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3 phase induction generator


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Hello all.

My name is Andy. I am a final year Bach' Eng Technology student at MIT in New Zealand.

I am presently developing a 3 phase induction generator from the commonly available 3 ph motor. I have the machine working ok and supplying a load stand alone. ie, disconnected from mains. Excitation being provided from a parallel connected set of capacitors. The whole scheme needs a small amount of residual magnetism in the core (rotor) to initiate charging, and commence generation.

Having now overloaded the machine numerous times and carried out locked rotor tests, I have managed to damage this essential residual magnetism. I have done numerous starts on the m/c with the rotor in the same position to restore the field, but the out put of the machine has slowly died away with successive overloads.

Question: Can anyone tell me how to restore the residual magnetism of the core to its original strength?:)

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Hi Andy!


Whilst familiar with the concept of converting a standard 3ph motor to an alternator as per your description, I have always believed that any residual magnetism could not be relied upon to 'start' alternation. In general, good quality motor rotors with reasonably pure 'soft iron' rotor laminations will not retain any permanent magnetism.


I believe such systems requires the motor to be run up to a speed (near) sychronous with an available supply (single phase, connected to only two phases will do), then connected to said supply, followed by connection of the capacitors you describe to provide reactive power, followed by disconnection from the supply. The result is a free running asynchronous generator (the terminal Voltage being controlled by the load and value of the capacitors, but limited by iron saturation).


BTW, if the capacitance is below a 'critical' value, the Voltage will decay to zero. The capacitors must be able to supply AT LEAST as much reactive power as the machine absorbs when running off mains as a motor.


Does this help?





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Because the steel used in the rotor has a grain, there is the potential for it to have a small residual magnetism. This is dependant on the grain orientation when the rotor was assembled. If the laminations are pressed out of the same sheet with the same die, and are not rotated or randomised before assembly, there will be a bias in the grain orientation.

To restore the magnetic field, I would suggest that you need to lock the rotor in one place and provide a strong DC magnetic field to the stator by passing a high current DC through the stator windings. Probably best achieved by a high power DC pulse stream. Applying AC to the stator will degauss the rotor rather than create residual magnetism.

I would suggest a cpacitor bank charged up, and then discharged across a stator winding through a rectifier, could do the trick.

Best regards,

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  • 11 months later...

Hello, Winding heating would be a concern, but how about using a dc welder ( the type with coarse and fine current controls ) to supply the dc required ??

Steve ;f;;p;

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