Using an induction motor as a generator

[paul6050]

I want to use a 10 HP, 230 VAC, single phase induction motor as a generator by turning it faster than synchronous speed while it is connected to the grid. I'm using a 40 HP gasoline engine to turn the motor.

I'm using an induction motor as the generator so the current is automatically synchronous to the grid. I have to use a single phase motor because 3 phase power is not available to me.

I plan on limiting the in-rush current by turning the motor at synchronous speed before supplying current to the run windings. The start windings will be disconnected.

If I do this, I am afraid I'll get a jolt in the motor because when the current is first applied to the main windings, it will try to grab the rotor and act like a brake for a split second until the magnetic fields are established in the rotor.

To prevent this, I am wondering if I applied a low voltage current to the run windings (12 VAC) to establish the magnetic fields in the rotor and then switched to 230 VAC, would I eliminate the jolt?

I have done all my testing using a 4HP engine and a 1hp motor. I don't want to damage my new 10HP motor!

Any thoughts on my ideas or plan?

 

[electricpete]

I don't know the direct answer, but you may be creating problems with that approach.

It is well-known that a normal induction motor which has power removed and quickly re-applied while running may experience extreme torques due to the fact that the motor internal voltage is out-of-phase with the voltage being reapplied. Your self-excited motor may create a similar situation, unless you either: A - Limit the terminal voltage at the motor to <~30% prior to closing (limits vector difference in voltages to 130% which is established threshhold) or B - Use some type of sync check to avoid closing out of phase.

 

[dpc]

My advice would be - don't do it. Induction generators are a pain in the neck, and using a 40 hp generator to drive a 10 hp generator can't be too efficient.

Having said that, I think if you get the motor close to synchronous speed **un-excited**, then connected to the utility source, you should be OK. That is how larger induction machines are generally started - no synchronzing equipment is used.

However, the larger question is what happens after you are on line. You'll need a good governor on your engine to regulate power output, and without some sort of excitation source, your running power factor will be terrible.

dpc

 

[busbar]

IIRC there two methods of starting induction generators in wind power. One is to close the unit breaker after the wind has the rotor near synchronous speed—or—“motor” the machine up to speed by closing the unit breaker. A very basic induction-generator discussion is at:

http://www.usmotors.com/

 

[SteveKW2]

I think you are going to have problems with high current at synchronous speed. The CEMF developed by the rotor is due to the &quot;slip.&quot; No slip, no magnetic field in the rotor to limit the current in the stator.
I think you would be better to start the motor from the line power then just then engage the engine at faster than synchronous. I doubt the current rush would be any greater than the starting current. If everything is bolted down that jolt shouldn't hurt anything.

 

[Lewish]

In a hydro generation site, where an induction motor is used as a generator, the standard procedure is to start the motor off the grid and let it come up to normal speed turning the turbine. Then you open the wicker gates and start powering the turbine. After that, the control system takes over and you are feeding the grid.
On a single phase motor, you have to be certain that the motor is turning the same direction when powered by the gasoline engine as it would be if it is powered by the grid. You will also need a different value capacitor for excitation. &quot;Motors As Generators for Micro-Hydro Power&quot; by Nigel Smith is an excellent resource for information about doing what you want. If you have access to Amazon.com, it is normally in stock there.

 

[gordonl]

I think establishing a field would be detrimental because you would need to ensure your grid and gen were in phase not just at the right speed.

When there is no slip, there is no stator current because there is no changing magnetic field. I beleive Pete's suggestion is wise because if there is residual magnetism in the motor creating a rotor field you could possibly create higher than expected torque. I'm not sure about the 30% value but it seems reasonable.

You may want reverse power protection, if your motor ran out of gas the &quot;gernerator&quot; would crank it in reverse at full speed, and I suspect your motor may not like it.

As someone else suggested, I imagine this will be more expensive on gas than the power it produces.

 

[paul6050]

Thanks for all the great feedback! Here are my findings in regards to your comments.

DPC- I don't seem to need a very precise governor because the harder I try to over-speed the motor, the more current it puts out. The load is steady because it is the grid!

BUSBAR - I'll have to look for that book.

STEVEKW2 - I never saw any current spikes when energizing the field windings with the motor turning at synchronous speed with the 1 hp motor. My induction amp meter just rose to the no-load current rating of the motor.

LEWISH - The motor is being over driven, that is turning it faster than synchronous speed. I am not fighting the motor. But you are right about running out of gas. Then I would be buying electricity to turn the engine!!!

I'll be burning natural gas and the numbers workout that I can make money doing this. But I won't know for sure until I put one together. The fuel consumption is pretty linear with the load. 7.5 cubic feet of gas per hour per HP output.

When you over drive the motor, you get the same current output as you would have to put in at full load.

Right now, it appears if I run the motor a little faster than synchronous, I can energize the field with full voltage without a problem.

Thanks for all your help. Paul

 

[busbar]

Paul6050-- The 'motor as induction-generator' information is on line at

http://www.usmotors.com/products/profacts/1-204.htm

 

[dpc]

The issue on the governor is not speed control - but rather load control. A 40 hp engine driving a 10 hp generator may be a little touchy.

I'm not sure where you are located, but there are strict requirements for operating in parallel with a public utility, even for an induction machine. Also this is covered under applicable sections of the National Electrical Code. Better safe than sorry.

Also, you should keep an eye on your bus voltage as you vary the output of the generator.

Good luck!

 

[paul6050]

Thank you Busbar for pointing me to the US Motor site. That is the one motor company I have not delt with on this project. Others that have tried this before me used Baldor and Leeson motors.

DPC - You're right, I need a governor to prevent the gas engine from running away if I loose my connection to the grid. I'm using a cruise control system to control the engine speed. Any faults like over current, over voltage or over speed will open a relay and be like like hitting the brake pedal in your car. The cruise control drops out and the engine goes to idle!

I live in Wisconsin and the electric company looks forward to my generator getting on line. They have a whole program devoted to decentralized generation. It's called C.O.G.S. Customer Owned Generating Systems.

 

[resqcapt19]

Gordon,
&quot;You may want reverse power protection, if your motor ran out of gas the &quot;gernerator&quot; would crank it in reverse at full speed, and I suspect your motor may not like it.&quot;

While the &quot;generator&quot; would motor and dive the prime mover, it wouldn't be in reverse. It would be in the same direction at synchronous speed.
Don(resqcapt19)

 

[Lewish]

What the book by Nigel Smith gives that the website doesn't is how to calculate your excitation requirements based on the parameters of the motor. It also provides suggestions on load controllers and protection equipment. And the book is cheap. I pickup one up for about $10.
What part of the country are you in that natural gas is cheap enough to run a piston engine and make money generating electricity? Just curious. In most areas it takes a dual cycle turbine to justify gas costs.

 

If you start around sync speed there should be little problem. I have done this to provide load on small mg sets.
Monitor positive power to keep the thing going the correct power direction. Dont excite the machine before energizing. You need about 10 seconds to lose a magnetic field on a rotor, and in that time the rotor could be subjected to large torque transients. If you are worried about damage, put a breaker on your feed which is small enough that you will pop it if you get anywhere near heavy currents. You motor is designed to start across the line, at considerably higher currents than you will run, your starting could be the easist this type of motor will ever see. You do need to have droop in your governer for the gas motor so that it will not hog load and try to provide the 40 hp into the 10 hp generator. The cruse control system will have to be soft enough that it will droop to line speed. Most small generator motor sets tend to be designed to operate seperate from the line, and are stiff, and do not vary much. You will need some kind of protection to prevent reverse motoring and to shut down your system on over load or if the line is lost, to prevent feeding a fault on the grid, like an undervoltage trip. It sounds like fun, good luck.

 

[nosmo]

At the time of the last coal miners strike in the UK in the 70s electricity went on &quot;RATION&quot; companies were limited to a 3 day working week, The company I worked for happened to have a Motor/alternater set used for testing machines going for export requiring 60HTZ instead of the 50Hrtz the UK enjoys. I suggested we make a natural gas engine driven generator to suppliment our power usage by using the alernator on this available equipment together with the addition of a MORRIS MINOR 1.5L engine as the prime mover. This idea was immediately approved. and I cobbled together a machine to supply power to the factory. early tests were dissasterious, the engine coupled by belts and pulleys to the alternator was the first problem, the belts flew off when a heavy load was aplied in the factory. this was due to leaving the original rubber engine mounts to support the engine. there was anothe problem. controlling the speed of the engine to provide the correct frequncy (50HTZ).
This problem was semi solved by making a small differential
gearbox consisting of a bunch of thrust bearings with a synchronous clock motor at each end. providing the two clock motors ran at the same speed the output shaft did not rotate, if one motor ran slower than the other the output shaft would rotate accordingly. this was simple and effective being connected to the throttle of the engine.
One clock motor was conected to a 12 vdc 50hrz &quot;shaver inverter&quot; as a referance frequncy.
The other clock motor was connected to a phase of the alternator.
After an 8 hour shift the electric clocks in the building were only 2 minuits fast.
I was not satisfied with the performance and decided to put the generator on line, this is a NO NO as far as the utility companies are concerned, because of the dangers of supplying electricity outside the building and onto lines which in their estimation would be dead in the case of an adjacent fault.
I connected three incandescent lamps accross the three contacts which would connect my generator to the grid. when all three went out I threw the switch and locked frequency with the power station, it was a smooth operation no belts flew off.
I then opened up the engine to full throttle and the rev counter remained steady at mains frequncy. I went round and studied the companies WATT METER supprisingly it was running backwards nicely, jerking forwards on occasions when a heavy load came on in the plant.
IF this alternator had been a large 3 phase motor coupled to the engine with a sprag clutch I think it would be running to this day because the costs of natural gas, even after the heatlosses of the internal combustion engine are taken into account are still cheaper than electricity.
certainly in the winter those heat losses could be used for space heating, I began experiments in blowing the exhaust of the engine into a tank of water. everything looked extremey prommising as the water temp rose rapidly,.
During the lunch brake some joker emptied a box of washing powder into my hot well, this ended the heating experiment.

 

[jbartos]

Suggestion: Considering the 40HP gas engine and 10HP asynchronous generator, would not it be prudent to consider an additional generating capacity to be added to take the full advantage of your prime mover? If additional generators are added, then one could consider an AC-DC-AC power electronic converter which would eliminate some difficulties that you currently have with the asynchronous generator. Also, DC section may have a stationary battery so that you could sell more energy when it is more expensive, i.e. in the peak hours.