# Electric motors

Nowadays there is a huge variety of different types of electric motors. In plain words they are divided into alternating and direct current motors (there are universal ones as well) and AC motors can be divided into synchronous and asynchronous (induction) motors. DC motors, on the other hand, can be divided into brushed and brushless. We will discuss briefly each of the four types of motors emphasizing on their application.

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##### Synchronous machine

Rotating electromagnetic field between the poles of an electric  machine

The AC synchronous electric machine is most frequently used as a generator (also called alternator). It is called synchronous because the rotating magnetic field generated by the stator winding is synchronous with the frequency of rotation of the rotor. A characteristic of the synchronous generator is that there is DC excitation. This excitation current is fed to the exciting coil, which is in the rotor, through slip rings and brushes. The current induces a rotating magnetic field in the rotor, where there are permanent magnets and it starts rotating. The magnetic field of the rotor can be generated by induction, by permanent magnets or by DC rotor exciting coils. Electric current from the magnetic field of the rotating rotor is induced in the stator windings. Normally the stator windings are three and they are symmetrical in order to create a three-phase voltage.
The synchronous machine is used as a motor through the use of powerful key transistors of the IGBT, MOSFET type and thyristors GTO, IGCT, SGCT having the ability to switch-off by a control electrode, when excess power is needed.

##### Asynchronous (induction) motor

Components of the induction motor                                                                                                                              Induction motor with a squirrel-cage rotor

The AC induction electric machine can be single-phase or three-phase and the three-phase one can operate as a single phase one. It is most commonly used as a motor. It may have a squirrel-cage or a wound rotor. The feeding of the stator coil induces magnetic field in the rotor and arouses forces whose vectors are tangential (the core force), radial (directed towards the axis), and when the rotor is canted – axial (it is used in electric hoists, in blackouts they also serve as an emergency brake). In a linear induction motor, the stator and the rotor are unrolled and the radial component repels the rotor from the stator creating levitation and the tangential one generates the movement speed. It is used in high-speed trains of the Maglev type.
The slippage of the magnetic field of the rotor with respect to the magnetic field of the stator is called slip. When the induction motor is started, it causes the occurrence of large currents that are compensated through different constructive decisions.
The asynchronous machine is used as a generator comparatively rarely.

With three-phase asynchronous motors, when switching the positions of two of the phases, the direction of the motor rotation changes.
The single-phase motor consists of stator and starter coil. The two coils are phase offset by 90 degrees. A suitable starting capacitor (approx. 70 microfarads per kw) responsible for the rotation of the rotor is connected to the starting coil. There are two-phase and polyphase induction motors but they have only limited application.

##### DC electric motor

Principle of operation of the DC motor                                                                                                                                                                               Components of the DC motor

These are the first widely used motors. The name shows clearly that these motors are fed with direct current. They are divided into brushed and brushless. Brushed motors are equipped with brushes and a commutator. Very often the brushed DC motors are replaced by induction motors, due to the relatively rapid wear of their brushes. Brushless motors are another option. They operate on the principle of frequency adjustment and auto-synchronization by modifying the vector of the stator magnetic field according to the position of the rotor. Their switching is done by electronics. DC motors have the highest torque of all electric motors, at low rotation speeds and the same size and weight. Nowadays there is rapid development of brushless DC electric motors. In order to get different ratios between speed and torque, different modes of connection are used: series-wound, shunt-wound, and compound-wound. DC motors with speed reducers are used to change the torque. The result is higher efficiency.
Universal motors

Universal motors can operate on both AC and DC power. They are most effective at a reduced frequency (about 25 Hz). Speed control of universal motors is done by thyristors and transistors. At normal frequency a universal motor cannot achieve the efficiency of a pure DC motor. Its advantage is that its speed is easily adjustable and for this reason this type of motor is often used in hand electric tools.

##### Other motors

There is an enormous variety of different types of electric motors, depending on their various structural features, number of phases, and their applications. Some of them are the linear (unrolled) motors, stepper motors, servomotors, Schrage-Richter motors, etc.