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Transcript
Chapter 19 AC Motors Objectives After studying this chapter, you will be able to: • Describe the construction of various types of AC motors • Explain the principle of operation of various types of three-phase motors and single-phase motors Objectives (cont’d.) • Discuss the reasons for the difference in the values of starting and running currents in AC motors AC Motor Construction • Induction motor is most common AC motor – Two main components: stator and rotor – Stator has electromagnets secured to frame – Rotor made of steel lamination in shape of cylinder • Windings placed in slots on rotor surface • Squirrel-cage rotor winding – Heavy copper bars welded to end rings AC Motor Construction (cont’d.) • Wound-rotor motor – Copper wire wound into rotor slots – Connected in wye – Ends connected to slip rings on shaft Three-Phase Motor Theory • Two-pole, three phase stator winding – When energized from three-phase source, three-phase currents are 120 degrees apart and continuously changing in value and direction – The effect of this variation in strength and direction produces a rotating field – See Figure 19-4C Three-Phase Motor Theory (cont’d.) • Speed of the rotating magnetic field – Synchronous speed of two-pole induction motor is 60 revolutions per second when supplied with 60 Hz. • Lower speeds obtained with more poles Three-Phase Motor Theory (cont’d.) • Rotor speed – Depends on synchronous speed and load – Rotor speed lags behind synchronous speed one percent (no load) to five percent (full load) • Direction of rotor rotation – Depends on the phase sequence – Revolves in same direction as stator field Three-Phase Motor Theory (cont’d.) • Torque – Varies with the strength of the stator and rotor fields – Also affected by phase relationship between the two fields Three-Phase Motor Theory (cont’d.) • Slip – Difference between synchronous speed and rotor speed – May be stated in rpm or a percentage Three-Phase Motor Starting and Running Current • Starting current is limited to four to six times the full load current • Loading a squirrel-cage motor – For large motors, reduced voltage starters are used to limit the starting current to a lower value that will not cause line drops Three-Phase Motor Starting and Running Current (cont’d.) • Double squirrel-cage rotor – Has two squirrel-cage windings – Inner winding has low resistance and high inductance – Outer winding has high resistance and low inductance – Results in large current and high power factor in outer winding; high starting torque Types of Three-Phase Motors • Multispeed squirrel-cage motors – Number of poles may be varied by changing external connections • Wound-rotor induction motors – Provides high starting torque at low current value – Accelerates smoothly under heavy loads – Poor speed regulation when operating with resistance in the rotor circuit Types of Three-Phase Motors (cont’d.) • Adjustable-speed induction motor (brushshifting motor) – Provides a wide range of speeds, depending on position of the brushes • High-frequency motors – Operate at speeds greater than can be obtained from a 60Hz supply Types of Three-Phase Motors (cont’d.) • Synchronous motors – Operates at constant speed from no load to full load – Power factor can be controlled by varying amount of current in the exciter winding – Generally used for driving loads requiring constant speeds and infrequent starting and stopping Single-Phase Motors • Disadvantages when compared to polyphase motors – Torque developed is not as smooth – Larger in physical size and less efficient Starting Single-Phase Motors • Phase-splitting principle: one method of making a single-phase motor self-starting – Stator has two windings: main and auxiliary – Two currents out of phase by less than 90 degrees • Resistance split-phase motor – Resistance connected in series with auxiliary winding, resulting in better starting torque Starting Single-Phase Motors (cont’d.) • Capacitor split-phase motors – Three classes: 1) low starting torque, 2) capacitor start, induction run, 3) capacitor start, capacitor run • Reversing split-phase motors – Reverses direction of rotor rotation by reversing the polarity of one of the fields Starting Single-Phase Motors (cont’d.) • Shaded-pole motors – Started by a low-resistance, short-circuited coil placed around one tip of each pole – Have poor starting torque • Repulsion motors – Starts based on the principle that like poles repel Starting Single-Phase Motors (cont’d.) • Series AC motors – More complex structure and heavier per horsepower – More expensive than DC motors of the same rating – Operating characteristics very similar to DC motors Universal Motors • Series motor that will operate on both AC and DC – Manufactured in small sizes with fractional horsepower ratings – Used for vacuum cleaners, portable electrical tools, and small household appliances Selection of Single-Phase Motors • Shaded-pole motors adequate for small fans, measuring instruments and other applications requiring little starting torque • Split-phase motors can be used for large machines that start without a load (small lathes, drills, grinders) • Capacitor and repulsion motors for machines that start under load Torque Motors • Have greatest torque output when stalled • Applications: opening and closing valves, dampers, doors, gates, windows, etc. • Large torque motors operate on threephase – Small ones versions of universal motor • Not intended for continuous duty Dual-Voltage Windings • Motors that can operate on either of two voltages – Accomplished by dividing each phase into two sections (either delta or wye) Multispeed Induction Motors • Synchronous speed of induction motor depends on supply frequency and number of poles • Changing speed by varying frequency requires use of variable frequency drive and means of adjusting motor current • Solid-state controller can be used to accomplish this Summary • Induction motor made up of stator and rotor • Types of three-phase motors include multispeed squirrel-cage, wound-rotor induction, adjustable speed induction, high-frequency and synchronous motors • Single-phase motors are larger in size and less efficient than polyphase motors Summary (cont’d.) • Universal motors can operate on either AC or DC • Dual voltage windings can operate on either of two voltages • Multispeed induction motors offer varying speeds by changing frequency or number of poles