EMT 462 ELECTRICAL SYSTEM TECHNOLOGY Chapter 4: AC Meters By: En. Muhammad Mahyiddin Ramli

Chap 4: AC Meters 2 Alternating Current Waveform Sinusoidal wave Square wave Triangle wave

Chap 4: AC Meters 3 Alternating Current Waveform

Chap 4: AC Meters 4

5 Average and RMS Value V avg = 0 V rms = 0.707V p Sine Wave Full Wave V avg = 0.636V p V rms = 0.707V p

Chap 4: AC Meters 6 Con’t V avg = 0.318V p V rms = 0.5V p Half Wave

Chap 4: AC Meters 7 Five Principal Meter Movements Used In AC Instrument 1. Electrodynamometer 2. Iron Vane 3. Electrostatic 4. Thermocouple 5. D’Arsonval with rectifier

Chap 4: AC Meters 8 Application of Meter Movements Meter Movement DC UseAC UseApplications ElectrodynamometerYES Standards meter, wattmeter, frequency meter “Indicator” applications such as in automobiles Iron VaneYES “Indicator” applications such as in automobiles ElectrostaticYES Measurement of high voltage when very little current can be supplied by the circuit being measured ThermocoupleYES Measurement of radio frequency ac signal D’ArsonvalYESYES with rectifier Most widely used meter movement for measuring direct current or voltage and resistance

Chap 4: AC Meters 9 PMMC Instrument on AC The PMMC instrument is polarized (terminals +ve & -ve) - it must be connected correctly for positive (on scale) deflection to occur. When an AC with a very low frequency is passed through a PMMC, the pointer tends to follow the instantaneous level of the AC As the current grows positively, the pointer deflection increases to a maximum at the peak of the AC As the instantaneous current level falls, the pointer deflection decreases toward zero. When the AC goes negative, the pointer deflected (off scale) to the left of zero This kind of pointer movement can occur only with AC having a frequency of perhaps 0.1Hz or lower

Chap 4: AC Meters 10 PMMC Instrument on AC  At 50Hz or higher supply frequencies - the damping mechanism of the instrument and the inertia of the meter movement prevent the pointer from following the changing instantaneous levels.  The average value of purely sinusoidal AC is zero.  Therefore, a PMMC instrument connected directly to measure 50Hz AC indicates zero average value.  It is important to note that although a PMMC instrument connected to an ac supply may indicating zero, there can actually be very large rms current flowing in its coils

Chap 4: AC Meters 11 Two Types of PMMC Meter Used In AC Measurement 1. Half wave rectification 2. Full wave rectification

Chap 4: AC Meters 12 D’Arsonval meter movement used with half wave rectification To convert alternating current (AC) to unidirectional current flow, which produces positive deflection when passed through a PMMC, the diode rectifier is used. Several types of rectifiers are selected such as a copper oxide rectifier, a vacuum diode, or semiconductor or “crystal diode”.

Chap 4: AC Meters 13 Con’t  For example, if the output voltage from a half wave rectifier is 10V rms so the dc voltmeter will provide an indication of approximately 4.5V dc  Therefore, the pointer deflected full scale when 10V dc signal is applied.  When we apply a 10Vrms sinusoidal AC waveform, the pointer will deflect to 4.5V  This means that the AC voltmeter is not as sensitive as DC voltmeter.  In fact, an AC voltmeter using half wave rectification is only approximately 45% as sensitive as a dc voltmeter.

Chap 4: AC Meters 14 Con’t  Actually, the circuit would probably be designed for full-scale deflection with a 10V rms AC applied, which means the multiplier resistor would be only 45% of the value of the multiplier resistor for 10V dc voltmeter. Since we have seen that the equivalent dc voltage is equal to 45% of the rms value of the ac voltage. S ac = 0.45S dc

Chap 4: AC Meters 15 Example 3.1 Compute the value of the multiplier resistor for a 10Vrms ac range on the voltmeter shown below: AC voltmeter using half wave rectification RSRS E in = 15V rms I fs = 1mA R m = 300Ω

Chap 4: AC Meters 16 Solution Method 1  The sensitivity of meter movement R s = S dc × Range dc – R m = 1k × - R m = 1k × 0.45(10) – 300 = 4.2k 

Chap 4: AC Meters 17 Con’t Method 2  The AC sensitivity for half way rectifier S ac = 0.45S dc = 0.45(1k) = 450  /V R s = S ac × Range ac – R m = 450 × 10 –300 = 4.2k 

Chap 4: AC Meters 18  D’Arsonval meter movement used with full wave rectification  Electrodynamometer, Iron-vane meter, thermocouple meter  Loading effects of AC Voltmeter

Chap 4: AC Meters 19 D’Arsonval Meter Movement Used With Full Wave Rectification 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.

Chap 4: AC Meters 20 Consider the following circuit: AC voltmeter using full wave rectification

Chap 4: AC Meters 21 Con’t When the 10Vrms of AC signal is applied to the circuit above, where the peak value of the AC input signal is And the average full wave output signal is Therefore, we can see that a 10Vrms voltage is equivalent to 9Vdc for full-scale deflection.

Chap 4: AC Meters 22 Con’t S ac = 0.9 S dc Or This means an ac voltmeter using full wave rectification has a sensitivity equal to 90% of the dc sensitivity

Chap 4: AC Meters 23 Example 3.2 Compute the value of the multiplier resistor for a 10V rms ac range on the voltmeter in Figure 1-2. AC voltmeter circuit using full wave rectification

Chap 4: AC Meters 24 Solution The dc sensitivity is: The ac sensitivity is: S ac = 0.9S dc = 0.9 (1k) = 900  /V

Chap 4: AC Meters 25 Con’t Therefore the multiplier resistor is: = 900 x 10V rms – 500 = 8.5k  R s = S ac x Range – R m

Chap 4: AC Meters 26 Notice Note: Voltmeters using half wave and full wave rectification are suitable for measuring sinusoidal ac voltages only.

Chap 4: AC Meters 27 Electrodynamometer Movement Fixed Coil Moving Coil Fixed Coil Source  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.

Chap 4: AC Meters 28 Electrodynamometer Movement 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: Θ m – angular deflection of the pointer E – rms value of source voltage K m – instrument constant (degrees/watt) l – rms value of source current cos θ – power factor

Electrodynamometer Work Principle Chap 4: AC Meters 29

Chap 4: AC Meters 30 Iron-Vane Meter Movement I 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. 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.

Chap 4: AC Meters 31 Iron-Vane Meter Movement 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.

Iron-Vane Meter Movement Chap 4: AC Meters 32

Chap 4: AC Meters 33 Loading Effect 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. S ac = 0.45S dc S ac = 0.9S dc

Chap 4: AC Meters 34 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.

Chap 4: AC Meters35 Success is the ability to go from one failure to another with no loss of enthusiasm. - Sir Winston Churchill