True three phase power system, motor/generator.

We spill a lot of ink on 3 phase power sources, RPC's, PP's, static and whatever. I have yet to see a current article on using a motor/generator system. It would not be as cheap as a RPC but cheaper than a Phase Perfect. Best part is that the output is true three phase balanced power using equipment that will probably last 50 years and be rebuild-able.

A very quick check on ebay reveals that 30 kw generators can be had for a few hundred dollars to less than $2000. Using an RPC type motor to drive should result in a reasonable price.

Has this come up before and if so what am I missing?

Tom

Sounds wonderful except for the fact that if you are wanting the whole 30KW, does not the single phase prime mover need to be capable of delivering MORE than 30KW of turning effort? That is one whopper of a single phase motor.

Stamford's 22KW "head" (just the "generating" device) ships at 398 Lbs - suggesting a serious piece of equipage. .

TDegenhart said:

We spill a lot of ink on 3 phase power sources, RPC's, PP's, static and whatever. I have yet to see a current article on using a motor/generator system. It would not be as cheap as a RPC but cheaper than a Phase Perfect. Best part is that the output is true three phase balanced power using equipment that will probably last 50 years and be rebuild-able.

A very quick check on ebay reveals that 30 kw generators can be had for a few hundred dollars to less than $2000. Using an RPC type motor to drive should result in a reasonable price.

Has this come up before and if so what am I missing?

Tom

Click to expand...

The prime mover is not a single phase motor, it is a 3 phase motor run from single phase power as in RPC.

Tom

TDegenhart said:

The prime mover is not a single phase motor, it is a 3 phase motor run from single phase power as in RPC.

Tom

Click to expand...

Which is fairly inefficient. As an RPC, a motor needs to supply only 1/3 of the load power. Since the motor runs on only part of its windings, the actual overload point is somewhere between say, 50% and 66% of marked rating. So you need a considerably larger motor.
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I HAVE seen it done. A motor shop I saw had a big single phase motor running a generator for exactly that purpose. I do not recall what the power was, but generator was the physical size of about a 25HP motor or so, and the single phase drive motor was similar.

With an adequate motor, it is fine. Due to various factors with generators, you would need to de-rate it to start the load motors, but if sized to drive several, it should be fine.

3 phase motor driving generator

My shop uses a 20 hp 3 phase motor to drive a Kato 20 kw synchronous generator. The motor runs on single phase. The maximum steady load that the generator supplies is about 8 kw. The worst case load occurs when the 10 hp Fanuc AC spindle drive is started at maximum RPM. The 6MB control has no provision for a soft start or stop. The manual lathe draws maximum power when the clutch is engaged with the spindle gear set at the 1000 rpm maximum turning a 90 lb chuck. I have learned to engage the clutch slowly when using the high RPM.

The Fanuc spindle drive uses regenerative braking. It works well with this generator.

The idle power consumption of the motor generator set is 2.3 kw. That is with no load except the large fans on the motor and generator. The generator is electrically isolated from the motor by a rubber drive coupling. A lightning strike at the power pole should not, in theory, cause damage to the Fanuc electronics. I have not had the luck as yet to put this to a test. The last strike I had shorted out the pole transformer. The power company has installed a set of "voltage fuses" to connect the lines to ground when there is a over voltage.

Robert

TDegenhart said:

We spill a lot of ink on 3 phase power sources, RPC's, PP's, static and whatever. I have yet to see a current article on using a motor/generator system.

Click to expand...

I worked in a shop that had one to drive an early model Reishauer that ran at some odd voltage and frequency. It screamed like a banshee and howled like a coyote, simultaneously. At a high volume. I can't say they are all loud but I'd be careful about that.

Robert, is the motor also synchronous? If it is an induction motor, you will have some slippage and resultant lower than 60 cycle output. Commercial units are made with a synchronous motor driving a generator. As I have said in the past, I foolishly did not take one home when I had the chance. This one had a 3 phase motor driving a 120/208 generator used to run a lot of computers in a stock brokerage. I'm sure lightning protection was one of the considerations.

A couple of years ago I posted something about using a synchronous motor for an RPC and got a response from someone who worked for an RPC manufacturer. I guess he monitors this forum for laughs, of which there are plenty.

If I were doing it, I would look for a single phase generator for a motor and pony it up to where it would lock in, using it to drive a 3 phase generator.

Bill

9100 said:

Robert, is the motor also synchronous? If it is an induction motor, you will have some slippage and resultant lower than 60 cycle output. Commercial units are made with a synchronous motor driving a generator. As I have said in the past, I foolishly did not take one home when I had the chance. This one had a 3 phase motor driving a 120/208 generator used to run a lot of computers in a stock brokerage. I'm sure lightning protection was one of the considerations.

A couple of years ago I posted something about using a synchronous motor for an RPC and got a response from someone who worked for an RPC manufacturer. I guess he monitors this forum for laughs, of which there are plenty.

If I were doing it, I would look for a single phase generator for a motor and pony it up to where it would lock in, using it to drive a 3 phase generator.

Bill

Click to expand...

The Kato synchronous generator I use came from a Synchronous Kato motor generator set that was used to convert 60 hz power to 50 hz power using pulleys and a timing belt. These motor generator sets were sold to companies that were required to demonstrate to EU countries that their products would work on 50 hz current. The motor generator sets showed up in salvage yards in San Jose in the 1980's and were sold for scrap prices.


I removed the synchronous motor and used it as a phase converter at another shop. The generated phase from the motor was routed with the two power lines to a delta-wye wired transformer. The transformer reduced the voltage difference between the phases to a few percent at all loads and provided a neutral line. The 3 phase power that was produced was as good as what the power company could provide. The excitation of the synchronous motor was adjusted so that the motor was running at a 100% power factor. This was a very efficient system.


When I needed a phase converter for my shop I used the remaining synchronous generator and a 20 hp induction motor to drive it. The motor does slip. My shop runs at 57 hz. None of the machines care. The motor is a Delco U frame TEFC design. It was not designed for high efficiency. The design priority was a very long life. I chose this motor because it was free. A high efficiency induction motor would allow a significant reduction in the 2.3 kw idle power of the motor generator set. If I were doing this work over again I would have kept the Kato motor generator set intact and just replaced the pulleys and drive belt with a direct drive coupling.

Robert

The slip will indeed result in a slight change of frequency.

However, if the motor-generator is derated to take care of starting overloads, then the slip in normal operation will be small, and not cause an issue. A synchronous motor would be ideal, and a single phase generator might do well. You would need to get it going, but there are ways to do that.

To me this is so Rube Goldbergish, I say just buy a new Phase Perfect and be done with it.

No noise, no heat, uses almost no power when "on" but not being called upon for the 3 phase....cut it on at beginning of workday and cut if off at end (or leave it on 24/7 but I would worry about lighting power surges in this area when away so I cut mine off)

It's output is every bit as good as true 3 phase in my experience....and I've had true 3 phase in various shops over the decades so I have experience in that regard.

True that.

I suppose the only reason for considering it is that it is nearly bullet-proof (pretty much literally is, from at least some angles), and does not require advanced tech parts. Possible to cobble up from out of the scrap yard. Survivalist kind of stuff. Not quite in the tinfoil hat area, but.....

Hey, the topic came up, no harm in answering.....

TDegenhart said:

The prime mover is not a single phase motor, it is a 3 phase motor run from single phase power as in RPC.

Tom

Click to expand...

Yes, Before I built my RPC, I ran a 10 hp 3 phase compressor on single phase
with a starter box (cedarburg kit) and I got 3/4 of the hp out of it,
I simply put a smaller pulley on it.

With your intended set-up, you could go one step further, and add some run
capacitors to balance things out a bit more.

Reliability

I grew up in the age of true "solid state", copper, silver and magnet steel. I have lived through the transition from old to new and recall the ad's of many years ago about how silicon would live forever. The reality is silicon will fail and unfortunately, often without warning. We replace contactors with solid state relays, but where safety is supreme, we backup the SSR with a mechanical contactor.

I knew there would be kick back, otherwise solid state would not have advanced to where it is. We are now in the throw-it-away era. On another thread, the poster is complaining about Phase Perfect's not lasting forever and are next to impossible to repair. I looked at this as a throw back to a system that worked and keeps working. When it needs love and kiss's it will tell you before it walks out the door. SS simply packs her bags and leaves with no warning and refuses to come back.

So what is better, the high class trophy wife or the girl next door that will stick with yo through thick or thin, that holds dear the "until death do us part"?

Tom

There is no doubt which is likely to be cheaper, at least sourced from the junkyard.

And, it may well be more reliable over a long time.

Robert R said:

The generated phase from the motor was routed with the two power lines to a delta-wye wired transformer. The transformer reduced the voltage difference between the phases to a few percent at all loads and provided a neutral line.

Robert

Click to expand...

I have proposed this several times with little or no response. The outputs of the transformer are composites of all the phases blended together, spreading the differences between the phases. I have been working on a transformer to enhance the effect but haven't been able to do much lately due to the usual crises. I made one and had it working but ran into problems with resonances between the windings and capacitors.

Two places I have seen both solid state and rotary converters are the plating supplies at Rock Island Arsenal and the supplies for the 300 VDC used to run the trains in a coal mine. Despite the need for maintenance, the platers much preferred the rotary MGs because they could watch things like brush wear and schedule replacements between jobs as opposed to having a run interrupted by a failure in a solid state unit.

The mine trains were fed by an overhead wire like a streetcar. One section of the mine was solid state and the other MGs, the two separated because the SS units could not handle the spikes from contactors while the mechanical units were oblivious to them. 15 MGs, each conservatively rated at 300 V, 1,000 amps could throw out some bodacious spikes. One time a miner hooked up a unit backward on the DC side, plus to minus. All it did was kick out the directional relay which opened the main contactor. It did make a righteous flash. Silicon would probably have needed total replacement.

If you think I have little love for things like switchers, you would be right. I use them, but I will never learn to like them.

Bill

I have a 2 hp synchronous motor made from a marathon 2 hp 76% efficient 3 phase motor, its more effective and probably efficient at generating 3 phase than my 5 hp 92% efficient induction motor.


As far as I know it is relatively easy to synchronize a three phase generator to mains by connecting it through a resistor, instead of waiting for the phases to synchronize. so you can use a regular induction motor to get it to 58hz and then just drop it online, wait 2 seconds and short out the resistor. this is basically what i recall doing with my 2 hp induction turned synchronous motor.


For another project I rewound a generac 5KW generator head for 3 phase 145/240v, I have not yet tried using it as a phase convertor. Recently I discovered my generac 5.5kw generator fully drives the rotor and stator into saturation. its rotor was driven at 150vac, 4.5 amps, something like 50vac was enough to get nominal volts. (Volts and amps measured before the rectifier.)

So anyhow if you do find an unregulated 3 phase generator head with no electronics in it, it is probably designed to regulate the voltage via saturation.

There are significant differences between converting a 3 phase induction motor to synchronous vs using a salient pole generator, which may not have any shorted turns inserted in the poles. obviously you can turn an induction motor rotor into a salient pole machine, and you could machine off the aluminum ring connecting the cage. but you can only do this once and its a lot of work to go back and insert the dampening winding. unless of course you melt out the aluminum cage and start over.

Robert R said:

When I needed a phase converter for my shop I used the remaining synchronous generator and a 20 hp induction motor to drive it. The motor does slip. My shop runs at 57 hz. None of the machines care. The motor is a Delco U frame TEFC design. It was not designed for high efficiency. The design priority was a very long life. I chose this motor because it was free. A high efficiency induction motor would allow a significant reduction in the 2.3 kw idle power of the motor generator set. If I were doing this work over again I would have kept the Kato motor generator set intact and just replaced the pulleys and drive belt with a direct drive coupling.

Robert

Click to expand...

The thinking man's phase perfect. It could die if some solid-state component failed - oh wait. Doesn't have any. Never mind.

johansen said:

As far as I know it is relatively easy to synchronize a three phase generator to mains by connecting it through a resistor, instead of waiting for the phases to synchronize. so you can use a regular induction motor to get it to 58hz and then just drop it online, wait 2 seconds and short out the resistor. this is basically what i recall doing with my 2 hp induction turned synchronous motor.

Click to expand...

To pull a generator into synch with another AC source, a transformer winding equal to the sum of the voltages i.e. two 120 V generators connected to each end of a 240 V winding with a center tap, taking the output current from the tap, drawing current will suck the two generators into synch. Since the winding will allow the two generators to be 180 degrees apart without drawing significant current, you can't get in trouble. When you draw current, the two halves of the winding interact to pull the two together. This works on two different size generators. When drawing near full load, adjust the governors to make a generator lag or lead to vary its share of the current.

Bill

9100 said:

I have proposed this several times with little or no response. The outputs of the transformer are composites of all the phases blended together, spreading the differences between the phases. I have been working on a transformer to enhance the effect but haven't been able to do much lately due to the usual crises. I made one and had it working but ran into problems with resonances between the windings and capacitors.

.......

If you think I have little love for things like switchers, you would be right. I use them, but I will never learn to like them.

Bill

Click to expand...

There should not be that much interaction, as the 3 phase transformer really does not have any particular interactive action. It is more a way to save material. Of course there IS a magnetic connection, but......

You WOULD get added impedance in the pass through legs, which should help balance the outputs, since otherwise only the generated leg has added impedance.

And you get a true neutral if you want it.

Switchers work, but do have issues of various sorts, depending on what type they are.

There is a saying: "If you can't fix it, you don't own it.". That has application to the Phase Perfect vs RPC or motor-generator debate.

Maybe I was not taught correctly but I was under the impression that in a linear device the principle of superposition applied, that is, in a three phase transformer, the individual phases can be treating independently of the others. You lost me with this blending of the phases.

Tom