Single phase generator supplying 3 phase loads

[asparkofhopein33916]

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?

 

[Carlton412]

No, it doesn't work that way. These kinds of scenarios can lead up to someown getting killed. You need a 3 phase generator of the proper capacity, a transfer switch and a qualified electrical contractor to make this happen safely.

 

[jumper]

Nope.

 

[asparkofhopein33916]

Would a rotary phase converter be adjustable to 240 v

 

[jumper]

The school does not have 3 phase?

 

[asparkofhopein33916]

Yes, I was referring to connecting the phase inverter to the load side of the 1 phase generator.

 

[jumper]

Where is jraef? He does this stuff.

 

[hillbilly1]

Yes, I was referring to connecting the phase inverter to the load side of the 1 phase generator.

Yes you can, if the generator is large enough for the connected load, but your load would have to be run off of the converter full time. Might get a little funky though, with the normal input 208 single phase, and the generator 240 volt single phase. Of course you could use a transformer to correct one or the other. It probably would be cheaper and easier to sell the single phase generator and switch and buy a three phase one.

 

[asparkofhopein33916]

No, it doesn't work that way. These kinds of scenarios can lead up to someown getting killed. You need a 3 phase generator of the proper capacity, a transfer switch and a qualified electrical contractor to make this happen safely.

Thanks for the the advice. I am an Electrical Contractor but I don't pretend to know everything. If you can't be helpful just ignore my posts

 

[asparkofhopein33916]

Yes you can, if the generator is large enough for the connected load, but your load would have to be run off of the converter full time. Might get a little funky though, with the normal input 208 single phase, and the generator 240 volt single phase. Of course you could use a transformer to correct one or the other. It probably would be cheaper and easier to sell the single phase generator and switch and buy a three phase one.

Thanks, I have had trouble finding a NG/LPG in the right size and voltage in 3 phase. I know connecting a thrtee phase would be the simplest solution but I don't have a $12,000.00 budget. I may experiment with this one and see if I can get it to work. I haven't worked much with converters. Is the output voltage adjustable or always the same as input Voltage.

 

[gar]

111014-0858 EDT

asparkofhopein33916:

You need to provide more information. What are the power requirements of each of the motors, their voltages, and number of phases? Are there more than one cooler and freezer?

.

 

[asparkofhopein33916]

111014-0858 EDT

asparkofhopein33916:

You need to provide more information. What are the power requirements of each of the motors, their voltages, and number of phases? Are there more than one cooler and freezer?

.

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.

 

[iMuse97]

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.

What are you suggesting in the highlighted piece above? Did you mean single phase? You need the converter (rotary phase converter). Locate this after the transfer switch with utility power feeding line side one of the transfer and your jennie feeding line side two. The ATS load side will feed the line of the converter, and you can power your 3PH loads from there. If the input voltages to the phase converter are close that should be good enough, given your comment that the refrigeration equipment is rated for 208/240, per your comment.

 

[asparkofhopein33916]

What are you suggesting in the highlighted piece above? Did you mean single phase? You need the converter (rotary phase converter). Locate this after the transfer switch with utility power feeding line side one of the transfer and your jennie feeding line side two. The ATS load side will feed the line of the converter, and you can power your 3PH loads from there. If the input voltages to the phase converter are close that should be good enough, given your comment that the refrigeration equipment is rated for 208/240, per your comment.

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?

 

[gar]

111014-2336 EDT

asparkofhopein33916:

I do not believe you understand the theory of how a three phase motor works, or for that matter a two phase, or single phase. Tomorrow I will try to explain some of this, or maybe one of the English posters will since they get up earlier than I do.

.

 

[hurk27]

I agree with Gar, first you have no way to keep the generator in sync with the utility source to allow this to even remotely work, second you will still have the problem when you loose the utility power as the 3-phase loads will single phase and go into over load, what this sounds like is you only have a 2 pole transfer switch along with the single phase generator, to make this work safely and function your going to have to locate a 3-pole ATS and place your phase converter ahead of the transfer switch, the only other way to do it with what you have will be to put the phase converter at the load and feed the ATS with single phase using the phase converter for both utility and generator supply, you can not just use one leg from one source and the other two from another source that is not in sync, even if you could sync them then it would be a code violation to do so, you have two options:

1. run the phase converter at the load and feed only single phase from both sources or:
2. use a 3-pole ATS and place the phase converter on the generator side of the 3-pole ATS?

One thing, many static type phase converters do not handle high torque loads very well, then also tend to run very un-balanced between the phases, you might want to look at a roto-phase type as it will do much better and will give you the required start up torque needed.

Read more about this here: http://www.phaseconverterinfo.com/phaseconverter_rotary.htm

 

[gar]

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.

.

 

[jeremysterling]

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.

What you are attempting is technically and theoretically very difficult if not impossible to execute.

Also, I'm trying to understand how your proposed standby system uses one leg of the utility when the purpose of the standby is to provide power upon loss of said utility?

In other words, if one leg of the utility is present, typically the other two are available, so why try to combine that single utility leg with two legs from a separately derived system?

 

[asparkofhopein33916]

What you are attempting is technically and theoretically very difficult if not impossible to execute.

Also, I'm trying to understand how your proposed standby system uses one leg of the utility when the purpose of the standby is to provide power upon loss of said utility?

In other words, if one leg of the utility is present, typically the other two are available, so why try to combine that single utility leg with two legs from a separately derived system?

You are quite correct. After giving it some more thought I realized that when Utility power was interupted the direct power leg would also be interupted therefore shutting the motopr down. I suppose a rotoary phase converter on the load side of the generator is my only solution unless I can locate a LPG 3 phase generator with stand by capability that is 5KW. I do have a 30 KVA transformer 480 Delta /208/120 Wye so I have voltage options with the generator. Thank you for your input.

 

[asparkofhopein33916]

I agree with Gar, first you have no way to keep the generator in sync with the utility source to allow this to even remotely work, second you will still have the problem when you loose the utility power as the 3-phase loads will single phase and go into over load, what this sounds like is you only have a 2 pole transfer switch along with the single phase generator, to make this work safely and function your going to have to locate a 3-pole ATS and place your phase converter ahead of the transfer switch, the only other way to do it with what you have will be to put the phase converter at the load and feed the ATS with single phase using the phase converter for both utility and generator supply, you can not just use one leg from one source and the other two from another source that is not in sync, even if you could sync them then it would be a code violation to do so, you have two options:

1. run the phase converter at the load and feed only single phase from both sources or:
2. use a 3-pole ATS and place the phase converter on the generator side of the 3-pole ATS?

One thing, many static type phase converters do not handle high torque loads very well, then also tend to run very un-balanced between the phases, you might want to look at a roto-phase type as it will do much better and will give you the required start up torque needed.

Read more about this here: http://www.phaseconverterinfo.com/phaseconverter_rotary.htm

Thanks for the link. I had located a converter on Grainger's website prior to contemplating this project. I will most likely be using the converter on the load side. Will there be any adverse effects if I run both motors from one converter? Also as I understood it, Static converters are only good for starting motors not to run them continuosly.