Download phase advancers

PHASE ADVANCERS
It is a particular type of a.c. exciter which is connected to the
rotor circuit of an Induction Motor to improve the p.f.
The principle used is that of injection through the slip rings of the
motor a current which is leading with regard to the rotor voltage.
This current relieves the stator circuit of the duty of magnetizing
the m/c, thus improving the p.f.
TWO types – (i) EXPEDOR (ii) SUSCEPTOR
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Constructional Details – Phase Advancer
Stator: Made of Sheet laminations. Does not contain any wdgs.
Provides Magnetic path for the Flux.
Rotor: Similar to that of a d.c. armature with commutator segments.
3 Brushes are arranged on the commutator, 1200E apart.
c.s view of a Phase
Advancer
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EXPEDOR 2 types (i) Scherbius Advancer (ii) Kapp Vibrator
SCHERBIUS ADVANCER
– Device for phase compensation.
 Device which develops a voltage in the rotor which is a
function of secondary current of IM (I2) & has some phase
relationship to it.
 Same generates or absorbs an e.m.f which resembles an
impedance rise or drop.
Arrangement of an
Expedor / Series Exciter
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Working Principle
• The slip rings of the Induction motor is connected to the three
brushes of the expedor.
• The Rotor or armature of the Exciter is driven by an auxiliary motor.
 At the starting instant, the exciter is taken out of the ckt.
 Using a 3 pole changeover SW, SRIM is started with rotor rheostat starter.
 When the motor attains rated speed, the SW is thrown to the exciter side.
 Now a 3 phase low frequency slip currents flow from SRIM to the armature
of Exciter.
 Armature current creates a rotating magnetic field in the exciter armature.
 The speed of the r.m.f will depend upon the rotor frequency of the SRIM.
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Case I: When the armature is stationary – the armature just acts like
a choke in series with the rotor ckt. p.f. of SRIM gets reduced.
Case II: When the armature is driven in the same direction as its own
field with speed less than field speed,
(i) Now the relative speed will decrease.
(ii) E.m.f induced in the armature decreases because freq.decreases
(iii) The effective reactance decreases. Overall p.f. improves.
(iV) Exciter Armature still acts as a choke.
Case III: When the armature is driven in the same direction as its own
field but with speed same as field speed,
(i) Induced e.m.f is zero.
(ii) The armature has no effect on the operation of SRIM.
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Case IV: When the armature is driven in the same direction as its own
field but with speed greater than field speed
(i) The phase of the induced e.m.f is reversed.
(ii) Now the armature of the exciter acts as a capacitor
instead of a choke. Overall p.f. of the SRIM improves.
Equivalent ckt of an IM with Expedor
Thus as long as the armature is driven at a speed greater than the field speed,
and in the same direction, the exciter will improve the p.f. of the SRIM.
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EXPEDOR
The phase compensating e.m.f (provided by exciter) is proportional to
the rotor current, and the advancer behaves as if a variable capacitor
is connected to the rotor circuit of the SRIM.
Injects an e.m.f to the rotor circuit of the SRIM, then the rotor current
is advanced in phase, and the consequent reaction on the stator advances
the phase of the stator current also.
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SUSCEPTOR ADVANCER / FREQUENCY CONVERTER ADVANCER
/ PARALLEL EXCITER - Device for phase compensation
 Device which develops a voltage in the rotor which is a function
of secondary (rotor) open circuit e.m.f & has some phase relationship to it.
 Same affects the magnetizing current of the machine & consequently
its magnetic susceptance.
• The Exciter is coupled mechanically to the SRIM, so that its speed
cannot be varied.
• The three phase brush gear on the commutator of the Exciter is
connected to the SRIM rotor circuit through slip rings.
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Working Principle
3Ø
supply
‘f’
Arrangement of an
Susceptor / Parallel Exciter
‘sf ‘
 3 phase supply at line frequency fed to the exciter will set up a rotating
magnetic field at speed NS relative to armature of exciter.
The arrangement is such that the rotation of the exciter and its field
are in opposite direction. The speed of the field relative to fixed brushes
will now be the slip speed.
 The magnitude of induced e.m.f depends on the relative speed &
the frequency of induced e.m.f. appearing at the brushes
will then be slip frequency.
i.e. The exciter is behaving as a frequency converter.
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 Thus the exciter commutator can be in direct connection with the
SRIM slip rings as both frequencies are same.
 Thus this arrangement provides an injection of e.m.f at slip frequency
to the rotor circuit of SRIM.
 The phase of the e.m.f at the commutator depends on the angular position
of the brushes & if the brush position is altered, the phase of this e.m.f
will be altered by a corresponding amount.
 Thus by controlling the position of the brushes on the commutator, the phase
of the injected e.m.f into the SRIM rotor circuit can be controlled ,
thereby altering the p.f. of the SRIM.
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The Susceptor behaves like a Capacitor connected in series
with the magnetizing reactance.
Equivalent ckt of an IM with Susceptor
Note: Regulating machines which are used only for p.f. improvement,
without speed control are called Phase Advancers.
If used only for speed control, it is called Slip Regulators.
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WALKER ADVANCER
 Similar to Scherbius Advancer,
except that it has a stator winding
which is connected in series with
the brushes.
 Exciter also produces an e.m.f.
which is approx. proportional to I2
but the angle at which it is injected
into the secondary ckt of SRIM can
be varied by moving the brushes
relative to the stator winding.
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 Two sets of windings are needed.
 Firstly a compensating winding connected in series with the brushes &
secondly an exciting winding also in series with the brushes.
 The exciter winding is responsible for producing flux in the machine.
 The compensating winding is designed to neutralize the Armature
m.m.f. entirely.
 The compensating winding must occupy a definite position relative to
brushes since the axis of armature m.m.f. at any moment is dependent
on brush position.
 The m/c can be designed with the exciting winding so arranged that the
e.m.f can be injected at any desired angle relative to current.
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