EMT 462 ELECTRICAL SYSTEM TECHNOLOGY Chapter 2 AC

EMT 462 ELECTRICAL SYSTEM TECHNOLOGY Chapter 2: AC Machines By: En. Muhammad Mahyiddin Ramli Chap 2: AC Machines

AC Machines: Introduction 2 major classes: a) Asynchronous machines / induction machines : – Motors or generators whose field current is supplied by magnetic induction (transformer action) into their field windings. b) Synchronous machines : – Motors or generators whose field current is supplied by a separate dc power source. Note: 1) Induction motor has the same physical stator as a synchronous machine, with a different rotor construction. 2) The fields circuit of most synchronous and induction machines are located on their rotors. Motors = ac electrical energy mechanical energy Generators = mechanical energy ac electrical energy Chap 2: AC Machines 2

AC Machinery Fundamentals A SIMPLE LOOP IN A UNIFORM MAGNETIC FIELDS. n n n A rotating loop of wire within the magnetic field. Magnetic field produced by a large stationary magnet produce-constant and uniform magnetic field, B. Rotation of the loop induced a voltage in the wire. V eind ө n Current flows in the loop, a torque will be induced on the wire loop. eind = Фmax ω sin ωt • Thus, the voltage generated in loop is a sinusoid whose magnitude is equal to the product of the flux and rotation speed of the machine Chap 2: AC Machines 3

AC Machinery Fundamentals THE ROTATING MAGNETIC FIELD n When two magnetic fields are present in a machine, a torque will be created which will tend to line up the two magnetic fields. n Magnetic field is produced by the stator and rotor of an ac machine. n Then a torque will be induced in the rotor cause the rotor to turn and align itself with the stator magnetic field. n The induced torque in the rotor would cause the rotor to constantly “ chase “ the stator magnetic field around in circle - the basic principle of all ac motor operation. But how the stator rotate? Chap 2: AC Machines 4

AC Machinery Fundamentals AC MACHINE POWER LOSSES The efficiency of an AC machines is defined as: Four types of losses in AC machines: q q Electrical or copper losses (I 2 R losses) Core losses Mechanical losses Stray load losses Chap 2: AC Machines 5

AC Machinery Fundamentals VOLTAGE REGULATION AND SPEED REGULATION VR is a measure of the ability of a generator to keep a constant voltage at its terminals as load varies. It is defined as follow: SR is a measure of the ability of a motor to keep a constant shaft speed as load varies. Chap 2: AC Machines 6

INDUCTION MOTOR Chap 2: AC Machines 7

Induction Motors Induction motors are the motor frequently encountered in industry. It simple, rugged, low-priced and easy to maintain. It run essentially constant speed from zero to full-load. The speed is frequency-dependent and consequently these motors are not easily adapted to speed control Induction machines is called induction because the rotor voltage (which produces the rotor current and the rotor magnetic field) is induced in the rotor winding rather than physically connected by wires. Chap 2: AC Machines 8

INDUCTION MOTOR CONSTRUCTION A 3 -phase induction motor has two main parts : • A stationary stator (stationary part of the machine) • Revolving rotor (rotating part of the machine) The rotor is separated from the stator by a small air gap (the tolerances is depending on the power of the motor). Chap 2: AC Machines 9

Two types of rotor which can placed inside the stator. a) Squirrel-cage induction motor (also called cage motors) b) Wound-rotor induction motor a) Cage rotor b) Wound rotor induction motors more expensive- maintenance Chap 2: AC Machines 10

Two types of rotor which can placed inside the stator. a) Squirrel-cage induction motor (also called cage motors) b) Wound-rotor induction motor a) Squirrel cage – the conductors would look like one of the exercise wheels that squirrel or hamsters run on. Chap 2: AC Machines 11

Two types of rotor which can placed inside the stator. a) Squirrel-cage induction motor (also called cage motors) b) Wound-rotor induction motor b) Wound rotor – have a brushes and slip ring at the end of rotor. (Y-connected) Chap 2: AC Machines 12

INDUCTION MOTOR CONSTRUCTION q. Cage induction Motor rotor consists of a series of conducting bars laid into slot carved in the face of rotor and shorted at either end by large shorting ring q. A wound rotor has a complete set of three-phase winding that are mirror images of the winding on the stator. The three phases of the rotor windings are usually Y-connected, the end of the three rotor wires are tied to slip ring on the rotor shaft. Rotor windings are shorted through brushes riding on the slip rings. Wound-rotor induction motors are more expensive than the cage induction motors, they required much more maintenance because the wear associated with their brushes and slip rings. Chap 2: AC Machines 13

INDUCTION MOTOR CONSTRUCTION Small cage rotor induction motor Large cage rotor induction motor Chap 2: AC Machines 14

INDUCTION MOTOR CONCEPT INDUCED TORQUE IN AN INDUCTION MOTOR The speed of the magnetic field’s rotation in a cage rotor induction motor (Figure 7. 6, Chapman) is given by: Where nsync = synchronous speed [r/min] fe = System frequency [Hz] p = number of poles This equation shows that the synchronous speed increases with frequency and decrease with the number of poles. The three-phase of voltages has been applied to the stator, and three-phase set of stator current is flowing. These currents produce a magnetic field BS , rotating counterclockwise direction. Chap 2: AC Machines 15

Basic Induction Motor Concepts This rotating magnetic field BS passes over the rotor bars and induces a voltage, eind in them: where, v = velocity of the bar relative to the magnetic field B = magnetic flux density vector l = length of conductor in the magnetic field • It is the relative motion of the rotor compared to the stator magnetic field that produces induced voltage in a rotor bar. The rotor current flow produces a rotor magnetic field, BR. • The induce torque in the machine is given by: • The voltage induced in a rotor bar depends on the speed of the rotor relative to the magnetic fields Chap 2: AC Machines 16

Basic Induction Motor Concepts THE CONCEPT OF ROTOR SLIP Slip speed is defined as the differences between synchronous speed and rotor speed: Where nslip = slip speed of the machines nsync = speed of the magnetic field nm = mechanical shaft speed of motor The other term used to describe the relative motion is slip, which is relative speed expressed on a per unit or a percentage basis. The slip is defined as : Chap 2: AC Machines 17

Basic Induction Motor Concepts • The previous equation also can be expressed in term of angular velocity (radians per second) as : • If the rotor turns at synchronous speed, s=0 ; if the rotor is stationary (locked or stop) , s=1. All normal motor speeds fall somewhere between those limits. • As for mechanical speed • These equation are useful in the derivation of induction motor torque and power relationship. Chap 2: AC Machines 18

Basic Induction Motor Concepts THE ELECTRICAL FREQUENCY ON THE ROTOR q The induction motor works by inducing voltages and current in the rotor of the machine-called a rotating transformer. q Like a transformer; primary (stator) induced a voltage in the secondary (rotor) q Unlike a transformer, the secondary frequency not necessarily the same as primary. q If the rotor of a motor is locked so that it cannot move, the rotor will have the same frequency as the stator. q If the rotor turns at synchronous speed, the frequency on the rotor will be zero. q For nm=0 r/min & the rotor frequency fr=fe slip, s = 1 nm=nsync & the rotor frequency fr=0 slip, s = 0 - For any speed in between, the rotor frequency is directly proportional to the difference between the speed of the magnetic field nsync and the speed of the rotor nm. Chap 2: AC Machines 19

Basic Induction Motor Concepts THE ELECTRICAL FREQUENCY ON THE ROTOR Since the slip of the rotor is defined as : Then the rotor frequency can be expressed as : Substituting between these two equation become : But nsync = 120 fe/P, so Therefore, fr = frequency rotor; fe = frequency stator Chap 2: AC Machines 20

Example 2. 1: Induction Motor A 208 -V, 10 -hp, four-pole, 60 -Hz, Y- connected induction motor has a fullload slip of 5%. a) What is the synchronous speed of this motor? b) What is the rotor speed of this motor at the rated load? c) What is the rotor frequency of this motor at the rated load? d) What is the shaft torque of this motor at the rated load? - Taken from Chapman’s Book, Example 7 -1, pg. 387. Chap 2: AC Machines 21

The Equivalent Circuit of An Induction Motor a) Transformer model of the transformer action-induction of voltages and currents in the rotor circuit of an IM is essentially a transformer operation. as in transformer model – certain resistance, self inductance in primary (stator) windings; magnetization curve and etc. b)Rotor circuit model - The greater the relative motion between the rotor and the stator magnetic fields, the greater the resulting rotor voltage and frequency. Locked-rotor or blocked-rotor –the largest relative motion when the rotor is stationary. c) Final Equivalent circuit - Refer the rotor part of the model over the stator side. Chap 2: AC Machines 22

The Equivalent Circuit of An Induction Motor A) TRANSFORMER MODEL IDEAL TRANSFORMER STATOR Symbol aeff Description Effective turn ratio – ratio of the conductors per phase on the stator to the conductors per phase on the rotor ROTOR Xm Resistance losses (correspond to iron losses, windage and friction losses) E 1 Primary internal stator voltage R 1 Stator Resistance ER Secondary internal rotor voltage X 1 Stator Leakage Reactance Rotor Resistance Rc Magnetizing reactance RR XR Rotor Reactance Chap 2: AC Machines 23

The Equivalent Circuit of An Induction Motor A) TRANSFORMER MODEL • Induction motor operates on the induction of voltage and current in its rotor circuit from the stator circuit (transformer action). • An induction motor is called a singly excited machine, since power is supply to only the stator circuit. • The flux in the machine is related to the integral of the applied voltage E 1. • The curve of magnetomotive force versus flux (magnetization curve) for this machine is compared to a similar curve for a power transformer. Chap 2: AC Machines 24

The Equivalent Circuit of An Induction Motor B) THE ROTOR CIRCUIT MODEL Suppose the motor run at a slip s, meaning that the rotor speed is ns (1 -s), where ns is the synchronous speed, then this modify the values of VOLTAGE and CURRENT on the primary and secondary side. The frequency of the induced voltage at any slip will be given fr = sfe Assuming ER 0 is the magnitude of the induced rotor voltage at LOCKED ROTOR condition the actual voltage induced because of slip (s) is, ER = s. ER 0 The resistor is not frequency sensitive, the value of RR remain the same. The rotor inductance is frequency sensitive (X= L=2 f. L) then XR = s. XR 0 Figure 6 shows the equivalent circuit when motor is running at a slip (s). Chap 2: AC Machines 25

The Equivalent Circuit of An Induction Motor B) THE ROTOR CIRCUIT MODEL Equivalent circuit of a wound-rotor when it at locked or blocked condition The frequency of the voltages and currents in the stator is f, but the frequency of the voltages and currents in the rotor is sf. Chap 2: AC Machines 26

The Equivalent Circuit of An Induction Motor B) THE ROTOR CIRCUIT MODEL j. XR=js. XR 0 Then, resulting rotor equivalent circuit as below. The rotor current flow can be found as : ER = s. ER 0 RR The rotor circuit model of an induction motor ZReq Chap 2: AC Machines 27

The Equivalent Circuit of An Induction Motor B) THE ROTOR CIRCUIT MODEL Then the rotor equivalent circuit become: js. XR 0 ER 0 ZReq The rotor circuit model with all the frequency (slip) effects concentrated in resistor RR Chap 2: AC Machines 28

The Equivalent Circuit of An Induction Motor C) THE FINAL EQUIVALENT CIRCUIT • Remember, in transformer, the voltages, currents and impedances on the secondary side of the device can be referred to PRIMARY side by turn ratio of the transformer : • The same transformation can be used for the induction motor’s rotor circuit by using effective turn ratio aeff : Chap 2: AC Machines 29

The Equivalent Circuit of An Induction Motor C) THE FINAL EQUIVALENT CIRCUIT The rotor circuit model that will be referred to the stator side as shown below The per-phase equivalent circuit of an induction motor. Chap 2: AC Machines 30

Power and Torque in Induction Motors Input is 3 phase system of voltages and currents. Electrical to mechanical power conversion Output is mechanical. PSCL – Losses In Stator Windings / I 2 R PRCL=I 2 R Pcore – Hysteresis & Eddy Current The power flow diagram of an induction motor – shows the relationship between the input electric power and output mechanical power. Chap 2: AC Machines 31

Example 2. 2: Power-in in Induction Motors An induction motor draws 60 A from a 480 V, 60 Hz, 50 -hp, three-phase line at a power of 0. 85 lagging. The stator copper losses are 2000 W and the rotor copper losses are 700 W. The rotational losses include 600 W of friction and wind-age, 1800 W of PAG = Pin – PSCL - PCORE core and negligible of stray load losses. Calculate the following quantities: (a)The air-gap power, PIN = √ 3 VTIL cos θ = √ 3 (480 V)(60 A)(0. 85) (b)The power converted, Pconv = 42. 4 k. W (c)The output power, (d)The efficiency of the motor PAG = Pin – PSCL – PCORE = 42. 4 k. W – 2 k. W – 1. 8 k. W Solution: = 38. 6 k. W (a)The air-gap power, Chap 2: AC Machines 32

(cont’d) PCONV = PAG – PRCL (b)The power converter, Pconv = 38. 6 k. W – 700 W = 37. 9 k. W POUT = PCONV – PF&W – PMISC = 37. 9 k. W – 600 W – 0 W = 37. 3 k. W (c) The output power. h = P out P in X 100 % = 37. 3 k. W x 100% = 88% 42. 4 k. W (d) The efficiency, Chap 2: AC Machines 33

Power and Torque in Induction Motors The per-phase equivalent circuit of an induction motor Input current Where Chap 2: AC Machines 34

Torque-speed characteristics (a) (b) a) A typical induction motor torque-speed characteristic curve. b) Showing the extended operating ranges (braking region and generator region) Chap 2: AC Machines 35

Torque-speed characteristic 1. The induced torque of the motor is zero at synchronous speed. 2. The torque speed curve is nearly linear between no-load and full-load. In this range, the rotor resistance is much larger than the rotor reactance. So, the rotor current increasing linearly. 3. There is maximum possible torque that cannot be exceeded (called pullout torque or breakdown torque) is 2 -3 times the rated full-load torque. 4. Starting torque on motor is slightly larger than full-load. 5. The torque on the motor for a given slip varies as the square of the applied voltage. 6. If the rotor of the induction motor is driven faster than synchronous speed, then the direction of the induced torque in the machine reverse and become generator. 7. If motor turning backward, relative to the direction of the magnetic field, the induced torque will stop the machine very rapidly and will try to rotate it in the other direction (called plugging). Chap 2: AC Machines 36

Speed Control of Induction Motors 1. By pole changing 2. By line frequency control 3. By line voltage control 4. By changing the rotor resistance Chap 2: AC Machines To vary the synchronous speed which is the speed of the stator and rotor magnetic field To vary the slip of the motor for a given load 37

Induction Motor Ratings • The most important ratings: • Output power • Voltage • Current • Power factor • Speed • Nominal frequency *Refer Chapman pg. 465 *Note: 1 h. p = 746 Watts Chap 2: AC Machines 38

SYNCHRONOUS MACHINES Chap 2: AC Machines 39

INTRODUCTION (Review) Transformer – energy transfer device. (transfer energy from primary to secondary) - form of energy remain unchanged. (Electrical) (DC/AC) Machines – electrical energy is converted to mechanical or vice versa. Motor operation The field induced voltage, E permits the motor to draw power from the line to be converted into mechanical power. This time, the mechanical output torque is also developing. The induced voltage is in opposition to the current flow-called counter emf. Chap 2: AC Machines 40

INTRODUCTION (Review) Generator operation The field induced voltage, E is in the same direction as the current and is called the “generated voltage”. The machine torque opposes the input mechanical torque that is trying to drive the generator, and it is called the counter torque. n n Generally, the magnetic field in a machine forms the energy link between the electrical and mechanical systems. The magnetic field performs two functions: q q Magnetic attraction and repulsion produces mechanical torque (motor operation) The magnetic field by Faraday’s Law induces voltages in the coils of wire. (generator operation) Chap 2: AC Machines 41

SYNCHRONOUS MACHINES CONSTRUCTION Have an outside stationary part, (stator) The inner rotating part (rotor) The rotor is centered within the stator. Air gap - the space between the outside of the rotor and the inside of the stator Origin of name: syn = equal, chronos = time Synchronous machines are called ‘synchronous’ because their mechanical shaft speed is directly related to the power system’s line frequency. Chap 2: AC Machines 42

SYNCHRONOUS MACHINES CONSTRUCTION STATOR § The stator of a synchronous machine carries the armature or load winding which is a three-phase winding. § The armature winding is formed by interconnecting various conductors in slots spread over the periphery of the machine’s stator. § When current flows in the winding, each group produces a magnetic pole having a polarity dependent on the current direction, and a magnetomotive force (mmf) proportional to the current magnitude. Chap 2: AC Machines 43

SYNCHRONOUS MACHINES CONSTRUCTION ROTOR § 2 types of rotors - cylindrical (or round) rotors - salient pole rotors. Salient pole rotor less expensive than round rotors and rotate at lower speeds § The rotor carries the field winding. The field current or the excitation current is provided by an external dc source. § Synchronous machine rotors are simply rotating electromagnets built to have as many poles as are produced by the stator windings. § Dc currents flowing in the field coils surrounding each pole magnetize the rotor poles. The magnetic field produced by the rotor poles locks in with a rotating stator field, so that the shaft and the stator field rotate in synchronism. Chap 2: AC Machines 44

SYNCHRONOUS MACHINES 1) Generator The rate of rotation of the magnetic fields in the machine is related to the stator electrical frequency, given as: The internal generated voltage of a synchronous generator is given as, This equation shows the magnitude of the voltage induced in a given stator phase. Chap 2: AC Machines 45

Equivalent Circuit of a synchronous Generator The per phase equivalent circuit Chap 2: AC Machines 46

Synchronous Generator – voltage regulation § If the generator operates at a terminal voltage VT while supplying a load corresponding to an armature current Ia, then; § In an actual synchronous machine, the reactance is much greater than the armature resistance, in which case; § Among the steady-state characteristics of a synchronous generator, its voltage regulation and power-angle characteristics are the most important ones. As for transformers, the voltage regulation of a synchronous generator is defined at a given load as; Chap 2: AC Machines 47

Phasor diagram of a synchronous generator The phasor diagram is showing the relationship among the voltages within a phase (Eφ, Vφ, j. XSIA and RAIA) and the current IA in the phase. Unity P. F (1. 0) Chap 2: AC Machines 48

Phasor diagram of a synchronous generator Lagging P. F Leading P. F. Chap 2: AC Machines 49

Power and Torque in Synchronous Generator In generators, not all the mechanical power going into a synchronous generator becomes electric power out of the machine The power losses in generator are represented by difference between output power and input power shown in power flow diagram below. Pconv Chap 2: AC Machines 50

Losses in Synchronous Generator Rotor - resistance; iron parts moving in a magnetic field causing currents to be generated in the rotor body - resistance of connections to the rotor (slip rings) Stator - resistance; magnetic losses (e. g. , hysteresis) Mechanical - friction at bearings, friction at slip rings Stray load losses - due to non-uniform current distribution Chap 2: AC Machines 51

Synchronous Generator § The input mechanical power is the shaft power in the generator given by equation: § The power converted from mechanical to electrical form internally is given by: § The real electric output power of the synchronous generator can be expressed in line and phase quantities as: and reactive output power: Chap 2: AC Machines 52

Synchronous Generator In real synchronous machines of any size, the armature resistance RA is more than 10 times smaller than the synchronous reactance XS (Xs >> RA). Therefore, RA can be ignored Simplified phasor diagram with armature resistance ignored. Chap 2: AC Machines 53

SYNCHRONOUS MACHINES 2) MOTOR Equivalent circuit of synchronous motor: Equivalent circuit Chap 2: AC Machines 54

Power and Torque in Synchronous Motor The power-flow diagram of a synchronous machines Chap 2: AC Machines 55

Example 3. 3 : Synchronous Generator. A three-phase, wye-connected 2500 k. VA and 6. 6 k. V generator operates at full-load. The per-phase armature resistance Ra and the synchronous reactance, Xd, are (0. 07+j 10. 4). Calculate the percent voltage regulation at: (a) 0. 8 power-factor lagging, and (b) 0. 8 power-factor leading. Chap 2: AC Machines 56

Solution: Chap 2: AC Machines 57

Success is the sum of small efforts, repeated day in and day out… - Robert Collier Chap 2: AC Machines 58