Synchronous Motor : Types and Applications

In the electrical systems, we use either in industries, power stations or domestic needs, motors and generators have become a common thing. With the demand for high energy efficient and less power consuming systems, the invention of new models of these electrical devices is seen. The basic calculating factor for motors and generators reliable operation is the Power factor. It is the ratio of applied power over the required power. Usually, the total powered consumed at the industries and factories are calculated based on the power factor. So, power factor should always be maintained at unity. But due to the rise of reactive power in these devices power factor decreases. To maintain power factor at unity many methods are introduced. The synchronous motor concept is one of them.

What is Synchronous Motor?

The definition of synchronous motor states that ” An AC Motor in which at steady state, rotation of the shaft is in sync with the frequency of applied current”. The synchronous motor works as AC motor but here the total number of rotations made by the shaft is equal to the integer multiple of the frequency of the applied current.

The synchronous motor doesn’t rely on induction current for working. In these motors, unlike induction motor, multiphase AC electromagnets are present on the stator, which produces a rotating magnetic- field. Here rotor is of a permanent magnet which gets synced with the rotating magnetic- field and rotates in synchronous to the frequency of current applied to it.

Synchronous Motor Design

Stator and rotor are the main components of the synchronous motor. Here stator frame has wrapper plate to which keybars and circumferential ribs are attached. Footings, Frame mounts are used to support the machine. To excite field windings with DC, slip rings and brushes are used.

Cylindrical and round rotors are used for 6 pole application. Salient pole rotors are used when a larger quantity of poles is required. Construction of the synchronous motor and synchronous alternator are similar.

Synchronous Motor Working Principle

Working of synchronous motors depends on the interaction of the magnetic field of the stator with the magnetic field of the rotor. The stator contains 3 phase windings and is supplied with 3 phase power. Thus, stator winding produces a 3 phased rotating Magnetic- Field. DC supply is given to the rotor.

The rotor enters into the rotating Magnetic-Field produced by the stator winding and rotates in synchronization. Now, the speed of the motor depends on the frequency of the supplied current.

Speed of the synchronous motor is controlled by the frequency of the applied current. The speed of a synchronous motor can be calculated as

Ns=60f/P=120f/p

where, f = frequency of the AC current (Hz)
p = total number of poles per phase
P = total pair number of poles per phase.

If the load greater than breakdown load is applied, the motor gets desynchronized. The 3 phase stator winding gives the advantage of determining the direction of rotation. In case of single-phase winding, it is not possible to derive the direction of rotation and the motor can start in either of the direction. To control the direction of rotation in these synchronous motors, starting arrangements are needed.

Starting Methods of Synchronous Motor

The moment of inertia of rotor stops the large-sized synchronous motors from self-starting. Because of this inertia of the rotor, it is not possible for a rotor to get in sync with the stator’s magnetic-field at the very instance power is applied. So some additional mechanism is required to help the rotor get synchronized.

Induction winding is included in the large motors which generate sufficient torque required for acceleration. For very large motors, to accelerate the unloaded machine, pony motor is used. Changing stator current frequency, electronically operated motors can accelerate even from the zero speed.

For very small motors, when the moment of Inertia of the rotor and the mechanical load are desirably small, they can start without any starting methods.

Types of Synchronous Motor

Depending upon the method of magnetization of the rotor, there are two types of synchronous motors –

• Non-excited.
• Direct current Excited.

Non-excited Motor

In these motors, the rotor is magnetized by the external stator field. The rotor contains a constant magnetic field. High retentive steel such as cobalt steel is used to make the rotor. These are classified as a permanent magnet, reluctance, and hysteresis motors.

• In Permanent magnet synchronous motors, a permanent magnet is used along with steel for rotor design. They have a constant magnetic field in the rotor, so induction winding cannot be used for starting. Being used as gearless elevator motors.

• In Reluctance motor, the rotor is made up of steel casting with projecting tooted poles. To minimize the torque ripples, rotor poles are less than stator poles. Contains Squirrel Cage Winding to provide starting torque to the rotor. Used in instrumentation applications.
• Hysteresis motors are self-starting motors. Here the rotor is a smooth cylinder made-up of high coercivity magnetically hard cobalt steel. These motors are expensive and are used where precise constant speed is required. Generally used as servomotors.

DC Current Excited Motor

Here the rotor is excited using the DC current supplied directly through slip rings. AC induction and rectifiers are also used. These are usually of large sizes such as larger than 1 horsepower etc.

Applications of Synchronous Motors

usually, synchronous motors are used for applications where precise and constant speed is required. Low power applications of these motors include positioning machines. These are also applied in robot actuators. Ball mills, clocks, record player turntables also make use of synchronous motors. Besides these motors are also used as servomotors and timing machines.

These motors are available in a fractional horseshoe size range to high power industrial size range. While used in high power industrial sizes, these motors perform two important functions. One is as an efficient means of converting AC energy into mechanical energy and the other is Power factor correction. Which application of servomotor have you come across?