ECET 211 Electric Machines & Controls Lecture 5-4 Electric Motors. Lecture 5-4 Electric Motors

Transcription

1 ECET 211 Electric Machines & Controls Lecture 5-4 Electric Motors (4 of 4) Text Book: Chapter 5 Electric Motors, Electric Motors and Control Systems, by Frank D. Petruzella, published by McGraw Hill, 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 5-4 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

2 Motors Selection based on General-purpose applications vs Specific tasks General purpose motor, Inverter-duty motors Technical Requirements: Mechanical load, Motor output horsepower, Torque, Speed Voltage, Frequency, Phase, Starting (current), Efficiency, Power factor, Motor temperature, Service Factor, Duty cycle, Frame size Government mandates EPAct (Energy Policy Act) 1992 Premium Efficiency - EISA (Energy Independent and Security Act) 2007: all applicable motors manufactured or imported into the U.S. after Dec must meet the Premium Efficiency guidelines Prof. Paul Lin 3 References Premium Efficiency Motor Selection and Application Guide A Handbook for Industry (136 pages), Advanced Manufacturing Office, U.S. Dept. of Energy, k_web.pdf EISA 2007 (Energy Independent and Security Act) Motor losses and loss reduction techniques MotorMaster+ software The Motor Guide (Low-power standard motors) ABB Group, (135 pages), a5c/$file/motor+guide+gb+02_2005.pdf Prof. Paul Lin 4 2

3 References Guide for AC Motor Selection (Small size, standard AC motors), Selection Procedure: (1) Required specifications (2) Calculate the operating speed (3) Calculate the required torque (4) Select a motor and gearhead (5) Confirm the speed Prof. Paul Lin 5 Mechanical Power Rating Torque Current Code Letter Efficiency Energy-Efficient Motors Frame Size Full-Load Speed Load Requirements Motor Temperature Ratings Duty Cycle Motor Enclosure Metric Motors Prof. Paul Lin 6 3

4 Mechanical Power Rating is expressed in either horsepower (HP) or watts (W) 1 hp = 746 W P = ω r *T Torque = (P * 5252)/n r ; P in HP, n r in RPM HP = (Torque * n r )/5252 Torque in lb-ft ; Speed in rpm Torque Motor torque is the twisting force exerted by the shaft of a motor. Figure 5-70 motor s torque-speed shows how a motor s torque production varies throughout the different phases of its operation. Prof. Paul Lin 7 Torque Figure 5-70 motor s torque-speed curve Locked-rotor torque (LTR) or Starting torque is produced by a motor when it is initially energized at full voltage. Pull-up torque (PUT) is the minimum torque generated by a motor as it accelerates from standstill to operation speed. Breakdown torque or pull-out torque is the maximum amount of torque a motor can attain without stalling. Full-load torque is produced by a motor functioning at rated speed and horsepower. Prof. Paul Lin 8 4

5 Example 1. A motor delivers 300 hp at 1800 rpm. How much torque does it produce? Answer: P = ω r T ω r = 2π n r / 60; n r in revolution/minute T = P/ω r =60 P/(2πn r ) N m T = (5252 * HP)/ n r lb ft T = 5252 x 300 hp/1800 = lb ft T = P/ω r =60 P/(2πn r ) N m 1 HP = kw P = 300 hp = kw T = P/ω r =60 P/(2πn r ) = 60 * 223,800/(2π * 1800) = 1187 N m Prof. Paul Lin 9 Example 2: An elevator is required to lift a load of 1,000 kg to an altitude of 30 m. (a) How much energy must the motor provide? (neglecting losses in the hoist assembly) Answer (a): W out = m g h = 1000 kg 9.81N/kg 30m = 295,300 N m 0.3 MJ = kwh Use: 1 N m = 1 Joule 1 kwh = 3.6 MJ (b) What size horsepower would be required to lift the elevator from ground up for 30 meters if the total time required is to be 45 seconds. Answer (b): W in = W out /η = W out = kwh P avg = W in /Δt = kwh/(45 sec/3600sec) = 0.083/ = 6.64 kw kw => hp conversion = 6.64 kw/0.745 kw/hp = 8.9 hp => chose 9-10 hp motor Prof. Paul Lin 10 5

6 Example 3: Assume the hoist assembly is 66% efficient. (a) If the time required to lift from bottom to the top is to be 45 sec. What size horsepower would be required. Answer (a): W in = W out /η = 0.083/0.66 = kwh P avg = W in /Δt = kwh/(45 sec/3600sec) = 0.126/ = kw kw => hp conversion = kw/0.745 kw/hp = 13.5 hp (b) If the time required to lift from bottom to the top is to be 60 sec. What size horsepower would be required. Answer (b): P avg = W in /Δt = kwh/(60 sec/3600sec) = 0.126/ = 7.55 kw kw => hp conversion = 7.55 kw/0.745 kw/hp = 10.1 hp Prof. Paul Lin 11 Exercise Question 1: This exercise question is about he potential energy and horsepower calculation for a hydraulic car-lifting system for use in a auto repair shop. If the maximum weight of the lifting system is 5,000 pound, and it is expected to lift up 6 ft in 10 second, find (a) The potential energy need to lift the car. (b) Power needed to lift in 10 sec. (c) Input power to the motor for used in the hydraulic system. Neglecting all the losses. Answer (a) W= m*g*h (joule) (b) P lift = W/t (watts) (c) P in_lift = P lift /η Convert Pin from kw to hp : 18.2 hp, chose 20 hp Prof. Paul Lin 12 6

7 Mechanical Power Rating Torque Current Code Letter Efficiency Energy-Efficient Motors Frame Size Full-Load Speed Load Requirements Motor Temperature Ratings Duty Cycle Motor Enclosure Metric Motors Prof. Paul Lin 13 Current Full-load current (nameplate current) The amount of current (amperes) the motor can be expected to draw under full load (torque) condition. Used to determine the size of overload sensing element for motor protection Lock-rotor current: starting inrush current Service-factor current The amount of current the motor will draw when it is subjected to a overload equal to the service factor on the nameplate of the motor. Prof. Paul Lin 14 7

8 Code Letter (NEMA) Assigned to motors for calculating the lock-rotor current based on the kilovolt-amperes per nameplate horsepower. LR current (single-phase motors) = (Code letter value * hp * 1000)/Rated voltage LR current (three-phase motors) = (Code letter value * hp * 577)/Rated voltage Locked-Rotor Code, kva/hp A G B H C J D K E L F M Prof. Paul Lin 15 Design Letter (NEMA) NEMA defines four standard motor designs for AC motor: A, B, C, and D. The design letter denotes the motor s performance characteristics relating to torque, starting current, and slip. Efficiency η = mechanical power output / electrical power input Power losses = core loss + stator and rotor resistance loss (copper losses) + mechanical losses + stray loss Prof. Paul Lin 16 8

9 Energy-Efficient Motors Efficiencies ranges between 75 and 98 percent Energy-efficient motors are manufactured with higher-quality materials and techniques. Figure 5-66 Typical energy efficient motor Prof. Paul Lin 17 Load Requirements Constant-torque Loads Conveyor Variable-torque Loads Centrifugal fans, pumps, and blowers Constant-horsepower loads Lethe, drilling and milling machines High-inertia loads: Large fans, blower, punch presses, and commercial washing machine Prof. Paul Lin 18 9

10 Load Requirements Must be considered in selecting the correct motor for a given application. A conveyor load example Figure 5-67 Constant-torque load Prof. Paul Lin 19 Variable-Torque Load Example Applications: Centrifugal fans, pumps, and blowers Figure 5-68 Variable-torque load Prof. Paul Lin 20 10

11 Constant Horsepower Load Application Examples: Lathe, Drilling and Milling Machines Figure 5-69 Constant-horsepower load Prof. Paul Lin 21 Motor Temperature Ratings: Ambient temperature, Temperature rise, Hot-spot allowance, and insulation class Duty Cycle: Continuous duty, Intermittent duty Motor Enclosure Figure 5-71 Motor enclosures Open drip- proof (ODP), Totally enclosed, fan-cooled (TEFC) Totally enclosed, non-ventilated (TENV), Hazardous location Figure 5.72 Explosion-proof motor Prof. Paul Lin 22 11

12 Metric Motors Replacement for a metric (IEC) motor installed on imported equipment 1) Get an exact replacement 2) Other considerations IEC (kw) vs HP IEC frame size metric dimension Frequency may be 50 Hz vs 60 Hz Speed, rpm Frequency 50 Hz Poles Sync Ns Full-load Nr Frequency 60 Hz Sync Ns Full-load Nr 2 3,000 2,850 3,600 3, ,500 1,425 1,800 1, , ,200 1, Figure 5.73 IEC motors Prof. Paul Lin 23 Procedure and Checklist References: Part 7 Motor Installation Horizontal AC Small Industrial Motor Motor Installation and Maintenance Instruction, /gepc/files/product/gei-56128(nema_ _horizmotor).pdf AC & DC Motor Installation & Maintenance Instruction, nter/files/lb5001 Installation Foundation Mounting Motor and Load Alignment Motor Bearings Procedure and Checklist Power Supply, Conductor Sizing, Wiring and Connections Electrical Wiring and Connections Grounding Conductor Size Voltage Levels and Balance Built-in Thermal Protection Operation Steps Prior to Starting Initial Start Jogging and Repeated Starts Prof. Paul Lin 24 12

13 Part 7 Motor Installation Foundation Minimum vibration and proper alignment between motor and load Concrete for large motors and driven loads Mounting Figure 5-74 Common type of motor mounting Motor and Load Alignment Figure 5-75 Laser alignment kit Direct-drive motors: 1:1 speed ratio Coupling - gears or pulley/belts Prof. Paul Lin 25 Part 7 Motor Installation Motor and Load Alignment (continue) Formula for calculating speed and pulley sizes for Belt-Driven System: Motor rpm/ Equipment rpm = Equipment pulley diameter/motor pulley diameter Example 5-5: What size of pulley is needed for the load? Figure 5-76 Solution: 1725/1150 = Equipment-pulley/2 Equipment-pulley = 3-inch Prof. Paul Lin 26 13

14 Part 7 Motor Installation Motor and Load Alignment (continue) Figure 5-77 Servicing a V beltdrive system Prof. Paul Lin 27 Motor Bearings Figure 5-78 Part 7 Motor Installation Prof. Paul Lin 28 14

15 Procedure and Checklist Part 7 Motor Installation Power Supply, Conductor Sizing, Wiring and Connections Electrical Connections: NEMA standards NEC Article 430 State & Local Code Grounding Equipment grounding conductor Figure 5-79 Motor shaft grounding ring Prof. Paul Lin 29 Procedure and Checklist Part 7 Motor Installation Power Supply, Conductor Sizing, Wiring and Connections Conductor Size (motor branch circuit conductor) Article 430 of the NEC Based on the motor full-load current, and increased where required to limit voltage drop. Undersized wire between the motor and the power source will limit starting abilities and cause overheating of the motor. Prof. Paul Lin 30 15

16 Procedure and Checklist Part 7 Motor Installation Power Supply, Conductor Sizing, Wiring and Connections Conductor Size (motor branch circuit conductor) Example 5-6. What size THW CU (Thermoplastic Insulated Wire, Copper wire) conductors are required for a single 15 hp, three-phase, 230 V squirrel-cage motor? Step 1. Full-load current (FLC) rating of the motor => conductor size. NEC 2008 Table : 230V, 15 hp => FLC 42 amperes. Step 2. NEC required branch circuit conductor supplying a single motor to have an ampacity not less than 125 percent of the motor FLC. Rated ampacity = 42 A x 125% = 52.5A Step 3. According to table (B)(16) => conductor size 6 AWG THW CU (55A with 60 C insulation) Prof. Paul Lin 31 Voltage Levels and Balance Voltage Levels Part 7 Motor Installation Voltage level with maximum deviation of 5 to 10 percent. Large voltage variation can have negative effects on torque, slip, current, efficiency, power factor, temperature, and service life. Unbalanced motor voltage Unbalanced current => overheating of the motor s stator windings and rotor bars, shorter insulation life, and wasted energy. Acceptable voltage unbalanced no more than 1 percent Voltage unbalance exceeds 5 percent => not to operate the motor. Percent voltage unbalance = (Max voltage deviation from the voltage average)/ Average voltage x 100 Prof. Paul Lin 32 16

17 Part 7 Motor Installation Example 5-7 What is the percent voltage unbalance for a threephase supply voltage of 480V, 435V, and 445V (Figure 5-80) Solution: Average voltage = ( )/3 = 453V Maxi deviation from the average voltage = = 27V Percentage voltage unbalance = Max voltage deviation/average x 100 = 27/453 x 100 = 5.96% Prof. Paul Lin 33 Built-in Thermal Protection Overload relay Thermal protectors inside the motor that sense motor windings temperature Figure 5-81 Built-in thermal motor protection Automatic reset Manual reset Part 7 Motor Installation Resistance temp. detectors Prof. Paul Lin 34 17

18 Part 8 Motor Maintenance and Troubleshooting Motor Maintenance Schedule Periodic Inspections Brush and Commutator Care Testing Windings Insulation 600V and below 1.5 MΩ 2,300 V 3.5 MΩ 4000V 5.0 MΩ Keep Your Motor Clean Keep Your Motor Dry Check Lubrication Check for Excessive Heat, Noise, and Vibration Excessive Starting is a Prime Cause of Motor Failures Prof. Paul Lin 35 Part 8 Motor Maintenance and Troubleshooting Troubleshooting Motors Digital Multimeter (DMM) Clamp-on ammeter Mega-ohmmeter Infrared (IR) thermometer Tachometer Oscilloscope Prof. Paul Lin 36 18

19 Summary & Conclusion Questions? Contact Prof. Lin through: Prof. Paul Lin 37 19