Lecture 17Electro Mechanical System1 Locked-rotor torque To produce a starting torque in a single-phase motor, we must somehow create a revolving field. This is done by adding an auxiliary winding. When the main and auxiliary windings are connected to an ac source, the main winding produces a flux s, while the auxiliary winding produces a flux a. If the fluxes are out of phase, so that a either lags or leads s, a rotating field is set up. To obtain the desired phase shift between s and a, we add an impedance Z in series with the auxiliary winding. The choice of impedance gives rise to various types of split –phase motor.
Lecture 17Electro Mechanical System2 Locked-rotor torque A special switch is also connected in series with the auxiliary winding. It disconnects the winding when the motor reaches about 75 percent of synchronous speed.
Lecture 17Electro Mechanical System3 Resistance split-phase motor Main winding of a motor is made of relatively large wire to reduce the I 2 R losses. It also has a relatively large number of turns. Under locked-rotor conditions, the inductive reactance is high and the resistance is low. The locked-rotor current I s lags considerably behind the applied voltage E. Auxiliary winding has small number of turns of fine wire. Higher resistance and lower reactance than main winding. The locked-rotor current I a is more nearly phase with E. Resulting phase angle α between I a and I s produces the starting torque.
Lecture 17Electro Mechanical System4 Capacitor-start motor Capacitor-start motor is like a split-phase motor, except that the auxiliary winding has almost similar number of turns A capacitor and a centrifugal switch are connected in series with the auxiliary winding Capacitor is chosen so that I a leads I s by about 80°, which is 25° more than split-phase motor For equal starting torques, current in the auxiliary winding is only half of split-phase motor During starting period the auxiliary winding of capacitor motor heats up less quickly. The locked-rotor line current I L is smaller, 4 to 5 times the rated full-load current.
Lecture 17Electro Mechanical System5 Efficiency and Power factor The efficiency and power factor of a fractional horse power single phase motors are usually low. At full-load 186W(1/4 hp) motor has an efficiency of about 60%. Low power factor is mainly due to large magnetizing current which ranges from 70 to 90 % of full load current. At no load these motors have a substantial temperature rise.
Lecture 17Electro Mechanical System6 Capacitor-run motor Capacitor-run motor is essentially a 2-phase motor that receives its power from a single-phase source. Two windings, one is directly connected to the source. Other winding is connected to the source, but in series with a paper capacitor. The capacitor-fed winding has large no. of turns of small wire, compared to directly connected winding Used to drive fixed loads. It has a high power factor due to capacitor and no centrifugal switch is required. However, the starting torque is low. Fluxes a and s created by the two windings are equal and out of phase by 90°.
Lecture 17Electro Mechanical System7 Reversing the direction of rotation To reverse the direction of rotation of a motor, we have to interchange the leads of either main or auxiliary windings. If equipped with centrifugal switch, its rotation cannot be reversed while motor is running For capacitor run motor, the direction can be changed while motor is running, since both winding are in circuit all the time For small motors a double throw switch is used
Lecture 17Electro Mechanical System8 Shaded – pole motor Popular for rating <0.05hp(~40 W) Very simple design. A small squirrel cage motor with auxiliary winding composed of copper ring Flux is composed of 1, 2, 3 and all are in phase 1 links the short circuited ring on left hand pole and produces a large current I a Current I a produces flux a which lags behind 2 & 3 Combined action of a and( 2 + 3 ) produces weak revolving field Direction of rotation is clock wise, from unshaded to shaded ring Similarly flux 2 produces current I a & b which lags behind 2 aa
Lecture 17Electro Mechanical System9 Universal motor Can operate on both ac and dc, the resulting torque-speed is same Very similar to a dc series motor When connected to ac source, ac current flows from armature and series field The field produces flux that reacts in the current flowing in the armature to produce torque Armature current and the flux reverses simultaneously so the torque always acts in the same direction No revolving flux is produced