On what principle does a synchronous motor work?

A synchronous motor is one in which the rotor normally rotates at the same speed as the revolving field in the machine. The stator is similar to that of an induction machine consisting of a cylindrical iron frame with windings, usually three-phase, located in slots around the inner periphery. The difference is in the rotor, which normally contains an insulated winding connected through slip rings or other means to a source of direct current (see figure).

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The principle of operation of a synchronous motor can be understood by considering the stator windings to be connected to a three-phase alternating-current supply. The effect of the stator current is to establish a magnetic field rotating at 120 f/p revolutions per minute for a frequency of f hertz and for p poles. A direct current in a p-pole field winding on the rotor will also produce a magnetic field rotating at rotor speed. If the rotor speed is made equal to that of the stator field and there is no load torque, these two magnetic fields will tend to align with each other. As mechanical load is applied, the rotor slips back a number of degrees with respect to the rotating field of the stator, developing torque and continuing to be drawn around by this rotating field. The angle between the fields increases as load torque is increased. The maximum available torque is achieved when the angle by which the rotor field lags the stator field is 90°. Application of more load torque will stall the motor.

One advantage of the synchronous motor is that the magnetic field of the machine can be produced by the direct current in the field winding, so that the stator windings need to provide only a power component of current in phase with the applied stator voltage—i.e., the motor can operate at unity power factor. This condition minimizes the losses and heating in the stator windings.

The power factor of the stator electrical input can be directly controlled by adjustment of the field current. If the field current is increased beyond the value required to provide the magnetic field, the stator current changes to include a component to compensate for this overmagnetization. The result will be a total stator current that leads the stator voltage in phase, thus providing to the power system reactive volt-amperes needed to magnetize other apparatuses connected to the system such as transformers and induction motors. Operation of a large synchronous motor at such a leading power factor may be an effective way of improving the overall power factor of the electrical loads in a manufacturing plant to avoid additional electric supply rates that may otherwise be charged for low power-factor loads.

Three-phase synchronous motors find their major application in industrial situations where there is a large, reasonably steady mechanical load, usually in excess of 300 kilowatts, and where the ability to operate at leading power factor is of value. Below this power level, synchronous machines are generally more expensive than induction machines.

The field current may be supplied from an externally controlled rectifier through slip rings, or, in larger motors, it may be provided by a shaft-mounted rectifier with a rotating transformer or generator.

A synchronous motor with only a field winding carrying a direct current would not be self-starting. At any speed other than synchronous speed, its rotor would experience an oscillating torque of zero average value as the rotating magnetic field repeatedly passes the slower moving rotor. Normally, a short-circuited winding similar to that of an induction machine is added to the rotor to provide starting torque. The motor is started, either with full or reduced stator voltage, and brought up to about 95 percent of synchronous speed, usually with the field winding short-circuited to protect it from excessive induced voltage. The field current is then applied and the rotor pulls into synchronism with the revolving field.

This additional rotor winding is usually referred to as a damper winding because of its additional property of damping out any oscillation that might be caused by sudden changes in the load on the rotor when in synchronism. Adjustment to load changes involves changes in the angle by which the rotor field lags the stator field and thus involves short-term changes in instantaneous speed. These cause currents to be induced in the damper windings, producing a torque that acts to oppose the speed change.

For more information, please visit Types of Induction Motors.

Protection for synchronous motors is similar to that employed with large induction motors. Temperature may be sensed in both the stator and field windings and used to switch off the electric supply. Considerable heating occurs in the rotor-damper winding during starting, and a timer is frequently installed to prevent repeated starts within a limited time interval.

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?

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