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Single Phase Induction Motors.ppt
- 1. Single-Phase Motors These motors, have output less than one horse-power or one kilowatt, hence are called fractional horse- power or fractional kilowatt motors. AC single-phase, fractional kilowatt motors perform variety of services in the homes, offices, business concerns, factories etc. Almost in all the domestic appliances such as refrigerators, fans, washing machines, hair driers, mixer grinders etc., only 1-phase induction motors are employed.
- 2. Classification of Single-phase Motors 1. Single-phase induction motors (i) split-phase type (ii) capacitor type (iii) Shaded-pole type 2. AC series motors or universal motors 3. Repulsion motors (i) Repulsion-start induction-run motors (ii) Repulsion-induction motors 4. Synchronous motors (i) Reluctance motors (ii) Hysteresis motors.
- 3. Single-phase Induction Motors A single-phase induction motor is very similar to a 3- phase squirrel cage induction motor in construction. • Similar to 3-phase induction motor it consists of two main parts namely stator and rotor. 1. Stator: It is the stationary part of the motor. It has three main parts, namely. (i) Outer frame (ii) Stator core and (iii) Stator winding. (i) Outer frame: It is the outer body of the motor. Its function is to support the stator core and to protect the inner parts of the machine. • Usually, it is made of cost iron. • To place the motor on the foundation, feet are provided in the outer frame as shown
- 4. (ii) Stator core: • The stator core is to carry the alternating magnetic field which produces hysteresis and eddy current losses. • To minimise these losses high grade silicon steel stampings are used to build core. • The stampings are assembled under hydraulic pressure and are keyed to the outer frame. • The stampings are insulated from each other by a thin varnish layer. • The thickness of the stamping usually varies from 0.3 to 0.5 mm. • Slots are punched on the inner periphery of the stampings to accommodate stator winding.
- 5. (iii) Stator winding: • The stator core carries a single phase winding which is usually supplied from a single phase AC supply system. • The terminals of the winding are connected in the terminal box of the machine. • The stator of the motor is wound for definite number of poles, as per the need of speed.
- 6. 2. Rotor: It is the rotating part of the motor. • A squirrel cage rotor is used in single phase induction motors.
- 7. • It consists of a laminated cylindrical core of some high quality magnetic material. • Semi-closed circular slots are punched at the outer periphery. • Aluminium bar conductors are placed in these slots and short circuited at each end by aluminium rings, called short circuiting rings • Thus, the rotor winding is permanently short circuited.
- 8. The rotor slots are usually not parallel to the shaft but are skewed. Skewing of rotor has the following advantages: (a) It reduces humming thus ensuring quiet running of a motor (b) It results in a smoother torque curves for different positions of the rotor (c) It reduces the magnetic locking of the stator and rotor (d) It increases the rotor resistance due to the increased length of the rotor bar conductors.
- 9. Nature of Field Produced in Single Phase Induction Motors
- 12. Torque Produced by Single-phase Induction Motor
- 13. • The two revolving fields will produce torques in opposite directions. Let the two revolving fields be field No. 1 and field No. 2 revolving in clockwise and anticlockwise direction. • The clockwise field produces torque in clockwise direction, whereas, the anticlockwise field produces torque in anticlockwise direction. • The clockwise torque is plotted as positive and anticlockwise as negative. • At stand still, slip for both fields is one. At synchronous speed, for clockwise direction, the field-1 will give condition of zero slip but it will give slip = 2 for field No. 2. • At synchronous speed in a counter clockwise direction, will give condition of zero slip for field -2 but slip = 2 for field No. 1. • The resultant torque developed in the rotor is shown by the curve passing through zero position as shown in Fig. 11.4.
- 14. • The resultant torque it is observed that the starting torque (torque at slip = 1) is zero. And except at starting there is always some magnitude of resultant torque, • At position 1-1’, the torque developed by field-1 is dominating, therefore, motor will pick-up the speed in clockwise direction. • At position 2-2’, the torque developed by field-2 is dominating, therefore, motor will pick-up the speed in anti-clockwise direction). • This shows that if this type of motor is once started (rotated) in either direction it will develop torque in that direction and rotor will pick-up the required speed.
- 15. • The above analysis shows that single phase induction motor with single winding develops no starting torque • but if the rotor is rotated in any direction by some auxiliary means in will develop torque in the same direction and will start rotating in that direction. • So the problem is to find out the auxiliary means to give the starting torque to the motor.
- 16. Methods to make Single-phase Induction Motor Self-starting • A single-phase induction motor inherently is not self-starting. • To make it self-starting, some method is required to be evolved to produce a revolving magnetic field in the stator core. • This may be obtained by converting a single-phase supply into two-phase supply which can be achieved by using an additional winding (this additional winding may be or may not be disconnected once the motor starts and picks- up the speed) or by creating another field (by induction) using a short circuiting band or ring. • Accordingly, depending upon the method used to make a 1-phase induction • motor self-starting, single-phase induction motors can be classified as: 1. Split-phase motors: These motors are started by employing two-phase motor action through the use of an auxiliary winding called starting winding. 2. Capacitor motors: These motors are started by employing two-phase motor action through the use of an auxiliary winding with capacitor. 3. Shaded-pole motors: These motors are started by the interaction of the field produced by a shading band or short circuiting ring placed around a portion of the pole structure.
- 20. The direction of rotation of a 1-phase (split phase) induction motor can be reversed by reversing (interchanging) the connections of either starting winding or running winding.
- 21. Capacitor Motors
- 24. Performance and characteristics: Speed is almost constant with in 5% slip. This type of motor develops high starting torque about 4 to 5 times the full load torque. It draw low starting current. The direction of rotation can be changed by interchanging the connection of either starting or running winding.
- 26. Capacitor run motors (fan motors): • A paper capacitor is permanently connected in the starting winding. In this case, electrolytic capacitor cannot be used since this type of capacitor is designed only for short time rating and hence cannot be permanently connected in the winding. • Both main as well as starting winding is of equal rating. Performance and characteristics. • Starting torque is lower about 50 to 100% of full load torque. • Power factor is improved may be about unity. Efficiency is improved to about 75%.
- 28. Capacitor start and capacitor run motors • In this case, two capacitors are used one for starting purpose and other for running purpose. • For starting purpose an electrolytic type capacitor (Cs) is used which is disconnected from the supply when the motor attains 75% of synchronous speed with the help of centrifugal switch S. • Whereas, a paper capacitor CR is used for running purpose which remains in the circuit of starting winding during running conditions. • This type of motor gives best running and starting operation. • Starting capacitor CS which is of higher value than the value of running capacitor CR.
- 29. Performance and characteristics: • Such motors operate as two phase motors giving best performance and noiseless operation. • Starting torque is high, starting current is low and give better efficiency and higher p.f. • The only disadvantage is high cost.
- 30. Shaded Pole Motor Construction • Shaded pole motor is constructed with salient poles in stator. • Each pole has its own exciting winding • A 1/3rd portion of each pole core is surrounded by a copper strip forming a closed loop called the shading band • Rotor is usually squirrel cage type.
- 32. • When a single phase supply is given to the stator (exciting) winding, it produces alternating flux. • When the flux is increasing in the pole, a portion of the flux attempts to pass through the shaded portion of the pole. • This flux induces an emf and hence current in the shading band or copper ring. • As per Lenz’s law the direction of this current is such that it opposes the cause which produces it i.e., increase of flux in shaded portion. Hence in the beginning, the greater portion of flux passes through unshaded side of each pole and resultant lies on unshaded side of the pole.
- 33. • When the flux reaches its maximum value, its rate of change is zero, thereby the emf and hence current in the shading coil becomes zero. • Flux is uniformly distributed over the pole face and the resultant field lies at the centre of the pole. • After this the main flux tends to decrease, the emf and hence the current induced in the shading coil now tends to increase the flux on the shaded portion of the pole and resultant lies on the shaded portion of the pole • Hence, a revolving field is set up which rotates from unshaded portion of the pole to the shaded portion of the pole as marked by the arrow head • Thus, by electromagnetic induction, a starting torque develops in the rotor and the rotor starts rotating. • After that its rotor picks up the speed.
- 36. Split Phase Capacitor Start Capacitor run Capacitor start and run Shaded pole Characteristi cs Low and medium starting torque High Starting torque Medium torque High starting torque Low starting torqu Starting torque 100% to 250% of rated value 250% to 400% of rated value 100% to 200% of rated value 200% to 300% of rated value 40% to 60% of rated value Braking torque Upto 300% Upto 350% Upto 250% Upto 250% Upto 10% Power factor 0.5 to 0.65 0.5 to 0.65 0.75 to 0.9 0.75 to 0.9 0.25 to 0.4 Efficiency 55% to 65% 55% to 65% 60% to 70% 60% to 70% 25% to 40% Rating 0.5 to 1 HP 0.125 to 1 HP 0.125 to 1 HP 0.125 to 1 HP Upto 40W Applications Fans, blowers, Centrifugal pumps, washing machines Compressors, Pumps, Conveyors, Refrigerators, AC and Washing Machines Fans, blowers, Centrifugal pumps, Compressors, Pumps, Conveyors, Refrigerators Fans, turn Tables, Hair driers, Motion picture projectors, Blowers,