# Why does an induction motor work as generator when running at a speed greater than synchronous speed?

We know that induction motor works as a generator that means they converts the mechanical energy it receives into electrical energy and this energy is received by the stator. For creating its own magnetic field it takes reactive power and supply active power. I can't understand what is the internal fact of this phenomenon? How does it receive reactive power and supply active power?

• Not an expert so leaving a comment. There is always residual magnetism in the cores of the rotor and stator windings. When the motor is spun like a generator, at some point the residual magnetism gives rise to an AC current in the rotor. Then it starts generating. The speed difference is due to "slip" which you can look up yourself. Slip just means that the rotating field in the stator does not rotate at the same speed as the rotor. The difference in speed gives rise to torque when used as a motor. – mkeith Nov 15 '14 at 16:33

When normally running from an AC supply, an induction motor runs at synchronous speed minus slip speed. The slip speed is determined by the amount of mechanical load attached. More load and the slip increases to allow the current induced in the rotor to rise and enable more power to be provided to the load.

If you were able to spin the motor and spin it at exactly synchronous speed the current taken by the stator would be zero (apart from the ever-present magnetization current). The voltage induced in the rotor would also be zero.

So we have the scenario that (ignoring magnetization currents), the stator current is zero at synch speed and rises (almost linearly) to some "full value" at maximum mechanical load.

Why should it be a surprise that spinning the motor at faster than synchronous speed (mentioning "sync-speed" absolutely implies it is connected to an AC supply of course) it becomes a generator?

Answering a bit more, the reactive "power" in the rotor does not contribute to output power when driven as a generator. Output power (into say a 3ph supply) is input mechanical power minus generator losses. The magetization currents in rotor and stator are just a means to an end.

• I still don't get how a rotor can induce current in the windings when it is not made with permanent magnets (i.e. if you were to spin the motor without it being powered). Perhaps, when the rotor is energize it behaves like an electromagnet? – user148298 Apr 7 '17 at 5:40

If you think of an induction motor as a transformer you will notice that the primary coils are in the stator and the secondary coils are in the rotor.

The secondary coils are shorted out so that current can flow within those coils by the induced current when an AC current is applied to the primary coils in the stator. This current in the secondary coils produces a magnetic field. Because of the inductance of the secondary coil, this magnetic field resists change.

When you apply a force slowing down the rotor, due to this resistance of the magnetic field to change, the magnetic field will lag behind and therefor the magnetic field in the stator will pull the magnetic field in the rotor producing a force against the force trying to slow down the rotor.

If you attempt to speed up the rotor, the exact opposite will occur. The magnetic field will then lead the magnetic field in the stator. this will induce a current in the stator which is in the direction that the current in the stator already is going. The harder you push the rotor, the more this current will be. This current results in the generation of an alternating current.

My answer is a bit different. The reason a motor can drive something is that the two magnets on each pole are slightly "to one side" so rotation with torque results. It you force the two magnets to be in the opposite relationship, the driven leading the driver, the magnetic field is pushed in a way that ramps the voltage up as the two poles come nearer to being centered over one another. Sort of like a spark coil.

Because the device is synchronous, that condition is never reached. The two are held in relative position by the magnetic opposition of the two like poles.

Power fed into the stator tries to bring the armature up to speed. At synchronous speed, no work can be done. Spinning the armature faster causes the stator to try to slow the armature back to synchronous speed. It does that be producing power and shoving it into the grid which is running at a lower voltage. The grid is a large sink. It takes work to maintain this condition so it is a 'generator'.

You can inject DC into the stator and generate power which is dumped as DC into a resistor. This is how AC motors can be stopped quickly. A radial arm saw motor is the most accessible implementation of this. It you disconnect the resistor it slows down like a normal motor instead of very quickly. After turning off the motor it coasts then the resistor cuts in with dramatic effect. It generates power which is used to oppose it's own rotation.

• Your last paragraph is not correct. You stop an induction motor by injecting DC current into the stator coil. Simply putting a resistor across the stator windings does nothing to help the motor slow down. – Dwayne Reid Mar 24 '15 at 23:21