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?
Best Answer
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
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.