Difference Between Synchronous Motor and Induction Motor

Synchronous Motor vs Induction Motor

Both Induction motors and synchronous motors are AC motors used to convert electrical energy to mechanical energy.

More about Induction Motors

Based on the principles of electromagnetic induction, the first induction motors were invented by Nikola Tesla (in 1883) and Galileo Ferraris (in 1885), independently. Because of its simple construction and rugged use and low construction and maintenance costs, induction motors were the choice over many other AC motors, for heavy equipment and machinery.

Construction and the assembly of the induction motor are simple. The two main parts of the induction motor are the stator, and the rotor. Stator in the induction motor is a series of concentric magnetic poles (usually electromagnets), and the rotor is a series of closed windings, or aluminium rods arranged in a way similar to a squirrel cage, hence the name squirrel cage rotor. The shaft to deliver the torque produced is through the axis of the rotor. The rotor is placed within the cylindrical cavity of the stator, but not electrically connected to any external circuit. No commutator or brushes, or other connecting mechanism is used to supply current to the rotor.

As any motor, it uses magnetic forces to rotate the rotor. The connections in the stator coils are arranged in a way that opposite poles are generated at the exact opposite side of the stator coils. At the start-up phase, magnetic poles are created in a periodically shifting manner along the perimeter. This creates a change in the flux across the windings in the rotor and induces a current. This induced current generates a magnetic field in the rotor windings, and the interaction between the stator field and the induced field drives the motor.

Induction motors are made to operate in both single and poly-phase currents, latter for heavy duty machines which require a large torque. The speed of the induction motors can be controlled by either using the number of magnetic poles in the stator pole or regulating the frequency of the input power source. The slip, which is a measure to determine the motor’s torque, gives an indication of the motor efficiency. The short-circuited rotor windings have small resistance, resulting in a large current induced for small slip in the rotor; therefore, it produces a large torque.

At the maximum possible load conditions, for small motors slip is about 4-6% and 1.5-2% for large motors, hence induction motors are considered to have a speed regulation and are considered constant-speed motors. Yet the rotational speed of the rotor is slower than the input power source frequency.

More about Synchronous Motor

Synchronous motor is the other major type of AC motor. Synchronous motor is designed to operate without any difference in the rotation rate of the shaft and the frequency of the AC source current; the period of rotation is an integral multiple of AC cycles.

There are three main types of synchronous motors; permanent magnet motors, hysteresis motors and reluctance motors. Permanent magnets made of neodymium-boron-iron, samarium-cobalt, or ferrite is used as the permanent magnets on the rotor. Variable-speed drives, where the stator is supplied from a variable-frequency, variable-voltage is the main application of permanent magnet motors. These are used in devices which needs precise speed and position control.

The hysteresis motors have a solid smooth cylindrical rotor, which is casted of a high coercivity magnetic “hard” cobalt steel. This material has a wide hysteresis loop, that is, once it is magnetized in a given direction, it requires a large reverse magnetic field in the opposite direction to reverse the magnetization. As a result, the hysteresis motor has a lag angle δ, which is independent of speed; it develops constant torque from startup to synchronous speed. Therefore, it is self-starting and doesn’t need an induction winding to start it.

Induction Motor vs Synchronous Motor

• Synchronous motors operate at synchronous speed (RPM=120f/p) while induction motors operate at less than synchronous speed (RPM=120f/p – slip), and slip is almost zero at zero load torque and the slip increases with the load torque.

• Synchronous motors require DC current to create the field in the rotor windings; induction motors are not required to supply any current to the rotor.

• Synchronous motors require slip rings and brushes to connect the rotor to the power supply. Induction motors don’t require slip rings.

• Synchronous motors require windings in the rotor, while induction motors are most often constructed with conduction bars in the rotor or use short circuited windings to form a “squirrel cage.”