Topic 1 Three phase induction motors contents: Revise the principles of electromagnetism • Phase sequence • Three phase induction motor action • Induction motor construction • stator construction • rotor construction • Speed and slip • Line and phase voltages • Terminal block markings • Motor nameplate details • Motor rating • Star and Delta connections • Electrical isolation • Electrical testing and fault finding • Rotation reversal • Maintenance requirements
Topic Three phase induction motors • • • • • • • • • • • • • • • • Revise the principles of electromagnetism Phase sequence Three phase induction motor action Induction motor construction stator construction rotor construction Speed and slip Line and phase voltages Terminal block markings Motor nameplate details Motor rating Star and Delta connections Electrical isolation Electrical testing and fault finding Rotation reversal Maintenance requirements Electromagnets • It was discovered that when a current flows in a conductor, it creates a magnetic field around the conductor • The strength of the magnetic field is proportional to the current When current flows in a coil, the resultant magnetic fields around each conductor combine to create a magnet In this case the magnetic lines of force are entering the bottom and leaving the top This would make the bottom a south and the top a north Right hand grip rule • Fingers follow the direction of the current through the coil, and the thumb points to the north pole Core material • By placing steel into the centre of the coil, it increases the amount of magnetism created • The type of steel inserted also varies the amount of magnetism • Coils are therefore classified by their core material B/H curve Silicon Steel Flux density Cast steel Iron Air Magnetising force Inductance is an important component in electrical systems • • • • • • • It is used in: Motors Relays/contactors Valves Transformers Electromagnets Sensors Three phase generation requires three winding sets • These are evenly distributed around the stator • These are referred to as A, B and C or Red White and Blue (they were Red, Yellow and Blue) Yellow has been used to represent white phase If the magnet spins clockwise • The phase sequence shall be ABC If the magnet spins anti clockwise • The phase sequence shall be CBA What would the values be if one leg short circuits 10Ω 20Ω 10Ω 10Ω 0Ω 10Ω Click to edit Master text styles Second level Third level Fourth level Fifth level Nameplate details Manufacturer name and manufacturer-specific ordering code This code will typically contain information about frame size and mounting arrangement Rated volts - The voltage at which the motor is designed to operates and give optimum performance Full load amps –.used to facilitate cable sizing, starter selection and motor protection Rated frequency in Hertz, and rated full load speed in revs/minute at that frequency Rated temp rise or insulation class – Industry standard specification of the thermal tolerance of the motor insulation Rated power (kW) Electrical connection and associated operating parameters – i.e three phase motors can be connected in a ‘star’ (Υ) or ‘delta’ (Δ) configuration In a star configuration the current flowing from the supply is reduced as is the torque Power factor for the motor Efficiency class – IEC nameplates quote EFF ratings at full load It is a measure of how well the motor converts electrical energy to mechanical power ... machine moulded into the laminations Copper rotor bars Three windings 12 0° apart A1 phase supply 12 0° apart N s Rate of rotation • On a pole per phase machine as shown, one revolution will occur for... phase If the magnet spins clockwise • The phase sequence shall be ABC If the magnet spins anti clockwise • The phase sequence shall be CBA This can be represented as a phasor phase line and phase. .. test between windings (10 00V) Not less than 1MΩ A – M, B – N, C – L all same continuity A, B and C to earth @ 500V not less than 1MΩ A – B, B – C, C –A @ 10 00V not less than 1MΩ A – B, B – C, C