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Transformer vs RPC or something else?

Pondering

Plastic
Joined
Apr 2, 2024
Location
Edmonton
I made a mistake. I live in a Canadian residence with 120V 60Hz single phase power, that has 240V to run the dryer and oven. I bought a used 240V air compressor, ran ok, but did not make enough compressed air fast enough.

I ordered a bigger air compressor from China, built to my (incorrect) specifications. I wanted a 7.5hp motor (5.5kw) that was 240V, 60Hz, and they said that is 3 phase only; but I could use a 11kw VFD to convert my single phase 240 to 3 phase, and they would build it in. I agreed. I was told when they tested it the compressor was drawing 38 amps at 240V, so 9120kw. However, when the compressor arrived, the 240 input is Live, Neutral, Ground. My residential 240V is Live Live ground. Live and learn.

I had an electrician look at it, and he told me I need to buy either a 240V to 240V transformer or an RPC (wired straight to the motor). I have spent the last couple of weeks contacting transformer companies and asking for a quote on a 240V to 240V 15kva transformer, but they seem to be very uncommon. I might have a line on one, I'll know the asking price later this week. I also could buy a 25hp RPC and try to wire that in instead. If I do that, and attach the RPC to the motor input, does that mean the VFD will run in reverse, converting the supplied 3 phase power to single phase? Or will that burn it out and I need to bypass the VFD completely?

Assuming the prices of the RPC and transformer are similar, which is a better choice? Or is there another option I am not aware of?

Extra info that may or may not be important:
1. the new compressor is an oil free scroll type, not a piston type like the old one, so it is supposedly a lot quieter, and can start up slower, using less of a spike power draw.
2. One transformer company offered to build me a 15kva 120V to 240V transformer, but my electrician said that would be a 76amp draw from the panel, and not allowed, so I need to stick to 240V inputs for whichever I choose.
 
The thing you want is called an "isolation transformer". Here's one that might work, but will either put out 219 or 263 V depending on how you wire it. https://www.ebay.com/itm/275921298325

However, I'm curious what prevents connecting "neutral" of compressor to "live" of grid. Is neutral connected to chassis in the compressor? It shouldn't be. Does neutral go straight through to the motor rather than being switched?
 
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@pcd I have attached the compressor wiring diagram my electrician looked at when he told me I need either a transformer or an RPC.
.
 

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  • Compressor Wiring Diagram.pdf
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The reason isn't obvious from the diagram. Which is not to say that there isn't one.

Have you asked the compressor manufacturer about whether it can be run off of line/line/ground?
 
The reason isn't obvious from the diagram. Which is not to say that there isn't one.

Have you asked the compressor manufacturer about whether it can be run off of line/line/ground?
Yes I did that as soon as the electrician told me no. The manufacturer said:
"That's dangerous, sir
Also not suggest you do that."

then later he added:

"Called my engineer, from your safety site and long time running, better to configure a transformer..

If you have the ability and ensure that your safety, you can customized the compressor.
We will assist you to handle it."

I said:
"by customize you mean run on 3 phase power?"

And he said:
"Yes, You need to adjust the line yourself to make this compressor work in three phase electricity

The safest way is to equip a transformer, which is more safe and worry-free
You can buy it from your local place, it may be cheaper"

Then we had a discussion I wont post here about connecting the live live to live neutral and he responded:
"Better to use the RPC or transformer
You have a very professional electrician. you also very knowledge, should know what to do better, my friend"

The manufacturer and their engineer said it was a bad idea, so here I am, wondering which choice is cheaper/easier/better in the long run.
 
Then it sounds like there's insufficient isolation between neutral and ground in the VFD or MAM890.

Re:
> If I do that, and attach the RPC to the motor input, does that mean the
> VFD will run in reverse, converting the supplied 3 phase power to single
> phase?

The VFD (phase converter) is unlikely to convert 3 phase power to single phase. Hard to say whether it will be damaged. Probably not, but there's no reason to leave it connected, so I would disconnect it from the motor if you go the RPC route.

If you power the motor directly via an RPC that leaves some questions open. How are you going to to control the motor? Can the MAM890 tolerate connection of pin 10 to line, or not?

Now, if it turned out that the VFD can handle line/line/ground and that it's the MAM890 that's the problem your life would be much easier because then you could use a small transformer for just the MAM890.

If you go the RPC route you're going to need to do some significant re-wiring of the compressor controls including adding a contactor and setting up the right interface between the MAM890 and the contactor.

If you power the whole thing off of an isolation transformer it should be plug and play.
 
I made a mistake. I live in a Canadian residence with 120V 60Hz single phase power, that has 240V to run the dryer and oven. I bought a used 240V air compressor, ran ok, but did not make enough compressed air fast enough.

I ordered a bigger air compressor from China, built to my (incorrect) specifications. I wanted a 7.5hp motor (5.5kw) that was 240V, 60Hz, and they said that is 3 phase only; but I could use a 11kw VFD to convert my single phase 240 to 3 phase, and they would build it in. I agreed. I was told when they tested it the compressor was drawing 38 amps at 240V, so 9120kw. However, when the compressor arrived, the 240 input is Live, Neutral, Ground. My residential 240V is Live Live ground. Live and learn.

I had an electrician look at it, and he told me I need to buy either a 240V to 240V transformer or an RPC (wired straight to the motor). I have spent the last couple of weeks contacting transformer companies and asking for a quote on a 240V to 240V 15kva transformer, but they seem to be very uncommon. I might have a line on one, I'll know the asking price later this week. I also could buy a 25hp RPC and try to wire that in instead. If I do that, and attach the RPC to the motor input, does that mean the VFD will run in reverse, converting the supplied 3 phase power to single phase? Or will that burn it out and I need to bypass the VFD completely?

Assuming the prices of the RPC and transformer are similar, which is a better choice? Or is there another option I am not aware of?

Extra info that may or may not be important:
1. the new compressor is an oil free scroll type, not a piston type like the old one, so it is supposedly a lot quieter, and can start up slower, using less of a spike power draw.
2. One transformer company offered to build me a 15kva 120V to 240V transformer, but my electrician said that would be a 76amp draw from the panel, and not allowed, so I need to stick to 240V inputs for whichever I choose.
is there a way to either pull the motor off and replace it with something that will bolt up, most motors are all universal by the Frame size and mounting. Or have a motor shop see if it can be rewound for $1000

A new transformer is in the range of $2500!
 
You need to get the exact manufacturer's manual for the VFD that came with the compressor. It will tell you if you can power this with 240 volts line-to-line. The designations on the incoming power terminals may not mean much and may be tailored for the location they expect the drive to be used. You can easily purchase a VFD locally that *is* rated for line-line-ground connection.
 
It makes very little sense that it would not work on a balanced "220V" as supplied in NA.

If the unit is "CE" as well as "C/UL" (Canada/US), then it was tested for isolation on both wires, so there should be no issue. It should withstand 220V to ground on either wire, while you will only be applying 120V to ground.

Any isues with the VFD will almost certainly be with overvoltage protection (MOVs most likely).

Of course, this is a chinese unit. No telling whether the UL and CE are real, or fake.

AND, when you have a product that is labeled for connection a particular way (line to neutral) and you wire it up a different way, then you may have invalidated the listing for the unit.


While I really doubt there is a problem, you are making assumptions etc if you wire it a different way from the instructions, and in case of any issues it may be "on you"..
 
I made a mistake. I live in a Canadian residence with 120V 60Hz single phase power, that has 240V to run the dryer and oven. I bought a used 240V air compressor, ran ok, but did not make enough compressed air fast enough.

I ordered a bigger air compressor from China, built to my (incorrect) specifications. I wanted a 7.5hp motor (5.5kw) that was 240V, 60Hz, and they said that is 3 phase only; but I could use a 11kw VFD to convert my single phase 240 to 3 phase, and they would build it in. I agreed. I was told when they tested it the compressor was drawing 38 amps at 240V, so 9120kw. However, when the compressor arrived, the 240 input is Live, Neutral, Ground. My residential 240V is Live Live ground. Live and learn.

I had an electrician look at it, and he told me I need to buy either a 240V to 240V transformer or an RPC (wired straight to the motor). I have spent the last couple of weeks contacting transformer companies and asking for a quote on a 240V to 240V 15kva transformer, but they seem to be very uncommon. I might have a line on one, I'll know the asking price later this week. I also could buy a 25hp RPC and try to wire that in instead. If I do that, and attach the RPC to the motor input, does that mean the VFD will run in reverse, converting the supplied 3 phase power to single phase? Or will that burn it out and I need to bypass the VFD completely?

Assuming the prices of the RPC and transformer are similar, which is a better choice? Or is there another option I am not aware of?

Extra info that may or may not be important:
1. the new compressor is an oil free scroll type, not a piston type like the old one, so it is supposedly a lot quieter, and can start up slower, using less of a spike power draw.
2. One transformer company offered to build me a 15kva 120V to 240V transformer, but my electrician said that would be a 76amp draw from the panel, and not allowed, so I need to stick to 240V inputs for whichever I choose.
Sell the compressor and jist buy the right one.
Sounds easier and cheaper

DV compressors are made in Canada and where very affordable when I got mine (2019)
Bought a 5ph single phase screw with air dryer for 8000? Canadian
 
Most VFD's sold in North America list L1 to N or similar to below for 240VAC single phase input. The VFD wants to see 240VAC single phase across the line. If there was built in surge suppression then that could be an issue. The other concern is that both legs if split phase 240 should be fused or have a breaker, there is no input power protection breaker/fusing. Contact the manufacturer to see if there are any issues with running it as such before you run out and change things.
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Today Larson electronics said they have what I need in stock, but they want just over $3000 US for it. I have attached the wiring diagram. I see a line on the input for 240V and another line on the output side for 240V. Anyone able to confirm this is exactly what I need?

Also, for transformers, is "temperature rise 135C" mean the temperature around it while in use would be 9120(my draw in kw) / 15000kw x 135(temp at full load)=82C? That would make my basement very warm in the summer. If so, would I be better off paying extra to get a larger size so it doesn't get as warm?
 

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  • 152416.pdf
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There are a number of used single phase 15 kva transformers on Ebay that have series/ parallel windings that can provide 240 volt to 240 volt isolation. The prices are in the $650 to $750 range.

Or they can be purchased new for $1100.

A 7.5 KVA transformer should be adequate for your application and they are much less expensive to purchase or ship. The 15KVA transformer weighs 180lbs. The 7.5KVA transformer weights 110 lbs.


European and Asian 240 volt electronics can often be adapted to the U.S. standard by removing a internal grounding strap that connects the 240 volt neutral to the case ground. The manufacturer of the VFD can provide instructions as to where the grounding strap is located.

The isolation transformer does have a second purpose. It filters out the 4 to 10 Khz switching frequency noise that ends up being applied to the incoming power lines. The smaller 7.5 KVA transformer will limit the current surge during motor startup. That can be an advantage if the VFD does not have a soft start feature.
Keep in mind that you are spending $500 to $1100 for a transformer to adapt a VFD that sells for $275. It may be less expensive to select a different VFD.
 
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There are a number of used single phase 15 kva transformers on Ebay that have series/ parallel windings that can provide 240 volt to 240 volt isolation. The prices are in the $650 to $750 range.

Or they can be purchased new for $1100.

A 7.5 KVA transformer should be adequate for your application and they are much less expensive to purchase or ship. The 15KVA transformer weighs 180lbs. The 7.5KVA transformer weights 110 lbs.


European and Asian 240 volt electronics can often be adapted to the U.S. standard by removing a internal grounding strap that connects the 240 volt neutral to the case ground. The manufacturer of the VFD can provide instructions as to where the grounding strap is located.

The isolation transformer does have a second purpose. It filters out the 4 to 10 Khz switching frequency noise that ends up being applied to the incoming power lines. The smaller 7.5 KVA transformer will limit the current surge during motor startup. That can be an advantage if the VFD does not have a soft start feature.
Keep in mind that you are spending $500 to $1100 for a transformer to adapt a VFD that sells for $275. It may be less expensive to select a different VFD.
Wow those are a LOT cheaper than new! But how do I tell if they are still good? Do transformers wear out over time, or can they be degraded by environment? Or do they pretty much last forever?
 
The dry type transformers rarely fail. The used ones tend to be scratched and dented due to their weight and difficulty in handling when they are salvaged.

In response to your temperature question:
They are roughly 97% efficient. A transformer operating at 10 KVA will produce about 300 watts of heat.
 
The dry type transformers rarely fail. The used ones tend to be scratched and dented due to their weight and difficulty in handling when they are salvaged.

In response to your temperature question:
They are roughly 97% efficient. A transformer operating at 10 KVA will produce about 300 watts of heat.
So if I change my formula to: "temperature rise 135C" mean the temperature around it while in use would be 9120(my draw in kw) / 15000kw x 135(temp at full load) x 0.03 (inefficiency percent) =2.5C above room temp?

So then I don't need to massively oversize it to reduce heat problems.

Ok, looked up definition:
( from https://www.copper.org/environment/sustainable-energy/transformers/education/trans_efficiency.html )

What Is Temperature Rise of a Transformer?​

All devices that use electricity give off waste heat as a byproduct of their operation. Transformers are no exception. The heat generated in transformer operation causes temperature rise in the internal structures of the transformer. In general, more efficient transformers tend to have lower temperature rise, while less efficient units tend to have higher temperature rise.

Transformer temperature rise is defined as the average temperature rise of the windings above the ambient (surrounding) temperature, when the transformer is loaded at its nameplate rating.

Standard Ratings and Overload Capacity​

Dry-type transformers are available in three standard temperature rises: 80C, 115C, or 150C. Liquid-filled transformers come in standard rises of 55C and 65C. These values are based on a maximum ambient temperature of 40C. That means, for example, that an 80C rise dry transformer will operate at an average winding temperature of 120C when at full-rated load, in a 40C ambient environment. (So-called hot spots within the transformer may be at a higher temperature than average.) Since most dry transformers use the same insulation on their windings (typically rated at 220C), irrespective of the design temperature rise, the 80C rise unit has more room for an occasional overload than a 150C rise unit, without damaging the insulation or affecting transformer life.


So after checking I think my first formula was more correct, and the windings will be 82C above ambient temp, but the actual heat put into the room would be the inefficiency loss of 3%, so 9120x003=273.6 watts. And then if it is the inefficiency loss that is radiated heat, then oversizing it is pointless as the load is constant when under use.

Is that more correct?
 
So after checking I think my first formula was more correct, and the windings will be 82C above ambient temp, but the actual heat put into the room would be the inefficiency loss of 3%, so 9120x003=273.6 watts. And then if it is the inefficiency loss that is radiated heat, then oversizing it is pointless as the load is constant when under use.
That's roughly accurate. Just because there's a hot thing in the room does not mean the entire room becomes that heat.

Note that the heat lost and temperature rise is usually proportional to the square of load/current, so a transformer operating at half power produces roughly a quarter of the heat. This is in addition to standing/no-load losses, which are present regardless of how much load is on the transformer.

300W is roughly 1000BTU/hour, if you're wanting to compare it to e.g. AC/heater sizing.
 
You can also buy these transformers new for around $1000 US, shipping is $200 to Canada. There are a range of types, these are major manufactures for transformers.
I emailed automation direct the day you gave me this link. Today they answered and said:
"In a case where the transformer would not be used for its intended purpose, the best approach is to contact a licensed electrician to inspect and certify that the application will be adequate. "

So what in the world is a transformer's intended purpose, if not to transform power from one type of voltage to another?
 








 
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