EMT462 Electrical System Technology

EMT462 Electrical System Technology
LECTURE VII mohd hafiz ismail level II jejawi EMT462 Electrical System Technology hafizism february 2007

INDUCTION MACHINE EMT462 Electrical System Technology hafizism february 2007

Induction Machine The machines are called induction machines because of the rotor voltage which produces the rotor current and the rotor magnetic field is induced in the rotor windings. Induction generator has many disadvantages and low efficiency. Therefore induction machines are usually referred to as induction motors. EMT462 Electrical System Technology hafizism february 2007

Induction Machine There are two different types of induction motor rotors that can be placed inside the stator: 1. Squirrel cage – the conductors would look like one of the exercise wheels that squirrel or hamsters run on. 2. Wound rotor – have a brushes and slip ring at the end of rotor The magnetic field's rotation of induction motors is given by EMT462 Electrical System Technology hafizism february 2007

Induction Machine 1. Squirrel cage – the conductors would look like one of the exercise wheels that squirrel or hamsters run on. EMT462 Electrical System Technology hafizism february 2007

Induction Machine 2. Wound rotor – have a brushes and slip ring at the end of rotor EMT462 Electrical System Technology hafizism february 2007

Induction Machine - Operation The stator’s rotating field cuts the rotors conductors thereby inducing voltages in the rotor circuit. The induced (Faraday) voltages cause currents to flow in the rotor. The rotor’s currents produce a rotating (rotor) field which is always aligned (travels with) the stator’s rotating field. The whole process is essentially that of a transformer. The induction motor is sometimes referred as a rotating transformer . EMT462 Electrical System Technology hafizism february 2007

Induction Machine - Operation Speed of rotation (synchronous speed) P is the number of magnetic poles designed into the machine, fe is the power line frequency. EMT462 Electrical System Technology hafizism february 2007

The Concept of Rotor Slip The voltage induced in a rotor bar of an induction motor depends on the speed of the rotor relative to the magnetic fields 1. Slip speed – defined as the difference between synchronous speed (magnetic field's speed) and rotor speed. nslip = nsync – nm nslip = slip speed of the machine nsync = speed of the magnetic fields nm = mechanical shaft speed of motor EMT462 Electrical System Technology hafizism february 2007

The Concept of Rotor Slip 2. Slip – defined as the relative speed expressed on a per-unit (or sometimes as percentage) basis If the rotor turns at synchronous speed, s = 0, while if the rotor is stationary (standstill), s = 1. Mechanical speed (rotor's speed) can be expressed in term of synchronous speed and slip as follow: EMT462 Electrical System Technology hafizism february 2007

The Electrical Frequency on the Rotor The rotor frequency can be expressed as fr = sfe where fr = rotor frequency s = slip fe = electrical frequency Alternative to find fr is defined as below EMT462 Electrical System Technology hafizism february 2007

Induction Motor – Power and Torque Example 5 A 220V, three-phase, two pole, 50Hz induction motor is running at a slip of 5 percent. Find: The speed of the magnetic fields in revolutions per minute The speed of the rotor in revolutions per minute The slip[ speed of the rotor The rotor frequency in hertz EMT462 Electrical System Technology hafizism february 2007

Induction Motor – Equivalent Circuit Same as a transformer Stator is connected to the ac source, and the rotor’s voltage and current are produced by induction. The primary of the transformer corresponds to the stator of the machine, whereas the secondary corresponds to the rotor Stator and Rotor as Coupled Circuits EMT462 Electrical System Technology hafizism february 2007

Induction Motor – Equivalent Circuit EMT462 Electrical System Technology hafizism february 2007

Induction Motor – Power and Torque The power flow diagram EMT462 Electrical System Technology hafizism february 2007

Induction Motor – Equivalent Circuit EMT462 Electrical System Technology hafizism february 2007

Induction Motor – Power and Torque The output power can be found as Pout = Pconv – PF&W – Pmisc The induced torque or developed torque: EMT462 Electrical System Technology hafizism february 2007

Induction Motor – Power and Torque Exercise 1 A 480-V, 50Hz, 50hp, three phase induction motor is drawing 60A at 0.80 PF lagging. The stator copper losses are 2 kW, and the rotor copper losses are 700W. The friction and windage losses are 600W, the core losses are 1800 W, and the stray losses are negligible. Fine the following quantities: a. The air gap power PAG b. The power converted Pconv c. The output power Pout d. The efficiency of the motor EMT462 Electrical System Technology hafizism february 2007

Induction Motor – Power and Torque Exercise 2 A 460 V, 25-hp, 60Hz, four pole, Y-connected induction motor has the following impedances in ohms per phase referred to the stator circuit: R1 =0.641Ω R2 =0.332Ω X1 =1.106Ω X2 =0.464Ω Xm =26.3Ω The total rotational losses = 110 W, Rotor slip = 2.2% at rated voltage and frequency. Find the motor's i) Speed, ii) Stator Current, iii) Power factor, iv) Pconv, v) Pout vi)  ind, vii) load and viii) Efficiency EMT462 Electrical System Technology hafizism february 2007

hafizism february 2007

4.0 DC Meters. 4.1 Introduction to Meters. 4.2 Analogue Meter 4.3 Introduction to DC Meters. 4.4 D’Arsonval Meter Movement in DC Meters. 4.5 Ayrton Shunt. 4.6 Ammeter Insertion Effect. 4.7 Ohmmeter. EMT462 Electrical System Technology hafizism february 2007

4.1 Introduction to Meters. A meter is any device built to accurately detect and display an electrical quantity in a form readable by a human being. (i) Pointer (analogue). (ii) Series of lights (analogue). (iii) Numeric display (digital). In this chapter students will familiarized with the d’Arsonval meter movement, its limitations and some of its applications. Electrical meters; (i) DC, AC average quantities: -Voltmeter -Ammeter -Ohmmeter (ii) AC measurements: -Oscilloscope EMT462 Electrical System Technology hafizism february 2007

Cont’d… A meter is any device built to accurately detect and display an electrical quantity in a form readable by a human being. In the analysis and testing of circuits, there are meters designed to measure the basic quantities of voltage, current, and resistance. Most modern meters are "digital" in design, meaning that their readable display is in the form of numerical digits. Older designs of meters are mechanical in nature, using some kind of pointer device to show quantity of measurement. The first meter movements built were known as galvanometers, and were usually designed with maximum sensitivity in mind. Figure 4.1: Galvanometer. Figure 4.2: Voltmeter EMT462 Electrical System Technology hafizism february 2007

Cont’d… A very simple galvanometer may be made from a magnetized needle (such as the needle from a magnetic compass) suspended from a string, and positioned within a coil of wire. Current through the wire coil will produce a magnetic field which will deflect the needle from pointing in the direction of earth's magnetic field. An antique string galvanometer is shown in Figure 4.1. The term "galvanometer" usually refers to any design of electromagnetic meter movement built for exceptional sensitivity, and not necessarily a crude device such as that shown in Figure 4.1. Practical electromagnetic meter movements can be made now where a pivoting wire coil is suspended in a strong magnetic field, shielded from the majority of outside influences. Such an instrument design is generally known as a permanent-magnet, moving coil, or PMMC movement. EMT462 Electrical System Technology hafizism february 2007

4.2 Analogue Meters. The analogue meters are mostly based on moving coil meters. The typical structure consists of a wire wound coil placed between two permanent magnets, Figure 4.3. When current flows through the coil in the presence of a magnetic field, a force is exerted on the coil; F = Bil This force is directly proportional to current flowing in the coil. If the coil is free to rotate, the force causes a deflection of the coil that is proportional to the current. By adding an indicator (e.g. needle) and a display, the level of current can be measured. Figure 4.3: Analogue Meter EMT462 Electrical System Technology hafizism february 2007

Cont’d… For a given meter, there is a maximum rated current that produces full-scale deflection of the indicator; FSD rating. By adding external circuit components, the same basic moving coil meter can be used to measure different ranges of voltage or current. Most meters are very sensitive. That is, they give full-scale deflection for a small fraction of an amp for example a typical FSD current rating for a moving coil meters is 50 μA, with internal wire resistance of 1 kΩ. With no additional circuitry, the maximum voltage that can be measured using this meter is 50 x 10-6x 1000V = 0.05V. Additional circuitry is needed for most practical measurements. EMT462 Electrical System Technology hafizism february 2007

4.3 Introduction DC Meters. The meter movement will have a pair of metal connection terminals on the back for current to enter and exit. Most meter movements are polarity-sensitive, one direction of current driving the needle to the right and the other driving it to the left. Some meter movements are polarity-insensitive, relying on the attraction of an unmagnetized, movable iron vane toward a stationary, current-carrying wire to deflect the needle. Such meters are ideally suited for the measurement of alternating current (AC). A polarity-sensitive movement would just vibrate back and forth uselessly if connected to a source of AC. EMT462 Electrical System Technology hafizism february 2007

Cont’d… An increase in measured current will drive the needle to point further to the right. A decrease will cause the needle to drop back down toward its resting point on the left. Most of the mechanical meter movements are based on electromagnetism ; electron flow through a conductor creating a perpendicular magnetic field, A few are based on electrostatics; the attractive or repulsive force generated by electric charges across space. EMT462 Electrical System Technology hafizism february 2007

(a) Permanent Magnet Moving Coil (PMMC).
Cont’d… (a) Permanent Magnet Moving Coil (PMMC). Figure 4.4: Permanent Magnet Moving Coil (PMMC) Meter Movement. In the PMMC-type instruments, Figure 4.4. Current in one direction through the wire will produce a clockwise torque on the needle mechanism, while current the other direction will produce a counter-clockwise torque. EMT462 Electrical System Technology hafizism february 2007

(b) Electrostatic Meter Movement.
Cont’d… (b) Electrostatic Meter Movement. In the electrostatics, the attractive or repulsive force generated by electric charges across space, Figure 4.5. This is the same phenomenon exhibited by certain materials; such as wax and wool, when rubbed together. If a voltage is applied between two conductive surfaces across an air gap, there will be a physical force attracting the two surfaces together capable of moving some kind of indicating mechanism. That physical force is directly proportional to the voltage applied between the plates, and inversely proportional to the square of the distance between the plates. Figure 4.5: Electrostatic Meter Movement. EMT462 Electrical System Technology hafizism february 2007

Cont’d… The force is also irrespective of polarity, making this a polarity-insensitive type of meter movement. Unfortunately, the force generated by the electrostatic attraction is very small for common voltages. It is so small that such meter movement designs are impractical for use in general test instruments. Typically, electrostatic meter movements are used for measuring very high voltages; many thousands of volts. One great advantage of the electrostatic meter movement, however, is the fact that it has extremely high resistance, whereas electromagnetic movements (which depend on the flow of electrons through wire to generate a magnetic field) are much lower in resistance. EMT462 Electrical System Technology hafizism february 2007

Cont’d… Some D'Arsonval movements have full-scale deflection current ratings as little as 50 µA, with an (internal) wire resistance of less than 1000 Ω. This makes for a voltmeter with a full-scale rating of only 50 millivolts (50 µA X 1000 Ω). Figure 4.6: Voltmeter. EMT462 Electrical System Technology hafizism february 2007

Figure 4.7: D’Ársonval Meter Movement Used in Ammeter Circuit
D’Arsonval Meter Movement in DC Meter. The basic d’Arsonval meter movement has only limited usefulness without modification. By modification on the circuit it will increase the range of current that can be measured with the basic meter movement. This is done by placing the low resistance in parallel with the meter movement resistance Rm. The low resistance shunt (Rsh) will provide an alternate path for the total meter current I around the meter movement. The Ish is much greater than Im. Where Rsh = resistance of the shunt Rm = internal resistance of the meter movement (resistance of the moving coil) Ish = current through the shunt Im = full-scale deflection current of the meter movement I = full-scale deflection current for the ammeter Figure 4.7: D’Ársonval Meter Movement Used in Ammeter Circuit EMT462 Electrical System Technology hafizism february 2007

Cont’d… The voltage drop across the meter movement is Vm = ImRm Since the shunt resistor is in parallel with the meter movement, the voltage drop across the shunt is equal to the voltage drop across the meter movement. That is, Vsh = Vm The current through the shunt is equal to the total current minus the current through the meter movement:, Ish = I – Im Knowing the voltage across, and the current through, the shunt allows us to determine the shunt resistance as Rsh = Vsh/Ish = ImRm/Ish = (Im/Ish)(Rm) = Im/(I – Im)*Rm Ohm EMT462 Electrical System Technology hafizism february 2007

Example 4.1: D’Arsonval Movement. A D'Arsonval meter movement having a full-scale deflection rating of 1 mA and a coil resistance of 500 Ω: Solution: Ohm's Law (E=IR), determine how much voltage will drive this meter movement directly to full scale, EMT462 Electrical System Technology hafizism february 2007

Example 4.2: D’Arsonval Meter. Calculate the value of the shunt resistance required to convert a 1-mA meter movement, with a 100 Ohm internal resistance, into a 0- to 10 mA ammeter. Solution: Calculate Vm. Vm is in parallel with Vsh. KCL . EMT462 Electrical System Technology hafizism february 2007

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