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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 (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 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 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. DEE_JRN 6 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. DEE_JRN 7 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. DEE_JRN 8 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. DEE_JRN 9 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. DEE_JRN 10 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. DEE_JRN 11 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. DEE_JRN 12 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. DEE_JRN 13