Presentation on theme: "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."— Presentation transcript:
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 1DEE_JRN
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, E apart. c.s view of a Phase Advancer 2DEE_JRN
3 EXPEDOR 2 types (i) Scherbius Advancer (ii) Kapp Vibrat or 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_JRN4 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_JRN5 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_JRN6 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. 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. Equivalent ckt of a motor with Expedor