single-phase-induction-motor.ppt
- 1. 9/7/2022 360 Chapter 7 Induction Motor 1 EEE 360 Energy Conversion and Transport George G. Karady & Keith Holbert Chapter 7 Induction Motors
- 2. 9/7/2022 360 Chapter 7 Induction Motor 2 Lecture 22
- 3. 9/7/2022 360 Chapter 7 Induction Motor 3 Construction
- 4. 9/7/2022 360 Chapter 7 Induction Motor 4 7.6 Single Phase Induction Motor 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
- 5. 9/7/2022 360 Chapter 7 Induction Motor 5 7.6 Single Phase Induction Motor
- 6. 9/7/2022 360 Chapter 7 Induction Motor 6 7.6 Single Phase Induction Motor • This “single-phase” motors are truly two- phase machines. • The motor uses a squirrel cage rotor, which has a laminated iron core with slots. • Aluminum bars are molded on the slots and short-circuited at both ends with a ring. Stator with laminated iron core Slots with winding Bars Ring to short circuit the bars Starting winding + _ Main winding Rotor with laminated iron core + _ Figure 7.42 Single-phase induction motor.
- 7. 9/7/2022 360 Chapter 7 Induction Motor 7 7.6 Single Phase Induction Motor Figure 7.10 Squirrel cage rotor
- 8. 9/7/2022 360 Chapter 7 Induction Motor 8 7.6.1 Operating principle
- 9. 9/7/2022 360 Chapter 7 Induction Motor 9 7.6 Single Phase Induction Motor • The single-phase induction motor operation can be described by two methods: – Double revolving field theory; and – Cross-field theory. • Double revolving theory is perhaps the easier of the two explanations to understand • Learn the double revolving theory only
- 10. 9/7/2022 360 Chapter 7 Induction Motor 10 7.6 Single Phase Induction Motor 7.6.1 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
- 11. 9/7/2022 360 Chapter 7 Induction Motor 11 7.6 Single Phase Induction Motor • The interaction between the fields and the current induced in the rotor bars generates opposing torque. • Under these conditions, with only the main field energized the motor will not start • However, if an external torque moves the motor in any direction, the motor will begin to rotate. Starting winding Main winding +t -t Main winding flux Figure 7.43 Single-phase motor main winding generates two rotating fields, which oppose and counter-balance one another.
- 12. 9/7/2022 360 Chapter 7 Induction Motor 12 7.6 Single Phase Induction Motor 7.6.1 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 (5%) positive slip • Reverse rotating field generates a larger (1.95%) negative slip sy m sy pos n n n s ) ( sy m sy neg n n n s ) (
- 13. 9/7/2022 360 Chapter 7 Induction Motor 13 7.6 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. Tm_start s ( ) 3 Irot_t s ( ) 2 Rrot_t s 2 nsy
- 14. 9/7/2022 360 Chapter 7 Induction Motor 14 7.6 Single Phase Induction Motor 7.6.1 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.
- 15. 9/7/2022 360 Chapter 7 Induction Motor 15 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 • The calculations are demonstrated on a numerical example
- 16. 9/7/2022 360 Chapter 7 Induction Motor 16 7.6 Single Phase Induction Motor Forward rotating field Xsta/2 Vsta Ista Rrot(1-spos)/(2spos) Rsta/2 Rc/2 Xm/2 Xrot/2 Rrot/2 Reverse rotating field Xsta/2 Rrot (1-sneg )/(2sneg ) Rsta/2 Rc /2 Xm /2 Xrot/2 Rrot/2 Ipos Ineg Figure 7.44 Equivalent circuit of a single-phase motor in running condition.
- 17. 9/7/2022 360 Chapter 7 Induction Motor 17 7.6 Single Phase Induction Motor The results of the calculations are: – Input power: – Developed or output power: * sta sta in I V S neg neg rot pos pos rot dev s s R s s R P 1 2 1 2 2 2 neg pos I I
- 18. 9/7/2022 360 Chapter 7 Induction Motor 18 7.6 Single Phase Induction Motor nm 400rpm 410rpm 1780rpm 400 600 800 1000 1200 1400 1600 1800 0 100 200 300 400 500 Pmech nm W Pmot_dev nm W Pmech nrated W nm rpm Pdev_max nmax Operating Point Pmech_max Stable Operating Region Figure 7.47 Single-phase motor mechanical output power and electrically developed power versus speed.
- 19. 9/7/2022 360 Chapter 7 Induction Motor 19 7.6.2.2 Starting torque
- 20. 9/7/2022 360 Chapter 7 Induction Motor 20 7.6 Single Phase Induction Motor • The single-phase motor starting torque is zero because of the 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° out- of-phase can generate the necessary rotating magnetic fields which start the motor. • Therefore, single-phase motors are built with two perpendicular windings.
- 21. 9/7/2022 360 Chapter 7 Induction Motor 21 7.6 Single Phase Induction Motor • 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.
- 22. 9/7/2022 360 Chapter 7 Induction Motor 22 7.6 Single Phase Induction Motor • Figure 7.50 shows the connection diagram of a motor using a capacitor to generate the starting torque. • When the motor reaches the operating speed, a centrifugal switch turns off the starting winding. I C Main winding Rotor Centrifugal switch V Starting winding Figure 7.50 Single-phase motor connection.
- 23. 9/7/2022 360 Chapter 7 Induction Motor 23 7.6 Single Phase Induction Motor • The centrifugal switch is necessary because most motors use a cheap electrolytic capacitor that can only carry ac current for a short period. • A properly selected capacitor produces around 90° phase shift and large starting torque. I C Main winding Rotor Centrifugal switch V Starting winding Figure 7.50 Single-phase motor connection.
- 24. 9/7/2022 360 Chapter 7 Induction Motor 24 7.6 Single Phase Induction Motor Operating Point nm 0.1rpm 1rpm nsy 0 500 1000 1500 0 2 4 6 8 10 12 Tsm nm N m Tm nm N m T nm N m Tnom N m nm rpm Starting winding is disconnected Main winding generated torque Combined main and starting windings generated torque Figure 7.51 Torque–speed characteristic of a small single- phase induction motor.
- 25. 9/7/2022 360 Chapter 7 Induction Motor 25 7.6 Single Phase Induction Motor • A less effective but more economical method using shaded pole motors • The motor has two salient poles excited by ac current. • Each pole includes a small portion that has a short- circuited winding. This part of the pole is called the shaded pole. • The main winding produces a pulsating flux that links with the squirrel cage rotor. • This flux induces a voltage in the shorted winding.
- 26. 9/7/2022 360 Chapter 7 Induction Motor 26 7.6 Single Phase Induction Motor • The induced voltage produces a current in the shorted winding. • This current generates a flux that opposes the main flux in the shaded pole (the part of the pole that carries the shorted winding). • The result is that the flux in the unshaded and shaded parts of the pole will be unequal. • Both the amplitude and the phase angle will be different.
- 27. 9/7/2022 360 Chapter 7 Induction Motor 27 7.6 Single Phase Induction Motor • These two fluxes generate an unbalanced rotating field. The field amplitude changes as it rotates. • Nevertheless this rotating field produces a torque, which starts the motor in the direction of the shaded pole. • The starting torque is small but sufficient for fans and other household equipment requiring small starting torque. • The motor efficiency is poor but it is cheap
- 28. 9/7/2022 360 Chapter 7 Induction Motor 28 7.6 Single Phase Induction Motor • The motor has two salient poles excited by ac current. • Each pole includes a small portion that has a short-circuited winding. • This part of the pole is called the shaded pole Main winding Shorted coil Unshaded pole Unshaded pole Shaded pole Shaded pole Shorted coil Squirrel cage rotor Figure 7.52 Concept of single- phase shaded pole motor.
- 29. 9/7/2022 360 Chapter 7 Induction Motor 29 7.6 Single Phase Induction Motor Figure 7.53 Shaded pole motor for household fan.