Single phase generator supplying 3 phase loads

[asparkofhopein33916]

111015-1006 EDT

asparkofhopein33916:

How an AC synchronous motor works. I am describing a synchronous motor because it illustrates a most important concept, and synchronous is simplier to visualize.

Consider two separate shafts axially aligned, and axially restrained. Attach a bar magnet on the end of each shaft perpendicular to each shaft. Position the shafts so the magnets are close but not touching.

Consider one shaft the driver and the other the load. Rotate the driver shaft with no torque load on the load shaft. What does the load shaft do? The load shaft follows the drive shaft in perfect synchronization with no angular shift.

Next lock the drive shaft. Apply an increasing torque on the load shaft and now there will be an angular displacement between the two shafts that will increase with increasing torque. This continues until a breakaway point is reached.

This same kind of displacement occurs when the shafts are rotating. So the load shaft runs in exact synchronization with the driver shaft from a speed perspective, but has an angular displacement relative to torque load.

Next replace the driver shaft and its magnet with two fixed coils. Apply a DC current to the coils. The load shaft aligns itself with the magnetic field of the coils.

The load shaft has inertia so it does not quickly respond to changes in applied force.

Rapidly change the polarity of the current to the coils. For example + to - to + 60 times per second. The bar magnet will move to a mid position where it is being tugged in one direction for 1/120 second, and then in the other direction for 1/120 second. The load shaft slightly oscillates back and forth about a mean position. The oscillating magnetic field is at one angular position in space with a changing magnitude from + to - to + every 1/60 second.

By some means get the load shaft to rotate at close to 3600 RPM. Now the polarity of the oscillating magnetic filed is in synchronization with the magnetic field of the rotating magnet. This is like you providing a pulse of energy to a person on a swing at just the correct point in the swing cycle.

For a single phase motor there has to be a way to get it started. Once started then it can supply output power from the magnetic pulses of energy. A side point --- there is more output torque ripple from a single phase motor than a 3-phase motor.

Next to a change of thought pattern. Consider an X-Y plot. The input motion to the Y axis is A sin t, and the input to the X axis is B cos t. Let A = B. When t = 0, then x = A and y = 0. When t = 45 deg, then x = 0.707*A and y = 0.707*A. If you do this for many different angles you will see the result is a circle of radius A. If A and B are not equal, then the result is an ellipse. For different harmonically related frequencies and varying phase relationships the plots are called Lissajous figures.

In this circular plot of two sine waves perpendicular to each other the result is a constant amplitude vector whose angle is a function of t. If the two input frequencies were not the same, then the result would not be a constant amplitude vector.


Add two more coils to our earlier single phase motor, and mount these perpendicular to the first two coils. Now excite one pair of coils with A sin t, and the other with A cos t. This produces in space a constant amplitude rotating magnetic vector that the bar magnet on the load shaft will track.

If we go back to the original two shaft two magnet setup and make the magnets far apart initially, rotate the driver shaft at 3600 RPM, and gradually bring the magnets closer together, then there will be a rotational pull of the driver shaft on the load shaft. Initially there will be some pulsating slip, but it is dominantly in one direction, and some point the load shaft syncs with the driver shaft.

To create this approximately constant magnitude rotating magnetic field it is critical that the frequency to each coil be exactly the same, and in a controlled and fixed phase relationship to the other coils.


That is the motor theory.


Your one motor is about a 5 HP unit and you may be able to find a single phase input to 3 phase VFD that could power it. A separate VFD could supply the other 3 phase load.

I do not care for rotating phase inverters, but if you go to the used market they are not too expensive. These are simply built out of a 3 phase induction motor with some capacitors. If you went the rotating route I would prefer a 3 phase AC generator from a natural gas engine.

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Wow!! Now that's what I call some feedback ! It looks like a rotary converter on the load side of the generator is my best solution but if you know any vendors for small LPG (5Kw) 3phase generators I would prefer not to use a converter.

 

[Jraef]

1 freezer and 1 cooler. Freezer motor rated 3 phase 208/240 12.8 amp. Cooler motor rated 3 phase 208/240 6.2 amp. Evaporators wired together at 208 1 phase 3.6 amp total load 22.6 amp @ 208 v. Generator is 12 KW. Will be feeding transfer panel with 60a. I have located a converter that is rated 60/23 input/output. but I would rather just extend the existing 3 phase service to one leg of the motors and not use a converter. I know it will be hard to align the OCPs but all the equipment will be accesable to only qualified personal. The generator has both frequency and voltage adjustment capability so I may be able to get matching voltages or somewhere near. The generator will only be used for power outages which are rare.

OK so now that we have moved you off of the crazy idea part of this, we have something to work with.

Looks as though you have a 3HP freezer and a 1-1/2HP cooler, then a couple of 1/4HP single phase evaporators. So you have 5HP total. The rule of thumb on being able to start behind a generator kW that is 2X the total HP fits, so we know your existing generator is going to work. If you get a rotary phase converter for the 3 phase loads, you will need separate ATS units for them and it's going to get expensive again.

The alternative would be a single 3 phase ATS to feed a sub panel that feeds your entire freezer system, both from the utility side and the generator side. So sell the 1 phase ATS on eBay, buy a 3 phase ATS, then use your generator to feed the RPC, feed the RPC into the ATS emergency side, then feed your utility 3 phase to the Normal side of the ATS, and the output of the ATS to a 3 phase load center that splits it up to the separate loads. You can then use the generator you already have and the RPC you found, the only new big expense will be a 60+A 208V 3 phase ATS. That may cost you anywhere from $500 to $800 on fleabay, more for new. You may need to redo some of the utility side wiring to accomodate this, but that's just labor and a little hardware. Student labor is cheap...

 

[asparkofhopein33916]

OK so now that we have moved you off of the crazy idea part of this, we have something to work with.

Looks as though you have a 3HP freezer and a 1-1/2HP cooler, then a couple of 1/4HP single phase evaporators. So you have 5HP total. The rule of thumb on being able to start behind a generator kW that is 2X the total HP fits, so we know your existing generator is going to work. If you get a rotary phase converter for the 3 phase loads, you will need separate ATS units for them and it's going to get expensive again.

The alternative would be a single 3 phase ATS to feed a sub panel that feeds your entire freezer system, both from the utility side and the generator side. So sell the 1 phase ATS on eBay, buy a 3 phase ATS, then use your generator to feed the RPC, feed the RPC into the ATS emergency side, then feed your utility 3 phase to the Normal side of the ATS, and the output of the ATS to a 3 phase load center that splits it up to the separate loads. You can then use the generator you already have and the RPC you found, the only new big expense will be a 60+A 208V 3 phase ATS. That may cost you anywhere from $500 to $800 on fleabay, more for new. You may need to redo some of the utility side wiring to accomodate this, but that's just labor and a little hardware. Student labor is cheap...

Thanks for the post. I like your configuration.

 

[CFL]

I teach at tech school in fla. The culinary teacher asked if I could provide stand by power for his walk in cooler and freezer. The evapororators and the fans are 1 phase but the compressors are 3 phase. I have a 1 phase stand by generator with an automatic tranfer switch that was donated to the school. Is it possible to adjust the voltage regulator on the generator to supply 208 volt from the generator and run the other phase leg from the schools power?

What do you teach?!

 

[Hv&Lv]

Good Call. That is how I'm set up right now. I just didn't like the fact that I would have to run the compressor motors 24/7 through the converter when I had three phase available plus I could save about $2500.00 for the converter. By bringing existing 208v 3 phase system through the ATS panel I thought I could isolate one phase and run it directly to the motor saftety switches ( with proper OCP ) and have the generator supply he other two lines. Does that make any sense?

Ok, that is just not smart to try. It won't work, and to top it off, will not be allowed by any utility unless there are some sophisticated safety devices installed (anti-islanding) that will cost more than scrapping the generator you have and buying a three phase generator. even then, you will not be allowed to run in synch with the POCO with that configuration even if it were possible.
Also, I doubt you could get that to meet article 705.
By the way, maybe I missed something. If the utility power is on, why do you need generation? and if the utility power is off, how are(is) the other line(s) supplied?