ECET 211 Electric Machines & Controls Lecture 5-1 Electric Motors (1 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 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
Magnetism Electromagnetism Generators Motor Rotation Part 1 Motor Principle Prof. Paul Lin 3 Part 1. Motor Principle - Magnetism Magnetism In physics, magnetism is a force that can attract (pull closer) or repel (push away) things that have a magnetic material like iron inside them; magnetic lines of forces of a bar magnet shown by iron filling on paper; https://simple.wikipedia.org/wiki/magnetism Magnetism is the force that creates rotation for a motor to operate. Permanent Magnet Inherent magnetic force (field) Attract and hold magnetic materials such as iron and steel Figure 5-1 Magnetic field of a permanent bar magnet: Lines of flux travel from the N pole to the S pole Prof. Paul Lin 4 2
Part 1. Motor Principle - Magnetism Magnetism Demonstration of an electric current in a wire, by Christian Oersted, a Danish scientist, 1821; source - http://wwwistp.gsfc.nasa.gov/education/imagnet.html Prof. Paul Lin 5 Part 1. Motor Principle - Electromagnetism Magnetic field produced around a current carrying conductor. Figure 5-2 Magnetic field around a straight current carrying conductor Left-hand rule (electron flow, from to +) Reference: Chapter 6 Introduction to Motors and Generators of Electrical Power and Controls, by Timothy L. Skvarenina and William E. DeWitt, 2004, Prentice Hall Right-hand rule (current flow from + to -) Thumb current direction Rest of four fingers Flux lines Prof. Paul Lin 6 3
Part 1. Motor Principle - Electromagnetism Reference: The AC s and DC s of Electric Motor, Motor Mastery University, Regal Beloit America, Inc, https://www.centuryelectricmotor.com/ Motor-Mastery-University.aspx Magnetic Fields Magnetic flux (Φ), SI unit Webers, abbreviated Wb DC Flux DC circuits AC Flux AC circuits Flux density B = Φ/A, SI unit Wb/m 2, or Telsas Figure 5-3 Magnetic field produced by a current-carrying conductor Prof. Paul Lin 7 Part 1. Motor Principle - Generator An electric generator: uses magnetism to convert mechanical energy into electrical energy A mechanical torque is applied to turn conductors through a magnetic field and generate electric current. A voltage is induced in a conductor whenever the conductor is moved through a magnetic field so as to cut lines of force. Faraday s law of induction: Induced voltage: e = N dφ/ dt N / t (volts) Figure 5-4 Simplified AC generator Prof. Paul Lin 8 4
Part 1. Motor Principle - Generator DC Generator and Motor Generating DC is basically the same as generating AC. The only difference is the manner in which the generated voltage is supplied to the output terminal Commutator Any DC machine can act as a generator or as a motor. Figure 5-5 Simplified DC generator Prof. Paul Lin 9 Part 1. Motor Principle Motor Rotation An electric motor rotates as the results of the interaction of two magnetic fields. Figure 5-6 Motor principle Fixed stator N-S magnet Moving armature (electromagnet) Rotation force is produced as a result of Like poles repel ( N N; S S) Unlike poles repel (N S) Prof. Paul Lin 10 5
Part 1. Motor Principle Motor Rotation The interaction between Magnetic field produced by the current Permanent N- S field A force being experienced by the conductor Figure 5-7 A current-carrying conductor, placed in a magnetic field Figure 5-8 Right-hand motor rule Fixed stator N-S magnet Moving armature (electromagnet) Prof. Paul Lin 11 Part 1. Motor Principle Motor Rotation Developing Motor Torque (Rotational force) T = F*r Figure 59 Developing motor torque By a current-carrying coil or loop of wire placed in a magnetic field Prof. Paul Lin 12 6
Figure 5-11 Typical DC industrial motor application For applications with high torque and variable speed; Examples: Mine hoist, steel rolling mills, ship propulsion, cranes, conveyors, and elevators Ohio Electric Motors, http://www.ohioelectricmotors.com/dcmotors-used-in-electric-hoists-reels-andwinches-1597 Figure 5-12 Major components of a DC motor More complicated and expensive than that of an AC motor Major components: Commutator, brushes, and armature windings Cost for maintenance of the brush/commutator assembly Prof. Paul Lin 13 DC Motor Parameters Speed: Motor shaft rotational speed, in RPM Torque: Turning force supplied by the motor s shaft T = F * r (force acting on a radius) Units: lb-in (pound-inches) lb-ft (pound-ft) Horsepower: the rate at which work is done. 1 hp lifting 33,000 pounds of object to a height of 1 foot in 1 minute 1 hp = 746 watts of electric power DC Current Motors and Drives (1/50 to 3000 hp), http://www.baldor.com/mvc/downloadcenter/files/br600 Baldor Shekby Plant - DC Manufacturing Plant, 2:35 min, video, https://www.youtube.com/watch?v=i7wxetxqm30 Prof. Paul Lin 14 7
Permanent-Magnet DC Motors Use permanent magnets to supply the main field flux and electromagnets to provide the armature flux. Movement of the magnetic field of the armature is achieved by switching current between coils within the motor called commutation Figure 5-13 Permanentmagnet DC motor operation Youtube Video DC Motor, How it works, Learn Engineering, 4:49 min video, https://www.youtube.com/wat ch?v=latphanefqo Prof. Paul Lin 15 Permanent-Magnet DC Motors Figure 5-14 Permanentmagnet DC motor Figure 5-15 Armature commutation of switching effect Prof. Paul Lin 16 8
Permanent-Magnet DC Motors Figure 5-16 Reversing the direction of rotation of a PM motor Forward/Reverse Control Current direction through the armature Speed control: Vary the voltage value apply to the armature Higher voltage => Higher speed Lower voltage => Lower speed Prof. Paul Lin 17 Series DC DC Motors Figure 5-17 Series-type DC motor Series filed: low resistance field (Rs) and low resistance armature (Ra) At starting, when DC supply voltage E is applied to the motor, the starting current is high I = E/(Rs + Ra) High current => Strong magnetic fields inside the motor => produce high torque (Torque & I 2 ) Idea for starting very heavy mechanical loads Prof. Paul Lin 18 9
Series DC Motors Figure 5-18 Speed-torque characteristics curves for a DC series Speed Curve Speed varies widely between no load and rated load => cannot be used where a constant speed I required Run faster with a light load (low current); Run slower as the load increases Ability to start very heavy loads Application areas: cranes, hoists and elevators Caution: Never operate a series motor without a load Prof. Paul Lin 19 Shunt DC Motors Figure 5-19 Shunt-type DC motor Shunt field (F1, F2) higher resistance Armature and Shunt field connect in parallel Figure 5-20 Speed-torque characteristic for a shunt DC motor The current through the shunt field wining: constant, does not vary with the motor speed The torque will vary only with the current through the armature Prof. Paul Lin 20 10
Shunt DC Motors Figure 5-21 Separately excited motor (variable speed drive) Independent control of the field and armature DC controller => Armature voltage DC controller => Shunt field voltage Prof. Paul Lin 21 Compound DC Motors Figure 5-22 Compound-type DC motor Shunt field (F1, F2) higher resistance Armature and Series field connect in series Cumulative-compound connection Under load the series field flux and shunt field flux act in the same direction to strengthen the total field flux Prof. Paul Lin 22 11
Compound DC Motors Figure 5-23 DC cumulative-compound motor connections and speedtorque characteristics Long-shunt compound Short-shunt compound Prof. Paul Lin 23 Direction of Rotation Figure 5-24 DC series motor reversing motor starter Figure 5-25 DC shunt and compound motor starting Prof. Paul Lin 24 12
Motor Counter Electromotive Force (CEMF) Figure 5-26 Motor CEMF CEMF (back EMF) a form of resistance that opposes and limits the flow of armature current I A = (V MTR CEMF)/R A where I A = armature current V MTR = motor terminal voltage R A = armature circuit resistance Prof. Paul Lin 25 Motor Counter Electromotive Force (CEMF) Example 5-1 The armature of a 250V DC motor draws 15A when operating at full load and has a resistance of 2Ω. Determine the counter EMF (back EMF) produced by the armature when operating at full load. Solution: I A = (V MTR CEMF)/R A CEMF = V MTR ( I A * R A ) = 250 V (15 * 2) = 230V Prof. Paul Lin 26 13
Speed Regulation A measure of motor s ability to maintain its speed from no load to full load without a change is the applied voltage to the armature or fields Percent speed regulation = ((No-load speed Full-load speed)/fullload speed) * 100 Example 5-2 A DC shunt motor is running with a measured no-load speed of 1775 rpm. When full load is applied, the speed drops slightly to 1725 rpm. Find the percentage speed regulation. Solution: Percent speed regulation = ((1775 1725)/1725) * 100 = 2.9% Prof. Paul Lin 27 Varying DC Motor Speed Figure 5-29 DC motor speed Figure 5-30 Armature-controlled DC motor Prof. Paul Lin 28 14
Varying DC Motor Speed Figure 5-30 Armature controlled DC motor Prof. Paul Lin 29 DC Motor Drives Figure 5-32 The block diagram for a typical DC motor drive Figure 5-32 Typical DC motor drive unit Prof. Paul Lin 30 15
Summary & Conclusion Questions? Contact Prof. Lin through: Email: lin@ipfw.edu Prof. Paul Lin 31 16