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Lesson 6
Lesson 6
Lesson 6
Lesson 6
Lesson 6
Lesson 6
Lesson 6
Lesson 6
Lesson 6
Lesson 6
Lesson 6
Lesson 6
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Lesson 6

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  • 1. Ho Chi Minh University of IndustryLesson 6:SINGLE-PHASE AC INDUCTION MOTORSContinue:Reduced:Disconnected:Connected:Generate:Starting:commonshaded pole motor:Reading passage 1:A poly-phase induction motor will continue to rotateeven if one phase is disconnected, at reduced torque.However, a poly-phase motor at standstill will notgenerate any net starting torque if connected only to asingle-phase supply. The key to the design of single-phase motors, then, is to provide a rotating magneticfield to produce starting torque.A common single-phase motor is the shaded pole motor,which is used in devices requiring lower torque, such aselectric fans or other small household appliances. In thismotor, small single-turn copper "shading coils" createthe moving magnetic field. Part of each pole is encircledby a copper coil or strap; the induced current in the strapopposes the change of flux through the coil (Lenzs Page 22
  • 2. Ho Chi Minh University of IndustryLaw), so that the maximum field intensity moves acrossthe pole face on each cycle.Because it has but a single alternating current source, asingle-phase motor can only produce an alternatingfield: one that pulls first in one direction, then in theopposite as the polarity of the field switches. A squirrel-cage rotor placed in this field would merely twitch,since there would be no moment upon it. If pushed inone direction, however, it would spin.The major distinction between the different types ofsingle-phase AC motors is how they go about startingthe rotor in a particular direction such that thealternating field will produce rotary motion in thedesired direction. This is usually done by some devicethat introduces a phase-shifted magnetic field on oneside of the rotor. • Work in group to discuss about these details as said below :1. What will happen if a poly-phase induction motor is disconnected one phase?2. What will happen if a poly-phase induction motor is connected to single phase supply.3. What is shaded pole motor. Give the applications.4. The types of single-phase AC motors are depended on which feature? Page 23
  • 3. Ho Chi Minh University of Industry5. Application of single phase motors...................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................Reading passage 2: Split-Phase MotorsThe split phase motor achieves its starting capability byhaving two separate windings wound in the stator. Thetwo windings are separated from each other. Onewinding is used only for starting and it is wound with asmaller wire size having higher electrical resistance thanthe main windings. From the rotors point of view, thistime delay coupled with the physical location of thestarting winding produces a field that appears to rotate.The apparent rotation causes the motor to start. Acentrifugal switch is used to disconnect the startingwinding when the motor reaches approximately 75% ofrated speed. The motor then continues to run on thebasis of normal induction motor principles. Page 24
  • 4. Ho Chi Minh University of IndustryThe phase of the magnetic field in this startup windingis shifted from the phase of the mains power, allowingthe creation of a moving magnetic field which starts themotor. Once the motor reaches near design operatingspeed, the centrifugal switch activates, opening thecontacts and disconnecting the startup winding from thepower source. The motor then operates solely on therunning winding. The starting winding must bedisconnected since it would increase the losses in themotor. • Fill in the blanks by using information from passage 2:In the split phase motor, the startup winding is designedwith a higher ....................than the ....................winding. This creates an LR circuit which slightly shiftsthe phase of the current in the startup winding. Whenthe motor is starting, the startup winding is connected tothe power source via a set of spring-loaded contactspressed upon by the not-yet-rotating ..................... Thestarting winding is wound with fewer turns of smallerwire than the .................... winding, so it has a lowerinductance (L) and higher .................... (R). The lowerL/R ratio creates a small phase shift, not more thanabout 30 degrees, between the flux due to the.................... winding and the flux of the ....................winding. The starting direction of rotation may be.................... simply by exchanging the connections ofthe startup winding relative to the running winding. Page 25
  • 5. Ho Chi Minh University of Industry • Complete the diagram as shown below :Reading passage 3: Capacitor-Start MotorsCapacitor start motors form the largest single groupingof general purpose single phase motors. These motorsare available in a range of sizes from fractional through3HP. The winding and centrifugal switch arrangement isvery similar to that used in a split phase motor. Themain difference being that the starting winding does nothave to have high resistance. In the case of a capacitorstart motor, a specialized capacitor is utilized in a serieswith the starting winding. The addition of this capacitorproduces a slight time delay between the magnetizationof starting poles and the running poles. Thus theappearance of a rotating field exists. When the motorapproaches running speed, the starting switch opens and Page 26
  • 6. Ho Chi Minh University of Industrythe motor continues to run in the normal inductionmotor mode.This moderately priced motor produces relatively highstarting torque, 225 to 400% of full load torque. Thecapacitor start motor is ideally suited for hard to startloads such as conveyors, air compressors andrefrigeration compressors. Due to its general overalldesirable characteristics, it also is used for manyapplications where high starting torque may not berequired. The capacitor start motor can usually berecognized by the bulbous protrusion on the framewhere the starting capacitor is located. • True/ False:1. The winding and centrifugal switch arrangement is very dissimilar to that used in a split phase motor.2. The starting winding of capacitor start motors does not have to have high resistance.3. The capacitor is utilized in a series with the starting winding.4. A slight time delay between the magnetization of starting poles and the main poles. Page 27
  • 7. Ho Chi Minh University of Industry5. When the motor approaches running speed, the starting switch opens and the motor discontinues to run in thenormal induction motor mode. • Complete the diagram as shown below : Page 28
  • 8. Ho Chi Minh University of IndustryReading passage 4: Permanent-Split Capacitor MotorsAnother variation is the Permanent Split-Capacitor(PSC) motor. This motor operates similarly to thecapacitor-start motor described above, but there is nocentrifugal starting switch and the second winding ispermanently connected to the power source. Bychanging taps on the running winding but keeping theload constant, the motor can be made to run at differentspeeds. The capacitor of this motor is left in series withthe starting winding during normal operation. Thestarting torque is quite low, roughly 40% of full-load, soPSC motors are frequently used in air handlers, fans,and blowers and other cases where a variable speed isdesired. • Complete the diagram as shown below : Page 29
  • 9. Ho Chi Minh University of Industry Single-phase AC synchronous motors The shaded pole motor is the simplest of all single phase starting methods. In the shaded pole motor, the stator poles are notched and a copper short circuiting ring is installed around a small section of the poles. As a result of the alteration of the filed pole configuration, the build-up of the magnetic field is delayed in the portion of the pole surrounded by the copper shorting ring. From the rotors point of view, this makes the magnetic field seem to rotate from the main pole toward the shaded pole. This slight appearance of field rotation is adequate to start the rotor moving and, once started, it will accelerate up to full speed. The shaded pole motor is simple and inexpensive, but has low efficiency and a very low starting torque.Speed regulation is poor, and it must be fan-cooledduring normal operation. Shaded-pole motors are thus Page 30
  • 10. Ho Chi Minh University of Industryused in shaft-mounted fans and blowers, and also smallpumps, toys, and intermittently used household items. Page 31
  • 11. Ho Chi Minh University of IndustryGrammar: • Article : A / An............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................ • Prepositions: up, of, with, into....etc............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................ • Conjunctions:............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................ • Passive voice:........................................................................................................................................................................................................................................................................................................................................................................ Page 32
  • 12. Ho Chi Minh University of Industry............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................ • Comparision:........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................ Page 33

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