Ac Synchronous Machine And Its Application Engineering Essay

Synchronous machines are chiefly used as jumping current ( AC ) generators. They supply the electric power used by all sectors of modern societies: industrial, commercial, agricultural, and domestic. Synchronous machines are sometimes used as constant-speed motors, or as compensators for reactive power control in big power systems. This article explains the constructional characteristics and runing rules of the synchronal machine. Generator public presentation for stand-alone and grid applications is discussed. The effects of burden and field excitement on the synchronal motor are investigated. The runing behaviour of a synchronal machine is studied, and a reappraisal of assorted excitement systems provided.

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Synchronous motor

A synchronal electric motor is an AC motor distinguished by a rotor whirling with spirals go throughing magnets at the same rate as the jumping current and ensuing revolving magnetic field which drives it. Another manner of stating this is that it has zero faux pas under usual operating conditions. Contrast this with an initiation motor, which must steal in order to bring forth torsion. They operate synchronously with line frequence. As with squirrel-cage initiation motors, velocity is determined by the figure of braces of poles and the line frequence. Synchronous motors are available in sub-fractional self-excited sizes to high-horsepower direct-current aroused industrial sizes. In the fractional HP scope, most synchronal motors are used where precise changeless velocity is required. In high-horsepower industrial sizes, the synchronal motor provides two of import maps. First, it is a extremely efficient agencies of change overing ac energy to work. Second, it can run at taking or unity power factor and thereby supply power-factor rectification.

There are two major types of synchronal motors: non-excited and direct-current excited.

Non-excited motors are manufactured in reluctance and hysteresis designs, these motors employ a self-starting circuit and necessitate no external excitement supply.

Reluctance designs have evaluations that range from sub-fractional to about 30A horsepower. Sub-fractional HP motors have low torsion, and are by and large used for instrumentality applications. Moderate torsion, built-in HP motors use squirrel- coop building with toothed rotors. When used with an adjustable frequence power supply, all motors in the thrust system can be controlled at precisely the same velocity. The power supply frequence determines motor runing velocity.

Hysteresis motors are manufactured in sub-fractional HP evaluations, chiefly as servomotors and clocking motors. More expensive than the reluctance type, hysteresis motors are used where precise changeless velocity is required.

D C-excited motors made in sizes larger than 1A horsepower, these motors require direct current supplied through faux pas rings for excitement. The direct current can be supplied from a separate beginning or from a District of Columbia generator straight connected to the motor shaft.

Slip rings and coppices are used to carry on current to the rotor. The rotor poles connect to each other and travel at the same velocity – hence the name synchronal motor.

Synchronous motors fall under the class of synchronal machines which besides includes the alternator ( synchronal generator ) . These machines are normally used in parallel electric redstem storksbills, timers and other devices where right clip is required.

The velocity of a synchronal motor is determined by the undermentioned expression:

where V is the velocity of the rotor ( in revolutions per minute ) , f is the frequence of the AC supply ( in Hz ) and n is the figure of magnetic poles.


Two pole

Two pole:


Main characteristics of synchronal machine:

A synchronal machine is an ac machine whose velocity under steady-state conditions is relative to the frequence of the current in its armature.

Armature twist: on the stator, jumping current.

Field twist: on the rotor, dc power supplied to construct a rotating magnetic field.

Cylindrical rotor: for two- and four-pole turbine generators.

Salient-pole rotor: for multi-polar, slow-speed, hydroelectric generators and for most synchronal motors.

The rotor, along with the magnetic field created by the dc field current on the rotor, rotates at the same velocity as, or inA synchrony with, the revolving magnetic field produced by the armature currents, and a steady torsion consequences.

Synchronous motors have the undermentioned features:

A three-phase stator similar to that of an initiation motor. Medium electromotive force stators are frequently used.

A lesion rotor ( revolving field ) which has the same figure of poles as the stator, and is supplied by an external beginning of direct current ( DC ) . Both brush-type and brushless exciters are used to provide the DC field current to the rotor.

The rotor current establishes a north/south magnetic pole relationship in the rotor poles enabling the rotor to “ lock-in-step ” with the revolving stator flux.

Starts as an initiation motor. The synchronal motor rotor besides has a squirrel-cage twist, known as an Amortisseur twist, which produces torsion for motor starting.

Synchronous motors will run at synchronal velocity in conformity with the expression:

120 ten Frequency

Synchronous RPM =

Number of Poles

Example: the velocity of a 24 -Pole Synchronous Motor runing at 60 Hz would be:

120 ten 60 / 24 = 7200 / 24 = 300 RPM

Synchronous Motor Operation:

The squirrel-cage Amortisseur twist in the rotor produces Get downing Torque and Accelerating Torque to convey the synchronal motor up to rush.

When the motor velocity reaches about 97 % of nameplate RPM, the DC field current is applied to the rotor bring forthing Pull-in Torque and the rotor will pull-in -step and “ synchronise ” with the revolving flux field in the stator. The motor will run at synchronal velocity and produce Synchronous Torque.

After synchronism, the Pull-out Torque can non be exceeded or the motor will draw out-of-step. Occasionally, if the overload is fleeting, the motor will “ slip-a-pole ” and resynchronize. Pull-out protection must be provided otherwise the motor will run as an initiation motor pulling high current with the possibility of terrible motor harm.

Advantages of Synchronous Motors:

The initial cost of a synchronal motor is more than that of a conventional AC initiation motor due to the disbursal of the lesion rotor and synchronising circuitry. These initial costs are frequently off-set by:

Precise velocity ordinance makes the synchronal motor an ideal pick for certain industrial procedures and as a premier mover for generators.

Synchronous motors have speed / torsion features which are ideally suited for direct thrust of big HP, low-rpm tonss such as reciprocating compressors.

Synchronous motors operate at an improved power factor, thereby bettering overall system power factor and eliminating or cut downing public-service corporation power factor punishments. An improved power factor besides reduces the system electromotive force bead and the electromotive force bead at the motor terminuss.

Synchronous generator:

Speed of rotary motion of synchronal generator:

Electric power generated at 50 or 60 Hz, so rotor must turn at fixed velocity depending on figure of poles on machine

To bring forth 60 Hz in 2 pole machine, rotor must turn at 3600 r/min, and to bring forth 50 Hz in 4 pole machine, rotor must turn at 1500 r/min

Internal generated electromotive force of Ac generated machine.

magnitude of induced electromotive force in one stage determined in last subdivision: EA=a?s2 Iˆ NC I† degree Fahrenheit

Partss of Ac synchronal machine:

A synchronal motor is composed of the undermentioned parts:

The stator is the outer shell of the motor, which carries the armature weaving. This twist is spatially distributed for poly-phase AC current. This armature creates a rotating magnetic field inside the motor.

The rotor is the revolving part of the motor. it carries field twist, which may be supplied by a DC beginning. On excitement, this field weaving behaves as a lasting magnet.

The faux pas rings in the rotor, to provide the DC to the field twist, in the instance of DC excited types.


The operation of a synchronal motor is simple to conceive of. The armature twist, when excited by a poly-phase ( normally 3-phase ) twist, creates a rotating magnetic field inside the motor. The field twist, which acts as a lasting magnet, merely locks in with the revolving magnetic field and rotates along with it. During operation, as the field locks in with the revolving magnetic field, the motor is said to be in synchronism.

Once the motor is in operation, the velocity of the motor is dependent merely on the supply frequence. When the motor burden is increased beyond the interruption down burden, the motor falls out of synchronism i.e. , the applied burden is big plenty to draw out the field weaving from following the revolving magnetic field. The motor instantly stalls after it falls out of synchronism.

Get downing method of synchronal motor:

Synchronous motors are non self-starting motors. This belongings is due to the inactiveness of the rotor. When the power supply is switched on, the armature twist and field twists are excited. Instantaneously, the armature weaving creates a rotating magnetic field, which revolves at the designated motor velocity. The rotor, due to inertia, will non follow the revolving magnetic field. In pattern, the rotor should be rotated by some other agencies near to the motor ‘s synchronal velocity to get the better of the inactiveness. Once the rotor nears the synchronal velocity, the field twist is excited, and the motor pulls into synchronism.

The undermentioned techniques are employed to get down a synchronal motor:

A separate motor ( called pony motor ) is used to drive the rotor before it locks in into synchronism.

The field twist is shunted or induction motor like agreements are made so that the synchronal motor starts as an initiation motor and locks in to synchronism once it reaches velocities near its synchronal velocity.

Reducing the input electrical frequence to acquire the motor get downing easy, Variable-frequency thrusts can be used here which have Rectifier-Inverter circuits or Cycloconverter circuits.

Particular Properties:

Synchronous motors show some interesting belongingss, which finds applications in power factor rectification. The synchronal motor can be run at lagging, integrity or taking power factor. The control is with the field excitement, as described below:

When the field excitement electromotive force is decreased, the motor runs in dawdling power factor. The power factor by which the motor slowdown varies straight with the bead in excitement electromotive force. This status is called under-excitation.

When the field excitement electromotive force is made equal to the rated electromotive force, the motor runs at unity power factor.

When the field excitement electromotive force is increased above the rated electromotive force, the motor runs at taking power factor. And the power factor by which the motor leads varies straight with the addition in field excitement electromotive force. This status is called over-excitation.

The most basic belongings of synchro motor is that it can be use both as a capacitance or inductance. Hence in bend it improves the power factor of system.

The taking power factor operation of synchronal motor finds application in power factor rectification. Normally, all the tonss connected to the power supply grid tally in dawdling power factor, which increases reactive power ingestion in the grid, therefore lending to extra losingss. In such instances, a synchronal motor with no burden is connected to the grid and is run over-excited, so that the taking power factor created by synchronal motor compensates the bing lagging power factor in the grid and the overall power factor is brought near to 1 ( unity power factor ) . If unity power factor is maintained in a grid, reactive power losingss diminish to zero, increasing the efficiency of the grid. This operation of synchronal motor in over-excited manner to rectify the power factor is sometimes called as Synchronous capacitor.


Synchronous motors find applications in all industrial applications where changeless velocity is necessary.

Bettering the power factor as Synchronous capacitors.

Electrical power workss about ever use synchronal generators because it is of import to maintain the frequence invariable at which the generator is connected.

Low power applications include positioning machines, where high preciseness is required, and robot actuators.

Mains synchronal motors are used for electric redstem storksbills.

Record participant turntables.


Synchronous motors have the undermentioned advantages over non-synchronous motors:

Speed is independent of the burden, provided an equal field current is applied.

Accurate control in velocity and place utilizing unfastened cringle controls, e.g. hoofer motors.

They will keep their place when a DC current is applied to both the stator and the rotor twists.

Their power factor can be adjusted to integrity by utilizing a proper field current comparative to the burden. Besides, a “ capacitive ” power factor, ( current stage leads voltage stage ) , can be obtained by increasing this current somewhat, which can assist accomplish a better power factor rectification for the whole installing.

Their building allows for increased electrical efficiency when a low velocity is required ( as in ball Millss and similar setup ) .

They run either at the synchronal velocity else no velocity is at that place.


With the aid of the above paper now we can understand ac synchronal machine, its working, method, uses, advantages, disadvantages, application etc. We can besides explicate what sort of farther sweetenings are traveling to be, on the field of ac synchronal machine. Although of import information is been provided about ac synchronal motors, ac synchronal generator etc. And even on the combination of both of them.