SYNCHRONOUS MOTOR
Presented Present ed by, by, Vinay Shreyas S hreyas K.V K .V B.E, M.Tech, M.Tech, Lecturer, Lecturer, Dept. of EEE H.K.B.K.C.E
Synchronous Synchron ous Machines •
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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 •
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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
Stator Same as an induction motor’ motor ’s stator Stator is the outer part of the motor. Which carries the armature winding. This armature creates the rotating magnetic field inside the motor.
Some books may call this the Armature
Rotor
Wound simular to a wound rotor induction motor
When operating DC voltage is placed across this coil to produce an electromagnet
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. Some books may call this the Field Windings
INTRODUCTION •
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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 synchronous machines).
INTRODUCTION The main advantages of synchronous motor are: 1. Abil Ability ity to work work as as syn synch chro rono nous us cond conden ense serr for for powe powerr fact factor or improvement 2. High Higher er ef effici ficien ency cy tha than n that that of of an ind induc ucti tion on mot motor or 3. Lower cost. Disadvantages: 1. Starting proble blems ar are fa faced. 2. A damper win wind ding ing is is re 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
Cylindrical-Rotor Synchronous Generator
Stator
Cylindrical rotor
Operation Principle The rotor of the generator is driven by a prime-mover
A dc dc current is flowing flowi ng 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
Magnetic force is obtained from an external source (In an induction machine rotor’ rotor ’s magnetism is induced from the stator)
Rotor poles lock onto the RMF
Rotor operates at synchronous synchronous speed
N sync
1 2 0f
P
= Nrotor
Called “ Excitation”
INTRODUCTION 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. supply. The process of pulling rotor into step with the rotating field is synchronism. called pull-in or synchronism.
OPERATION OPERATION FROM FIXED FREQUENCY FREQUEN CY SUPPLY 1. START ARTING ING One widely used method is to start the synchronous motor a) As an induction induction motor motor with field unexcited and b) Damper winding serving as a squirrel-cage rotor. rotor.
STARTING METHODS a. Line voltage is applied to the armature terminals terminals and the field circuit left left unexcited. b. Motor Motor starts as an induct induction ion motor motor and while while it reaches reaches near 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, beginning, when voltage is applied, applied, the rotor is stationary. stationary. The rotating 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 synchronous machines Two common approaches approaches are used to supply a DC current to the field circuits on the rotating rotor: 1. Suppl Supply y the the DC DC pow power er fro from m an exter external nal DC source to the rotor by means of slip rings and brushes; 2. Suppl Supply y the the DC DC pow power er fro from m a spec special ial 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 •
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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.
Torqu orquee Ang Angle le Torque Ang Angle le Dependant on: • Load torque • Excitation • The magnetic link between the stator & the rotor rotor can be thought thought of of as a connecting spring.
Full Load
• The excitation can be used to strengthen the spring
Torqu orquee Ang Angle le If the load Torque fluctuates Or Changes rapidly • The magnetic “ Spring ” will bounce • Causing large fluctuations in supply current • Amortisseur windings are added to the rotor • Also known as hunting
Torqu orquee Ang Angle le
No Load
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 (AFD), ), variable-speed drives (VSD VSD), ), AC 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 mo tor 2. A controller 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 DC signal is then pulse width modulated (PWM) (PWM) and filtered to feed a clean power signal to the motor.
The PWM signal is then controlled to control the speed (i.e. the output) of the motor.
MODES OF VARIABLE VARIABLE FREQUENCY FREQUEN CY CONTROL Two Modes of Operation
a) True Synchr Synchron onous ous Mode Mode
b) Self Self Contr Controll olled ed Mode Mode
a) Tru ruee Synch ynchrronou onouss Mode: ode: o
o
It is supplied by a separate externally controlled frequency source. 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 VARIABLE FREQUENCY FREQUEN CY CONTROL b) Self Controlled Mode: o
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Stator supply frequency is changed with the rotor speed which remains always at synchronous speed i.e., for every rotation of the rotor with respect to a reference stator position, all the thyristor get their firing pulses. 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 •
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A self controlled synchronous motor drive employing a load commutate commutate thyristor thyristor inverter inverter is shown in the Fig. The drive employs two converters, which are termed here as source side converter and load side converter.
QUESTIONS….???????
