Transcript 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
DEE_JRN
1
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
DEE_JRN
2
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 & 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
DEE_JRN
3
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.
DEE_JRN
4
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.
DEE_JRN
5
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 a motor 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.
DEE_JRN
6