unit ii syncronous motor - rmd engineering college · unit ii syncronous motor author: suresh...
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Principle of operation – Torque equation – Operation
on infinite bus bars - V and Inverted V curves –Power input and power developed equations –Starting methods – Current loci for constant power input, constant excitation and constant power developed-Hunting – natural frequency of oscillations – damper windings- synchronous condenser.
SYLLABUS
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Construction
Stator identical to that of a three-phase induction motor – now called the “armature”
Energize from a three-phase supply and develop the rotating magnetic field
Rotor has a DC voltage applied (excitation)
Rotor could be a permanent-magnet type
CONSTRUCTION
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Operation
Magnetic field of the rotor “locks” with the rotating magnetic field – rotor turns at synchronous speed
PRINCIPLE OF OPERATION
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Supply is given tothe statorwinding toproduces therotating flux withsyncronousspeed.
Same Poles repeal with each other .
When both the stator androtor poles synchronisedwith each other the motorstarted to rotate insynchronous speed.
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The gross torque developed by the motor Tg=9.55*(Pm/N) N-m
Net output of the motor then can be obtained by subtracting
friction and windage i.e. mechanical losses from gross mechanical power
developed.
Pout = Pm – Mechanical losses
The shaft torque developed by the motor Tsh= 9.55*(Pout/Ns)
TORQUE EQUATION
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STRATING TORQUE
RUNNING TORQUE
PULL IN TORQUE
PULL OUT TORQUE
DIFFERENT TYPES OF TORQUE
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OPERATION ON INFINITE BUSBAR
Infinte Bus bar represents constant voltage and constant frequency.
Which can deliver or absorb real and reactive power. Without any limitation.
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The overexcited synchronous motor running under
no load condition is called syncronous condenser.
Applications of Synchronous Motor:
. Power Factor correction.
.Constant Load and Constant Voltage Drive.
.Voltage Regulation.
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A synchronous machine running without a
prime mover or a mechanical load
Depending on field excitation, it caneither absorb or generate vars
With a voltage regulator, it canautomatically adjust vars to maintainconstant voltage
Started as an induction motor and thensynchronized
Normally connected to tertiary windingsof transformers
Synchronous Condenser
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With constant mechanical load on the synchronous motor, the variation of
field current changes the armature current drawn by the motor and also its
operating power factor.
Normal Excitation : The armature current is minimum at a particularvalue of field current, which is called the normal field excitation. Theoperation power factor of the motor is unit at this excitation and thusthe motor is equivalent to a resistive type of load.elow under three
different modes of field excitation.
V AND INVERTER V CURVES
(EFFECT OF EXCITATION ON ARMATURE
CURRENT AND POWER FACTOR )
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Under Excitation : When the field current is decreased gradually
below the normal excitation, the armature current increase and theoperating power factor of the motor decreases. The power factor underthis condition is lagging. Thus, the synchronous motor draws a laggingcurrent, when it is under excited and is equivalent to an inductive load.
Over Excitation : When the field current is increased gradually beyondthe normal excitation, the armature current again increase and theoperating power factor increases. However, the power factor is leadingunder this condition. Hence, the synchronous motor draws a leadingcurrent, when it is over excited and is equivalent to a capacitive load.
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Phasor Diagrams for V and Inverted V curve
Er-Resultant Voltage
Eb-Back Emf.
V-Terminal Voltage
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Circuit Diagram
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The phenomenon of oscillation of the rotor about its final equilibrium
position is called Hunting.On the sudden application of load, the rotorsearch for its new equilibrium position and this process is knownas Hunting. The Hunting process occurs in a synchronous motor as wellas in synchronous generators if an abrupt change in load occurs.
Causes of HuntingThe various causes of hunting are as follows:-
Sudden changes of load.
Faults were occurring in the system which the generator supplies.
Sudden change in the field current.
Cyclic variations of the load torque
HUNTING
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OSCILLATIONS DUE TO HUNTING
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Effect of Hunting
The various effects of hunting are as follows:-
It can lead to loss of synchronism.
It can cause variations of the supply voltage producing undesirable
lamp flicker.
The possibility of Resonance condition increases. If the frequency of
the torque component becomes equal to that of the transient
oscillations of the synchronous machine, resonance may take place.
Large mechanical stresses may develop in the rotor shaft.
The machine losses increases and the temperature of the machine
rises.
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Reduction of Hunting
The following technique given below is used to reduce the
phenomenon of hunting.
Use of damper windings
Uses of flywheels
The prime mover is provided with a large and heavy flywheel. This
increases the inertia of the prime mover and helps in maintaining the
rotor speed constant.
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MERITS:
It runs at a constant speed . The only way to change its speed is to vary the supply frequency because Ns = 120f/P).
It is not inherently self-starting.
It can be operated under a wide range of power factors both lagging and leading. Hence, it can be used for power factor correction, in addition to supplying torque to drive loads.
Most synchronous motors are rated between 150 kW and 15 kW and run at speeds from 150 to 1800 rpm.
It is less costly in certain kW and speed ranges i.e. for 35 to 350 kW rating at speeds less than 500 rpm.
MERITS AND DEMERITS
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DEMERITS:
It is not self-starting. Special methods are adopted to make it self-starting.
It needs frequent maintenance.
External DC source is necessary for providing excitation.
Additional damper winding is necessary.
Hunting takes place if the load is changed suddenly
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Using Pony Motors:
Using Damper Winding.
As a Slipring Induction Motor.
Using Small DC Machine.
STARTING METHODS OF SYNCHRONOUS
MOTOR:
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Using pony motors
. In this method, the rotor is brought to the synchronous speed
with the help of some external device like small induction motor.
Such an external device is called 'pony motor'.
. Once the rotor attains the synchronous speed, the d.c.
excitation to the rotor is switched on. Once the synchronism is
established pony motor is decoupled. The motor then continues to
rotate as synchronous motor.
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Using Damper Winding
In a synchronous motor, in addition to the normal field winding, the
additional winding consisting of copper bars placed in the slots in the pole
faces. The bars are short circuited with the help of end rings. Such an
additional winding on the rotor is called damper winding.
This winding as short circuited, acts as a squirrel cage rotor winding of an
induction motor.
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As aSlipring Induction motor:
Using external rheostat starting torque is increased.
So the motor starts has induction motor.
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Using Small D.C. Machine
Synchronous motor are provided with a coupled d.c. machine.
This machine is used as a d.c. motor to rotate the synchronous
motor at a synchronous speed. Then the excitation to the rotor is
provided.
Once motor starts running as a synchronous motor, the same d.c.
machine acts as a d.c. generator called exciter. The field of the
synchronous motor is then excited by this exciter itself.
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Power Input to Synchronous Motor:
Pin = IqVcosα –IdVsinα
Mechanical Power Developed:
Pmech/phase= Eb(v/zs(cos(θ-α) )–Eb/zs cosθ)
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Maximum Power Developed:
Pmech max: EbV/Xs
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CURRENT LOCI FOR CONSTANT
POWER INPUT
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CURRENT LOCI FOR CONSTANT
EXCITATION
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CURRENT LOCI FOR CONSTANT
MECHANICAL POWER DEVELOPED