Transcript chapter 5 - UniMAP Portal

CHAPTER 5
AC Meter.
School of Computer and Communication
Engineering, UniMAP
Prepared By:
Amir Razif b. Jamil Abdullah
EMT 113: V-2008
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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.
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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.
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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.
4
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.
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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.
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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.
7
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.
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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 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.
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Cont’d…
 The peak value of 10 Vrms sine wave is,
E p  10Vrms *1.414  14.14V peak
E ave  E dc  0.318 * E p
or
E ave 
Ep

 0.45 * E rms
Edc
0.45Erms
Rs 
 Rm 
 Rm
I dc
I dc
 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,
S  0.45S
ac
dc
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Example 5.1: 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.
Figure 5.7: AC Voltmeter Using HalfWave Rectification.
Solution:
Find the sensitivity for a half wave rectifier.
S ac  0.45S dc  0.45 *
1
450

I fs
V
Rs  S ac * Range ac  Rm
.
450 10V

*
 300  4.2 K
V
1
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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.
 .
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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.
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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).
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Cont’d…
 From the circuit in Figure 5.9, the peak value of the 10 Vrms
signal with the half-wave rectifier is,
E p  1.414 * E rms  14.14V peak
 The average dc value of the pulsating sine wave is,
E ave  0.636 E p  9V
 Or can be compute as,
Eave  0.9 * Erms  0.9 *10V  9V
 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.
S ac  0.9 * S dc
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Example 5.2: 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.
Figure 5.10: AC Voltmeter Using FullWave Rectification and Shunt.
Solution:
(a) Calculate the current shunt and total current,
I sh
.
E m 1mA * 500


 1mA
Rsh
500
and
I T  I sh  I m  1mA  1mA  2mA
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Cont’d…
 The equivalent DC voltage is,
E dc  0.9 * 10Vrms  0.9 *10V  9.0V
RT 
Rs  RT  2 Rd 
E dc 9.0V

 4.5 K
IT
2mA
Rm Rsh
Rm  Rsh
 4500  2 * 50 
(b) The ac sensitivity,
(c.) The dc sensitivity,
S ac
500 * 500
 4.15K
500  500
RT
4500


 450 / V
Range
10V
S dc 
1
1

 500 / V
I T 2mA
or
 .
S dc 
S ac 450 / V

 500 / V
0. 9
0.9
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