Synchronous motors are a type of alternating current motor in which the rotation of the shaft is synchronized with the frequency of the feed stream; the rotation period is exactly equal to an integer number of cycles CA. Its rotation speed is constant and depends on the frequency of the mains voltage to which it is connected and the number of pole pairs of the motor, this speed being known as "synchronous speed". This type of engine has electromagnets in the stator of the motor create a

Rotating magnetic field in time synchronicity at this speed.

The mathematical expression that relates the speed of the machine with the above parameters is:

n = \ frac {60 \ cdot f} {P} = \ frac {120 \ cdot f} {p}

where:

**f: frequency of the network that is connected machine (Hz)**

**P: Number of pole pairs having the machine**

**p: Number of poles having the machine**

**n: Synchronous speed of the machine (RPM)**

**For example, if a machine has four poles (two pole pairs) connected to a 50 Hz grid, the machine will operate at 1500 rpm.**

They work much like an alternator. Within the family of synchronous motors must distinguish:

**Synchronous motors.**

Asynchronous motors synchronized.

The permanent magnet motors.

Synchronous motors are called so because the rotor speed and the speed of the stator magnetic field are equal. Synchronous motors are used in large machines with a variable load and need a constant speed.

**Braking phase synchronous motor**

Generally, the desired speed of this engine is adjusted by a rheostat. The synchronous motor, when it reaches the critical torque stop, not being the most orthodox way to do it. The critical torque is reached when the engine load exceeds the allocated torque. This causes overheating which can damage the engine. The best way is to vary the load until the current consumption of the network as low as possible, and then disconnect the motor.

Another way, and most common, is regulating rheostat thereby vary the intensity and can disconnect the motor safely.