Automation and Drives A&D SD Customer Service & Support A&D SD CST 23/11/04 1 of 14 10A MotorTheory.ppt Motor Theory Motor Construction Motor Theory Drives Training Foils © Siemens AG, A&D SD CS Congleton, 2004

Automation and Drives A&D SD Customer Service & Support A&D SD CST 23/11/04 2 of 14 10A MotorTheory.ppt Motor Theory Motor Construction Induction Motor Operation Rotor windings built into Iron Rotor Air Gap Stator windings in outer fixed Iron Core The Three phase ac voltage in the stator windings sets up a rotating magnetic field which crosses the air gap and induces current in the rotor winding. The resultant current acts with the field to produce a rotational force

Automation and Drives A&D SD Customer Service & Support A&D SD CST 23/11/04 3 of 14 10A MotorTheory.ppt Motor Theory Motor Construction Torque - Speed Chart of Induction Motor Constant Flux Range Field Weakening Range Maximum Torque Nominal Torque

Automation and Drives A&D SD Customer Service & Support A&D SD CST 23/11/04 4 of 14 10A MotorTheory.ppt Motor Theory Motor Construction Practical meaning of this graph If overloaded too much, the motor will stall ‘pull out’ but continue to draw high current. The motor will continue to run in overload, and may overheat At frequencies above ‘normal’, Maximum torque is greatly reduced At lower frequencies full torque is available

Automation and Drives A&D SD Customer Service & Support A&D SD CST 23/11/04 5 of 14 10A MotorTheory.ppt Motor Theory Motor Construction Voltage and Frequency Requirements A Linear Voltage to Frequency Curve is suitable for most Applications. A Quadratic curve can be used with pumps and fans to save energy by reduction of magnetising losses. Special curves can be programmed for special motors and applications. At higher frequencies higher voltage would be desirable, but is not usually possible. Vector and Flux Current control systems control the flux levels independently. Because an induction motor works like a transformer, the voltage must be reduced if the frequency is reduced.

Automation and Drives A&D SD Customer Service & Support A&D SD CST 23/11/04 6 of 14 10A MotorTheory.ppt Motor Theory Motor Construction Speed Torque Characteristics of Motors and Loads Pump and Fan Constant Torque Torque

Automation and Drives A&D SD Customer Service & Support A&D SD CST 23/11/04 7 of 14 10A MotorTheory.ppt Motor Theory Motor Construction Fan Bearings Connection Box Winding/ Stator Rotor Shaft Air Gap

Automation and Drives A&D SD Customer Service & Support A&D SD CST 23/11/04 8 of 14 10A MotorTheory.ppt Motor Theory Motor Construction Fan Terminal box Terminal board Screwed gland D-end ball bearing Featherkey Shaft end D-end end shield Enclosure Feet Fan cowl Mechanical Design

Automation and Drives A&D SD Customer Service & Support A&D SD CST 23/11/04 9 of 14 10A MotorTheory.ppt Motor Theory Motor Construction Squirrel-cage rotor Stator – Rotor and shaft inside Stator winding (copper) Laminated stator core with insulated sheet steel End rings of rotor winding (alu.) Laminated rotor core with insulated sheet steel Electrical Design - Live Parts

Automation and Drives A&D SD Customer Service & Support A&D SD CST 23/11/04 10 of 14 10A MotorTheory.ppt Motor Theory Motor Construction Electrical energy U,J Mechanical energy M,n Mode of functioning - Energy Converter A low-voltage motor converts electrical energy into mechanical energy.

Automation and Drives A&D SD Customer Service & Support A&D SD CST 23/11/04 11 of 14 10A MotorTheory.ppt Motor Theory Motor Construction Three-phase system: The three-phase system consists of three sinusoidal voltages, the amplitudes of which are offset by 120°. The system frequency is either 50 or 60 Hz, depending on the country. Transformer principle: The stator and rotor function as a transformer when they are at rest. l Primary winding: stator l Secondary winding:squirrel-cage rotor Three-phase asynchronous motor: l A voltage is applied to the stator winding as a result of which a current flows and a magnetic field is generated. The magnetic field is transmitted from the laminated stator core to the laminated rotor core via the air gap. l The rotating stator field induces a voltage in the rotor winding. l The voltage induced in the rotor causes a current to flow in the squirrel-cage rotor. l The rotating stator field together with the rotor field generate a torque. l The motor shaft rotates. As the motor shaft speed approaches the speed of the rotating stator field (synchronous operation) the potential torque is reduced. lTherefore the motor always runs a little slower than the applied frequency. This is known as the slip. Principles of a Three-Phase Asynchronous Motor

Automation and Drives A&D SD Customer Service & Support A&D SD CST 23/11/04 12 of 14 10A MotorTheory.ppt Motor Theory Motor Construction n s = f x 60 p n s = f x 60 p No. of poles No. of pole pairs n S at 50 Hzn S at 60 Hz min min min min min min min min -1 nS= synchronous motor speed f = system frequency p = number of pole pairs Mode of functioning - Number of Poles The mechanical constuction and the number of windings also determines the motor speed. Actual speed of is always less than the synchronous speed (slip), and is dependent on load

Automation and Drives A&D SD Customer Service & Support A&D SD CST 23/11/04 13 of 14 10A MotorTheory.ppt Motor Theory Motor Construction M N Rated-load torque M M Motor torque M L Load torque M B Accelerating torque M A Locked-rotor torque M K Breakdown torque M S Pull-up torque n N Rated speed n S Synchronous speed Motor and Load Torque Characteristics

Automation and Drives A&D SD Customer Service & Support A&D SD CST 23/11/04 14 of 14 10A MotorTheory.ppt Motor Theory Motor Construction Siemens AG Standard Drives Martin Brown A&D SD CS Congleton