The principle of synchronous motors

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Synchronous motors belong to AC motors, and their stator windings are the same as those of asynchronous motors. Its rotor rotates at the same speed as the rotating magnetic field generated by the stator windings, which is why it is called a synchronous motor. For this reason, the current in a synchronous motor leads the voltage in phase; that is, a synchronous motor is a capacitive load. For this reason, synchronous motors are often used to improve the power factor of power supply systems. Synchronous motors generally come in two structural types: 1. The rotor is excited using direct current. The rotor of this motor is shown in Figure 1. As can be seen from the figure, its rotor is of the salient-pole type; the magnetic field coils mounted on the pole cores are connected in series, resulting in alternating opposite polarities, and there are two leads connected to the two slip rings mounted on the shaft. The magnetic field coil is excited by a small DC generator or battery; in most synchronous motors, the DC generator is mounted on the motor shaft to supply the excitation current to the rotor pole coils. Since this synchronous motor cannot start automatically, a squirrel-cage winding is also installed on the rotor to serve as a means for starting the motor. The squirrel-cage windings are arranged around the rotor, and its structure is similar to that of an asynchronous motor. When a three-phase AC power supply is applied to the stator windings, a rotating magnetic field is generated within the motor. The squirrel-cage windings cut through these magnetic flux lines, resulting in induced currents that cause the motor to rotate. After the motor starts rotating, its speed gradually increases to a value slightly lower than that of the rotating magnetic field. At this point, the rotor’s magnetic field coils are excited by direct current, resulting in the formation of magnetic poles on the rotor. These magnetic poles attempt to keep up with the rotating magnetic poles on the stator, thereby increasing the speed of the motor’s rotor until it rotates in sync with the rotating magnetic field. 2. Synchronous motors whose rotors do not require excitation: Synchronous motors with unexcited rotors can be used with single-phase power supplies as well as with multi-phase power supplies. In this type of motor, one variant has a stator winding similar to that of a split-phase or multi-phase motor, along with a squirrel-cage rotor whose surface is cut into planes, as shown in Figure 2. It is therefore a salient-pole rotor, whose magnetic poles are made of a type of magnetized steel and can maintain their magnetism over time. The squirrel-cage winding is used to generate starting torque, and when the motor reaches a certain speed, the rotor poles synchronize with the current frequency of the stator coils. The polarity of the salient poles is induced by the stator; therefore, their number should be equal to the number of poles on the stator. When the motor reaches its intended speed, the squirrel-cage windings cease to function, and rotation is maintained by the rotor and magnetic poles following the stator’s magnetic poles in a synchronized manner.