# Using 15hp induction motor as generator



## MacroPc (Jul 6, 2021)

Hi guys, new here. I have seen similar topics about this but not quite exactly what Im thinking of doing, so here it goes.

I found a couple of 460v 3ph electric motors for only $125(for both). Originally I thought they were dual voltage and was planning on using a 220v 1ph to 3ph VFD but looking at nameplate in person I realized both motors are only 460v.

I was going to pass since I can't use these as I planned but then I thought about using the 15hp motor as a generator. I spoke to some engineers at work and they assured me I _can_ do this but not much more info on that (such as efficiency, loss, hz, etc...).

Ok so my plan is: to take a smaller 220v single phase motor and attach a pulley and belt to it and the shaft of the 15hp motor. Powering the smaller motor will turn the shaft of the 15hp motor and essentially turning it into a generator. If I do that with say a 5hp motor, same (or slightly above) the rpms of the 15hp motor will that give me 460v 3ph (forgot the amperage but lets just guess and say 10 amps)?

Another concern I have is will the AC output of 15hp motor be 60hz? Or will that depend on rpm or will it just need to be regulated (somehow?)?

I am a Controls Electrician and I wire panels up to 480v 3ph but thats with a transformer and that power readily available. Not too mention I wire panels and motors off of a print, not necessarily needing to understand it all to do my job, hehe.


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## iowagold (Jan 22, 2020)

so is this just a science project or do you plan on using this for a back up power plan?

in reality you are better off buying a generator ready to go.
here is why.
all of the hard problems are solved.

power production is a loss thing. all parts involved are a minus...


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## MacroPc (Jul 6, 2021)

Not a backup, more of giving me access to 3 phase power without having to buy a phase converter and/or a VFD for every peice of 3 phase equipment I have/build/buy. I would like to be able to basically build this generator onto a wheeled cart so i can move my 3 phase power around the shop.


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## MacroPc (Jul 6, 2021)

If I have to modify the induction motor by installing magnets with it, what kind of output power could I expect to get out of this? Im assuming that the size and strength of magnets would make a big difference.

But do I even need to do that? This topic gets pretty confusing online and also talking to engineers at work. Some people say yes itll work, some say itll only work if I modify motor, others say it wont, other say not well - and Ive got the same results looking on the web.

I saw several people using them as is, several use them by installing magnets, more using capacitors... but none show or explain what kind of output power one can expect or what really determines the output (size and strength of magnets, size of capacitors, RPM of generator motor etc...)


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## ToolLover (Jan 13, 2020)

Well MacroPc, it looks like you need to do more research. What you are asking is old engineering. Meaning it is so far back that most of the few guys like you that are here have long forgotten how it was done. Best team up with one of the engineers you asked and go from there. I am sure that there are books on the subject, just cannot remember where I put them.
I once had transformer configurations that covered multiple pages. I cannot remember that info either. Time takes its toll.
You should have bought a phase converter.


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## iowagold (Jan 22, 2020)

yea a vfd is the way to go for sure..
pricey!
unless you can rescue a drive from the scrap yard!
toshiba makes a good drive.
been to school for them back in the day on those for service.
they are state of the art good.

so you need to ask or post how much 3 phase power do you need...
and how do you plan on powering that?

most of the time you are better off with a rental cat generator on a trailer.
(that is what we do here when we need 3 phase 50hz for export machinery)
or just buy a 3 phase generator already built.
look at LARGE military generators as well.
sometimes you can find one with low hours that needs a bit of tlc for the restoration.

sometimes you can get 3 phase power on your property for not too much 
if you are close to the main lines or close to an industrial grid..


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## MacroPc (Jul 6, 2021)

If I found a phase converter for that cheap I would have. I bought these because even if all else fails they are still worth more in scrap than I paid.

I thought it would be useful to me in many ways to learn how this is done and to have a usable peice of equipment when/if it gets done. 

I am in the "posting and asking on forums" stage in my research as I have found many ways this is possible but not the specifics Im looking for.


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## MacroPc (Jul 6, 2021)

I think you guys are missing the point here. I dont have a need for a generstor right now. Im looking to learn how this works and what I can or cant expect out of a build like this or what to do to it to get what I expect.

This started out as "What could I do with a 15hp 460v 3ph induction motor" - and this is what I thought of doing. Im just looking to learn more about this because Im a hobbyist that recycles crap and builds new stuff. Just looking for knowledge here.


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## ToolLover (Jan 13, 2020)

I found some phase converter info here that may get you in the right direction.


https://www.youtube.com/results?search_query=phase+converters+from+single+to+3+phase


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## MacroPc (Jul 6, 2021)

I watched that exact same video this morning. Like I said Ive seen it used many different ways but that doesnt answer my questions.


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## FlyFisher (Jun 30, 2018)

For starters, running the "motor" backwards as a "generator" you are not going to get your 15hp motor rating.

The power available post-motor driven as RPC is going to be significantly less than the input power.

That is, you are going to be driving this off of 240v single phase. You will have a motor running on 240v single phase that drives the input shaft (which was the output shaft) of the converted "motor". Just to get the single phase motor to turn and the converted 3-phase motor to turn you will be eating up an enormous amount of power - that unloaded current is your "idle current". You may be looking at 10-15 amps at 240v just in idle current. 

Now add in the motor load you are trying to run on 3-phase. I assume those motors are anywhere between 1hp to 3hp (thinking knee mill, good size metal lathe). 

Not knowing exact numbers, just thinking out loud through theory - I think you are going to need something way more than a 5hp motor to drive your 15hp 3-phase motor as an RPC for even a 1hp 3-phase load. Then what you are going to run in to is you don't typically find that large of motor that can run on single phase 240v. The amount of current they draw is prohibitive - which is why those larger motors go 3 phase - much more efficient and then when you get up higher in voltage on top of that you significantly reduce the current draw.

Are you going to have a circuit available that can supply, say, 100 amps at 240v? 

For what it is worth, a 7.5hp air compressor I'm looking at is recommended to be on an 80 amp circuit. I do not think a 7.5hp motor driving your 15hp 3-phase motor is going to net very respectable results. But it is a place to start.

If this is more an experiment and exercise in learning - then just throw some mud at the wall and see what you get. 

Otherwise, I agree with others that a VFD is going to be the easiest method of getting 3-phase power. 

Note - VFD's are great for variable speed drives also. Think lathes and mills in being able to dial in spindle speed. HOWEVER - not all 3-phase AC motors are kind to variable frequency. So don't expect that you can take any mill or lathe with a stock motor on it, slap a VFD on, and voila have a good infinitely adjustable RPM machine. You can burn up motors that are designed solely for 60hz trying to drive them at other frequencies. Research motors and insulation classifications - the higher insulation protection motors are better places to find motors that will run better on VFD's. 

The above having been said - just because a motor is not good to run on other frequencies doesn't mean you CAN NOT use a VFD to drive them - it just means you need to set the VFD to 60hz. IE - use it as your 3-phase 60hz supply, and don't vary the frequency to vary the motor RPM's.


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## MacroPc (Jul 6, 2021)

Thanks flyfisher. That shed some light on what Im thinking.
I know all about VFDs, Im currently using 2, one 10hp 7.5kw on my hydraulic press and a 2hp one on my power hammer (for blacksmithing - new hobby). 

My "hopeful" goal was to be able to just have one "cart/generator/power supply" I could wheel around and have one or 2 pcs of equipment plugged into it at any given time (assuming this contraption could deliver). 
Perhaps this next question might be elementary to some but Im afraid Im going to ask it anyways. I understand the concept of not being able to generate more power than put in but what I dont quite understand is how drawing power from the generator motor increases the... resistance? on its shaft thats getting spun by the idler motor? Are the magnetic fields somehow increased to the rotor?

The way I was explained to was that the generator motor could only output so much, regardless how much juice would be trying to draw from it. 

Or maybe a simpler way to ask would be how does the shaft get harder to spin by drawing from it? If its spun at say 1800rpm and its outputting 240volts (hypothetically) and 20 amps does that get affected by a load?


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## FlyFisher (Jun 30, 2018)

MacroPc said:


> Thanks flyfisher. That shed some light on what Im thinking.
> I know all about VFDs, Im currently using 2, one 10hp 7.5kw on my hydraulic press and a 2hp one on my power hammer (for blacksmithing - new hobby).
> 
> My "hopeful" goal was to be able to just have one "cart/generator/power supply" I could wheel around and have one or 2 pcs of equipment plugged into it at any given time (assuming this contraption could deliver).
> ...


As to the elementary question - as this is a generator forum this is a good concept to wrap your mind around and the concept works the same in a generator as it does a motor, just either direction.

Lets take a look at the generator side of it (alternator if you will). If there is no electrical load, but your voltage and frequency are there, you have an input load on what is spinning the alternator just to generate that voltage and frequency. This would be the mechanical load of the alternator - keeping the spinning mass of the armature moving, the bearing losses, etc. That is just to get the voltage and frequency of the power output.

Now introduce your "load". What happens electrically is that you add resistance to the windings of the alternator. The power being pushed through the alternator windings is from the windings being passed through magnetic fields. If there is a "load" then there is "resistance" on those windings. As the load increases the resistance increases. That means it becomes harder and harder for the windings to pass through the magnetic fields. This loading is what drags the power source down.

In any alternator design there will be a max load rating. What this is based on is largely the heat generated. The windings are usually enamel coated copper. That has a temperature limit. if you heat up the enamel too much you burn it. Once the coating is gone the windings short out. So the goal is to keep the temperature below a danger threshold of the enamel burning off of the winding wire. Therefore, you have a current limit on the alternator that will be in that "safe zone".

The mechanical resistance range that you are going to see in order to drive the alternator to its specs are what the input power is with no load all the way up to the full electrical load. Your input power source has to be able to drive to that upper end of the electrical load.

An RPC (rotary phase converter) is an extremely inefficient means of producing 3-phase power because your idle input power to start with is so high. You have to overcome all of the mechanical losses just to have a starting point of the power "there".

Just because the power gets "there" doesn't mean you have all the power in the world. Your electrical load will add to the resistance on top of all the mechanical losses. So you have to add them together.

If your input power source does not have the power to push through the mechanical losses AND the electrical load it isn't going to work. The RPM of the RPC is going to sag, if it doesn't stall entirely, and that sag in RPM is going to drop the voltage and frequency of the output power.

Keep in mind that motor start up loads can be very hard. If you have a load that is borderline on the capacity of your RPC when running it could stall the RPC on start up. There are ways to soften the start up load, so this might be an option. Something to be aware of.

As to the current and the temperature going up in the windings - that theory is the same in the reverse in a motor. If you don't put a suitable circuit breaker in place so that you can pull more current than the motor is rated for you can load it so much that you burn the windings. There is also a duty cycle that gets in to this equation also. Look at an off-road recovery winch. You may have a winch capable of 15,000lb line pull, but only 10-15% duty cycle. If you keep your finger on the button to run it trying to get out of a mud hole you might get a hard pull for a good period of time then the smoke will roll and you'll be SOL. The low duty cycle means you need to allow it time to cool down so you get the windings back in to a safe temperature and able to take the hard current hit again (for a short period of time). The higher the duty cycle of a motor or alternator the heavier and heavier it will be built for the same rated load. There is no substitute for mass and size when it comes to current handling for long periods of time. Thats the same reason on large generators you see 2 different ratings for Prime Use vs Stand By use*. Prime is usually 10-20% down. Yeah, the alternator can supply more than that, but it puts the alternator in danger of over-temp and reducing the lifespan of the windings.

*The derate between Stand By and Prime is not the same as the derate between fuel types. The fuel type just means there is a different energy density which means the engine will have a different amount of power. Even if an alternator is rated to prime use at a particular load, if you switch to natural gas and don't have the power to drive to that prime use load you won't get there - nor does the alternator care, other than it will be happier as it won't run hard and at the upper end of its' temp range.


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## MacroPc (Jul 6, 2021)

Now that answered my questions (not exactly but some of my thoughts were on the assumption of it "working" the way I thought).

I really appreciate the time you took explaining things. You definately saved me alot of time and frustration, along with a few $ Im sure. 

Now to figure out what im going to do with these 200 and 300 lb motors... any ideas? 

Ive looked into getting 3 phase power installed in my garage/shop and Im leaning towards its not worth it (we plan on buying a new house as soon as we find one we want). I know it would increase the value of our home but really only to a select group of people and honestly my house/garage isnt worth it - but thats another story.

So, not being able to power either of these motors (I only have a 100amp box that im **** near maxing out already when running my equipment and the air conditioning) does anyone have any cool stories or ideas to use these for? I got them cheap so I dont mind tearing them apart if I have something cool to do with the parts. Otherwise Ill probably try and sell them and if that fails Ill scrap em n get my money back. These things are outrageously enormous and heavy as **** so Id rather not move them multiple times ^_^.

Thanks again FlyFisher and everyone else.


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## FlyFisher (Jun 30, 2018)

MacroPc said:


> (I only have a 100amp box that im **** near maxing out already when running my equipment and the air conditioning)


Ever heard the saying of trying to get 10lbs of $hit in a 5lb bag? If all you have is 100 amp service you are too short on power to do much as you have discovered.

Someone earlier in the thread mentioned looking in to large surplus military generators that output 3-phase. I think that is a logical decision based on the circumstances. Yes, you will have to feed it fuel. However, you can't simply "upgrade service" from your power company. 

For example - we have 200 amp service here - twice your 100 amps. In load testing generators, we can run the house on 50 amps - central air, portable room AC, and 2x refrigerators included. So we have 150 amps over top that 50 amp amount. That 150 amps could run a decent RPC. You'll crank up your kwh usage quickly when it runs, but you could do it.

However, in your case you don't have that. You have a 100 amp box. You don't simply swap out the service disconnect breaker with a 200 amp breaker and upgrade the meter. The box likely isn't going to be rated for it. That means that you will have to have an electrician upgrade your box. To do that is to rewire every circuit in the house in to a new box/breaker set. That is not a quick task and an electricians time is good money. The service upgrade from the utility company - line and meter - may not even cost you anything, if it does it should be reasonably priced. The utility company will be paid off the usage. So the expensive part is going to be the box upgrade.

If you were going to stay put in that location for years and years then it might be worthwhile to get actual pricing on the upgrade, but if you are going to relocate I don't think it is a justified upgrade. 

If you are max'ing out your utility feed then you need another feed. That is where the large military generator comes in. That can be your supplemental power. Note that a lot of large generators have the ability to select the voltage and output configuration - single phase 120/240, 3 phase delta, 3 phase wye, etc, etc. You'll have to check specific generator specs to see what is available. The output depends on how the phases of the alternator are wired - series, parallel, what is neutral, what is hot, etc and there are a bazillion ways to wire them. You could get a lot of backup power for your house as well as the ability to run your 3-phase equipment (at different times - you have to select the output of generators that have selectable output while there is no load, if not off entirely).


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## MacroPc (Jul 6, 2021)

I have looked into getting another power feed installed in the garage. My electric service company would install the meter only after my new breaker box is installed. I dont have more need than available power/amperage right now but Im sure if I get anything more that would be running concurrently with what I am running now I would start running into problems.


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## iowagold (Jan 22, 2020)

pretty easy just set a new 3 phase panel in and the outside stuff (3 phase meter socket and head).
you can always just do a drop cord socket bucket off that 3 phase box.
and when you get set up in the shop run the hard conduit lines to the locations for the equipment.
just make sure they will let you have 2 meters on the same address.


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## ToolLover (Jan 13, 2020)

A note on getting three phase installed in your garage:
During the late 60's I needed three phase in my shop.
After some time the utility company (Duke Power) agreed to run an extra line and supply an open Delta transformer configuration with high leg for my shop. No 120 VAC and I had to buy a 30 amp transformer.
Unbeknown to me there was a hidden catch that I was not aware of.
My future power bills would be based on the highest bill and remain at that amount for the next year.
So, I paid dearly for my bid to get three phase in my shop.
Sometimes you get more that you expect. Beware!


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## MacroPc (Jul 6, 2021)

Well thats a bs move if I ever saw one! I know that if I got a 2nd meter installed for 3 phase I would get charged at least $15 a month on that meter just for the "equipment use".
So when we sell the house the new owner might not want that installed and Im not sure how the power company would react to taking the meter out so I dont think Im going to get it installed in this garage. Thats why I wanted a "cart generator" so I could have access to 3 phase without additional equipment.

My new house will either already have or get a separate breaker box installed in the shop! I have 3 requirements for new house, gotta be in the country so I can shoot stuff, gotta be a ranch style house cus I have a bad knee and hate stairs, and lastly I need at bare minimum a 2 car garage but preferably a garage and a separate building to turn into a shop. I have alot of tools and equipment so I need a place to work on stuff. As it is right now my garage is so packed just to do ANYTHING I have to pull the Harley out and move at least 1 stand (with the anvil on it) just to work on something as small as a microwave.

I cant wait to move... ^_^

Do you have any ideas or suggestions for these 2 electric motors I have?


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## ToolLover (Jan 13, 2020)

There is no need to go to such extreme lengths to solve your problem.
The solution is staring you in the face:


Amazon.com : phase converter 1 phase to 3 phase


Choose one and forget engineering your own phase converter.


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## MacroPc (Jul 6, 2021)

I already have a static phase converter in my cart on amazon, lol. The point was to use these motors for something, not strictly making a phase converter/generator. The generator was just my idea for them. Now that Ive learned its not really worth it, Im looking for another idea for the motors.


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## FlyFisher (Jun 30, 2018)

ToolLover said:


> Unbeknown to me there was a hidden catch that I was not aware of.
> My future power bills would be based on the highest bill and remain at that amount for the next year.
> So, I paid dearly for my bid to get three phase in my shop.
> Sometimes you get more that you expect. Beware!


Several years ago I interviewed with a major generator company for a position (they approached me, not the other way around). It was an interesting experience in a lot of ways, but didn't work out. In that experience I learned one of their major markets was "peak shaving".

In the commercial world what you describe is exactly correct. Industrial facilities are charged for power usage based on their "peak" usage, not their "consumed" usage.

On top of that electric companies will sometimes divide up usage based on time periods. So during "peak hours" the rates go up. You don't just see this in commercial, but you also see it in residential. However, I don't think I have ever heard of "peak usage" being the rule of a bill for residential, period, before. Thats a new one for me - if the facility/property was "residential".

Where the peak shaving generator market sits is that large companies may choose to run large generators for industrial processes that, otherwise, would ramp up their draw from the power grid. So instead of having that draw boost their power rate across the board for the year they keep their grid power usage minimal and fire up a generator to take the heavier processes.

Thinking through the concept - think of a generator in the multi-hundred kw range - think the size of a semi trailer or larger and the expense of the generator, maintenance, and the fuel usage. For that to be a cost reduction to the company over the utility fees is mind-boggling to me. But again - that was a major market to the company I was talking to. 

With respect to the direct point - I would be real curious how the power was classified - maybe there wasn't any way the utility company could classify 3-phase as "residential".

It would also be interesting to know if a single meter could be used for 3 phase and single phase. I suppose that is dependent on the voltage and configuration. The 120/208 wye combo is common, but that wouldn't allow the conventional 240v single phase. 480v 4-wire delta would give 240v split phase (edit - that would be 480v split phase down to 240v, so double the voltage of conventional household power) on the phase with neutral, but not 120v without a step down transformer. I guess the bigger issue is if you load 1 of the 3 phases (with your house load, for example) you throw off the balance of the other 2 phases.


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