Transcript chapter 4 - ac meter

CHAPTER 4
AC Meters.
Prepared By:
Rosemizi B. Abd. Rahim
1
4.1 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.
2
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 4.1.
Figure 4.1: D’Arsonal Electromechanical
Meter Movement.
3
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 4.2.
 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 4.2: Rectified D’Arsonal
Electromechanical Meter Movement.
4
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 4.3.
 The electrostatic meter movements
are capable of measuring very high
voltages without need for range
resistors or other, external apparatus.
Figure 4.3: Iron-Vane Electromachanical
Meter Movement.
5
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 4.4.
Figure 4.4: AC Voltmeter with Resistive
Divider.
6
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 4.5.
Figure 4.5: AC Voltmeter with Capacitive
Divider.
7
4.2 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 4.6 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 4.6: DC Voltmeter Circuit
Modified to Measure AC Voltage.
8
Cont’d…
 The peak value of 10 V rms sine wave is,
E p  10Vrms  (10)( 2V peak )  14.14V peak
Edc  Eave 
Edc 
Ep

2 Erms

 0.318E p
 0.45Erms
Shown that a dc voltmeter will provide 45% of the ac input
Rs 
Edc
0.45Erms
 Rm 
 Rm
I dc
I dc
 From the above equation, the sensitivity of dc voltmeter is
S ac  0.45S dc
9
4.3 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 4.9.
Figure 4.9: Full Wave Bridge Rectifier
Used in AC Voltmeter Circuit.
10
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).
11
Cont’d…
 From the circuit in Figure 4.9, the peak value of the 10 Vrms
signal with the half-wave rectifier is,
E p  2 Erms  14.14V peak
 The average dc value of the pulsating sine wave is,
Eave 
2E p

 0.636 E p  9V
Eave  (0.636)( 2Erms )  0.9Erms
 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
12
Example 4.2: D’Arsonval Meter Full-Wave Rectifier.
Each diode in the full-wave rectifier circuit in Figure 4.10 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.
c) The equivalent DC sensitivity.
Figure 4.10: AC Voltmeter Using FullWave Rectification and Shunt.
Solution:
(a)
The multiplier Rs
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
13
Cont’d…Example

The equivalent DC voltage is,
E dc  0.9 * 10Vrms  0.9 *10V  9.0V
RT 
a) The multiplier Rs
(b) The ac sensitivity,
E dc 9.0V

 4.5 K
IT
2mA
Rs  RT  2 Rd 
Rm Rsh
Rm  Rsh
 4500  2 * 50 
S ac 
500 * 500
 4.15K
500  500
RT
4500

 450 / V
Range
10V
(c.) The dc sensitivity,
S dc 
1
1

 500 / V
I T 2mA
or
S dc 
S ac 450 / V

 500 / V
0 .9
0 .9
14
Quiz on Chapter 4
1)
Calculate the ac and dc sensitivity and the value of the multiplier resistor required to
limit current to the full-scale deflection current in the circuit shown in Figure below.
All diodes have forward resistance of 300 and an infinite reverse resistance.
D2
D1
Ein = 20Vrms
D3
D4
Rs
Rsh = 250
Ifs = 1mA
Rm = 250
15
Quiz on Chapter 4
2)
A DC multimeter can be designed by using a movement full-scale meter connecting
with Rshunt , Rmultiplier1 , Rmultiplier2 , R and a battery as shown in Figure
below.
Determine the value of each resistor to make the circuit become a dc multimeter.
that it can be used to measure 10V voltage, 5A current and 0Ω resistance.
(assume the full scale meter is 1 mA, 500 Ω and 9V battery)
16