ATE1230: Electrical Machines
Electrical Machines
Module 3: AC Machines
PREPARED BY
Academic Services Unit
April 2012
© Institute of Applied Technology, 2012
Module 3: AC Machines
Module Objectives:
Upon successful completion of this module, students should be able to:
· Describe the basic construction and applications of AC induction motors and discuss the various types and applications of single phase induction motors.
· Describe the basic construction and working principle of three phase induction motors and calculate the synchronous speed.
· Define the term slip and calculate the percentage slip
· Discuss the working principles , construction , applications of synchronous motors.
· Describe the principle of magnetic induction as it applies to ac generators and state the differences between the two basic types of ac generators.
· Build a basic dynamo using a DC motor.
Module Contents:
Topic / Page No.3.1 / Introduction to AC Machines / 3
3.2 / AC Induction Motors Fundamentals / 4
3.3 / Single-Phase Induction Motors / 9
3.4 / Three-Phase Induction Motors / 14
3.5 / Synchronous Motors / 15
3.6 / AC Generators / 16
3.7 / Lab Activity 1 / 20
3.8 / Review Exercise / 21
3.1 Introduction to AC Machines
Alternating Current (AC) is the world standard for driving motors and other electrical equipment. Nowadays, apart from lighting devices, electric motors represent the largest loads in industry and commercial installations. Their function, to convert electrical energy into mechanical energy, means they are particularly significant in economic terms, and hence, they cannot be ignored by installation or machinery designers, installers or users.
There are many types of motor in existence, but 3-phase asynchronous or induction motors, and in particular squirrel cage motors, are the most commonly used in industry and in commercial buildings applications above a certain power level. Moreover, although they are ideal for many applications when controlled by contactor devices, the increasing use of electronic equipment is widening their field of application. Simple and rugged design, low-cost, low maintenance and direct connection to an AC power source are the main advantages of AC induction motors
On the other hand the use of synchronous motors, known as brushless or permanent magnet motors, combined with converters is becoming increasingly common in applications requiring high performance levels, in particular in terms of dynamic torque (on starting or on a change of duty), precision and speed range.
In this course, you will be studying the main parts and working principles of the following machines:
1. Induction Motors
2. Synchronous Motors.
3. AC Generators
Task 1:
Watch the following videos to help you better understand the construction and principle of operation of AC machines:
http://www.youtube.com/watch?v=HWrNzUCjbkk&feature=related
http://www.youtube.com/watch?v=Q4FlUP-kJe8
http://www.youtube.com/watch?v=uYfTzCa71SE
http://www.youtube.com/watch?v=nRMZ3K2pzcE
3.2 AC Induction Motors FundamentalsBASIC CONSTRUCTION
Like most motors, an AC induction motor has a fixed outer portion, called the stator and a rotor that spins inside with a carefully engineered air gap between the two.
Virtually all electrical motors use magnetic field rotation to spin their rotors. A three-phase AC induction motor is the only type where the rotating magnetic field is created naturally in the stator because of the nature of the supply. DC motors depend either on mechanical or electronic commutation to create rotating magnetic fields. A single-phase AC induction motor depends on extra electrical components to produce this rotating magnetic field.
Two sets of electromagnets are formed inside any motor. In an AC induction motor, one set of electromagnets is formed in the stator because of the AC supply connected to the stator windings. The alternating nature of the supply voltage induces an Electromagnetic Force (EMF) in the rotor (just like the voltage is induced in the transformer secondary) as per Lenz’s law, thus generating another set of electromagnets; hence the name – induction motor. Interaction between the magnetic field of these electromagnets generates twisting force, or torque. As a result, the motor rotates in the direction of the resultant torque. The three basic parts of an AC motor are the rotor, stator, and enclosure.
Stator
The stator is made up of several thin laminations of aluminum or cast iron. They are punched and clamped together to form a hollow cylinder (stator core) with slots as shown in Figure 3.1. Coils of insulated wires are inserted into these slots as shown in Figure 3.2. Each grouping of coils, together with the core it surrounds, forms an electromagnet (a pair of poles) on the application of AC supply. The number of poles of an AC induction motor depends on the internal connection of the stator windings. The stator windings are connected directly to the power source. Internally they are connected in such a way, that on applying AC supply, a rotating magnetic is created.
Figure 3.1: Stator construction / Figure 3.2: Stator windingsRotor
There are two types of induction motor rotor – the wound rotor or slip ring rotor and the squirrel cage rotor. The cage rotor consists of a laminated cylinder of silicon steel with copper or aluminium bars slotted in holes around the circumference and short-circuited at each end of the cylinder as shown in Figure 3.3 a, b and c . In small motors the rotor is cast in aluminium. Better starting and quieter running are achieved if the bars are slightly skewed.
Figure 3.3a: A typical squirrel cage rotorFigure 3.3b: Skewed rotor conductors
Figure 3.3c: Arrangement of conductor bars in a cage rotor
Enclosure
The enclosure consists of a frame (or yoke) and two end brackets (or bearing housings). The stator is mounted inside the frame. The rotor fits inside the stator with a slight air gap separating it from the stator. There is no direct physical connection between the rotor and the stator.
The enclosure also protects the electrical and operating parts of the motor from harmful effects of the environment in which the motor operates. Bearings, mounted on the shaft, support the rotor and allow it to turn. A fan, also mounted on the shaft, is used on the motor shown below for cooling. Exploded views of squirrel cage rotor motor and slip ring motor rotor are shown in Figures 3.4 and 3.4 respectively. .
Figure 3.4: Exploded view of a squirrel cage rotor motorFigure 3.5: Exploded view of a slip-ring rotor motor
Types of AC Induction Motors
Generally, induction motors are categorized based on the number of stator windings. They are:
• Single-phase induction motor
• Three-phase induction motor
3.3 Single-Phase Induction MotorsThere are probably more single-phase AC induction motors in use today than the total of all the other types put together. It is logical that the least expensive, lowest maintenance type motor should be used most often. The single-phase AC induction motor best fits this description.
As the name suggests, this type of motor has only one stator winding (main winding) and operates with a single-phase power supply. In all single-phase induction motors, the rotor is the squirrel cage type.
The single-phase induction motor is not self-starting. When the motor is connected to a single-phase power supply, the main winding carries an alternating current. This current produces a pulsating magnetic field. Due to induction, the rotor is energized. As the main magnetic field is pulsating, the torque necessary for the motor rotation is not generated. This will cause the rotor to vibrate, but not to rotate. Hence, the single-phase induction motor is required to have a starting mechanism that can provide the starting “kick” for the motor to rotate.
The starting mechanism of the single-phase induction motor is mainly an additional stator winding (start/auxiliary winding) as shown in Figure 3.6. The start winding can have a series capacitor and/or a centrifugal switch. When the supply voltage is applied, current in the main winding lags the supply voltage due to the main winding impedance. At the same time, current in the start winding leads/lags the supply voltage depending on the starting mechanism impedance (i.e. the total impedance of the capacitor and starting coil). Interaction between magnetic fields generated by the main winding and the starting mechanism generates a resultant magnetic field rotating in one direction. The motor starts rotating in the direction of the resultant magnetic field.
Once the motor reaches about 75% of its rated speed, a centrifugal switch disconnects the start winding. From this point on, the single-phase motor can maintain sufficient torque to operate on its own.
Except for special capacitor start/capacitor run types, all single-phase motors are generally used for applications up to 3/4 hp (horsepower) only. Depending on the various start techniques, single-phase AC induction motors are further classified as described in the following sections.
Figure 3.6: single-phase ac induction motor with and without a start mechanismSplit-Phase AC Induction Motor
The split-phase motor is also known as an induction start/induction run motor. It has two windings: a start and a main winding as shown in Figure 3.7. The start winding is made with smaller gauge wire and fewer turns, relative to the main winding to create more resistance, thus putting the start winding’s field at a different angle than that of the main winding which causes the motor to start rotating. The main winding, which is of a heavier wire, keeps the motor running the rest of the time. Good applications for split-phase motors include small grinders, small fans and blowers and other low starting torque applications with power needs from 1/20 to 1/3 hp.
Figure 3.7: Typical split-phase ac induction motorCapacitor Start AC Induction Motor
This is a modified split-phase motor with a capacitor in series with the start winding as shown in Figure 3.8. The capacitor provides a start “boost” by increasing the phase shift between the start winding and the main winding. Like the split-phase motor, the capacitor start motor also has a centrifugal switch which disconnects the start winding and the capacitor when the motor reaches about 75% of the rated speed.
Figure 3.8: Typical capacitor start AC induction motorPermanent Split Capacitor (Capacitor Run) AC Induction Motor
A permanent split capacitor (PSC) motor as shown in Figure 3.9 has a run type capacitor permanently connected in series with the start winding. The typical starting torque of the PSC motor is low, from 30% to 150% of the rated torque. Permanent split-capacitor motors have a wide variety of applications depending on the design. These include blowers with low starting torque needs, and intermittent cycling uses such as adjusting mechanisms, gate operators and garage door openers.
Figure 3.9: Typical PSC motorCapacitor Start/Capacitor Run AC Induction Motor
This motor as shown in Figure 3.10 has a start type capacitor in series with the auxiliary winding, like the capacitor start motor, for high starting torque. Like a PSC motor, it also has a run type capacitor that is in series with the auxiliary winding after the start capacitor is switched out of the circuit. This allows high overload torque.
It is able to handle applications too demanding for any other kind of single-phase motor. These include woodworking machinery, air compressors, high-pressure water pumps, vacuum pumps and other high torque applications requiring 1 to 10 hp.
Figure 3.10: Typical capacitor start/run induction motor3.4 Three- Phase Motors
If a three-phase supply is connected to three separate windings equally distributed around the stationary part or stator of an electrical machine, an alternating current circulates in the coils and establishes a magnetic flux.
Working principles of three phase induction motors:
When a three-phase supply is connected to insulated coils set into slots in the inner surface of the stator or stationary part of an induction motor as shown in Figure 3.11, a rotating magnetic flux is produced. The rotating magnetic flux cuts the conductors of the rotor and induces an emf in the rotor conductors. This induced emf causes rotor currents to flow and establish a magnetic flux which reacts with the stator flux and causes a force to be exerted on the rotor conductors, turning the rotor.
Figure 3.11: Three phase windings3.5 Synchronous Motors
Another type of AC motors is the synchronous motor. One type of synchronous motor is constructed somewhat like a squirrel cage rotor as shown in Figure 3.12. In addition to rotor bars, coil windings are added. The coil windings are connected to an external DC power supply by slip rings and brushes. On start, AC is applied to the stator and the synchronous motor starts like a squirrel cage rotor. DC is applied to the rotor coils after the motor reaches maximum speed. This produces a strong constant magnetic field in the rotor which locks in step with the rotating magnetic field. The rotor turns at the same speed as synchronous speed (speed of the rotating magnetic field). There is no slip. Variations of synchronous motors include a permanent magnet rotor. The rotor is a permanent magnet and an external DC source is not required. These are found on small horsepower synchronous motors.
Figure 3.12: AC motor as the synchronous motor3.6 AC Generators
Synchronous generators or alternators are used to convert mechanical power derived from steam, gas, or hydraulic-turbine to ac electric power. They are the primary source of electrical energy we consume today.
Regardless of size, all electrical generators, whether dc or ac, depend upon the principle of magnetic induction. An emf is induced in a coil as a result of:
1. a coil cutting through a magnetic field, or
2. a magnetic field cutting through a coil.
As long as there is relative motion between a conductor and a magnetic field, a voltage will be induced in the conductor. The part of a generator that produces the magnetic field is called the field, while the part in which the voltage is induced is called the armature. For relative motion to take place between the conductor and the magnetic field, all generators must have two mechanical parts — a rotor and a stator.
There are two basic types of AC generators:
· ROTATING-ARMATURE ALTERNATORS
· ROTATING-FIELD ALTERNATORS
1. ROTATING-ARMATURE ALTERNATORS
The rotating-armature alternator is similar in construction to the dc generator in that the armature rotates in a stationary magnetic field as shown in Figure 3.13. In the dc generator, the emf generated in the armature windings is converted from ac to dc by means of the commutator. In the alternator, the generated ac is brought to the load unchanged by means of slip rings. The rotating armature is found only in alternators of low power rating and generally is not used to supply electric power in large quantities.