ECET 211 Electric Machines & Controls Lecture 5-1 Electric Motors (2 of 4) Text Book: Chapter 5 Electric Motors, Electric Motors and Control Systems, by Frank D. Petruzella, published by McGraw Hill, 2015. Paul I-Hai Lin, Professor of Electrical and Computer Engr. Tech P.E. States of Indiana & California Dept. of Computer, Electrical and Information Technology Purdue University Fort Wayne Campus Prof. Paul Lin 1 Lecture 5-1 Electric Motors Chapter 5. Electric Motors Part 1. Motor Principles Part 2. Direct Current Motors Part 3. Three-Phase Alternating Current Motors Part 4. Single-Phase Alternating Current Motors Part 5. Alternating Current Motor Drives Part 6. Motor Selection Part 7. Motor Installation Part 8. Motor Maintenance and Troubleshooting Prof. Paul Lin 2 1
Part 3 Three-Phase AC Motors Rotating Magnetic Field Induction Motor Wound-Rotor Induction Motor Three-Phase Synchronous Motor Prof. Paul Lin 3 Youtube videos Part 3 Three-Phase AC Motors Baldor Athens, Georgia Plant (AC Induction Motor small HP, higher volume ), 11:59 minutes, video, https://www.youtube.com/watch?v=cbfe- Bt7RjY Baldor King Mountain, North Carolina Plant (Large AC Motors: 300-3500 Hp), 12: 17 min, video, https://www.youtube.com/watch?v=h0geugzewiu Baldor Columbus, Mississippi Plant (Steel Band & Cast Iron Frame, AC Motors, Fixed & Adjustable Speed NEMA 250T-360T Frames), 10:23 minutes, Video, Baldor Gainesville, Georgia Plant, (Adjustable Speed AC & DC Motors, Navy, Nuclear and Mine Motors), 11:54 min video, https://www.youtube.com/watch?v=lltu1thkezg Baldor Fort Smith, Arkansas Motor Plant (Rotor), 6:57 minutes, https://www.youtube.com/watch?v=rreyh7h7vha TES Generators and Motors Production of Electric Machines, 10:55 minutes, video, https://www.youtube.com/watch?v=5mu42tzhy8m Prof. Paul Lin 4 2
Part 3. Three-Phase AC Motors Rotating Magnetic Field Stator s magnetic field is made to rotate electrically around and around in a circle Rotor s magnetic field is to made to follow the rotation of the stator s field pattern by being attracted and repelled by the stator field Figure 5-34 Rotating magnetic field Prof. Paul Lin 5 Part 3. Three-Phase AC Motors Rotating Magnetic Field Synchronous speed The synchronous speed, Ns, of an AC motor is the speed of the stator s magnetic field rotation. Ns = 120*f/P Ns rmp; f Hz, P = # of pole wound in each of the single-phase windings Actual rotor peed This is the speed at which the shaft rotates. It is the motor s nameplate speed. Example 5-3: Determine the synch. Speed of a 4-pole AC motor connected to a 60 Hz electrical supply. Figure 5-35 Rotating magnetic field Prof. Paul Lin 6 3
Part 3. Three-Phase AC Motors Rotating Magnetic Field Slip The rotor does not revolve at synchronous speed, but tends to slip behind. The slip is required for the induction motor to run. No slip => no relative motion => no induced voltage The rotor induced voltage has a frequency of f r = s*f Percentage slip = (Ns Nr)/Nr x 100 Example 5-4: Determine the slip of an induction motor having a synchronous speed of 1,800 rpm and rated actual speed of 1750 rpm. (Answer: 2.78%) Prof. Paul Lin 7 Part 3. Three-Phase AC Motors Induction Motor Rotor Induced Current No external voltage is applied to its rotor The AC current in the stator induces a voltage across an airgap and into the rotor winding to produce rotor current and associated magnetic field. Figure 5-36 Induced rotor current Prof. Paul Lin 8 4
Part 3. Three-Phase AC Motors Induction Motor Stator Coil Groupings - Phases A three-phase motor stator windings consists of three separate groups of coils, called phases, and designated A, B, and C. The phases are displaces from each other by 120 electrical degrees, and contain the same number of coils, connected for the same number of poles. Figure 5-37 Stator coils for a Y- connected four-pole 3Φ induction motor Prof. Paul Lin 9 Part 3. Three-Phase AC Motors Squirrel-Case Induction Motor Figure 5-38 Squirrel-case induction motor Rotor Constructed using a number of single bar short-circuited by end rings and arranged in hamster-wheel or squirrel-cage configuration. Induced rotor current establish their own magnetic field, which interacts with the stator magnetic field to produce a torque. The resultant production of torque spins the rotor in the same direction as the rotation of the magnetic field produced by the stator Prof. Paul Lin 10 5
Part 3. Three-Phase AC Motors Squirrel-Case Induction Motor Rotor Resistance and the Starting Torque The resistance of the squirrel-cage rotor has an important effect on the operation of the motor. High resistance rotor => develops a high starting torque at low starting current Low resistance rotor => develops low slip and high efficiency at full load NEMA-Type Squirrel-case Induction Motors Figure 5-39 Typical squirrel-case motor speed curve Prof. Paul Lin 11 Part 3. Three-Phase AC Motors Squirrel-Case Induction Motor - NEMA-Type Squirrel-case Induction Motors Figure 5-39 Typical squirrel-case motor speed curve NEMA Design B: Standard type normal starting torque, low starting current, and low slip at full load. For fans and blowers applications that required normal starting torque NEMA Design C: Higher than standard resistance => improves the rotor power factor at start, providing more starting torque For pump and other applications that requires a high starting torque NEMA Design D: Rotor resistance higher than Design C => produce maximum starting torque Applications: Cranes, hoists Prof. Paul Lin 12 6
Part 3. Three-Phase AC Motors Squirrel-Case Induction Motor Figure 5-39 Power Circuit for Reversing a Three-Phase IM Forward Running F Forward magnetic contactor L 1 T1, L2 T2, L3 T3 Reverse Running R Reverse magnetic contactor L 1 T3, L2 T2, L3 T1 Prof. Paul Lin 13 Part 3. Three-Phase AC Motors Squirrel-Case Induction Motor Figure 5-41 Motor power factor (PF): ranged from 70% to 90% Using wattmeter, ammeter, and a voltmeter P = 3 V L *I L * cosθ = 1.732 * V L *I L *PF Prof. Paul Lin 14 7
Part 3. Three-Phase AC Motors Squirrel-Case Induction Motor Power Quality Lock-rotor current (starting current) Motor draws a high inrush current Up to 6 times of their nameplate full-load current Can create voltage sags or dips in the power lines, which may cause light flickers and problems with other operating equipment. Prof. Paul Lin 15 Part 3. Three-Phase AC Motors Squirrel-Case Induction Motor Figure 5-43 Dual-speed, three-phase squirrel-case single-winding motor Prof. Paul Lin 16 8
Part 3. Three-Phase AC Motors Squirrel-Case Induction Motor Figure 5-44 Typical connections for dual-voltage wye and delta series and parallel Prof. Paul Lin 17 Part 4. Single-Phase AC Motors For home and business appliances operate on single-phase AC power. Larger in size, for the same horsepower, than a three-phase motor Produced torque: pulsating, and irregular Lower power factor and lower efficiency Motor sizes from fractional (1/8, 1/4,1/3, 1/2, 3/4 hp) to 10 hp range The three basic types Split-phase Split-phase capacitor Shade-pole Prof. Paul Lin 18 9
Part 4. Single-Phase AC Motors Split-Phase Motor Figure 5-49 Split-phase induction motor Run (main) winding Starting winding Starting winding is displaced in the stator 90 electrical degrees from the running winding Centrifugal switch open to disconnect the starting winding when the motor speed reaches about 75% of full-load speed Prof. Paul Lin 19 Part 4. Single-Phase AC Motors Split-Phase Motor Figure 5-50 Rotating magnetic field of a split-phase motor Run (main) winding Starting winding Prof. Paul Lin 20 10
Part 4. Single-Phase AC Motors Split-Phase Capacitor Motor Figure 5-52 Dual-voltage splitphase motor connections (NEMA standard) Low voltage operation: Two run windings and start windings => all connected in parallel High voltage operation: Two run windings connect in series Start winding connect in parallel with one of the run windings Prof. Paul Lin 21 Part 4. Single-Phase AC Motors Split-Phase Capacitor-Start Motor Figure 5-53 Capacitor start motor A modified split-phase motor A capacitor connected in series with the starting winding crates a phase shift of approximately 80 degrees between the starting and running winding. It provides more than double the starting torque with 1/3 less starting current than the split-phase motor. Capacitor short circuit? => Start winding always connected, draw excessive current Capacitor open circuit?=> Motor cannot start Prof. Paul Lin 22 11
Part 4. Single-Phase AC Motors Split-Phase Capacitor-Start Motor Figure 5-54 Two-speed capacitor start motor Double speed (pole change) Low speed (8 pole): 900 rpm High speed (6 pole): 1200 rpm Prof. Paul Lin 23 Part 4. Single-Phase AC Motors Split-Phase Permanent Capacitor Motor Figure 5-55 Reversible permanent-capacitor motor connection Has a run-type capacitor permanently connected in series with the start winding. Low starting torque, not suited for hard-to-start applications Applications: Fans, blowers with low starting torque Intermittent cycling uses such adjusting mechanisms, gate operators, garage door openers that needs instant Prof. Paul Lin 24 reversing 12
Part 4. Single-Phase AC Motors Capacitor-Start/Capacitor-Run Motor Figure 5-56 Capacitor-start/capacitorrun motor During starting time: Start capacitor (electrolytic type) and Run capacitor (oil type) connected in parallel During run time: Start capacitor is disconnected Applications (starting hard loads): woodworking. Machinery, air compressors, high-pressure water pumps, vacuum pumps, other high torque applications Available in sizes from 1/2 to 25 hp. Prof. Paul Lin 25 Part 4. Single-Phase AC Motors Shaded-Pole Motor Figure 5-57 Shaded-pole motor (1/20 to 1/6 hp) Only one main winding, and no start winding or switch. Rotor squirrel cage Starting uses a continuous copper loop around a small portion of each motor pole Currents in the copper loop delay the phase of magnetic flux in that part of the pole enough to provide a rotating field. Typical applications: fan, can openers, blowers, and electric razors. Prof. Paul Lin 26 13
Part 4. Single-Phase AC Motors Universal Motor Figure 5-58 Universal motor Constructed as a series-type DC motor with a wound series field (on stator), and a wound armature (on the rotor) Can be operated with either DC or AC Figure 5-59 Motor speed and direction controls - drill Prof. Paul Lin 27 Email: lin@ipfw.edu Summary & Conclusion Questions? Contact Prof. Lin through: Prof. Paul Lin 28 14