EMT462 Electrical System Technology LECTURE VIII mohd hafiz ismail hafizism@gmail.com 04-9798330 level II jejawi EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology 4.5 Ayrton Shunt. The purpose of designing the shunt circuit is to allow to measure a current I that is some number n times larger than Im, Figure 4.8. The number n is called a multiplying factor and relates total current and meter current as the Ayrton Shunt. I = nIm Substituting for I in previous equation, yields Rsh = RmIm/(nIm-Im) = Rm/(n-1) Ohm Advantage: (i) it eliminates the possibility of the meter movement being in the circuit without any shunt resistance. (ii) May be used with a wide range of meter movements. Figure 4.8: Aryton Shunt. EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Cont’d… The individual resistance values of the shunts are calculated by starting with the most sensitive range and working toward the least sensitive range. The shunt resistance is, Rsh = Ra + Rb + Rc On this range the shunt resistance is equal to Rsh and can be computed by the equation, Rsh = Rm/(n-1) The equation needed to compute the value of each shunt, Ra, Rb, and Rc, can be developed from the circuit in Figure 4.8. Since the resistance Rb + Rc is in parallel with Rm + Ra, the voltage across each parallel branch should be equal and can be written as VRb + Rc = VRa + Rm EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Cont’d… In current and resistance terms we can write (Rb + Rc) (I2-Im)=Im (Ra +Rm) or I2(Rb + Rc) - Im(Rb + Rc)= Im[Rsh-(Rb + Rc)+Rm] Multiplying through by Im on the right yields I2(Rb + Rc) - Im(Rb + Rc) = ImRsh- Im(Rb + Rc)+ImRm This can be rewritten as Rb+ Rc = Im (Rsh+ Rm)/I2 Having already found the total shunt resistance Rsh, we can now determine Ra as Ra = Rsh – (Rb + Rc) The current I is the maximum current for the range on which the ammeter is set. The resistor Rc can be determined from Rc = Im(Rsh+ Rm)/I3 The resistor Rb can now be computed as, Rb = (Rb + Rc) – Rc EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Example 4.3: Aryton Shunt. Compute the value of the shunt resistors for the circuit below. I3 = 1A, I2 = 100 A, I1 = 10 mA, Im = 100 uA and Rm = 1K Ohm. Solution: The total shunt resistance is found from This is the shunt for the 10 mA range. When the meter is set on the 100-mA range, the resistor Rb and Rc provide the shunt . The total shunt resistance is found by the equation. EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Cont’d…Example The resistor Rc , which provides the shunt resistance on the 1-A range can be found by the same equation, however the current I will now be 1A. The resistor Rb is found from the equation below; The resistor Ra is found from; Verify the above result. . EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Cont’d… (b) Voltmeter Design. Consider a moving coil meter with FSD rating of 1 mA and coil resistance, Rc, of 500 Ω. The maximum voltage required to produce FSD is 0.5 V. The voltage range is increased by adding a series resistor, The voltage that can be applied to the – and + terminals before FSD current flows is then increased to: VFSD= IFSD(Rc+ Rm) Rm is called a multiplier resistor because it multiplies the working range of the meter. Alternatively, it may be thought of as dividing the measured voltage across the moving coil meter. Figure 4.9: Voltmeter. EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Cont’d… For a given required FSD voltage, say VFSD, the multiplier resistance, Rm, is chosen as: Rm= (VFSD/ IFSD) –Rc For example, to provide a voltmeter with FSD reading of 10 V with the given meter (IFSD = 1 mA, Ri= 500 Ω): Rm = (10 / 1 x 10-3) –500 = 9.5kΩ. With exactly 10 V applied, there will be exactly 1 mA of current flowing, thereby producing full-scale deflection. There is only the maximum allowed voltage of 0.5V dropped across the moving coil meter. The scale of the meter must be changed to indicate the new range of the circuit. EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology 4.6 Ammeter Insertion Effect. We frequently overlook the error caused by inserting an ammeter in a circuit to obtain a current reading. All ammeters contain some internal resistance. By inserting the ammeter in the circuit means increase the resistance of the circuit and result in reducing current in the circuit. Refer to the circuit in Figure 4.10, Ie is the current without the ammeter. Suppose that we connect the ammeter in the circuit (b), the current now becomes Im due to the additional resistance introduced by the ammeter. Figure 4.10: (a) Expected Current Value in a Series Circuit (b) Series Circuit with Ammeter. EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Cont’d… From the circuit; Placing the meter in series result in; Divide the above equations yields; Insertion error, EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Example 4.4: Ammeter Insertion Effects. A current meter that has an internal resistance 78 Ohm is used to measure the current through resistor Rc in Figure 4.10. Determine the percentage of error of the reading due to ammeter insertion. Solution. The Thevenin equivalent resistance. The ratio of meter current to the expected current is, Solving for Im yields, EMT462 Electrical System Technology hafizism february 2007

4.7 Ohmmeter. The d’Arsonval meter movement can be used with the battery and resistor to construct a simple ohmmeter. Figure 4.11 is the basic ohmmeter circuit, Introduce Rx between point X and Y so that we can calculate the value of resistance. Figure 4.11: Basic Ohmmeter Circuit. EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Cont’d… P represent the ratio of the current I to the full scale deflection Figure 4.12: Basic Ohmmeter Circuit with Unknown Resistor Rx Connected Between. EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Example 4.5: Ohmmeter. A 1mA full-scale deflection current meter movement is to used in an ohmmeter circuit. The meter movement has an internal resistance, Rm, of 100 Ohm, and a 3-V battery will be used in the circuit. Mark off the meter face for reading resistance. e to ammeter insertion. Solution. Value of Rz, which will limit current to full-scale deflection is, Value of Rz, with 20% full-scale deflection is, EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Cont’d…Example Value of Rz, with 40% full-scale deflection is, Value of Rz, with 50% full-scale deflection is, Value of Rz, with 75% full-scale deflection is, The ohmmeter is nonlinear due to the high internal resistance of the ohmmeter. EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology 5.0 AC Meters. 5.1 Introduction to AC Meters. 5.2 D’Arsonval Meter Movement with Half-Wave Rectification. 5.3 D’Arsonval Meter Movement with Full-Wave Rectification. EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology 5.1 Introduction to AC Meters. Five principal meter movement that are commonly used in ac instruments; (i) Electrodynamometer. (ii) Iron-Vane. (iii) Electrostatic. (iv) Thermocouple. (v) D’Arsonval (PMMC) with rectifier. The d’Arsonval meter is the most frequently used meter movement, event though it cannot directly measure alternating current or voltage. In this chapter it will discuss the instruments for measuring alternating signal that use the d’Arsonval meter movement. EMT462 Electrical System Technology hafizism february 2007

(a) AC voltmeters and ammeters Cont’d… (a) AC voltmeters and ammeters AC electromechanical meter movements come in two basic arrangements: (1) Based on DC movement designs. (2) Engineered specifically for AC use. Permanent-magnet moving coil (PMMC) meter movements will not work correctly if directly connected to alternating current, because the direction of needle movement will change with each half-cycle of the AC. Permanent-magnet meter movements, like permanent-magnet motors, are devices whose motion depends on the polarity of the applied voltage, Figure 5.1. Figure 5.1: D’Arsonal Electromechanical Meter Movement. EMT462 Electrical System Technology hafizism february 2007

Cont’d… (b) DC-style Meter Movement for AC application. If we want to use a DC-style meter movement such as the D'Arsonval design, the alternating current must be rectified into DC, Figure 5.2. This can be accomplished through the use of devices called diodes. The diodes are arranged in a bridge, four diodes will serve to steer AC through the meter movement in a constant direction throughout all portions of the AC cycle: Figure 5.2: Rectified D’Arsonal Electromechanical Meter Movement. EMT462 Electrical System Technology hafizism february 2007

Cont’d… (c) Iron-Vane Electromechanical. The AC meter movement without the inherent polarity sensitivity of the DC types. This design avoid using the permanent magnets. The simplest design is to use a non-magnetized iron vane to move the needle against spring tension, the vane being attracted toward a stationary coil of wire energized by the AC quantity to be measured, Figure 5.3. The electrostatic meter movements are capable of measuring very high voltages without need for range resistors or other, external apparatus. Figure 5.3: Iron-Vane Electromachanical Meter Movement. EMT462 Electrical System Technology hafizism february 2007

Cont’d… (d) AC Voltmeter with Resistive Divider. When a sensitive meter movement needs to be re-ranged to function as an AC voltmeter, series-connected "multiplier" resistors and/or resistive voltage dividers may be employed just as in DC meter design, Figure 5.4. Figure 5.4: AC Voltmeter with Resistive Divider. EMT462 Electrical System Technology hafizism february 2007

Cont’d… (e) AC Voltmeter with Capacitive Divider. Capacitors may be used instead of resistors, though, to make voltmeter divider circuits. This strategy has the advantage of being non-dissipative; no true power consumed and no heat produced. Refer to Figure 5.5. Figure 5.5: AC Voltmeter with Capacitive Divider. EMT462 Electrical System Technology hafizism february 2007

5.2 D’Arsonval Meter Movement with Half-Wave Rectification. In order to measure the alternating current with the d’Arsonval meter movement, we must rectify the alternating current by use of diode rectifier . Figure 5.6 is the DC voltmeter circuit modified to measure AC voltage. The forward diode, assume to be ideal diode, has no effect on the operation of the circuit . For example if the 10 V sine-wave input is fed as the source of the circuit, the voltage across the meter movement is just the positive half-cycle of the sine wave due to the rectifying effect of the diode. Figure 5.6: DC Voltmeter Circuit Modified to Measure AC Voltage. EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Cont’d… The peak value of 10 V rms sine wave is, If the output voltage from the half-wave rectifier is 10V only, a dc voltmeter will provide an indication of approximately 4.5 V. From the above equation, EMT462 Electrical System Technology hafizism february 2007

. Example 4.1: D’Arsonval Meter Half-Wave Rectifier. Compute the value of the multiplier resistor for a 10 Vrms ac range on the voltmeter shown in Figure 5.7. Solution: Find the sensitivity for a half wave rectifier. . Figure 5.7: AC Voltmeter Using Half-Wave Rectification. EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Cont’d… Commercially produced ac voltmeters that use half-wave rectification have an additional diode and shunt as shown in Figure 5.8, which is called instrument rectifier. . Figure 5.8: Half-Wave Rectification Using an Instrument Rectifier and a Shunt Resistor. EMT462 Electrical System Technology hafizism february 2007

5.3 D’Arsonval Meter Movement with Full-Wave Rectification. The full-wave rectifier provide higher sensitivity rating compare to the half-wave rectifier. Bridge type rectifier is the most commonly used, Figure 5.9. Figure 5.9: Full Wave Bridge Rectifier Used in AC Voltmeter Circuit. EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Cont’d… Operation; (a) During the positive half cycle (red arrow), currents flows through diode D2, through the meter movement from positive to negative, and through diode D3. - The polarities in circles on the transformer secondary are for the positive half cycle. - Since current flows through the meter movement on both half cycles, we can expect the deflection of the pointer to be greater than with the half wave cycle. - If the deflection remains the same, the instrument using full wave rectification will have a greater sensitivity. (b) Vise versa for the negative half cycle (blue arrow). EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Cont’d… From the circuit in Figure 5.9, the peak value of the 10 Vrms signal with the half-wave rectifier is, The average dc value of the pulsating sine wave is, Or can be compute as, The AC voltmeter using full-wave rectification has a sensitivity equal to 90% of the dc sensitivity or twice the sensitivity using half-wave rectification. EMT462 Electrical System Technology hafizism february 2007

. Example 4.2: D’Arsonval Meter Full-Wave Rectifier. Each diode in the full-wave rectifier circuit in Figure 5.10 has an average forward bias resistance of 50 Ohm and is assumed to have an infinite resistance in the reverse direction. Calculate, (a) The multiplier Rs. (b) The AC sensitivity. © The equivalent DC sensitivity. Solution: (a) Calculate the current shunt and total current, . Figure 5.10: AC Voltmeter Using Full-Wave Rectification and Shunt. EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Cont’d…Example The equivalent DC voltage is, (b) The ac sensitivity, (c.) The dc sensitivity, . EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology hafizism february 2007