gen in question isnt outputting 240v so i dont know what your reply has to do with

I explained how transformer works correctly, no?

The page he referred to had multiple generators and options.

 

[wyliesdiesels]

I explained how transformer works correctly, no?

The page he referred to had multiple generators and options.

but the OPs generator is 120v not 240v

 

[mm08822]

I thought this was very unusual until ...

What he really needs is 240VAC split phase output ! 240VAC is required to run well pumps, stove, water heater, etc. 17A is probably enough to run a pump, but not a hot water heater or stove. Of course this does depend on what loads are on the 120VAC circuits.

In THEORY, you can use a 240/120 transformer wired backwards to make 240VAC (without a neutral), which should be fine for a pump. For safety, the transformer should be at least 2KVA rated. Not cheap (unless you can find a used one on eBay).

Qualified electricians please comment !

If half of the gen output is 3750 watts (VA), then a 4KVA xformer minimum would be needed hanging off of the existing 35a cb.

A output winding with a neutral (center tap) would be required to bond and reference the circuit to ground. Has nothing to do with a load needing a neutral or not.

 

[dcg9381]

@chipdog4 what's the actual use case here?

 

[chipdog4]

He’s been using it for power outages. Keeping his and his neighbors fridges and freezer cold.
He’s on city water and has furnaces.

Our parents are close to him and have the bigger 12,500 watt 240 version he can grab if the temps are frigid.

Thanks everyone. I can put this pipe dream to rest now.

 

[dave*99]

He’s been using it for power outages. Keeping his and his neighbors fridges and freezer cold.
He’s on city water and has furnaces.

Our parents are close to him and have the bigger 12,500 watt 240 version he can grab if the temps are frigid.

Thanks everyone. I can put this pipe dream to rest now.

So I have to ask - what 240V loads does he need to feed?

 

[Brazen Bull]

There is no free lunch.

Who pays the fuel bill for the Sun than?

 

[Bert_]

Yes, I am, but that is not the OP's gen the question is posed to.

The gen is offered in a 120/240v option, per website OP provided a link to.

Not sure why anyone would want a straight 120v unit. The 120/240 unit gives more flexibility.

I've got a 4000w 120/240 generator. I keep thinking about opening it up and seeing if I could convert to 120v only.

The advantage of 120v only is that you don't have to worry about current balancing.

 

[mm08822]

I've got a 4000w 120/240 generator. I keep thinking about opening it up and seeing if I could convert to 120v only.

The advantage of 120v only is that you don't have to worry about current balancing.

If the need is there then do it. You may also want to consider 2 other options:
1) add ammeters to each leg for monitoring
2) consider a 2pdt or 3pdt s/s to switch between 120 vs 120/240. (Could only switch when gen off)

option would be safer for the gen, IMO.

 

[Max]

Who pays the fuel bill for the Sun than?

Depending on your beliefs, either the Universe or God. Take your pick.

 

[Bert_]

If the need is there then do it. You may also want to consider 2 other options:
1) add ammeters to each leg for monitoring
2) consider a 2pdt or 3pdt s/s to switch between 120 vs 120/240. (Could only switch when gen off)

option would be safer for the gen, IMO.

Those would be options.

The switch would need some sort of cover to prevent it from being bumped. But why have the switch if I don't need 240v?

A current meter would be helpful but still require balancing. Wired for 120/240 4000w is only 16A per leg, balancing is going to be pretty important

 

[mm08822]

Those would be options.

The switch would need some sort of cover to prevent it from being bumped. But why have the switch if I don't need 240v?

A current meter would be helpful but still require balancing. Wired for 120/240 4000w is only 16A per leg, balancing is going to be pretty important

As I said, if the need is there, convert it. I personally would want to keep the 120/240v output as I have fed PIBs several times other than random 120v needs.

 

[Brazen Bull]

If the need is there then do it. You may also want to consider 2 other options:
1) add ammeters to each leg for monitoring
2) consider a 2pdt or 3pdt s/s to switch between 120 vs 120/240. (Could only switch when gen off)

option would be safer for the gen, IMO.

I read an article of a fellow living off grid in his cabin had a big 120/240v gasoline generator.
He ran mostly 120v loads and on that branch on the generator it ran rough and the fuel economy sucked, eventually wearing the engine out in six months.
He bought the same generator and a massive 240-120v stepdown transformer and used the 240v branch and stepped it down. The engine ran smooth and it didn't guzzle fuel. The system was still in good shape after years as of the writing of the article.

 

[RiverRider]

I've got a 4000w 120/240 generator. I keep thinking about opening it up and seeing if I could convert to 120v only.

The advantage of 120v only is that you don't have to worry about current balancing.

What are you wanting to "balance?"

 

[mm08822]

What are you wanting to "balance?"

He doesn't want to balance anything.
Each winding can handle 16a @120v. Bert is saying it is less to be concerned with if he can use the pair of windings doubled up to get 32a @ 120v. Output circuitry wiring and ocp would need to adjusted to further reduce the management, but still need to protect each gen winding. It can be done.

 

[Brazen Bull]

He doesn't want to balance anything.
Each winding can handle 16a @120v. Bert is saying it is less to be concerned with if he can use the pair of windings doubled up to get 32a @ 120v. Output circuitry wiring and ocp would need to adjusted to further reduce the management, but still need to protect each gen winding. It can be done.

I think what he means is load balancing. On the generator head there are two 120v phases out of step with eachother and a neutral tap to get 240v and 120v when you want.
He wants to keep the load equal on both legs 120v legs.
Related, in three-phase stuff, the load on all three phases needs to be balanced or there are problems.

 

[mm08822]

I think what he means is load balancing. On the generator head there are two 120v phases out of step with eachother and a neutral tap to get 240v and 120v when you want.
He wants to keep the load equal on both legs 120v legs.
Related, in three-phase stuff, the load on all three phases needs to be balanced or there are problems.

No, he wants to parallel the 2 windings and do away with the split phase configuration.

Not true for the 3 phase loads, each winding can have different loads even none at all. However, balanced or close to is better.

 

[Brazen Bull]

Not true for the 3 phase loads, each winding can have different loads even none at all. However, balanced or close to is better.

The book I cracked open talking about polyphase current said it was a no-no.
Besides if you have no load on one phase wouldn't that be an uneven mechanical load on the engine too?

 

[mm08822]

The book I cracked open talking about polyphase current said it was a no-no.
Besides if you have no load on one phase wouldn't that be an uneven mechanical on the engine too?

Well in the real world, not every 3 phase gen drives only 3 phase loads. Plug a drill into a 120v recept on a 120/208v gen. It will run just fine.
Or run a 208v single phase motor only. Still good.

 

[Bert_]

What are you wanting to "balance?"

Tomorrow I have to wire up a 50kw generator. 120/240. Balancing that is pretty easy. If you actually load it up the load will be pretty diverse. Not to mention the 50kw is way more than enough generator, I didn't sell it.

On a little 4000w generator, balancing is more important, or at least it's more of a challenge. It would be very easy to have 2500w on one phase, 500w on the other. Total is only 3000w but one winding is getting overloaded. If I want the full 4000 Watts, I have to get an equal 16 amps on both phases.

One solution would be to get a bigger generator. I think this is what a lot of people do. If I got a 7500 then balancing really wouldn't matter because I wouldn't use much more than 1/2 capacity anyway.

I want to keep the 4000 though. It's a nice size that I can bring in my job trailer. That usually gives me an excuse to run it a few times a year. 4000w really is enough power. Plus I already own it.

 

[RiverRider]

Tomorrow I have to wire up a 50kw generator. 120/240. Balancing that is pretty easy. If you actually load it up the load will be pretty diverse. Not to mention the 50kw is way more than enough generator, I didn't sell it.

On a little 4000w generator, balancing is more important, or at least it's more of a challenge. It would be very easy to have 2500w on one phase, 500w on the other. Total is only 3000w but one winding is getting overloaded. If I want the full 4000 Watts, I have to get an equal 16 amps on both phases.

One solution would be to get a bigger generator. I think this is what a lot of people do. If I got a 7500 then balancing really wouldn't matter because I wouldn't use much more than 1/2 capacity anyway.

I want to keep the 4000 though. It's a nice size that I can bring in my job trailer. That usually gives me an excuse to run it a few times a year. 4000w really is enough power. Plus I already own it.

Okay...I see what you're concern is and it makes sense to me.

Have you considered just connecting a 2:1 step-down transformer across the 240v output? I've never done any alterations or modifications (that's kind of a no-no in aviation, generally speaking) to a generator/alternator setup, so I don't know what's involved and I suppose it might be easier and/or less expensive than a step-down solution. I suppose that depending on the make & model there may be windings and other internals that you can just swap into the machine, so I don't know.

I hope you come up with the best solution, so good luck.

 

[RiverRider]

The book I cracked open talking about polyphase current said it was a no-no.
Besides if you have no load on one phase wouldn't that be an uneven mechanical load on the engine too?

That is correct.

As an illustration, residential electric power is supplied as the output of a 240v center-tapped source. The center tap is the "neutral" which allows you to use either phase as 120v power. When the two phases are viewed individually with the neutral as a reference, they are 180 degrees out of phase. If you connect the two 180-degree opposite phases together you short the 240v source. Makes big smoke.

I don't believe the motor or engine driving the generator would ever know the difference between phases being loaded in a balanced or unbalanced manner.

 

[Bert_]

Okay...I see what you're concern is and it makes sense to me.

Have you considered just connecting a 2:1 step-down transformer across the 240v output? I've never done any alterations or modifications (that's kind of a no-no in aviation, generally speaking) to a generator/alternator setup, so I don't know what's involved and I suppose it might be easier and/or less expensive than a step-down solution. I suppose that depending on the make & model there may be windings and other internals that you can just swap into the machine, so I don't know.

I hope you come up with the best solution, so good luck.

Yes a step down would definitely work. That does add cost and a heavy transformer.

If there is 4 leads brought out of the generator winding it will be an easy conversion. If they only brought out 3 wires then it will probably be more trouble than it's worth.