AC Motors 29.1 AC Induction Motors• Simplest and most rugged electric motor • Consists of _____________and ____________• AC in the primary member (stator) ___________current in the secondary member (rotor)• Combined electromagnetic effects of the stator and rotor currents produce the force (torque) to create rotation. AC Motor Constructionhttp://www.sea.siemens.com/step/templates/lesson.mason?motors:3:1:1 LaminateAC Motor - Stator AC Motor - Rotor_______________ cage Three-Phase Electrical Power• Uses three sets of armature windings to produce three separate outputs• Armature windings are physically separated _______ from each other, and therefore, each phase is _______ apart from another• __________ power may be generated by a generator of a given size and weight Single-Phase v. Three-PhaseAC Motors 29.2 Delta and Wye Connections AC Motor Speed• The magnetic field rotates at the synchronous speed of the motor• Determined by the number of poles in the stator and the frequency of the AC power – ns= synchronous speed (in RPM), – f = frequency (in Hz), and – p = the number of poles AC Motor Speed• Synchronous speed is the absolute upper limit of motor speed. • When running, the rotor always rotates slower than the magnetic field (or no torque!)• The speed difference, or slip, is normally referred to as a % of synchronous speed: – s = slip (in %), – ns= synchronous speed– na= actual speed AC Motor - Speed vs. Torque0255075100% Rated Torque% Synchronous Speed100Locked rotor/starting torqueMinimumaccelerationtorqueService factorload / torqueDashed line applicable toinitial acceleration only Single-phase AC Motors• Single phase AC motors require a "trick" to generate a 2nd "phase" to develop ______________ _____________• Three common methods:– split-phase (auxiliary winding is rotated 90°)– capacitor– shaded-pole Split-Phase AC Motor01002003000 25 50 75 100% Synchronous Speed% Rated Torqueswitch opens02550751000 100 200 300% Rated Torque% Synchronous Speedswitch opensMotor starts with both main and auxiliary windingA centrifugal switch opens and removes the aux windingAC Motors 29.3 Disadvantages• Repeated start/stop cycles _________ the windings (high start resistance)• Less useful for large inertial loads• Requires ________ __________ to handle starting currentsSplit-Phase AC MotorAdvantages• Operate at ________ speed, 4 pole, 60 Hz:– 1780 RPM (no load)– 1700/1725 RPM at full load• ____________ at low speed• Rapid acceleration• Relatively low cost Single-Phase Capacitor MotorsACRotorMainwindingAuxiliary windingCapacitor• Permanent split capacitor (__________)• Capacitor-start (later switched out)• Start-capacitor, run-capacitor (switched) Permanent Split Capacitor (PSC)01002003000 25 50 75 100% Synchronous Speed% Rated Torque02550751000 100 200 300% Rated Torque% Synchronous Speed Disadvantages• More _____________ for same HP• ______________ performance when starting• Need to always use manufacturer's desired capacitor valuePermanent Split Capacitor (PSC)Advantages• ________, smoother than split phase• Reduced starting current– Longer life– Higher reliability• Capable of ________ start/stop cycles Shaded Pole AC Motor01002003000 25 50 75 100% Synchronous Speed% Rated Torque02550751000 100 200 300% Rated Torque% Synchronous Speed Disadvantages• Low starting and running torque• Low _____________• Available in sub-fractional to ~ 1/4 hp sizesShaded Pole AC MotorAdvantages• _________in design and construction• Suitable for ______ __________, high volume app's• Relatively quiet and free from vibration• "Fail safe" design -starts in only 1 directionAC Motors 29.4 NEMA - National Electrical Manufacturers Association• NEMA is responsible for several North American electric motor industry "standards"• ______ _________ (1/4 , 1/2 , 1 )• Frame size– diameter, length, shaft size, etc.• ____________ _______________• Housing/protection types and ratings IEC – International Electrotechnical Commision• IEC is responsible for European and Asian electric motor standards• Similar to (but not the same as!) NEMA standards Service Factors• A multiplier ( < ) applied to the rated ______________________• Indicate how much the motor can be overloaded without overheating• Generally used for– handling a known, occasional overload– provide a _________________________ where environment or service condition is not well known Motor Enclosures• DP - dripproof• DPFG - dripproof, fully guarded• SP - splashproof• FV - forced ventilation (separate/attached fan)• TENV - totally enclosed, non-ventilated• TEFC - totally enclosed, fan cooled• TEUC - totally enclosed, unit cooled (heat-X)http://www.sea.siemens.com/step/templates/lesson.mason?motors:4:3:1 AC Motor Efficiency• Efficiency, η• Small universal motors have η~ 30%• Large 3-phase motors have η~ 95%• Depends on actual motor load vs. rated load– efficiency best near rated load– efficiency drops rapidly for both under- and over-load