single phase induction motor.pptx

single phase induction motor in power elctronics

1. single-phase induction machine

2. INT R ODUC TION The single-phase induction machine is the most frequently used motor for refrigerators, washing machines, clocks, drills, compressors, pumps, and so forth. The single-phase motor stator has a laminated iron core with two windings arranged perpendicularly. One is the main and the other is the auxiliary winding or starting winding.

3. construction The single-phase motor stator has a laminated iron core with two windings arranged perpendicularly – one is the main and the other is the auxiliary winding or starting winding. The motor uses a squirrel-cage rotor, which has a laminated iron core with slots in it. Single-phase induction motor is not self-starting and requires some mechanism to assist it in the starting process.

4. Operating principle ope r at i on can be T he sin g l e - phase i n duction mo t or described by two methods: Double revolving field theory; and Cross-field theory. Double revolving theory is perhaps the easier of the two

5. Double revolving field theory A single-phase ac current supplies the main winding that produces a pulsating magnetic field. Mathematically, the pulsating field could be divided into two fields, which are rotating in opposite directions. The interaction between the fields and the current induced in the rotor bars generates opposing torque.

6. DOUBLE-FIELD REVOLVING THEORY 6 Electrical Machines

7. Resultant φ 𝑚 cos 2Ɵ flux φ r = 2 x = φ 2 2 𝑚 cos Ɵ 5

8. 2/12/2019 8 7.6 Single Phase Induction Motor Double revolving field theory The pulsating filed is divided a forward and reverse rotating field Motor is started in the direction of forward rotating field this generates small (0.5 %) positive slip Reverse rotating field generates a larger (1.95%) negative slip

9. 9 Single Phase Induction Motor 7.6.1 Double revolving field theory The three-phase induction motor starting torque inversely depends on the slip This implies that a small positive slip (0.01–0.03) generates larger torque than a larger negative slip (1.95–1.99 ) This torque difference drives the motor continues to rotate in a forward direction without any external torque.

10. 2/12/2019 7.6 Single Phase Induction Motor Double revolving field theory Each of the rotating fields induces a voltage in the rotor, which drives current and produces torque. An equivalent circuit, similar to the equivalent circuit of a three phase motor, can represent each field The parameters of the two circuits are the same with the exception of the slip.

11. 2/12/2019 11 7.6 Single Phase Induction Motor 7.6.1 Double revolving field theory The two equivalent circuits are connected in series. Figure 7.44 shows the equivalent circuit of a single-phase motor in running condition. The current, power and torque can be calculated from the combined equivalent circuit using the Ohm Law

12. 2/12/2019 12 7.6 Single Phase Induction Motor Figure 7.44 Equivalent circuit of a single-phase motor in running condition .

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14. DOUBLE-FIELD REVOLVING THEORY The torque–slip curve is shown below 14 Electrical Machines

15. Starting torque z e r o o f the Th e sin g l e - phase mo t or sta r t i n g t o r q ue is pulsating single-phase magnetic flux. The starting of the motor requires the generation of a rotating magnetic flux similar to the rotating flux in a three-phase motor. Two perpendicular coils that have currents 90° outof-phase can generate the necessary rotating magnetic fields which start the motor. Therefore, single-phase motors are built with two perpendicular windings.

16. The phase shift is achieved by connecting a resistance, an inductance, or a capacitance in series with the starting winding. Most frequently used is a capacitor to generate the starting torque.

17. Sin g le - p h a s e i n ductio n mo t ors a r e c l ass i f ie d a n d n a m ed according to the method employed to make them self-starting. Split-phase induction motor : ( started by two phase motor action through the use of an auxiliary or starting winding) Capacitor motors : ( started by two phase motor action through the use of an auxiliary and a capacitor) Capacitor-start induction run motor Capacitor run induction motor Capacitor-start and Capacitor run induction motor (3) Shaded-pole induction motor :(started by the motion of the magnetic field produced by means of shading coil around A portion of the pole structure)

18. Split phase induction motor 10 Two winding Main winding & auxiliary winding 90 degree phase angle Main winding – low resistance and high reactance Aux winding- high resistance and low reactance Switch = motor pickup its 75 % of its rated speed.

19. W ork i ng 11 Is lags by V because low reactance and high resistance Im lags V by very large angle due to high reactance Starting torque proportional to sin α Disconnect switch after reach its 75 % of rated speed

20. Centrifugal switch- operation It is connected in series with the auxiliary winding and mounted on the rotor shaft. When the motor started and reaches 75% of synchronous speed, CFS is mechanically opened and in running condition auxiliary winding remains cutout from the circuit. Therefore, the motor runs only with main winding.

21. Characteristics Starting torque – 1.5 times to 2 times the full load torque. Starting current – 6 to 8 times the full load current. Operating power factor – 0.55 to .65. Efficiency – 60-65% 5.Power rating-(1/20 to 1/2kW) 6. Speed range- 2875 rpm to 700 rpm

22. A p pl i c a ti o ns It has low starting current and moderate starting torque. It is used for easily started loads and typical applications include fans, saws, washing machines, grinders, blowers centrifugal pumps, oil burners, domestic refrigerators etc.

23. Capacitor induction motor Capacitor start motor Permanent capacitor motor Capacitor start capacitor run motor 15

24. Capacitor start motor 16 Electrolytic capacitor 90degree phase angle Im and Is Im lags V due to high reactance of main winding Due to capacitor Is lead V Im and Is angle 90 High starting torque Apps: Lathes, drilling machiine,fan etc

25. Characteristics Starting torque- 3.5 to 4.5 times the full load torque α-varies from 30˚ to 80˚-Starting torque can be improved Power factor varies from 0.65 to 0.75 Efficiency– 65 to 70% Power rating –1/10 to 3/4 kW

26. Requirements of capacitor Electrolytic capacitors are used as starting capacitors ,value around 50μF to a few hundred micro-farads. Capacitor designed for extremely short time duty service and is guaranteed for not more than 20 periods of operation/hr, each period not to exceed 3 seconds.

27. A p pl i c a ti o ns This motor has high starting torque and therefore is used for hard start loads such as pumps, compressors, refrigeration units, conveyors, certain machine tools etc.

28. Capacitor start, capacitor run motor 20

29. Characteristics Starting torque – 3 to 3.5 times the full load torque. Efficiency – 60% approx. Power factor-0.9 approx. Power rating- 1/8 to 3/4 HP Pull out torque- up to 260% of rated torque

30. A p pl i c a ti o ns It combines the advantages of capacitor start and permanent-capacitors and is used for hard to start loads such as compressors, refrigerators, pumps, conveyors etc. These motors are often employed requiring a quite operating motor such as in hospitals, studios.

31. Shaded-pole induction motor

32. Theory of operation Small motors requiring only small starting torque may be made self-starting by using low resistance copper bands called shading bands. The emf induced in the shading band causes a current to flow in the band. This current opposes the change of flux to which it is due and thus there is a time lag between the fluxes in the shaded and unshaded portion of the pole. The flux in the shaded portion of the pole lags the flux in the other portion. The results is like a rotating field moving in the direction from the unshaded to the shaded portion of the pole and a starting torque is produced.

33. Characteristics Low starting torque – 0.4 to 0.8 times the full load torque. Efficiency-35% approx. Powerfactor-0.45 approx. Pull out torque- 15% of full load torque. Power rating-1/300 to 1/20 HP (2.5W to 40W)

34. A p pl i c a ti o ns Because of their poor starting torque, low power factor and poor efficiency such motors are only suitable for low power applications such as for toys, portable fans , electric clocks, hair dryers, slide projectors, humidifiers, small business machines such as photo copying machines, vending machines, advertising displays etc .