Currents with frequencies close to or several times the power frequency (e.g. 150 Hz) can be driven by a standard induction motor to generate electricity with an AC generator with a suitable number of poles, but its output capacity is limited Medium frequency furnace.
Such devices should use high-speed rotation of a two-pole motor to keep the number of generator poles as small as possible and obtain the best output efficiency, but for general designs (as opposed to expensive turbine generators), the peripheral speed is limited to about 50*/second. This means that the maximum diameter of these motors is about 1/π=0.32 meters, thus limiting the output capacity.
As the frequency increases, the pole spacing also becomes very narrow. Small motors with a capacity of 400 can be made into salient poles, but for high-power and high-speed motors, the induction principle is used. The induction AC generator does not have a rotor sintering group, but a simple tooth structure as shown in Figure 7.3. Since the local magnetic winding and the AC winding are both on the stator, there is no centrifugal force acting on it.
The rotor can be made of a laminated surface of very durable perforated silicon steel sheets. The circumferential speed is allowed to increase to 92*/sec and the maximum diameter to 0.6 m (2930 rpm, i.e. 307 rpm/g). Since the output is proportional to D, this increases the output capacity by a factor of 3.4, other conditions being equal.
For several years, induction alternators have been constructed as unipolar motors () in which the rotor flux is bisected axially and the flux lines of the central field winding pass unidirectionally through the air gaps. This unidirectional flux is modulated by the teeth of the rotor and directed by the AC windings on the stator. These motors are obsolete, although they can still be found in production today. They are bulky, non-standard motors and have long time constants due to the pure iron core in the magnetic circuit, and have therefore been replaced by multi-pole motors.
Multi-pole induction alternators are similar in construction to ordinary AC motors, although they differ in the toothed rotor of the slot machine and the squirrel-cage induction group. The vertical rotor of the single-rotor motor is supported between two bearings, and the stator is made of stampings and installed in the combined rotor of the generator set in a standard frame. This frame is a structural part and therefore does not affect the magnetic flux.