Chapter 13 – AC Motors ppt

Chapter 13 – AC Motors AC motors convert AC electrical energy to Mechanical energy Chapter 13 – AC Motors • AC motors: the armature of rotor is a magnet (different to DC motors) the stator is formed by electromagnets (like in DC motors) Effects of AC Supply on Magnetic Poles • • • • • Consider the rotor to be a permanent magnet Current flowing through conductors energize the magnets and develop N and S poles The strength of electromagnets depends on current First half cycle current flows in one direction Second half cycle it flows in opposite direction As AC voltage changes, the poles alternate Using AC Supply to Make an Elementary Motor (1) • Consider the AC voltage at degrees, then, no current will flow, and there is no magnetism Using AC Supply to Make an Elementary Motor (2) • As voltage increases, current starts to flow and electromagnets gain strength and North and South poles appear • (Use left hand rule to find poles) • The rotor magnet is pushed CW, and the rotor and motor starts to rotate Using AC Supply to Make an Elementary Motor (3) • When voltage decreases, the current decreases also, the electromagnet loses the strength, and when V=0 there is no magnetism Using AC Supply to Make an Elementary Motor (4) • Now, AC voltage builds up as part of the negative cycle • Then, current flows in opposite direction, and the magnets reverse polarity • Therefore, the CW rotation continues Using AC Supply to Make an Elementary Motor (5) • This process is repeated over and over, as AC voltage goes through its cycles, and we have continuous rotation AC Motor Rotation The whole picture Limitation of the Elementary Motor • The initial position of the rotor determines the direction of the motor rotation • Indicate the rotation in the figures below: 10 Capacitor Start AC Motor (One phase + Capacitor) • Here the capacitor provides the phase difference • The difference is that the current in the star winding leads the current in the run winding (ICE) • Similar effect as with the inductor, but it creates a motor with higher starting power • Refrigerators, compressors, air conditioners 19 Three Types of Capacitor Start Motors Capacitor Start (disconnects capacitor after motor speed picks up) Capacitor Run (Keeps the capacitor connected during the operation of the motor, in order to keep the electric power consumption low) Capacitor Start-Run (uses two capacitors, one for starting and one for running This further improves Power Consumption) 20 Synchronous Speed • • • AC motors always rotate with the speed of their revolving magnetic field The speed of the revolving poles is the maximum possible speed of rotation of the motor It is called “Synchronous Speed” 21 Motor Construction The Stator • • The stator forms a hollow cylinder with coils of insulated wire inserted into slots of the stator core The coils, plus the steel core form the electromagnets 22 Motor Construction The Rotor • • • • • • There are two types of motor rotors: The wound rotor The squirrel cage The wound rotor has coils of wire wound in the slots of the rotor (Similar to generator coils) The “Squirrel cage” consists of bars of copper or aluminum electrically connected at each end with conducting rings As the rotor rotates inside a magnetic field, it receives electromagnetic induction, then current flows and form the rotor electromagnet 23 Types of Motor Enclosures ODP – Open Drip Proof TENV – Totally Enclosed Non-Ventilating TEFC – Totally enclosed Fan Cooled XP – Explosion Proof 24 Types of Motor Enclosures • ODP – Open Drip Proof – Air flows through motor (fan blades help flow) – Used in environments free from contaminants 25 Types of Motor Enclosures • TENV – Totally Enclosed Non-Ventilating – Protect motor from corrosive and harmful elements – Frame fins help to dissipate heat 26 Types of Motor Enclosures • TEFC – Totally enclosed Fan Cooled – Similar to TENV except has external fan for cooling 27 Types of Motor Enclosures • XP – Explosion Proof – Similar to TEFC but enclosures are cast iron 28 Slip • • • Slip is associated with synchronous speed If the motor turned at the same RPM as the magnetic field, there would be no relative motion between the rotor and the field Therefore, no current would be induced into the rotor, and no magnetic field would exist Rotor speed < synchronous speed Slip = synchronous speed – rotor speed % slip = ( Ns – Nr / Ns ) 100 29 Three Phase AC Motor • • It has three pairs of electromagnets, connected to one of the three phases of the power supply It provides a lot higher power that what single phase motor can deliver 30 AC Motor Data Plate • Each motor has a plate mounted on its frame, with electrical and mechanical information 31 32 33 .. .Chapter 13 – AC Motors • AC motors: the armature of rotor is a magnet (different to DC motors) the stator is formed by electromagnets (like in DC motors) Effects of AC Supply on... This phase difference makes motor work 17 Reactor Start AC Motor The Centrifugal Switch 18 Capacitor Start AC Motor (One phase + Capacitor) • Here the capacitor provides the phase difference • The... air conditioners 19 Three Types of Capacitor Start Motors Capacitor Start (disconnects capacitor after motor speed picks up) Capacitor Run (Keeps the capacitor connected during the operation