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Ac Synchronous Machine And Its Application Engineering Essay

Paper Type: Free Essay Subject: Engineering
Wordcount: 2402 words Published: 1st Jan 2015

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Synchronous machines are principally used as alternating 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 large power systems. This article explains the constructional features and operating principles of the synchronous machine. Generator performance for stand-alone and grid applications is discussed. The effects of load and field excitation on the synchronous motor are investigated. The hunting behavior of a synchronous machine is studied, and a review of various excitation systems provided.

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Synchronous motor

A synchronous electric motor is an AC motor distinguished by a rotor spinning with coils passing magnets at the same rate as the alternating current and resulting rotating magnetic field which drives it. Another way of saying this is that it has zero slip under usual operating conditions. Contrast this with an induction motor, which must slip in order to produce torque. They operate synchronously with line frequency. As with squirrel-cage induction motors, speed is determined by the number of pairs of poles and the line frequency. Synchronous motors are available in sub-fractional self-excited sizes to high-horsepower direct-current excited industrial sizes. In the fractional horsepower range, most synchronous motors are used where precise constant speed is required. In high-horsepower industrial sizes, the synchronous motor provides two important functions. First, it is a highly efficient means of converting ac energy to work. Second, it can operate at leading or unity power factor and thereby provide power-factor correction.

There are two major types of synchronous 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 require no external excitation supply.

Reluctance designs have ratings that range from sub-fractional to about 30 hp. Sub-fractional horsepower motors have low torque, and are generally used for instrumentation applications. Moderate torque, integral horsepower motors use squirrel- cage construction with toothed rotors. When used with an adjustable frequency power supply, all motors in the drive system can be controlled at exactly the same speed. The power supply frequency determines motor operating speed.

Hysteresis motors are manufactured in sub-fractional horsepower ratings, primarily as servomotors and timing motors. More expensive than the reluctance type, hysteresis motors are used where precise constant speed is required.

D C-excited motors made in sizes larger than 1 hp, these motors require direct current supplied through slip rings for excitation. The direct current can be supplied from a separate source or from a dc generator directly connected to the motor shaft.

Slip rings and brushes are used to conduct current to the rotor. The rotor poles connect to each other and move at the same speed – hence the name synchronous motor.

Synchronous motors fall under the category of synchronous machines which also includes the alternator (synchronous generator). These machines are commonly used in analog electric clocks, timers and other devices where correct time is required.

The speed of a synchronous motor is determined by the following formula:

where v is the speed of the rotor (in rpm), f is the frequency of the AC supply (in Hz) and n is the number of magnetic poles.


Two pole

Two pole:


Main features of synchronous machine:

A synchronous machine is an ac machine whose speed under steady-state conditions is proportional to the frequency of the current in its armature.

Armature winding: on the stator, alternating current.

Field winding: on the rotor, dc power supplied to built 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 synchronous motors.

The rotor, along with the magnetic field created by the dc field current on the rotor, rotates at the same speed as, or in synchronism with, the rotating magnetic field produced by the armature currents, and a steady torque results.

Synchronous motors have the following characteristics:

A three-phase stator similar to that of an induction motor. Medium voltage stators are often used.

A wound rotor (rotating field) which has the same number of poles as the stator, and is supplied by an external source of direct current (DC). Both brush-type and brushless exciters are used to supply 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 rotating stator flux.

Starts as an induction motor. The synchronous motor rotor also has a squirrel-cage winding, known as an Amortisseur winding, which produces torque for motor starting.

Synchronous motors will run at synchronous speed in accordance with the formula:

120 x Frequency

Synchronous RPM =

Number of Poles

Example: the speed of a 24 -Pole Synchronous Motor operating at 60 Hz would be:

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

Synchronous Motor Operation:

The squirrel-cage Amortisseur winding in the rotor produces Starting Torque and Accelerating Torque to bring the synchronous motor up to speed.

When the motor speed reaches approximately 97% of nameplate RPM, the DC field current is applied to the rotor producing Pull-in Torque and the rotor will pull-in -step and “synchronize” with the rotating flux field in the stator. The motor will run at synchronous speed and produce Synchronous Torque.

After synchronization, the Pull-out Torque cannot be exceeded or the motor will pull out-of-step. Occasionally, if the overload is momentary, the motor will “slip-a-pole” and resynchronize. Pull-out protection must be provided otherwise the motor will run as an induction motor drawing high current with the possibility of severe motor damage.

Advantages of Synchronous Motors:

The initial cost of a synchronous motor is more than that of a conventional AC induction motor due to the expense of the wound rotor and synchronizing circuitry. These initial costs are often off-set by:

Precise speed regulation makes the synchronous motor an ideal choice for certain industrial processes and as a prime mover for generators.

Synchronous motors have speed / torque characteristics which are ideally suited for direct drive of large horsepower, low-rpm loads such as reciprocating compressors.

Synchronous motors operate at an improved power factor, thereby improving overall system power factor and eliminating or reducing utility power factor penalties. An improved power factor also reduces the system voltage drop and the voltage drop at the motor terminals.

Synchronous generator:

Speed of rotation of synchronous generator:

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

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

Internal generated voltage of ac generated machine.

magnitude of induced voltage in one phase determined in last section: EA=√2 π NC φ f

Parts of ac synchronous machine:

A synchronous motor is composed of the following parts:

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

The rotor is the rotating portion of the motor. it carries field winding, which may be supplied by a DC source. On excitation, this field winding behaves as a permanent magnet.

The slip rings in the rotor, to supply the DC to the field winding, in the case of DC excited types.


The operation of a synchronous motor is simple to imagine. The armature winding, when excited by a poly-phase (usually 3-phase) winding, creates a rotating magnetic field inside the motor. The field winding, which acts as a permanent magnet, simply locks in with the rotating magnetic field and rotates along with it. During operation, as the field locks in with the rotating magnetic field, the motor is said to be in synchronization.

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Once the motor is in operation, the speed of the motor is dependent only on the supply frequency. When the motor load is increased beyond the break down load, the motor falls out of synchronization i.e., the applied load is large enough to pull out the field winding from following the rotating magnetic field. The motor immediately stalls after it falls out of synchronization.

Starting method of synchronous motor:

Synchronous motors are not self-starting motors. This property is due to the inertia of the rotor. When the power supply is switched on, the armature winding and field windings are excited. Instantaneously, the armature winding creates a rotating magnetic field, which revolves at the designated motor speed. The rotor, due to inertia, will not follow the revolving magnetic field. In practice, the rotor should be rotated by some other means near to the motor’s synchronous speed to overcome the inertia. Once the rotor nears the synchronous speed, the field winding is excited, and the motor pulls into synchronization.

The following techniques are employed to start a synchronous motor:

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

The field winding is shunted or induction motor like arrangements are made so that the synchronous motor starts as an induction motor and locks in to synchronization once it reaches speeds near its synchronous speed.

Reducing the input electrical frequency to get the motor starting slowly, Variable-frequency drives can be used here which have Rectifier-Inverter circuits or Cycloconverter circuits.

Special Properties:

Synchronous motors show some interesting properties, which finds applications in power factor correction. The synchronous motor can be run at lagging, unity or leading power factor. The control is with the field excitation, as described below:

When the field excitation voltage is decreased, the motor runs in lagging power factor. The power factor by which the motor lags varies directly with the drop in excitation voltage. This condition is called under-excitation.

When the field excitation voltage is made equal to the rated voltage, the motor runs at unity power factor.

When the field excitation voltage is increased above the rated voltage, the motor runs at leading power factor. And the power factor by which the motor leads varies directly with the increase in field excitation voltage. This condition is called over-excitation.

The most basic property of synchro motor is that it can be use both as a capacitor or inductor. Hence in turn it improves the power factor of system.

The leading power factor operation of synchronous motor finds application in power factor correction. Normally, all the loads connected to the power supply grid run in lagging power factor, which increases reactive power consumption in the grid, thus contributing to additional losses. In such cases, a synchronous motor with no load is connected to the grid and is run over-excited, so that the leading power factor created by synchronous motor compensates the existing lagging power factor in the grid and the overall power factor is brought close to 1 (unity power factor). If unity power factor is maintained in a grid, reactive power losses diminish to zero, increasing the efficiency of the grid. This operation of synchronous motor in over-excited mode to correct the power factor is sometimes called as Synchronous condenser.


Synchronous motors find applications in all industrial applications where constant speed is necessary.

Improving the power factor as Synchronous condensers.

Electrical power plants almost always use synchronous generators because it is important to keep the frequency constant at which the generator is connected.

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

Mains synchronous motors are used for electric clocks.

Record player turntables.


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

Speed is independent of the load, provided an adequate field current is applied.

Accurate control in speed and position using open loop controls, e.g. stepper motors.

They will hold their position when a DC current is applied to both the stator and the rotor windings.

Their power factor can be adjusted to unity by using a proper field current relative to the load. Also, a “capacitive” power factor, (current phase leads voltage phase), can be obtained by increasing this current slightly, which can help achieve a better power factor correction for the whole installation.

Their construction allows for increased electrical efficiency when a low speed is required (as in ball mills and similar apparatus).

They run either at the synchronous speed else no speed is there.


With the help of the above paper now we can understand ac synchronous machine, its working, method, uses, advantages, disadvantages, application etc. We can also explain what kind of further enhancements are going to be, on the field of ac synchronous machine. Although important information is been provided about ac synchronous motors, ac synchronous generator etc. And even on the combination of both of them.


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