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Ac motors

AC motor notes for semester 3 building services eng student

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Ac motors

  1. 1. An AC motor is an electric motor driven by an alternating current.<br />AC MOTORS<br />
  2. 2. TYPES OF AC MOTORS<br />There are two main types of AC motors, depending on the type of rotor used.<br /> The first type is the induction motor, which runs slightly slower than the supply frequency. The magnetic field on the rotor of this motor is created by an induced current.<br /> The second type is the synchronous motor, which does not rely on induction and as a result, can rotate exactly at the supply frequency or a sub-multiple of the supply frequency. The magnetic field on the rotor is either generated by current delivered through slip rings or by a permanent magnet. Other types of motors include eddy current motors, and also AC/DC mechanically commutated machines in which speed is dependent on voltage and winding connection.<br />
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  4. 4. BASIC PRINCIPLE<br />When the rotor rotates, the stator conductors are cut by the magnetic flux, hence they have induced e.m.f produced in them.<br />Because the magnetic poles are alternatively N and S, they induced an e.m.f and hence current in armature conductors which first flow in one direction and then in the order.<br />
  5. 5. CONSTRUCTION<br />ROTOR<br /> 1. Salient pole <br /> It is used in low and medium speed alternators<br /> 2. Smooth cylindrical <br /> It is used for steam turbine driven alternators. The rotor consists of a smooth solid forged steel cylinder having a number<br />STATOR<br /> In alternator, it is used for holding the armature stampings and windings in position<br />
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  7. 7. SINGLE PHASE MOTOR<br />To operated on single phase supply and for use in numerous application like ceiling fans, refrigerators, food mixers ect.<br />A single phase induction motor physically looks similar to that of a three phase induction motor except that its stator is provided with a single phase winding.<br />The rotor of any single phase induction motor is interchangeable with that of a polyphase induction motor<br />A single phase winding would produce no rotating magnetic field and no starting torque.<br />In the stator of a single phase motor is provided with an extra winding known as starting <br />
  8. 8. 3 PHASE MOTOR<br />Induction AC Motors: Induction ac motors are the simplest and most rugged electric motor and consists of two basic electrical assemblies: the wound stator and the rotor assembly. <br />The induction ac motor derives its name from currents flowing in rotor that are induced by alternating currents flowing in the primary member (stator).<br /> The combined electromagnetic effects of the stator and rotor currents produce the force to create rotation.<br />
  9. 9. AC motors typically feature rotors, which consist of a laminated, cylindrical iron core with slots for receiving the conductors.<br /> The most common type of rotor has cast-aluminum conductors and short-circuiting end rings. <br />This ac motor "squirrel cage" rotates when the moving magnetic field induces a current in the shorted conductors. <br />The speed at which the ac motor magnetic field rotates is the synchronous speed of the ac motor and is determined by the number of poles in the stator and the frequency of the power supply: ns = 120f/p, where ns = synchronous speed,    f = frequency, and     p = the number of poles.<br />
  10. 10. Synchronous speed is the absolute upper limit of ac motor speed. <br />If the ac motor's rotor turns exactly as fast as the rotating magnetic field, then no lines of force are cut by the rotor conductors, and torque is zero. <br />When ac motors are running, the rotor always rotates slower than the magnetic field. <br />The ac motor's rotor speed is just slow enough to cause the proper amount of rotor current to flow, so that the resulting torque is sufficient to overcome windage and friction losses, and drive the load. <br />The speed difference between the ac motor's rotor and magnetic field, called slip, is normally referred to as a percentage of synchronous speed: s = 100 (ns - na)/ns, where s = slip, ns = synchronous speed, and na = actual speed.<br />
  12. 12. STARTING METHODS FOR INDUCTION MOTORS<br />Squirrel cage motor<br /> Direct on line starting<br /> Starting current is high and may cause interference with supplies to other consumers.<br />
  13. 13. Auto transformers starting<br />Used to reduce the stator voltage.<br />
  14. 14. STAR DELTA STARTING<br />With this method for starting the connections to the stator phase winding are star connected<br />For the running the winding are switched <br />
  15. 15. SQUARE CAGE MOTOR<br />In this type a cooper or aluminium bars are placed in slots cut in the laminated iron rotor core.<br />This type of rotor has no external connections which means that slip rings and brushes are not needed<br />
  16. 16. AVANTAGES OF SQUIRREL-CAGE <br /> The advantages of squirrel –cage motors compared with the wound rotor type are that they :<br />Are cheaper and more robust<br />Have slightly higher efficiency and power factor.<br />Are explosion proof, since the risk of sparking is eliminated by the absence of slip rings and brushes<br />
  17. 17. ROTOR WOUND MOTOR<br />A wound rotor induction motor has a stator like the squirrel cage induction motor, but a rotor with insulated windings brought out via slip rings and brushes. <br />However, no power is applied to the slip rings. Their sole purpose is to allow resistance to be placed in series with the rotor windings while starting. This resistance is shorted out once the motor is started to make the rotor look electrically like the squirrel cage counterpart. <br />
  18. 18. ADVANTAGES OF WOUND ROTOR INDUCTION MOTORS<br />The advantages of the wound rotor motor compared with the cage type are that they :<br />Have a much higher starting torque<br />Have a much lower starting current<br />Have a means of varying speed by use of external rotor resistance.<br />

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AC motor notes for semester 3 building services eng student


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