Transcript AC Meters

ALTERNATING CURRENT METERS
Part 2
Edited by
Syarifah Norfaezah
Prepared by
Sanna Taking & Mohamad Nazri Abdul Halif
School of Microelectronic Engineering
D’Arsonval meter movement used
with full wave rectification
Fig.
2:
Full
bridge
rectifier used in an ac
voltmeter circuit
During the positive half cycle, 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, which allows current to flow only on every
other half cycle; if the deflection remains the same, the instrument using
full wave rectification will have a greater sensitivity.
2
Consider the circuit shown in Fig. 1-2
Fig. 1-2: AC voltmeter using full wave rectification
3
Cont.
When the 10Vrms of AC signal is applied to the circuit
above, where the peak value of the AC input signal is
E p  2 xE rms  1.414 x (10)  14.14V
And the average full wave output signal is
E ave  E dc  0.636xE p  0.636x14.14  9V
Therefore, we can see that a 10Vrms voltage is equivalent
to 9Vdc for full-scale deflection.
4
Cont.
Or
E avg  0.636E p  0.636( 2 xE rms )  0.9E rms
This means an ac voltmeter using full wave
rectification has a sensitivity equal to 90% of
the dc sensitivity
Sac = 0.9 Sdc
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Example 1-2
Compute the value of the multiplier resistor for a
10Vrms ac range on the voltmeter in Figure 1-2.
Fig. 1-2: AC voltmeter circuit using full wave rectification
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Solution 1-2
The dc sensitivity is
1
1
Sdc 

 1k / V
I fs 1mA
The ac sensitivity is
Sac = 0.9Sdc = 0.9 (1k) = 900 /V
7
Cont.
Therefore the multiplier resistor is
Rs
= Sac x Range – Rm
= 900 x 10Vrms – 500
= 8.5k
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Assignment
RS
Ein = 10Vrms
Ifs = 1mA
Rsh = 0.5kΩ
Rm = 0.5kΩ
Each diode in full-wave rectifier circuit above has an
average forward resistance of 50Ω and is assumed to
have an infinite resistance in the reverse direction.
Calculate:
•
•
•
Multiplier resistance, RS
AC sensitivity
The equivalent dc sensitivity.
9
Cont.
Note:
Voltmeters using half wave and full
wave rectification are suitable for
measuring only sinusoidal ac voltages.
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Electrodynamometer Movement
Source
Fixed Coil
Moving
Coil
Fixed Coil
• Most fundamental and versatile meter use today.
• Is a current-sensitive device – the pointer deflects up scale because of
current flow through moving coil.
• Most important applications: voltmeter and ammeter standard.
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Electrodynamometer (Cont..)
• The single-coil electrodynamometer movement consists of a fixed
coil divided into two equal halves.
• Both halves of the split fixed coil and the moving coil are
connected in series – current from the circuit being measured
passed through all the coils causing magnetic field around the
fixed coils. The moving coil rotates in this magnetic field.
• The electrodynamometer – handle much more current than d’
Arsonval movement. It can handle ~ 100mA.
• The electrodynamometer – have a very low sensitivity rating of ~
20 to 100 Ω/V.
• Most extensive application: Wattmeter.
• The magnetic torque that cause pointer deflect up scale:
 m  K m El cos 
Θm – angular deflection of the pointer
Km – instrument constant (degrees/watt)
cos θ – power factor
E – rms value of source voltage
l – rms value of source current
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Iron-vane Meter Movement
• The iron-vane meter movement
consists of a fixed coil of many turns
and two iron vanes placed inside the
fixed coil.
• it is widely used in industry.
I
• the current can be measured
passes through the winding of the
fixed coil setting up a magnetic field
that magnetized the two iron vanes
with the same polarity.
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Iron-vane Meter (Cont…)
• If one iron vanes is attached to the frame of a fixed coil –
the other iron vane will then be repelled by amount
related to the square of current.
• Although it is responsive to direct current (the hysteresis)
– the iron vanes causes appreciable error. (used only for
a very inexpensive indicators, i.e charge-discharge
indicators on automobiles).
• It is used extensively in industry for measuring ac when
errors on the order of 5% to 10% are acceptable.
• Iron-vane movement very sensitive to frequency change
(25 – 125 Hz) - it is because the magnetization of the
iron vane is nonlinear.
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Thermocouple Meter
I
Thermocouple
Source
Insulating
bead
d’ Arsonval
meter
movement
Heater
Basic thermocouple meter
• Usually consists a heater element – fine wire, a thermocouple,
and d’Arsonval meter movement.
• to measure a very high-frequency ac (very accurate well >
50Hz).
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Loading effects of AC Voltmeter
The sensitivity of ac voltmeters, using either
half wave or full wave rectification, is less
than the sensitivity of dc voltmeters.
Therefore, loading effect of an ac voltmeter
is greater than that of a dc voltmeter.
Sac = 0.45Sdc
Sac = 0.9Sdc
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Voltage and Current
Transformer Applications
Calibrating AC voltmeters and ammeters for different
full-scale ranges of operation is much the same as with
DC instruments: series "multiplier" resistors are used to
give voltmeter movement a higher range, and parallel
"shunt" resistors are used to allow ammeter
movements to measure currents beyond their natural
range. However, we are not limited to these
techniques as we were with DC: because we can use
transformers with AC too.
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Summary
For general purposes, the d’Arsonval movement – either halfwave or full-wave rectifier, is widely used.
Meter Movement
App. Voltage & Freq.
Reading Obtained
D’Arsonval
10Vrms, 60 Hz
0V
Iron vane
10Vrms, 60 Hz
10V
Electrodynamometer
10Vrms, 60 Hz
10V
Thermocouple
10Vrms, 60 Hz
10V
D’Arsonval with halfwave rectifier
10Vrms, 60 Hz
4.5V
D’Arsonval with fullwave rectifier
10Vrms, 60 Hz
9.0V
Iron vane
10Vdc
10V
Electrodynamometer
10Vdc
10V
Thermocouple
10Vdc
10V
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NEXT LECTURE
DC & AC BRIDGE: Part 1 (DC bridge)
• The Wheatstone bridge
• The Kelvin bridge
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