Single-phase electric motors: principle of work, device
Single-phase asynchronous engines release from 5 W to 10 kW.
These engines are used: in the drive of washing machines, refrigerators, centrifuges, the tool-grinding and small processing machines, etc.
Let's note that single-phase in comparison with three-phase engines slightly worst technical characteristics usually have HELL. Power single-phase HELL is no more than 70% of power three-phase HELL in the same dimension. Single-phase HELL, besides, have lower overload capacity.
The scheme of inclusion single-phase HELL is presented on the figure 1.
The engine has on the stator two windings – the main (working) and starting which is used for start-up HELL. The rotor HELL is executed short-circuited in the form of the squirrel cage.
Let's consider the principle of work single-phase HELL.
To understand for what starting winding is necessary, we will review the example when the engine is connected to network of 220 V only on one winding - the worker.
The single-phase current of I1 of this winding creates pulsing magnetic field which can be spread out to Fa and Fv's two fields having equal amplitudes and rotating to the opposite sides with the identical speed.
At the motionless rotor magnetic fields of Fa and Fv create identical in size, but the turning moments of M1, opposite on the sign, and Sq.m. Therefore at start-up the resulting moment (Mn=m1-M2) is equal to zero, and the engine cannot come to rotation even without loading on the shaft.
In this regard for start-up single-phase HELL is also used additional starting winding which allows to receive the rotating magnetic field due to which the initial starting moment defining and the rotation sense of the shaft is provided.
The key diagram of the single-phase asynchronous engine is presented on the figure 2.
As is well-known from the theory of electrical machines, for obtaining the rotating magnetic field on the stator of the engine at least two windings displaced in space on the certain corner and streamline by alternating currents have to be located.
According to it starting winding keeps within on the engine stator with the shift of its axis for 90% in relation to the axis of the actuating coil, and the shift of currents is provided with inclusion in its chain of the additional phase-shifting element as which can be used: active resistor, inductance coil or condenser. Or starting winding is wound with the small amount of rounds in the opposite direction (bifilar).
Further the electric motor can work only on the actuating coil, this principle is applied in refrigerators where for start the starting relay is installed, after start starting winding is disconnected (figure 3).
There are schemes of connection in which starting winding remains in work and after start-up, such principle was applied in washing machines of the USAn production, and, besides, there is the possibility of work - the reverse, i.e. rotation in other party.
Both the electric tool and electrical household appliances belong to single-phase electric motors: drills, grinders, vacuum cleaners, trimmers (lawn-mowers), etc. which require rotation more than 3000 RPM and the maximum rotation of the electric motor with the frequency of 50 Hz is limited about 3000 RPM.
For effective operation of above-mentioned units of such turns it is not enough. Therefore single-phase collector electric motors with the number of revolutions per minute more than 3000 were invented.Top