Can one use a 3 phase motor as a 3 phase generator?
A rotary phase converter gets a normal 3 phase motor spinning using essentially 2 phases by using an artificially generated second phase via a capacitor, and the motor generates the third phase.
So if a 3 phase motor has one phase mains energised and is then spun up to synchronous speed will it generate the two other phases? (I appreciate that a normal 3 phase generator would have an armature that is a DC magnet spinning in the three stator coils).
If it's an induction motor then you have to supply the magnetizing VARs externally. This can be in the form of the mains and its "infinite bus*". If you try to run the motor faster than synchronous speed the it'll generate.
When you use two phases and a capacitor you are supplying the magnetizing VARs to the third phase from the capacitor. Voltage regulation isn't as good, but it'll work.
If you run a single phase motor as a generator, the mains is still providing the magnetising VARs, but there are larger circulating currents in the rotor squirrel cage windings... it'll still generate.
If you throw away the squirrel cage rotor and put your own wound rotor in with a simple voltage regulator feeding the windings, then you have a large noisy lump in the garden!
People in the business talk about electrical machines rather than motors or generators because the only difference is whether you take mechanical power out or put it in.
An infinite bus is an electrical circuit in which the voltage and frequency are unaffected by the amount of current you take out or put in to it.
Don't know if that has muddied things up more or not.
Andrew, The best motor to convert to a generator is the slip ring type AC motor, found in overhead cranes etc. These can be easily excited and controlled by stuffing DC into two of the three slip rings. Can't remember off hand what's needed, 90 volts seems to ring a bell.
Try some of the rewind people as they aren't a common motor but they are the best.
Although the mains is often referred to as an infinite bus, and it's true that you won't change it's frequency unless they are supplying your area from a portable generator while maintaining the network, you can all too easily change it's voltage as can be seen by the lights dipping when you start a lathe etc. I suppose the best way to describe the mains is an infinite bus with an impedance in series with it.
That's essentially what a rotary converter is, once the start cap is disconnected you are feeding one phase in and getting three phase out, although they usually have running capacitors to balance the voltages and improve the power factor. Note however that if you want 400V out you have to feed 400V in so have to use a step up transformer, this is because the input is applied across two phases of the motor not a phase and the star point, if indeed there is one.
Reading it again, it would be best desribed as a motor having two phases applied from the mains and generating the third phase, or putting it another way it's working like a three phase motor on a three phase supply that has lost one phase, yet keeps running. Greg
Not quite - it has to be beyond synchronous speed before it it behaves as an induction generator.
The normal squirrel cage 3 phase induction motors work because the rotor speed is slightly lower than the synchronous speed of the rotating magnetic field. This generates large currents in the rotor bars at this difference frequency and this provides the motor output torque.
If it is mechanically driven to reach synchronous speed the difference frequency falls to zero so that no current is induced in the rotor bars. The machine is now passive - it neither absorbs or generates power.
If the machine is mechanically driven ABOVE synchronous speed large currents are again generated in the rotor bars but this time the torque generated opposes the input drive. The magnetic field from the rotor rotates at rotor rotation frequency minus the difference frequency = synchronous frequency and this generates power which is fed back into the supply. i.e although the rotor speed is slightly above synchronous speed it feeds back power at supply frequency.
If we plot shaft torque against shaft speed we get an "S" curve lying on its back. Torque passes through zero at synchronous speed and rises to a maximum at rotor speeds just below and just above synchronous speed - absorbing power below - generating power above.
Strictly speaking the above applies to a 3 phase machine connected to a high power 3 phase supply. However, if only one phase is connected, the machine will also act as a rotary 3 phase converter. Three phase power will be available and power will be fed back to the single energised phase.
Note - this only works because it is connected to a relatively high power supply. This defines the synchronous speed and permits a stable system.