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Wound Rotor Induction Motor As Generator


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#1 tazil

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Posted 01 May 2007 - 02:08 AM

Dear Members

I am currently doing a project on Wound rotor induction generator (3 phase, slip ring, 415V, 50Hz, 2.2kW). As I am from Fiji, I currently cant find this motor locally. Would anyone please help me locate a manufacturer or distributor who makes this motor.

Secondly, I am suppose to use this motor in generator mode to generate atleast 240V/phase. I know l'll have to provide power to rotor via slip ring. Would it be possible to use an inverter and 12V car battery to supply the rotor and generate 240V from stator?

Regards

#2 marke

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Posted 01 May 2007 - 08:10 AM

Hello tazil

Welcome to the forum.

I believe that there are some mixed concepts in your requirements.
An induction generator uses an induction motor connected to a supply and overdriven to generate power back into the supply. The induction motor is usually a standard cage induction motor, but could be an would rotor motor with the rotor shorted.

An alternator is like an induction motor, it has a winding on the rotor which is connected to an excitation source to provide the rotating magnetic field that generates the voltage.
The slip ring motor has a three phase winding and the alternator has a single winding.

If you are looking to generate power where there is an existing supply available and you wish to suppliment this, then an induction generator can be used. This can be a wind generator, pelton wheel and similar driving sources. There is no need for voltage regulation, but there is usually a mechanical override in case the "wind is too strong" and an electrical disconnect if the wind is not strong enough.

If there is no existing supply, or the generator is required to act as a standby supply, then you need to use an alternator. This will require excitation which is controlled to regulate the output voltage. It will also require speed control on the driving source to regulate the frequency.
The excitation is often taken from the output of the alternator (self excited) and rectified. At startup, the residual magnetism in the rotor is sufficient to generate a low output voltage. This is rectified and fed back into the rotor to increase the rotor field and output voltage. This intern increases the output voltage etc. Once the output voltage reaches the required voltage, the excitation is limited. Any change in load, will cause a change in the output voltage. The excitation needs to be adjusted to correct the output voltage. This is done by an AVR.
External excitation can be done using a controlled DC bias . The level of excitation is controlled by an AVR.

The self excited system is cheaper than external excitation, but suffers from very poor overload capacity.

Best regards,

#3 tazil

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Posted 01 May 2007 - 10:11 PM

Thank you very much for the quick reply.

Yes, the project is for a wind energy conversion system. I apologize for providing less details earlier on. I am suppose to be using the Doubly-Fed Induction Generator (3phase stator, 3 phase rotor) system whereby the 3 phase rotor windings is connected to an AC-DC-AC back-to-back PWM converter. Usually, this system is grid connected so rotor excitation is provided from the grid. The converter control leads to constant voltage and frequency output.

However, due to budget constraints, I am unable to get a back-to-back converter so have to look on to other methods for excitation. External excitation has to be used because the project specifies use of this generator in an isolated case such as a remote village with no power grid. Loading will be an issue so self-excitation falls short. Thats why I was asking if a 12v car battery (which could be charged by solar panels) could be used with a 12v DC to 240v AC inverter to energize the rotor.

Regards
Tazil

#4 jraef

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Posted 02 May 2007 - 02:48 AM

You are getting caught up in semantics. Look for a synchronous generator. It has the separate slip-ring fed excitation winding you are looking for.
"He's not dead, he's just pinin' for the fjords!"

#5 marke

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Posted 02 May 2007 - 10:34 AM

The Doubly-Fed Induction Generator is designed to connect to the AC supply at both ends. The sliprings are connected to the AC supply via a 4 quadrant inverter which is essentially an active front end inverter system with some clever control algorithms. This control system also controls the pitch of the blades.

The advantage of this system for wind generation, is that it will still function if the blades are running at the wrong speed, but I believe that it still requires the AC supply to operate.
If you are looking for a stand alone system, then I think that you need to consider a more standard approach with speed and voltage regulation.
As you are only looking at around 2.2KW, I would suggest that you use a DC output alternator at a suitable voltage, perhaps 48 volts, and you can use the wind generator to charge 48 volt batteries, and then a solid state inverter to step up to 230Volts with a transformer. You could also use solar cells to keep the batteries topped up during the day.

If you went a stage further and used 300VDC, you could use a standard single phase input VFD for motor control, suitably over sized with a 1:1 wound transformer with the primary windings delta connected and the secondary windings star connected to give you a three phase 400 volt MEN output.

I think that this would be more practical than the Doubly-fed Induction Generator with no supply.

Best regards,




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