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Induction motors do not have magnets in them, instead the magnetic field the outer coils generate creates electricity, and subsequently magnetic field, in the inner coil. This means those motors need no brushes as the inner coil is not connected to anything.

I was wondering, purely academically, if you could prime the motor with field first, by putting current through the outer coil and then spin it to generate electricity. Assuming you'd spin it correctly, I imagine the magnetic fields would maintain themselves using power absorbed from the motion until the system came to a halt.

Is this possible or nonsense?

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Induction motors make very good generators. They are the typical generator used in wind turbines for instance.

Let's look at the speed torque curve. Here's one from electrical4u.com

enter image description here

Speeds in the 0 to ns, standstill to synchronous, are normally the only speeds people consider.

Normally, an induction motor operates at full load with a few percent slip. Consider what happens as the torque delivered by the motor drops, the slip reduces, and less energy is drawn from the supply. As the torque drops to zero, the speed rises to synchronous speed, and no power is drawn from the supply.

If we now increase the speed above synchronous, driving the motor externally, torque is required to turn the motor, and energy is delivered to the supply.

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  • \$\begingroup\$ Thanks, didn't expect it to be this simple. How is the current extracted? I assume it's a DC current, so you can't just have diodes there. \$\endgroup\$ Nov 19 '19 at 15:46
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    \$\begingroup\$ The current is AC. Consider what the current would be when running as a motor with, for instance, 3% slip, and call this Im. Now operate as a generator running with -3% slip, or let's call it 3% shove! Neglecting losses, the generated current is -Im. Note this requires a stiff voltage supply, the generator is operating to push current into the grid. \$\endgroup\$
    – Neil_UK
    Nov 19 '19 at 15:51
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    \$\begingroup\$ Look up doubly fed induction generator. That is what is also called a type III wind turbine generator \$\endgroup\$ Nov 19 '19 at 17:17
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Induction motors can be used to generate power, but it is only simple if you are feeding power to the grid. In that case, the field excitation comes from the grid.

There are instructions on the internet explaining how to use capacitors. I believe you must first drive the motor to the proper speed without a load connected, Then you charge the capacitor(s) and connect them to the motor. Then connect the load. To get the proper voltage and maintain generator operation, the capacitor value must be selected to suit the motor and the load. Some sources on the internet give curves for capacitor selection. This is a method used for some home power projects. It is not suitable for use as a reliable power source.

For utility wind generation using induction motors, the generator is connected to a regenerative inverter. Power from the regenerative inverter is rectified and connected to a grid-tie inverter. Doubly-fed induction generators are more widely used. They are three-phase, wound-rotor machines with inverters connected to both the stator and rotor windings.

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