Bài giảng Electromechanical energy conversion - Chapter VIII: Single and Two - Phase Motors - Nguyễn Công Phương

NguyễnCôngPhương ELECTROMECHANICAL ENERGY  CONVERSION Single‐ and Two‐Phase Motors Contents I. Magnetic Circuits and Magnetic Materials II. Electromechanical Energy Conversion Principles III. Introduction to Rotating Machines IV. Synchronous Machines V. Polyphase Induction Machines VI. DC Machines VII.Variable – Reluctance Machines and Stepping Motors VIII.Single and Two – Phase Motors IX. Speed and Torque Control sites.google.com/site/ncpdhbkhn 2 Single- and Two-Phase Motors 1. Single-Phase Induction Motors: Qualitative Examination 2. Starting & Running Performance of Single- Phase Induction & Synchronous Motors 3. Revolving-Field Theory of Single-Phase Induction Motors 4. Two-Phase Induction Motors sites.google.com/site/ncpdhbkhn 3 Single-Phase Induction Motors: Qualitative Examination (1) • Structurally, the most common  Stator types of single-phase induction Iˆ winding motors resemble polyphase squirrel-cage motors except for Vˆ the arrangement of the stator Squirrel-cage windings.  rotor Fag1max Ftcos( ae )cos( e ) 1 F  Ftcos(  ) ag1max2 ae e 1 F  Ftcos(  ) ag1max2 ae e sites.google.com/site/ncpdhbkhn 4 Single-Phase Induction Motors: Qualitative Examination (2) sites.google.com/site/ncpdhbkhn 5 Single- and Two-Phase Motors 1. Single-Phase Induction Motors: Qualitative Examination 2. Starting & Running Performance of Single-Phase Induction & Synchronous Motors 3. Revolving-Field Theory of Single-Phase Induction Motors 4. Two-Phase Induction Motors sites.google.com/site/ncpdhbkhn 6 Starting & Running Performance of Single- Phase Induction & Synchronous Motors (1) • Single-phase induction motors are classified in accordance with their starting methods and are usually referred to by names descriptive of these methods. • Selection of the appropriate motor is based on – The starting- and running-torque requirements of the load, – The duty cycle of the load, and – The limitations on starting and running current from the supply line for the motor. • The cost of single-phase motors increases with their rating and with their performance characteristics such as starting-torque-to-current ratio. • Typically, in order to minimize cost, an application engineer will select the motor with the lowest rating and performance that can meet the specifications of the application. • Where a large number of motors are to be used for a specific purpose, a special motor may be designed in order to ensure the least cost. • In the fractional-kilowatt motor business, small differences in cost are important. sites.google.com/site/ncpdhbkhn 7 Single- and Two-Phase Motors 1. Single-Phase Induction Motors: Qualitative Examination 2. Starting & Running Performance of Single- Phase Induction & Synchronous Motors a) Split-Phase Motors b) Capacitor-Type Motors c) Shaded-Pole Induction Motors d) Self-Starting Synchronous-Reluctance Motors e) Hysteresis Motors 3. Revolving-Field Theory of Single-Phase Induction Motors 4. Two-Phase Induction Motors sites.google.com/site/ncpdhbkhn 8 Split-Phase Motors (1)  ˆ ˆ Imain ˆ Vˆ I Switch Iaux ˆ V Main winding Iˆ Iˆ Iˆ  aux main Auxiliary winding • Have 2 stator windings: – Main/run winding, and – Auxiliary/start winding. • The axes of these windings are displaced 90 electrical degrees in space. sites.google.com/site/ncpdhbkhn 9 Split-Phase Motors (2)  ˆ ˆ Imain ˆ Vˆ I Switch Iaux ˆ V Main winding Iˆ Iˆ Iˆ  aux main Auxiliary winding • The auxiliary winding has a higher resistance-to-reactance ratio than the main winding  the two currents will be out of phase. • The winding currents are equivalent to unbalanced two-phase currents, and the motor is equivalent to an unbalanced two-phase motor  a rotating stator field which causes the motor to start. • After the motor starts, the auxiliary winding is disconnected, usually by means of a centrifugal switch that operates at about 75 percent of synchronous speed. sites.google.com/site/ncpdhbkhn 10 Split-Phase Motors (3)  ˆ ˆ Imain ˆ Vˆ I Switch Iaux ˆ V Main winding Iˆ Iˆ Iˆ  aux main Auxiliary winding • The simple way to obtain the high resistance-to-reactance ratio for the auxiliary winding is to wind it with smaller wire than the main winding, a permissible procedure because this winding operates only during starting. • Split-phase motors have moderate starting torque with low starting current. • Typical applications include fans, blowers, centrifugal pumps, and office equipment. • Typical ratings are 50 to 500 watts; in this range they are the lowest-cost motors available. sites.google.com/site/ncpdhbkhn 11 Single- and Two-Phase Motors 1. Single-Phase Induction Motors: Qualitative Examination 2. Starting & Running Performance of Single- Phase Induction & Synchronous Motors a) Split-Phase Motors b) Capacitor-Type Motors c) Shaded-Pole Induction Motors d) Self-Starting Synchronous-Reluctance Motors e) Hysteresis Motors 3. Revolving-Field Theory of Single-Phase Induction Motors 4. Two-Phase Induction Motors sites.google.com/site/ncpdhbkhn 12 Capacitor-Type Motors (1)  Switch ˆ ˆ Iaux Iˆ Imain Vˆ Main Vˆ winding Iˆ C  aux ˆ Auxiliary winding ˆ I Imain • Capacitors can be used to improve motor starting performance, running performance, or both, depending on the size and connection of the capacitor. • The time-phase displacement between the two currents is obtained by means of a capacitor in series with the auxiliary winding. • The auxiliary winding is disconnected after the motor has started  the auxiliary winding and capacitor can be designed at minimum cost for intermittent service. sites.google.com/site/ncpdhbkhn 13 Capacitor-Type Motors (2)  Switch ˆ ˆ Iaux Iˆ Imain Vˆ Main Vˆ winding Iˆ C  aux ˆ Auxiliary winding ˆ I Imain • By using a starting capacitor of appropriate value, the auxiliary-winding current I ˆ at standstill can be made to aux ˆ lead the main-winding Imain current by 90 electrical degrees, as it would in a balanced two-phase motor. • In practice, the best compromise between starting torque, starting current, & cost typically results with a phase angle somewhat less than 90o. • Used for compressors, pumps, refrigeration and air-conditioning equipment, and other hard-to-start loads. sites.google.com/site/ncpdhbkhn 14 Capacitor-Type Motors (3)  Iˆ ˆ V Main winding  Auxiliary winding • Permanent-split-capacitor motor. • The capacitor and auxiliary winding are not cut out after starting. • The construction can be simplified by omission of the switch, and the power factor, efficiency, and torque pulsations improved. sites.google.com/site/ncpdhbkhn 15 Capacitor-Type Motors (4)  Switch Iˆ ˆ V Main winding  Auxiliary winding • Capacitor-start, capacitor-run motor. • Two capacitors are used: one for starting & one for running. • The small value of capacitance is permanently connected in series with the auxiliary winding. • The much larger value is connected in parallel with the running capacitor via a switch (opened when the motor comes up to speed). sites.google.com/site/ncpdhbkhn 16 Capacitor-Type Motors (5) Ex. 1  A 2.5-kW 120-V 60-Hz capacitor-start motor has the ˆ Switch ˆ Imain following impedances (at starting): Zmain = 4.5 + j3.7 Ω, I Z = 9.5 + j3.5 Ω. Find the value of starting capacitance ˆ aux V Main that will place the main & auxiliary winding currents in winding Iˆ C quadrature at starting?  aux Auxiliary winding 3.7 o main atan 39.6 4.5 ˆ Iaux  39.6oo 90.0 50.4 o Vˆ ZZjXjtotal aux c9.5 (3.5  X c ) Iˆ 3.5  X c o ˆ atan 50.4 Imain 9.5 X 15.0 c 1 C 177 F 11 2 60 ( 15.0) X   c CC260 sites.google.com/site/ncpdhbkhn 17 Single- and Two-Phase Motors 1. Single-Phase Induction Motors: Qualitative Examination 2. Starting & Running Performance of Single- Phase Induction & Synchronous Motors a) Split-Phase Motors b) Capacitor-Type Motors c) Shaded-Pole Induction Motors d) Self-Starting Synchronous-Reluctance Motors e) Hysteresis Motors 3. Revolving-Field Theory of Single-Phase Induction Motors 4. Two-Phase Induction Motors sites.google.com/site/ncpdhbkhn 18 Shaded-Pole Induction Motors • Usually has salient poles with one portion of each pole surrounded by a short-circuited turn of copper called a “shading coil”. • Induced currents in the shading coil cause the flux in the shaded portion of the pole to lag the flux in the other portion. • The result is similar to a rotating field moving in the direction from the unshaded to the shaded portion of the pole. • Currents are induced in the squirrel-cage rotor and a low starting torque is produced. • Their efficiency is low, but shaded-pole motors are the least expensive type of subfractional-kilowatt motor. • Ratings up to about 50 watts. sites.google.com/site/ncpdhbkhn 19 Single- and Two-Phase Motors 1. Single-Phase Induction Motors: Qualitative Examination 2. Starting & Running Performance of Single- Phase Induction & Synchronous Motors a) Split-Phase Motors b) Capacitor-Type Motors c) Shaded-Pole Induction Motors d) Self-Starting Synchronous-Reluctance Motors e) Hysteresis Motors 3. Revolving-Field Theory of Single-Phase Induction Motors 4. Two-Phase Induction Motors sites.google.com/site/ncpdhbkhn 20 Self-Starting Synchronous- Reluctance Motors • Anything which makes the reluctance of the air gap a function of the angular position of the rotor with respect to the stator coil axis will produce reluctance torque when the rotor is revolving at synchronous speed. • The stator may be polyphase or any one of the single-phase types described above. • The motor will start as an induction motor and at light loads will speed up to a small value of slip. sites.google.com/site/ncpdhbkhn 21 Single- and Two-Phase Motors 1. Single-Phase Induction Motors: Qualitative Examination 2. Starting & Running Performance of Single- Phase Induction & Synchronous Motors a) Split-Phase Motors b) Capacitor-Type Motors c) Shaded-Pole Induction Motors d) Self-Starting Synchronous-Reluctance Motors e) Hysteresis Motors 3. Revolving-Field Theory of Single-Phase Induction Motors 4. Two-Phase Induction Motors sites.google.com/site/ncpdhbkhn 22 Hysteresis Motors • The phenomenon of hysteresis can be used to produce mechanical torque. • The rotor is a smooth cylinder of magnetically hard steel, without windings or teeth. • The rotor is placed inside a slotted stator carrying distributed windings designed to produce as nearly as possible a sinusoidal space distribution of flux. • In single-phase motors, the stator windings usually are of the permanent-split-capacitor type. • The capacitor is chosen so as to result in approximately balanced two-phase conditions within the motor windings. • The stator then produces a primarily space- fundamental air-gap field revolving at synchronous speed. • Advantages: – It develops constant torque right up to synchronous speed. – It can synchronize any load which it can accelerate, no matter how great the inertia. – It is quiet & it produces smooth rotation of its load. sites.google.com/site/ncpdhbkhn 23 Single- and Two-Phase Motors 1. Single-Phase Induction Motors: Qualitative Examination 2. Starting & Running Performance of Single- Phase Induction & Synchronous Motors 3. Revolving-Field Theory of Single-Phase Induction Motors 4. Two-Phase Induction Motors sites.google.com/site/ncpdhbkhn 24