SYNCHRONOUS MOTOR

SYNCHRONOUS MOTOR

Presented by,

Vinay Shreyas K.V B.E, M.Tech,Lecturer, Dept. of EEE

H.K.B.K.C.E

Synchronous Machines

• Synchronous generators or alternators are used to convert mechanical power derived from steam, gas, or hydraulic-turbine to ac electric power

• Synchronous generators are the primary source of electrical energy we consume today

• Large ac power networks rely almost exclusively on synchronous generators

• Synchronous motors are built in large units compare to induction motors (Induction motors are cheaper for smaller ratings) and used for constant speed industrial drives

Construction

Basic parts of a synchronous generator:

• Rotor - dc excited winding • Stator - 3-phase winding in which the ac emf is

generated

The manner in which the active parts of a synchronous machine are cooled determines its overall physical size and structure

StatorSame as an induction motor’s stator

Some books may call this the Armature

Stator is the outer part of the motor. Which carries the armature winding. This armature creates the rotating magnetic field inside the motor.

Rotor Wound simular to a wound rotor induction motor

When operating DC voltage is placed across this coil to produce an electromagnet

Some books may call this the Field Windings

The rotor is the rotating part of the motor. It carries the field winding supplied by the D.C. source making the winding behaves has a permanent magnet.

INTRODUCTION

• In a synchronous generator, a DC current is applied to the rotor winding producing a rotor magnetic field. The rotor is then turned by external means producing a rotating magnetic field, which induces a 3-phase voltage within the stator winding.

• In a synchronous motor, a 3-phase set of stator currents produces a rotating magnetic field causing the rotor magnetic field to align with it. The rotor magnetic field is produced by a DC current applied to the rotor winding.

• Field windings are the windings producing the main magnetic field (rotor windings for synchronous machines); armature windings are the windings where the main voltage is induced (stator windings for synchronous machines).

INTRODUCTIONThe main advantages of synchronous motor are:

1. Ability to work as synchronous condenser for power factor improvement

2. Higher efficiency than that of an induction motor

3. Lower cost.

Disadvantages:

4. Starting problems are faced.

5. A damper winding is required.

Motor generator sets, air compressors, pumps, servo drives are examples of synchronous motor drives.

In the past synchronous motors were mainly used in constant speed applications.

But due to the development of semiconductor variable frequency sources, synchronous motor are now used in variable speed applications.

Various Types

Salient-pole synchronous machine

Cylindrical or round-rotor synchronous machine

Salient-Pole Synchronous Generator

Stator

Salient-pole rotor

Cylindrical-Rotor Synchronous Generator

Stator

Cylindrical rotor

Operation Principle

The rotor of the generator is driven by a prime-mover

A dc current is flowing in the rotor winding which produces a rotating magnetic field within the machine

The rotating magnetic field induces a three-phase voltage in the stator winding of the generator

Motor Speed Magnetic force is obtained from an external source

(In an induction machine rotor’s magnetism is induced from the stator) Rotor poles lock onto the RMF Rotor operates at synchronous speed

P

f120N sync = Nrotor

Called “ Excitation”

In S.M. D.C excitation is normally provided by the rotor circuit, which moves in synchronism with the rotation magnetic field, so that with the fixed frequency supply, the motor operates at constant speed.

1. S.M is not inherently self starting. 2. It has to be run upto near synchronous speed by some means before it can be synchronized to the supply.

The process of pulling rotor into step with the rotating field is called pull-in or synchronism.

INTRODUCTION

OPERATION FROM FIXED FREQUENCY SUPPLY

1. STARTING

One widely used method is to start the synchronous motor

a) As an induction motor with field unexcited and

b) Damper winding serving as a squirrel-cage rotor.

STARTING METHODS

a. Line voltage is applied to the armature terminals and the field circuit left unexcited.

b. Motor starts as an induction motor and while it reaches near by 95% of its synchronous speed, the dc field is excited.

b. At that moment stator and rotor poles get interlocked with each other and hence pull the motor into synchronism.

However two points should be noted.

1. At the beginning, when voltage is applied, the rotor is stationary. The rotating field of the stator winding induces a very large e.m.f. in the rotor during the starting period, though the value of this e.m.f. goes on decreasing as the rotor gathers speed.

2. During starting period there are many thousands of volts induced in the rotor. Normally field windings are meant for 110-250 V. • Hence the rotor windings have to be highly insulated for withstanding high

voltages.

STARTING METHODS

3. When full line voltage is switched to the armature, a very large current usually 5 to 7 times the full load armature current is drawn by the motor.• starting current can be reduced using auto-transformers

4. However, the voltage should not be reduced to a very low value because the starting torque of an induction motor varies approximately as the square of the applied voltage.• Usually a value of 50% to 80% of the full line voltage is

satisfactory.

Another method of starting is to use a low power auxiliary motor coupled to the synchronous motor shaft.

With the help of auxiliary motor, the rotor speed is brought near, synchronous speed and then dc field is switched on.

This method has a very low starting torque.

Construction of synchronous machines

Two common approaches are used to supply a DC current to the field circuits on the rotating rotor:

1. Supply the DC power from an external DC source to the rotor by means of slip rings and brushes;

2. Supply the DC power from a special DC power source mounted directly on the shaft of the machine.

Slip rings are metal rings completely encircling the shaft of a machine but insulated from it. One end of a DC rotor winding is connected to each of the two slip rings on the machine’s shaft. Graphite-like carbon brushes connected to DC terminals ride on each slip ring supplying DC voltage to field windings regardless the position or speed of the rotor.

Construction of synchronous machines

Slip rings

Brush

INTRODUCTION

• When a synchronous motor is used for driving a varying load, then a condition known as hunting is produced.

• We know that when a synchronous motor is loaded, its rotor falls in phase by the coupling angle α.

• As the load is increased, angle also increases, producing more torque. If load is suddenly decreased, it corresponds to a new angle α.

• In this process, the rotor starts oscillating.

• The amplitude of these oscillations is built up to a large value and may eventually become so great as to throw the machine out of synchronism.

• To stop the build of these oscillations, dampers (also known as squirrel cage winding) are employed.

Torque Angle

Full Load

Torque AngleDependant on:• Load torque• Excitation

• The magnetic link between the stator & the rotor can be thought of as a connecting spring.

• The excitation can be used to strengthen the spring

If the load Torque fluctuatesOr

Changes rapidly• The magnetic “ Spring ” will bounce

• Causing large fluctuations in supply current

• Amortisseur windings are added to the rotor

• Also known as hunting

Torque Angle

No Load

Torque Angle

SYNCHRONOUS MOTOR VARIABLE SPEED DRIVES

Synchronous Motor either runs at synchronous speed or not at all. i.e., while running it maintains a constant speed.

The only way to change its speed is to vary the supply frequency because

VARIABLE FREQUENCY CONTROL

System for controlling the rotational speed of an AC motor by controlling the frequency of the electrical power supplied to the motor.

It’s a specific type of adjustable speed drive.

Also known as adjustable-frequency drives (AFD), variable-speed drives (VSD), AC drives, micro drives or inverter drives.

Since the voltage is varied along with frequency, these are sometimes also called VVVF (variable voltage variable frequency) drives.

VFD SYSTEM DESCRIPTION

A variable frequency drive system generally consists of the following

1.An AC motor

2. A controller

3.An operator interface.

Inverter circuit is probably the most important section of the VFD - changes DC energy into three channels of AC energy that can be used by an AC motor.

VARIABLE FREQUENCY DRIVE

Typically, the power supply to the variable frequency controller will start out as 3-phase AC power. The AC signal will be rectified and converted to a DC signal.

The PWM signal is then controlled to control the speed (i.e. the output) of the motor.

The DC signal is then pulse width modulated (PWM) and filtered to feed a clean power signal to the motor.

MODES OF VARIABLE FREQUENCY CONTROL

Two Modes of Operation

a) True Synchronous Mode b) Self Controlled Mode

b) True Synchronous Mode:

o It is supplied by a separate externally controlled frequency source.

o Frequency is slowly increased such that the difference between the synchronous speed & rotor speed is small and the machine speed increases at a low rate to keep pace with the change in input frequency or synchronous speed.

MODES OF VARIABLE FREQUENCY CONTROL

b) Self Controlled Mode: o Stator supply frequency is changed with the rotor

speed which remains always at synchronous speedo i.e., for every rotation of the rotor with respect to a

reference stator position, all the thyristor get their firing pulses.

o As the rotor position is sensed, switching sequence of devices in the inverter used for the stator is determined by the control circuit.

SELF CONTROLLED SYNCHRONOUS MOTOR DRIVE

• A self controlled synchronous motor drive employing a load commutate thyristor inverter is shown in the Fig.

• The drive employs two converters, which are termed here as source side converter and load side converter.

QUESTIONS….???????