Download Special Machines EM-III

AC Machines
BOOSTER
Basic Function:-
Sometimes when we use electrical power we need different voltage level to main
supply. It is provided by Booster.
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Different voltage level can be obtained by MG set or Special D.C. Generator.
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V1, I1 Main Supply Voltage and Current (Motor Input)
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V2, I2 Output Parameter of Generator (Driven Generator Output)
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In this general method Total Losses is high,
PL= V1*I1
PL= V2*I2
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V2*I2
[(1-(n1n2)) / (n1n2)]
BOOSTER Cont.
In Booster:- Generator driven by Shunt motor, generates only ADDITIONAL VOLTAGE V
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Booster Output = V*
I2
- Where V is additional Voltage required.
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Total Losses = V* I2 [(1-(n1’n2’)) / (n1’n2’)].
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So from this booster use we can say the Total losses are less because V is used
instead of V2.
BOOSTER Cont.
Separately Excited Booster
BOOSTER Cont.
Separately Excited Booster Cont.
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This can be employed for step up or down the Voltage.
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The figure shows step up voltage arrangement for the same direction of current.
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For step down step down voltage the arrangement of all current should be
reversed, Now Booster B act as a generator for motor.
BOOSTER Cont.
Series Booster Figure
BOOSTER Cont.
Series Booster Cont.
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Series booster is mostly used in traction work.
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In feeder of traction the voltage drop kept in limits and C/S area of conductor is
very heavy without any voltage controlling device connections so its not
economical design.
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By use of series booster in feeder becomes economical and same time much
better voltage regulation at receiver end can be achieved.
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Series booster is driven at constant speed by shunt motor and can carry full load
current in feeder.
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Booster generates and add main voltage exactly equal to voltage drop in feeder.
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Potential of load end of feeder is kept same as positive bus bar of generator,
Simultaneously –ve booster will maintain the load end voltage of –ve feeder.
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Voltage drop in feeder will automatically compensated at all load current by
designing working of booster on initial straight line portion of magnetization work.
Balancer
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In 3 wire DC system, Correct potential maintain at neutral wire done by Balancer.
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It have 2 identical shunt wound, mechanically coupled machines in series across
+ve and –ve outer of system.
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It can be installed din main or convenient substation.
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Machine act as Motor in series at no load and generate equal back E.M.F., So
neutral point have potential just midway between two outer P and Q.
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Current in neutral divide at S point.
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Machine A act as generator because current flow in direction of its induced emf.
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Machine B act as motor because current flow in opposite direction to induced emf.
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So total load on two sides of machine is maintained constant or balanced, When
heavy load occur Machine act as generator and takes part of load.
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Total load is increased when machine acting as motor.
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As shown in figure field winding of two machine are connected in series across the
outer and carry exactly equal currents and induced emf E in two armature will
then have same value.
SRM (Switched Reluctance Motor)
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It has concentrated winding on stator poles and No winding on rotor teeth.
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The operating principle of SRM is nearly similar to variable reluctance motor
(VRM).
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In both type of motor switching inverter are required to stator supply to operate
motor, But in SRM some what design and control mode are different like it rotate
in continuous rotation where VRM in step manner.
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SRM construct in two ways i) Single Salient rotor ii) Double salient rotor
SRM (Switched Reluctance Motor) Cont.
SRM (Switched Reluctance Motor) Cont.
i)
Single salient rotor:-
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In single salient 2 phase SRM, stator has non salient cylindrical and rotor is 2 pole
salient type and contain NO winding.
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SRM consist of 2 salient pole.
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Inductance of each stator phase varies with rotor position and maximum when
stator phase winding axis coincide with rotor axis DUE TO SALIENT POLE ROTOR.
SRM (Switched Reluctance Motor) Cont.
ii) Double salient rotor:- Stator has 4 salient poles and Rotor has 2 salient pole without winding.
SRM (Switched Reluctance Motor) Cont.
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In this both type of SRM motor consist of 2-phase winding and 2 pole salient rotor.
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Torque is produced when, rotor pole to aline with stator pole to maximize the stator
flux linkages when winding of stator pole is excited by converter circuit.
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Movement of rotor synchronised in opposite direction with stator field.
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It is known as switch reluctance motor because position of rotor is sensed by rotor
sensor and stator excitation is supplied in switched mode from phase to phase as per
rotor position.
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In this flux linkages depend on inductance and it is depend on rotor position.
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So torque produced by motor when Kth phase of stator winding is excited is
TK = ð / ð θ [(1/2) LK θK] * iK^2
TK= (1/2) * iK^2 * [ð LK (θK) / ð θ ]
SRM (Switched Reluctance Motor) Cont.
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Torque is proportional to square of exciting current.
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Value of average torque is zero.
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When in inductance is increased exciting current applied at particular rotor
position FOR Avg. +ve torque development in motor.
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To excite phase winding at particular instant and de-excite later on at desired
instant by specially designed converter circuit are used.
SRM (Switched Reluctance Motor) Cont.
Application of SRM:-
Good power density
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Robust design of motor
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Simple stator excitation
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Reliable power control circuit
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Used in range of 100KW like low power servo motor-High power traction motor
applications.
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The variable speed drive application of SRM is now a days option against inverter
feed induction motor and DC motors.