ELECTRICAL TECHNOLOGY PGT 105 Week 5 Chapter 2b

ELECTRICAL TECHNOLOGY PGT 105 Week 5 – Chapter 2(b) Alternating-Current (AC) Meters

Contents • d’Arsonval Meter Movement Used with Half -Wave Rectification • d’Arsonval Meter Movement Used with Full. Wave Rectification • Electrodynamometer Movement • Iron-Vane Meter Movement • Thermocouple Meter • Loading Effects of AC Voltmeters • Peak-to-Peak-Reading AC Voltmeters

Introduction AC Waveform Square wave Sinusoidal wave Triangular wave

Introduction • Two types of PMMC meter used in AC measurement: q. Half-wave rectification q. Full-wave rectification

[1/7] d’Arsonval Meter Movement Used with Half. Wave Rectification

d’Arsonval Meter Movement Used with Half-Wave Rectification • In order to measure alternating current with d’Arsonval meter movement, we must rectify the alternating current using a diode rectifer to produce unidirectional current flow. • Unidirectional - positive deflection • If we add a diode to the dc voltmeter as in Figure 3. 1, we will have a circuit that is capable of measuring ac voltage.

d’Arsonval Meter Movement Used with Half-Wave Rectification Figure 3. 1: DC voltmeter circuit modified to measure AC voltage

d’Arsonval Meter Movement Used with Half-Wave Rectification • The peak value of the 10 V rms sine wave is where • The average value of the ac sine wave or the dc value or

d’Arsonval Meter Movement Used with Half-Wave Rectification • If the output voltage from a half-wave rectifier is 10 Vrms so the dc voltmeter will provide an indication of approximately 4. 5 Vdc. • When 10 Vrms sinusoidal ac waveform is applied, the pointer will deflect to 4. 5 V. • Ac voltmeter is not sensitive as dc voltmeter. • An ac voltmeter using half-wave rectification only approximately 45% as sensitive as a dc voltmeter.

d’Arsonval Meter Movement Used with Half-Wave Rectification • The circuit would probably be designed for full- scale deflection with 10 V rms alternating current applied, which means the multiplier resistor would be only 45% of the value of the multiplier resistor for a 10 V dc voltmeter. • Since we have seen that the equivalent dc voltage is equal to 45% of the rms value of ac voltage, the equation for computing the value of multiplier resistor

d’Arsonval Meter Movement Used with Half-Wave Rectification • Sensitivity of the ac meter Thus, or

[2/7] d’Arsonval Meter Movement Used with Full. Wave Rectification

d’Arsonval Meter Movement Used with Full-Wave Rectification • Full-wave rectifier is more desirable to use compare to half-wave – higher sensitivity rating. • Frequently full-wave rectification circuit used – the bridge-type rectifier. Figure 3. 2: Full-wave bridge rectifier used in an ac voltmeter circuit

d’Arsonval Meter Movement Used with Full-Wave Rectification Operation • During the positive half-cycle, current flows through diode D 2 through the meter movement from +ve to -ve and through diode D 3. • The polarities in circles on the secondary transformer are for the positive half-cycle. • Since current flows through the meter movement on both half-cycles, the deflection of the pointer should be greater compared to half-wave. • Which allow current to flow only on every other half-cycle.

d’Arsonval Meter Movement Used with Full-Wave Rectification AC Wave In Rectified) AC Wave Out (Full-Wave

d’Arsonval Meter Movement Used with Full-Wave Rectification • Consider the circuit in Fig. 3. 3 Figure 3. 3: Ac voltmeter circuit using full-wave rectification

d’Arsonval Meter Movement Used with Full-Wave Rectification • The peak value of the 10 V rms signal is computed as The average, or dc value or

d’Arsonval Meter Movement Used with Full-Wave Rectification • We can see that 10 V rms voltage ≈ 9 Vdc when full-wave rectification is used. • The pointer will deflect to 90% of full scale. • An ac voltmeter using full-wave rectification has a sensitivity equal to 90% of the dc sensitivity or twice the sensitivity of a circuit using half-wave rectification. • Thus, sensitivity of ac meter using full-wave rectifier,

[3/7] Electrodynamometer Movement

Electrodynamometer Movement Figure 3. 4: Electrodynamometer movement • Has the same basic operating principle as d’Arsonval meter movement, except the permanent magnet is replaced by fixed coil.

Electrodynamometer Movement • The two fixed coils and moving coil are connected in series such that the same current flows through each coil. • Current flow through the three coils in either direction causes a magnetic field between the fixed coils. • The movable coil rotates in this magnetic field. • Can be used in both AC and DC system to measure current. • Some voltmeters and ammeters use the electrodynamometer.

Electrodynamometer Movement • A shunt resistor Rsh is used with an electrodynamometer movement to expand current-measuring capabilities. • Rsh is normally placed in parallel with only the moving coil. Figure 3. 5: Electrodynamometer with a shunt resistor

Electrodynamometer Movement • In order to compute the value of shunt resistor, the resistance of the moving coil, Rm should be known. Shunt resistor or Where n = the multiplication factor Note: same equation as d’Arsonval meter movement

Electrodynamometer Movement • To be used in a voltmeter circuit, the multiplier resistor Rs is placed in series with the meter movement in the same way with the d’Arsonval meter movement. Note: same equation as d’Arsonval meter movement

[4/7] Iron-Vane Meter Movement

Iron-Vane Meter Movement • Can be used to measure both AC current and voltage. • Also measure DC current and voltage, by changing meter scale calibration. Figure 3. 6: Moving Iron-Vane meter movement

Iron-Vane Meter Movement • Operates on the principle of magnetic repulsion (penolakan) between like poles. • The measured current flows through a fixed coil – produce a magnetic field proportional to the magnitude of current. • Two iron vane attached to a pointer. • The two iron vanes consist of one fixed and one moveable vane.

Iron-Vane Meter Movement • The magnetic field produced by the current flows magnetizes the two iron vanes with the same polarity regardless of the direction of current through the coil. • Like poles repel (menolak) one another, the moving iron vane pulls away from the fixed vane and moves the meter pointer. • This motion exerts a force against a spring. • The distance the moving iron vane will travel against the spring depends on the strength of the magnetic field.

Iron-Vane Meter Movement • The strength of magnetic field depends on the magnitude of current flow. • When this type of movement is used to measure voltage, the field coil consists of many turns of fine wire used to generate a strong magnetic field with only a small current flow.

[5/7] Thermocouple Meter

Thermocouple Meter • Thermocouple meter consists of a heater element, usually made of fine wire, a thermocouple, and a d’Arsonval meter movement. • To measure AC and DC. • Its operation based on the action of a thermocouple (termogandingan / pengganding suhu). • Thermocouple – consists of two dissimilar metals, develops small potential difference (0 – 10 m. V) at the junction of the two metals. • The potential difference – a function of the junction temperature, causes current to flow through the meter movement

Thermocouple Meter Figure 3. 7: Schematic for a basic thermocouple meter

Thermocouple Meter • The thermocouple sense the temperature of the heater wire, which is the function of the current or voltage being measured. • Therefore, thermocouple and the heater must be thermally coupled but electrically isolated.

[6/7] Loading Effects of AC Voltmeters

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 (either half wave or full wave) is greater > than of a dc voltmeter. (Note: example will be discuss in tutorial)

[7/7] Peak-to-Peak. Reading AC Voltmeters

Peak-to-Peak-Reading AC Voltmeters • It is desirable to measure nonsinusoidal waveform. • One way of taking this measurement is with peak-to-peak-reading AC voltmeter. • Our interest is with the peak-to-peak detector. Figure 3. 8: Block diagram for peak-to-peak-reading ac

Peak-to-Peak-Reading AC Voltmeters • A circuit that is capable of detecting the peak- to-peak amplitude of ac signals, either sinusoidal or nonsinusoidal are peak-to-peak detector circuit.

Peak-to-Peak-Reading AC Voltmeters • Operation • During the -ve half-cycle of the input signal, diode D 1 is forward-biased and charges capacitor C 1. • During the +ve half-cycle, diode D 1 is reversedbiased. • The +ve going input signal and the voltage across C 1 are now of the same polarity. • Capacitor C 2 charges to the sum of these voltages through D 2. • The voltage across C 2 is now equal to the peak

Peak-to-Peak-Reading AC Voltmeters • Operation …cont • This voltage is now applied to an ordinary AC voltmeter • Peak-to-peak reading voltmeter are sometimes used to measure the waveforms that either nonsinusoidal or swing unevenly about a zero reference axis (e. g. 20 V positive and 5 V negative)

Peak-to-Peak-Reading AC Voltmeters Example peak detector circuit

The End