Topics Introduction Review: Principle of Operation of Electric Machinery AC machines Single phase induction motors Other special purpose motors Exercise Questions MECH 1200 Fundamentals of AC Electric Machinery

Introduction Two main types of AC Electric Machinery: - Synchronous Machines - Induction Machines NEMA: National Electric Manufacturers association. NEMA develops standards for electric motors, and classifies motors into 4 classes A,B, C, and D with class D motors being the ones that supply highest torques. Stator Rotor Image is in the public domain. Source: Wikimedia Commons, Wikimedia Foundation MECH 1200

Review: Principles of Operation of Electric Machines There are three methods by which electric motors work: 1.Having a stationary magnetic field in the stator and an alternating magnetic field in the rotor. - This method is employed in DC motors 2.Having a rotating magnetic field in the stator and a fixed magnetic field in the rotor. - This method is employed in synchronous motors, Stepper and other motors 3.Having a rotating magnetic field in the stator and an induced magnetic field in the rotor. - This method is employed in induction motors. MECH 1200

N S S N Review: Principle of Operation of DC and AC Synchronous Machines N S N S N S AC Synchronous Machines Stator magnetic field: Stationary Rotor magnetic field: Alternating DC Machines Stator magnetic field: Rotating Rotor magnetic field: Fixed (stationary w.r.t. rotor) S N S N N S MECH 1200

Construction of AC Machines Induction motor: – Stator: a rotating magnetic field exists in the stator windings. The stator windings are connected to a three phase power supply (except for single phase motors). – Rotor: An induced magnetic field exists in the rotor of a squirrel cage induction motor. Synchronous motor: – Stator: a rotating magnetic field from a three phase power supply. – Rotor: A fixed magnetic field in the rotor from a dc power supply. Therefore, it needs brushes, but no commutation as in dc motors. Rather, slip rings that stay in contact with the brushes are used. Note: Wound rotor induction motors require slip rings as in synchronous motors. MECH 1200

Terms MECH 1200

Induction Motor Speed and Torque Relationship Locked rotor torque: The starting torque of the motor. Full load torque: the torque supplied by the motor at rated speed, voltage and frequency. Torque Speed (ω) Full load torque Locked rotor torque Breakdown torque MECH 1200

Single Phase Induction Motors A single phase induction motor will have an extremely low starting torque, to solve this problem, some methods are used such as connecting a capacitor to it or start windings. The stator magnetic field does not rotate, but rather pulsates (i.e. gets bigger and smaller) Torque Speed MECH 1200

Starting Single Phase Induction Motors There are several methods to start a single phase induction motor: 1)Split-Phase: here an auxiliary winding is connected in parallel to the stator winding, then a centrifugal switch disconnects it when motor acquires speed. 2)Capacitor- Start: this is more expensive than the split-phase and has a much higher starting torque. A capacitor is connected in series with the auxiliary winding to make a rotating magnetic field in the stator. This motor is used in compressors, pumps, and air conditioners. 3)Capacitor start-capacitor run: since adding a capacitor improves the motor operation significantly, a permanent capacitor is used in this type, in addition to a bigger capacitor only at start and then disconnected by a centrifugal switch. Most expensive and gives the highest starting torque (~ 300% of full load torque) 4)Shaded pole: the stator poles of this motor are salient with a coil that is wrapped around the salient poles. These motors are the cheapest and produce the smallest amount of starting torque (about 50% of full load torque) they are also the least efficient. MECH 1200

Other Special Purpose Motors Stepper Motor: A special type of synchronous motors, that moves in steps every time a pulse is sent from its control unit. One type of stepper motors has a permanent magnet in its rotor. Reluctance Motor: A hybrid of the induction and synchronous motors, operates at synchronous speed, with a rotor of salient poles. Hysteresis Motor: a self starting synchronous motor, torque is generated through hysteresis. Brushless DC motor: A combination of a permanent magnet stepper motor with a position sensor and a control unit that gives the motor the pulses to work. It was designed to run off DC power when brushed motors are unacceptable such as in artificial heart applications or when radio frequency interference is unacceptable, or at low pressure environment such as at high altitudes. MECH 1200

Exercise Questions 1.What is the difference between the commutators and slip rings? Slip rings are used with brushes to connect the rotor windings to electric power in ac machines. The commutators are used to convert the dc current to alternating current in the rotor of dc machines. 2.Which motor needs slip rings in it a) series dc motor b) synchronous motor c) squirrel cage induction motor d) wound rotor induction motor e) brushless dc motor MECH 1200

3.The torque vs. speed characteristic shown below is for which type of motors? a)Series DC motor b)Shunt DC motor c)The synchronous motor d)The induction motor e)The stepper motor Speed (ω) Torque MECH 1200

4.The torque vs. speed characteristic shown below is for which type of motors? a) Series DC motor b) Shunt DC motor b) The synchronous motor c) The induction motor d) The stepper motor Speed (ω) Torque MECH 1200

5.How can we reverse the rotation of a three phase ac motor? By simply reversing any two of the three lines. 6.What is the definition of synchronous speed? The speed of rotation of the magnetic field in the stator. 7.Which machine is most suitable as an electric generator? The synchronous machine. 99% of electric generators in the world are synchronous generators. MECH 1200

11. Of all electric motors, what is the most commonly found motor in industry (about 85% of industrial motors are of this type)? a)Synchronous motor b)Universal motor c)Permanent magnet DC motor d)Three phase induction motor e)Single phase induction motor MECH 1200

12. Name three types of single phase induction motors: Split phase, Capacitor start, capacitor start-capacitor run, shaded pole 13.Rank the single phase motors above from best to worst: Capacitor start-capacitor run, then capacitor start, then split phase, then shaded pole. 14.Which motor can have a permanent magnet in its rotor? (two answers are correct) a)Permanent magnet dc motor b)Squirrel cage induction motor c)Stepper motor d)Brushless dc motor e)Single phase induction motor MECH 1200