What is the synchronous 3-phase motor

Synchronous motor



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Three-phase current

The voltage curve of a sinusoidal alternating voltage is usually not plotted against time in graphical representations, but against the number of degrees. As we will see in the chapters on generators, this results from the uniform rotary motion of the rotor of such machines:

Illustration 1:
Voltage curve of a sinusoidal alternating voltage:
One period results in 360 °.
Three-phase current, too Three-phase alternating current or Three-phase alternating voltage called, consists of three individual alternating voltages with the same frequency and a phase shift of 120 degrees each (= 360 ° / 3):

Figure 2:
3-phase three-phase current:

The three individual voltages are carried in separate wires with a common ground line ("negative pole"). With three-phase current, the individual voltages do not overlap. This superimposition (short circuit) would result in a voltage of zero volts at any point in time. A single live line of such an arrangement is also referred to as a phase.

Three-phase synchronous motor

Electric motors can be operated with the three phases of the three-phase current. For this purpose, each phase is connected to a pair of coils (or a single coil), which creates a magnetic field that changes in strength and polarity. The alternation of the magnetic fields creates a rotating magnetic field. In this, the magnetic poles "wander" around the motor axis. The electrically or permanently excited rotor is pulled along by this rotating magnetic field. The rotation takes place synchronous to the speed of rotation of the magnetic field. The speed of the motor is determined by the frequency of the three-phase current and the number of poles of the stator and rotor. If the frequency of the three-phase current is higher than the speed required by the design, the motor begins to tremble like a stepper motor that is driven too quickly and does not continue to rotate.
If a permanent magnet is used as the rotor, such a motor does not require any sliding contacts and therefore works almost free of wear. Such motors are called brushless.

Figure 3: (Start animation.)
3-phase synchronous motor:
With this arrangement of three pairs of coils and a two-pole rotor, the speed corresponds to the frequency of the applied alternating voltage. If 6 pairs of coils were installed, you would get half the speed.


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