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 (brush-shifting 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 three-phase 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