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.

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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.

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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?

Synchronous motor and synchronous machine can be used as a synchronous motor or as an alternator. Synchronous motors are available in a wide range, generally rated between 150kW to 15MW with speeds ranging from 150 to 1800 rpm.

and induction motor are the most widely used types of AC motor. Construction of a synchronous motor is similar to an alternator (AC generator) . A samecan be used as a synchronous motor or as an alternator. Synchronous motors are available in a wide range, generally rated between 150kW to 15MW with speeds ranging from 150 to 1800 rpm.

Construction of synchronous motor

construction of a synchronous motor (with salient pole rotor) is as shown in the figure at left. Just like any other motor, it consists of a stator and a rotor. The stator core is constructed with thin silicon lamination and insulated by a surface coating, to minimize the

The(with salient pole rotor) is as shown in the figure at left. Just like any other motor, it consists of a stator and a rotor. The stator core is constructed with thin silicon lamination and insulated by a surface coating, to minimize the eddy current and hysteresis losses . The stator has axial slots inside, in which three phase stator winding is placed. The stator is wound with a three phase winding for a specific number of poles equal to the rotor poles.

The rotor in synchronous motors is mostly of salient pole type. DC supply is given to the rotor winding  via slip-rings. The direct current excites the rotor winding and creates electromagnetic poles. In some cases permanent magnets can also be used. The figure above illustrates the construction of a synchronous motor very briefly.

Working of synchronous motor

synchronous machine as shown in figure below.

The stator is wound for the similar number of poles as that of rotor, and fed with three phase AC supply. The 3 phase AC supply produces rotating magnetic field in stator. The rotor winding is fed with DC supply which magnetizes the rotor. Consider a two poleas shown in figure below.

• Now, the stator poles are revolving with synchronous speed (lets say clockwise). If the rotor position is such that, N pole of the rotor is near the N pole of the stator (as shown in first schematic of above figure), then the poles of the stator and rotor will repel each other, and the torque produced will be anticlockwise.
• The stator poles are rotating with synchronous speed, and they rotate around very fast and interchange their position. But at this very soon, rotor can not rotate with the same angle (due to inertia), and the next position will be likely the second schematic in above figure. In this case, poles of the stator will attract the poles of rotor, and the torque produced will be clockwise.
• Hence, the rotor will undergo to a rapidly reversing torque, and the motor will not start.

But, if the rotor is rotated upto the synchronous speed of the stator by means of an external force (in the direction of

But, if the rotor is rotated upto the synchronous speed of the stator by means of an external force (in the direction of revolving field of the stator), and the rotor field is excited near the synchronous speed, the poles of stator will keep attracting the opposite poles of the rotor (as the rotor is also, now, rotating with it and the position of the poles will be similar throughout the cycle). Now, the rotor will undergo unidirectional torque. The opposite poles of the stator and rotor will get locked with each other, and the rotor will rotate at the synchronous speed.

Characteristic features of a synchronous motor

• Synchronous motor will run either at synchronous speed or will not run at all.
• The only way to change its speed is to change its supply frequency. (As Ns = 120f / P)
• Synchronous motors are not self starting. They need some external force to bring them near to the synchronous speed.
• They can operate under any power factor, lagging as well as leading. Hence, synchronous motors can be used for power factor improvement.

 Application of synchronous motor

• As synchronous motor is capable of operating under either leading and lagging power factor, it can be used for power factor improvement.  A synchronous motor under no-load with leading power factor is connected in power system where static capacitors can not be used.
• It is used where high power at low speed is required. Such as rolling mills, chippers, mixers, pumps, pumps, compressor etc.

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