Term paper

Of
BASIC ELECTRICAL AND ELECTRONICS
ECE131
Topic: AC synchronous machine and
its applications
Date of allotment : 17-9-2010
Date of review : 14-10-2010
Date of submission : 18-11-2010

Submitted to: Submitted by:

Mrs. Cherry Bhargav Karan Arora

Department of Roll no: RK6002B43

E.C.E Reg. No: 11001335

Section: K6002
 ACKNOWLEDGEMENT

First of all I would like to thank the Lovely Professionally University and take
the opportunity to do this project as a part of the B-Tech.

Many people have influenced the shape and content of this project, and many
supported me through it. I express my sincere gratitude to Lect. Mrs. Cherry
Bhargav for assigning me a project on AC synchronous machine and
its applications, which is an interesting and exhaustive subject.
He has been an inspiration and role model for this topic. His guidance and active
support has made it possible to complete the assignment.

I would also like to thank my friends who have helped and encouraged me
throughout the working of this project.

Last but not the least I would like to thank the Almighty for always helping me.

INTRODUCTION
 Synchronous motor
A synchronous electric motor is an AC motor distinguished by a rotor spinning with coils passing magnets at
the same rate as thealternating 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.

DC-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.
Parts:

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

Operation

The operation of a synchronous motor is simple to imagine. The armature winding, when excited by a poly-
phase (usually 3-phase)Supply, 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.

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 methods

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.
Uses

 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

Advantages

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, eg. 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 or they do not run at all.