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Lecture 17Electro Mechanical System1 Locked-rotor torque  To produce a starting torque in a single-phase motor, we must somehow create a revolving field.

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Presentation on theme: "Lecture 17Electro Mechanical System1 Locked-rotor torque  To produce a starting torque in a single-phase motor, we must somehow create a revolving field."— Presentation transcript:

1 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.

2 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.

3 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.

4 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.

5 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.

6 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°.

7 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

8 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 aa

9 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


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