Synchronous motor

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A synchronous motor is a single-phase or three-phase synchronous machine in motor operation , in which a constantly magnetized rotor (rotor) is synchronously driven by a moving magnetic rotating field in the stator . The running synchronous motor has a movement that is synchronous to the AC voltage. The speed is linked to the frequency of the alternating voltage via the number of pole pairs .

Movement diagram of a three-phase synchronous motor.

Layout and function

The field in the rotor is generated by permanent magnets ( self-excitation , e.g. magnetized ferrite cylinder as rotor) or external electromagnetic excitation (with field coil on the rotor, power supply via slip rings). The stator coils are sometimes operated by a frequency converter with a suitable, controlled alternating current , especially in large synchronous machines . This means that variable-speed drives with high performance can be implemented.

In normal operation, the synchronous motor does not slip . When loaded, the rotor magnetic field lags behind the stator magnetic field by a certain angle ( rotor angle ), which increases with increasing load. However, this is only possible up to a maximum moment at which the angle is 90 °. If the load torque exceeds this breakdown torque , the rotor stops.

When switched on, the stator rotating field immediately rotates with the synchronous speed. However, the runner needs some time to accelerate due to its moment of inertia . Therefore, a synchronous motor needs a starting aid, e.g. B. a starting cage. This is a short-circuit cage in the rotor through which the motor as a three-phase asynchronous machine starts up to synchronous speed. If the rotor almost reaches the synchronous speed, the excitation current of the rotor winding is switched on so that the rotor is drawn into the rotating stator field.

The direction of rotation of the motor is specified by the stator rotating field; two phases must be swapped for a change of direction .

Every permanent magnet synchronous motor can also work as a synchronous generator. Examples are bicycle and motorcycle alternators. Separately excited synchronous machines are used in power plants, generating sets and as a car alternator .

Synchronous motors can be operated with single-phase alternating current or with three-phase current (see also three-phase synchronous machine ). Two-phase synchronous motors are also found less frequently.

Advantages and disadvantages

One advantage of synchronous motors over commutated DC motors is that there is no need for the commutator carrying the operating current - only the significantly lower excitation power has to be transmitted to the rotor with slip rings; These are also omitted for permanently excited motors. This eliminates the wear and tear of the brushes and increases the efficiency.

One advantage of the synchronous motor over the asynchronous motor is the rigid coupling of the speed and the angular position to the operating frequency. Synchronous motors are therefore suitable for actuators and other applications in which a load-independent, stable speed is required. In addition, permanent magnet synchronous motors are more compact and efficient than asynchronous machines, especially for smaller machines. Phase shift operation is also possible with a three-phase synchronous motor . The disadvantage is the more difficult self-start on the three-phase network. One way to avoid this disadvantage is to install an additional squirrel cage in the rotor so that the motor can start up as an asynchronous motor.

Typical for synchronous motors are unwanted mechanical torsional vibrations of the rotor, which can be excited by uneven loading or energization. They can lead to the overturning torque being exceeded and cause an uneven torque. They are avoided with short-circuit windings (short-circuit cage or damper windings around the rotor poles). The rotor position is usually recorded for operation on the converter .

Single-phase synchronous motors

Synchronous motor with gear (turntable drive of a microwave oven ), diameter 50 mm

Single-phase synchronous motors need a start-up aid in order to “take step”, but permanent-magnet single-phase motors often start up by themselves in an undefined direction due to oscillating movements. Examples of this are small water pumps (lye pumps and aquarium pumps) and lemon squeezers.

For small drives there are also synchronous motors whose rotors are not magnetized and have teeth to concentrate the magnetic field of the (also toothed) stator. In principle, these are similar to reluctance motors . You also need start-up help.

Synchronous motor of a mechanical time switch

Miniaturized synchronous motors for synchronous clocks are small and start up automatically in the right direction. In the 1970s and 1980s, the motors were equipped with up to 25 pole pairs to increase their smoothness. The stator and the permanent magnet rotor therefore have up to 50 poles. They rotated at only 120 / min instead of 3000 / min. Today's synchronous watch motors have a lighter rotor and lighter gears with small teeth to improve smoothness. Although the field winding only generates one pair of poles, teeth and gaps in the stator lamination cause magnetic flux of different strengths that are sufficiently different to replace real complementary poles. That is why teeth and gaps together make up the number of poles in the permanent magnet rotor.

Single-phase synchronous motors can be found in a large number of small drives that require constant speed or a simple design:

Two-phase synchronous motors with auxiliary phase

Two-phase synchronous motors are often used to replace the less efficient capacitor motors. They have a better starting behavior and starting torque than single-phase synchronous motors and allow a defined direction of rotation and a reversal of the direction of rotation. Application examples are:

  • Pump drives
  • Valve actuators

Three-phase synchronous motors

With the development of suitable sensorless power electronic control, three-phase synchronous motors are also increasingly used for lower powers, for example as actuators. They have the advantage of a defined rotor position with high dynamics, high torque and high efficiency. Today they are the most important form of implementation of electric vehicle drives.

Brushless DC motors

Small permanent magnet synchronous motors with switching electronics are often referred to as brushless direct current motors , English brushless direct current with the abbreviation BLDC . The coil strands of the stator are controlled by a four-quadrant controller. The electronics for controlling the bridge is a regulated frequency converter .

See also


  • Gerd Fehmel, Horst Flachmann, Otto Mai: The master's examination in electrical machines . 12th edition, Vogel Buchverlag, Oldenburg and Würzburg, 2000, ISBN 3-8023-1795-5
  • Gregor D. Häberle, Heinz O. Häberle: Transformers and electrical machines in power engineering systems . 2nd edition, Verlag Europa-Lehrmittel, Haan-Gruiten, 1990, ISBN 3-8085-5002-3
  • Günter Springer: Expertise in electrical engineering . 18th edition, Verlag Europa-Lehrmittel, Wuppertal, 1989, ISBN 3-8085-3018-9

Web links

Commons : Synchronous Motors  - Collection of pictures, videos and audio files

Individual evidence

  1. Comments on three-phase synchronous machines