induction motor/generator

[gorwell]

Hi, I am hoping to install a grid connected hydro power scheme approx 10kw. Trying to get a handle on the induction motor/generator. I understand about paralell connection of a 3 phase motor to mains: connect it to my waterwheel, speed it up to 1500+rpm (UK grid,230/400v)and it should generate. Simple and elegant.
Now with single phase: will a 3 phase motor, connected to 1 phase when spun up to speed (obviously it won`t self start) act as a generator in the same way?
Will the generation be smooth or in any way lumpy due to 2 missing phases?
Motors I have are 3 ph star/delta (star start, delta run). If the above works I guess I would need to run the motor in star and use only one of the legs. Would continuous running in star damage the motor?
What would happen if you connected all three motor coils to the same phase? Would it generate more efficiently or in some way explode?

 

[Skogsgurra]

I think that it will generate. Yes, definitely. But never seen it. If you have a 400 V grid, then connect as usual but "forget" one phase.

Re. connecting the spare phase to grid: Don't! That will be the equivalent of shorting across two phases. Actually, if you measure voltage between your three motor terminals, you will see somthing very similar to a three-phase system. So similar that you can run three-phase loads off it.

Gunnar Englund

http://www.gke.org

 

[gorwell]

Thanks skogsgurra.
The incoming power is singlephase 230v + neutral.
Coils of motor are A1A2,B1B2,C1C2. Would I connect L1 to A1, Neutral to A2 (single leg of Y)?
And by doing this would 2 other phases be induced in B & C when motor is energised and overcranked?
If so would output power be the same on all phases?
I only want to generate in 1 phase at present.

 

[Skogsgurra]

OK, you will get one phase by connecting A1 to L1 and A2 to N.

The other two windings will produce phase shifted voltages with the same charateristics. Same voltage, same frequency, but shifted +120 and -120 degrees. Many single-phase to three-phase converters work just like that. All you need to do is connect the A2, B2 and C2 windings together to have a 400 V three-phase system available on A1, B1 and C1.

The extra phases will not have the same "punch" so you have to be careful when designing short-circuit protection. There is no guarantee that those phases have power enough to open the fuses if you get a short in the system.

Gunnar Englund

http://www.gke.org

 

[waross]

I would connect the motor in delta and insulate the third wire.
The motor winding connected across the line will provide rated current times line voltage for 1/3 the output of the motor rating.
The other two windings will act similar to an open delta transformer connection. The current will be the motor rated current. The resultant voltage of the two windings will be equal in magnitude and phase angle to the winding connected across the line. This will aid the first winding and the resulting output will be 2/3 of the motor output.
respectfully
A wye connection uses two windings inefficiently and does not use the third winding at all.
respectfully

 

[Skogsgurra]

Yes, that will utilize the motor better. And you can still get a three-phase system, albeit only at 230 V - not 400 V.

Don't understand the "insulate the third wire"? Wouldn't a classic delta work?

Gunnar Englund

http://www.gke.org

 

[waross]

Please forgive my poor wording.
When the motor is connected for delta, there will be three wires brought out for connection to the power source. Since this is a single phase installation, any two of the wires may be connected to the system and the third wire will not be used. I meant insulate it and forget it, rather than trying to find a place to connect it.
It is a classic delta connection.
I know that you know this Gunnar, but thank you for your tactful way of allowing me to explain myself to those who may have been confused by my wording.
Respectfully

 

[Skogsgurra]

It wasn't all that bad, Bill. ;-)

Gunnar Englund

http://www.gke.org

 

[gorwell]

Thanks guys, over next few days I will try it out.
More questions: the motor I will be using is as i said, 3 phase star delta. So on run the voltage in each phase is 400. Wiring as per suggestion means voltage at 230. does this mean that max power available will be (from 1 phase only) 230/400*max amps ? E.G from a 11 kw motor (ignore all losses) power available would be 230/400*11/3kw=2.1kw?
If so would it be better to get 230v delta wound motor (which in effect means getting a USA motor)
How about using a single phase motor?

 

[waross]

Power available from one phase will be (11kw/3phases x (230V/400V) = 2.108kw.
The other two phases will contribute an equal amount, for a total of 4.216 kw.
Don't count on a US motor being delta wound. Most of the motors that I see are star wound and with the common 9 lead motor, you cannot reconnect for delta.
How about an autotransformer?
respectfully

 

[gorwell]

Thanks waross. Is there any difference between an autotransformer and a transformer? I know the physical diff (2 coils or one) but is there a particular reason to use an autotransformer or is it cost/habit.
Being able to sort out some simple rules for people to connect small hydro to the grid will be a very good thing.It seems that most small scale producers/advisors are wedded to stand alone systems, which of course are useful, but for a vast number of potential producers being tied to the grid is actually far more useful: the grid soaks up unwanted power (and may even pay for it) which does away with complex ballast load systems, extra power is automatically fed in as necessary and the induction motor/generator is cheap, reliable and not mechanically damaged by overspeed. This forum saves C02.
Regards

 

[itsmoked]

gorwell; Autotransformers are smaller in the magnetic circuit and so are cheaper. They also tend to be stiffer.
No isolation is the trade-off.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Auto transformers.
The advantages are cost and size. An autotransformer for a 10% boost in voltage will be about 10% of the size of a conventional transformer. The actual size ratio will depend on whether the application is a step-up application, an autotransformer step-down, or a bucking connection step-down.
In regards to connecting to the grid. The main issues are related to the policies and requirements of the local power utility.
Typically, we ask an original poster for information as to the requirements of the local utility or refer the original poster to his local utility and then never hear back from them.
If any one ever posts the requirements of his local utility, I and others on the forum will enjoy suggesting different ways to meet the requirements. In the absence of information as to the requirements of the supply utility there is not much we can suggest.
respectfully

 

[gorwell]

In the UK there is a schedule whereby connecting to the grid at up to 16 amps per phase is pretty easy as long as simple protection and islanding prevention is used. This hopefully will encourage small generators, especially when it can be shown that using induction generators is cheap and reliable. Distributed generation, as long as standards are kept up, can only make the grid more robust.
As you say, anyone reading this must check with their local suppliers before making plans to coonect to the grid.
Regards

 

[waross]

I was recently reading some material concerning speed control of an induction motor by reduced voltage. The information indicated that while some types of single phase motors may be controlled by reduced voltage alone, the rotor excitation of a three phase motor is significantly different and both frequency and voltage must be reduced to control a three phase motor.
I suspect that there may be some differences in the ability to generate in island mode between three phase and single phase induction generators. I don't know this answer yet.
respectfully

 

[gorwell]

Using an induction motor/generator as a stand alone generator is quite common in small hydro setups but it requires capacitors etc and voltage is a bit suspect.
If I were to use a single phase motor instead of 3 phase this would cut out the voltage mismatch, correct? If so how would this be connected? Would I ignore/disconnect all start/run capacitors and just connect live and neutral to the main winding, or could I just connect the motor as a motor in the normal way and then crank it up to overspeed to get it to generate?
Single phase motors over 1kw or so can be expensive. Will two or three locked to each other (with a timing belt type drive, non slip) generate OK in paralell?

 

[waross]

I am not sure that a single phase motor will generate in island mode.
When most single phase motors are up to speed, the centrifugal switch has disconnected everything but the run winding. You just need the main windings to generate.
A 7.5 Hp. three phase delta wound motor will probably give the same output as a 5 hp. single phase motor.
For three phase motors generating single phase use delta motors.
Single phase motors in parallel, I don't think you need a timing belt. A vee belt should be good.
respectfully

 

[gorwell]

I`m not concerned about island mode: I can live with the odd power cut.
My take on it is that a 3 phase 400v delta motor 11kw run at 230v will provide 2.1kw accross 1 phase and another 2.1kw accross the other two phases. Using the 2.1kw from two other phases is problematic. With the first phase you are locked to the grid (supply is only sp) so any excess power is fed into grid and any excess needed is taken from the grid. In order that the other two phases can be used they will need a constantly monitored load tailored to the available output (output power from a waterwheel is only reasonably constant).
Using a single phase motor should overcome these problems.
Regards

 

[waross]

The maximum power output without overheating will be 2.1 kw + 2.1 kw. The actual power output will depend on the driving energy. The kw output will be almost equal to the power input. If the wind blows harder, the windmill will put out more power. If the wind blows too hard, the generator will put out too much power and will overheat.
respectfully

 

[itsmoked]

Wind huh?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com