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MECH1200 Electric Motors Fundamentals of DC Electric Machinery History MECH1200 • • Introduction • Principle of Operation of Electric Machinery • Principle of Operation of AC Synchronous Machines • Principle of Operation of DC Machines • Types of Electric Machines • Construction of DC Machines • DC Machine Torque Characteristics • DC Machine Speed Control • Exercise Questions MECH12 History - Very advanced mechanisms are developed in the middle ages. - Development of the steam engine leads to the industrial revolution in the mid 1700’s. - Michael Faraday runs the first electric motor experiment in 1821. A mechanical clock made in 1510 James Watt Faraday and Daniell in the lab Images are in the public domain. Source: Wikimedia Commons, Wikimedia Foundation MECH1200 Introduction What is an Electric Motor? • Electromechanical device that converts electrical energy to mechanical energy • Mechanical energy used to: • Rotate pump impeller, fan, blower • Drive compressors • Lift materials • Motors in industry: 70% of electrical load MECH1200 • Electric Machinery: - Electric Motors: convert electrical energy into mechanical energy. - Electric Generators: convert mechanical energy to electric energy. • The Electric machine is composed of: - Stator - Rotor MECH1200 The Stator is the stationary part of a motor. The Rotor is a moving component of an electromagnetic system in the electric motor How Does an Electric Motor Work? MECH1200 Principle of Operation of AC Synchronous Machines MECH1200 N S N Alternators such as the one shown above are three phase synchronous machines. In synchronous motors, the magnetic field in the stator rotates, and the rotor tries to follow the stator’s magnetic field, i.e. it tries to synchronize itself with it, and thence the name synchronous motor. NS N S S S N When Once A the Stationary the rotor rotor isthe rotates energized, field itaflowing rotates cycle, intechnique the itin to stator switches the ABut Every circuit how half that can cycle we usesmake amagnetic commutation current thehalf magnetic field the alternate rotor ismost used Principle of Operation of DC Machines polarity, An alternating so stable the magnets position: field are again in thethe repelling rotor switches to supply direction anthat alternating inmagnetic due the to rotor? current commutation, into watch: rotor. Commutators Brushes (To maintain electrical contact) N S Commutation Circuit MECH1200 MECH1200 Types of Electric Motors Electric Motors DC Motors AC Motors Permanent Magnet DC (PMDC) Series Motor Special Purpose Motors Stepper motor Induction Motor Brushless DC motor Hysteresis motor Shunt Motor Compounded Motor Synchronous Motor Reluctance motor Universal motor Separately Excited motor MECH1200 Classification of Motors Electric Motors Alternating Current (AC) Motors Synchronous Induction Single-Phase Three-Phase Direct Current (DC) Motors Separately Excited Series Self Excited Compound Shunt MECH1200 Three types of Motor Load Motor loads Description Examples Constant torque Output power varies but loads torque is constant Conveyors, rotary kilns, constant-displacement pumps Variable torque Torque varies with square loads of operation speed Centrifugal pumps, fans Constant power Torque changes inversely loads with speed Machine tools MECH1200 DC Motors • Parts of a DC motor: – – – – Armature with windings Brush assembly Field windings Commutator Construction of DC Machines MECH1200 • Field Windings: In DC machines, this is the winding in • • • • the stator. Note: Permanent magnet dc motors do not have field winding but rather a permanent magnet. Armature Windings: The windings in the rotor . Commutator segments: the method to convert the dc current to an alternating current in the rotor of the dc motor is called commutation. They are connected to the rotor windings, and stay in contact with the brushes. Brushes: pieces made of carbon or graphite and pushed against a spring to maintain electrical contact with the commutator segments. Interpole windings: A third set of windings are mounted on the stator, and connected in series with the rotor to reduce the sparking between brushes and the commutator. MECH1200 • If the armature and field windings are connected in series, then the machine that results is called the DC series machine. • If the armature and field windings are connected in parallel, then the machine that results is called the DC shunt machine. • If the armature is connected with a field winding in series and another field winding in parallel, then the resulting machine is called the compounded machine. • Note that the interpole windings are always connected in series with the rotor. MECH1200 • Name the type of each of the motors A, B, and C shown in the figure below: A: Shunt motor B: Series motor C: Compound motor MECH1200 DC Motor Construction MECH1200 Interpole Windings • Due to shifting of the magnetic flux lines, arcing and short-circuiting can result • Interpole windings counteract this shift and reduces the arcing • Are in series with field coils MECH1200 Counter (or Back) EMF N S Likewise, as the armature cuts through the magnetic field of the field coils (stator), a voltage is induced. (Think how a generator works.) S Recall from 1100: an induced voltage in a wire (or any conductor) results from relative motion of either the wire or the magnetic field. N MECH1200 Counter (or Back) EMF Combined Armature and Field Lines of Force MECH1200 Counter (or Back) EMF This induced voltage causes current to flow in the opposite direction of the applied DC current, thus “countering” it, reducing the overall current and voltage. Counter EMF current is “out of page” (+z direction) Armature current is “into page” (- z direction) MECH1200 Permanent Magnet DC Motors • Excellent starting torque • Good speed regulation • Limited to low HP loads • Torque is limited to 150% of rated torque. • Reverse direction of rotation by interchanging armature connections. MECH1200 Series Wound - DC Motors MECH1200 Series Wound - DC Motors • Highest starting torque • Not used with loads that are belt or chain coupled • Loads should always be directly coupled. • Reverse direction of rotation by interchanging armature connections. • Cannot be used without a load attached Examples: Locomotives, Cranes, Hoists.. MECH1200 Series Wound – DC Motors Runaway motor at low/no loads High starting torque MECH1200 Shunt Wound DC Motors • Best speed regulation • Open shunt field winding will cause motor speed to increase to dangerous levels. • Field loss relay should always be used. • Allows simplified control for reversing the motor. • http://www.youtube.com/watch ?v=VGmoKK5Iqrs MECH1200 Shunt Wound DC Motors • Field winding are connected in parallel to the armature windings. • Field and armature windings can be hooked to same or different sources. • “Separately excited” – independent control of armature and field. MECH1200 Shunt Wound DC Motors Self excited – When constant speed is desired. Field is independent of armature current. Doesn’t provide high starting torque Ex: Elevator, Centrifugal Pumps Separately excited – Can operate above its rated speed. Speed can be increased by weakening the field in the field windings. Results in lower torque Requires two power supplies Ex: Woodworking Machinery, Printing Press MECH1200 • Properties: Shunt Wound DC Motors Speed constant independent of load up to certain torque • Field winding parallel with armature winding • Current = field current + armature current Speed control: insert resistance in armature or field current MECH1200 Shunt Wound DC Motors • Reverse direction by switching either armature or field winding polarities MECH1200 Shunt DC Motor Terminal Characteristics Question: At which speed should we operate the motor to achieve maximum power? Hint: The power corresponds to the area of the rectangle under the operating point on the speed-torque line. Which of the three rectangles has the largest area? Speed (RPM) No load speed 3000 Maximum power is at 50% of no load speed 2400 1800 1200 Stall point 600 Torque (ft-lb) 5 1 0 Shunt DC Motor Speed Control MECH1200 • There are three methods to control the speed of a DC motor: 1. Changing the armature voltage. 2. Changing armature resistance (by adding a resistor in series with armature) 3. Change the field (stator) magnetic field Speed (ω) Speed (ω) As Va increases As Ra increases Torque Torque MECH1200 Compound Wound DC Motors • Starting torque : – Better than shunt-wound DC motor. – Not as good as series-wound DC motor. • Speed regulation – Better than series-wound DC motor – Not as good as with shunt-wound DC motor. • Reverse direction of rotation by interchanging armature connections. MECH1200 DC Motors DC compound motor Suited for high starting torque if high % compounding: cranes, hoists Field winding in series and parallel with armature winding Good torque and stable speed Higher % compound in series = high starting torque Series DC Motor Terminal Characteristics MECH1200 Speed (RPM) If the series motor is unloaded, speed increases significantly (this condition is called Runaway, and it must be avoided) Shunt motor Series motor Very high starting torque Torque (ft-lb) MECH1200 DC Motor Rating Plates MECH1200 DC STEPPER MOTORS MECH1200 DC STEPPER MOTORS • Voltage Rating – provides desired torque • Resistance-per-winding determines – the current draw of the motor – Maximum operating speed • Degrees per Step – Sets the number of degrees the shaft will rotate for each full step MECH1200 STEPPER MOTOR CONTROL • 4 signal wires fired in the correct sequence will turn the motor MECH1200 Stepper motor location MECH1200 How does the stepper motor fit in MECH1200 Stepper motor STEPS MECH1200 What are Poles in a Motor? •Winding(s) which produce the magnetic field(s) necessary to cause the rotor to turn. 3 Phase; 2 Pole Motor Brushless DC Motors • Rotor is permanent magnet • Stator is wired to provide rotating magnetic field MECH1200 MECH1200 Types of Motor Enclosures • ODP – Open Drip Proof – Air flows through motor (fan blades help flow) – Used in environments free from contaminants MECH1200 Types of Motor Enclosures • TENV – Totally Enclosed Non-Ventilating – Protect motor from corrosive and harmful elements – Frame fins help to dissipate heat MECH1200 Types of Motor Enclosures • TEFC – Totally enclosed Fan Cooled – Similar to TENV except has external fan for cooling Types of Motor Enclosures • XP – Explosion Proof – Similar to TEFC but enclosures are cast iron MECH1200 MECH1200 Hazardous Locations Division I – Hazardous material present in the air as a norm Division II - Hazardous material present in the air as an abnormal event MECH1200 Hazardous Locations/ Classes Summary • DC motors are: – – – – permanent magnet series-wound shunt-wound compound-wound MECH1200 Exercise Questions MECH1200 1. How does the rotor of a dc motor maintain electrical contact with its commutation circuit? By using brushes that are pushed against a spring. 2. Name two motors that do not need brushes for their rotor windings. Squirrel cage induction motor and the brushless DC motor, and some types of stepper motors 3. Which motor supplies the highest output torque to weight ratio? The series DC motor. Exercise Questions MECH1200 4. Why is it not recommended to use brushed dc motors in spacecraft and some aircraft applications? Because in the space environment there is no air to carry away the heat generated by the brushes of the dc motor, causing them to wear very quickly. The same thing applies in the low pressure environment of aircraft. Thus a brushless dc motor is used. 5. In an environment that contains explosive gases, such as in mines, which motor do you recommend using: a) series dc motor b) shunt dc motor c) induction motor d) universal motor MECH1200 6. Which motor is recommended for artificial heart applications a) Series dc motor b) Shunt dc motor c) Induction motor e) Brushless dc motor 7. What type of machines is used as an electric generator in vehicles to recharge the battery? The synchronous generator 8. What is added to the dc motor to reduce sparking in dc motor brushes? Interpole windings are added. MECH1200 9. Which machines have a more complex construction, DC or AC machines? DC machines are much more complex, due to existence of interpoles and commutation circuits. 10. Which motor is potentially dangerous to operate with no load on it? The series dc motor 11. Why series dc motors should not be used in belt drives? Because if the belt breaks, the motor will be unloaded. Series dc motors are dangerous to operate if unloaded, as their speed will increase significantly until the motor is damaged. 12. Which type of motors can operate on both ac and dc power supplies? The universal motor