Electrical Instruments and Measurement: Instruments used to measure, current, voltage, power energy, flux, frequency etc., are called electrical measuring instruments.
Classification of Electrical Instrument
Absolute Instrument: It gives the value of the parameters under measurement in terms of the instrument.
e.g., Tangent galvanometer, Rayleigh's current balance.
Secondary Instrument: It gives the value of parameters directly under measurement.
e.g., Voltmeter, thermometer, pressure gauge etc.
Note Absolute instruments are highly accurate than secondary instrument as they contain less few moving mechanical parts resulting in a lower operational of power consumption.
Analog Instrument: Its output varies continuously with respect to time all the while maintaining a constant relationship with the input.
Deflecting Instrument: It gives the value of the parameters under measurement in terms of the deflection of the pointer away from the zero position. e.g., PMMC
Null Deflection: Null deflecting instruments indicate their end of measurement with a zero deflection. e.g., Bridge circuit, DC potentiometer etc.
Note: Null deflecting instrument is highly accurate as the comparison to deflecting instrument as their operational power consumption at zero deflection is zero.
Indicating Instrument: It gives the instantaneous value of the parameters under measurement. e.g., Ammeter, voltmeter, wattmeter, galvanometer.
Integrating Instrument: It gives the sum total of the electrical parameter consumed over a specified period of time. e.g., Energy meter
Recording Instrument: e.g., Recording voltmeter.
- PMMC type instruments are used only in DC voltage and current.
- Induction type instruments are used only in AC (voltage and current) measurement.
- An electrodynamometer type instrument can be used to measure DC well as AC voltage.
Principle of Operation of Analog Instrument
- Electro Magnetic Effect Moving iron coil type, PMMC type, electrodynamometer type instrument.
- Induction Effect Energy meter
- Heating Effect Hotwire type, thermocouple, bolometer.
- Electrostatic Effect Electrostatic type instrument.
- Hall’s Effect Flux meter, Poynting vector type wattmeter.
Essentials of Indicating Instrument
Deflecting Torque/Force: Deflecting torque is proportional to quantity under measurement. The utility of this torque is to deflect the pointer away from the zero position.
Controlling Torque/Force (TC): It controls the deflection by bringing the pointer to rest at the steady-state position.
It brings the pointer back to zero position when the parameter under measurement is removed.
Note: If the control force is absent then the pointer will be deflected beyond the maximum scale.
Damping Torque/Force (TD): This torque is responsible for damping out the oscillation of the pointer due to inertia.
Construction Details of Indicating Instrument: Moving System Moving system of indicating produces the deflecting torque. Various types of supports for moving system as
- Used in galvanometer class instrument.
- Used only in the vertically held instrument.
Taut Suspension: It is used in PMMC type instrument require a low friction and high sensitivity.
Pivot and Jewel Bearing Type Supports: Weight of moving system decides the sensitivity of the instrument.
Note: Torque to weight ratio of the moving system should be high and equal to 0.1
Control System: There are two types of the mechanism is considered here. One is spring control and other is gravity control mechanism as given below.
Spring Control Mechanism
Control torque TCS = kθ
where k = spring constant
θ = deflection
as Td ∞ l
at steady state position Td = Tc; θ ∞ l
Spring control has a uniform scale
Gravity Control Mechanism
Tcg = K sin θ
Td ∞ l
at steady state position Td = Tcg
I ∞ sin θ
Where Td = Deflection torque
Tcg = Control torque of gravity control mechanism
Note: Gravity control mechanism has initially linear scale.
- If damping torque Td is absent then pointer oscillate around the mean position.
- Fatigue in spring is avoided by annealing and aging.
Damping System: Damping system is provided in the instrument which helps the moving system of the instrument to reach to the final position at the earliest.
Eddy Current Damping: Mechanism Used in a strong operating field. e.g., PMMC (Permanent Magnet Moving Coil instrument) type.
Air Friction Damping: Used when the operating field is weak as used in moving iron and electro dynamometer type instrument.
Fluid Friction Damping: Used in high voltage measurement. Used in the vertically mounted instrument. e.g., Electrostatic type instrument.
Measuring Current: Ammeters
To measure current, the circuit must be broken at the point where we want that current to be measured, and the ammeter inserted at that point. In other words, an ammeter must be connected in series with the load under test.
As it’s very important that the insertion of the ammeter into a circuit has little effect the circuit’s existing resistance and, thus, alter the current normally flowing in the circuit, ammeters are manufactured with very low values of internal resistance.
Because ammeters have a very low internal resistance, it is vitally important that they are never inadvertently connected in parallel with any circuit component —and especially with the supply. Failure to do so will result in a short-circuit current flowing through the instrument which may damage the ammeter (although most ammeters are fused) or even result in personal injury.
Measuring Voltage: Voltmeters
To measure potential-difference or voltage, a voltmeter must be connected between two points at different potentials. In other words, a voltmeter must always be connected in parallel with the part of the circuit under test.
In order to operate, a voltmeter must, of course, draw some current from the circuit under test, and this can lead to inaccurate results because it can interfere with the normal condition of the circuit. We call this the ‘loading effect’ and, to minimize this ‘loading effect’ (and, therefore, improve the accuracy of a reading), this operating current must be as small as possible and, for this reason, voltmeters are manufactured with a very high value of internal resistance —usually many megaohms.
Types of Instruments used for Ammeter and Voltmeter
PMMC (Permanent Magnet Moving Coil) Only for DC current measurement.
Moving Iron Type For both AC and DC
Electro Dynamometer Type For both AC and DC
Electro Thermic Type For both AC and DC
Also for hot wire type, thermocouple type and bolometer.
Induction Type Only for AC measurement.
Electrostatic Type Both AC and DC
Rectifier Type Both AC and DC
Permanent Magnet Moving Coil (PMMC) Type Instrument
It is used for measurement of DC only. Material used for magnet in PMMC is Alnico (AI + Ni + Co) and Alcomax (Al + Co + max….). The field strength in PMMC varies from 0.1 Wb/m2 to 1Wb/m2.
- Concentric magnetic construction is used to get longer angular movement of the pointer.
- Due to the strong operating field of the permanent magnet, the eddy current damping mechanism is used to produce the damping torque.
- The control torque in PMMC is provided with spiral-shaped hair type phosphor-bronze spring.
Td = NBIA due to magnetic field
TC = Kθ due to control spring
As θ ∞ I
The scale of the PMMC instrument is uniform scale
where N = Number of turns of the moving coil
B = Flux density of permanent magnet in Wb/m2
A = Area of the coil in m2
I = Current in amp
G = NBA = Displacement constant of the galvanometer
- The control spring in PMMC has dual utility, they not only produce controlling torque but also used to lead the current into the system.
- Due to strong operating torque at the permanent magnet, an eddy current damping is used to produce damping torque.
Advantages of PMMC
- High torque to weight ratio.
- High accuracy and sensitivity.
- Magnetic shielding not required due to a strong operating field.
- Low power consumption (25 – 200 μ W).
Disadvantages of PMMC
- High cost.
- Used for measurement of DC only.
- Limited current carrying capacity (100 mA) approx.
Sources of Errors
- Ageing effect of the permanent magnet (can be compensated by using a pre-edged magnet.
- Ageing effect of the spring.
- Temperature effect of the coil and the control spring.
Application of PMMC Instrument
Where, Multiplying factor of the shunt
Rsh = Shunt resistance (Ω)
Rm = Internal resistance of the movement (Ω)
Im = Ifs = Full scale deflection current of the movement (A)
I = Full-scale current of the ammeter including the shunt.
- Shunt should have a small and constant temperature coefficient.
- The materials used for the shunt in PMMC is Magnin as it gives small thermal emf with copper.
- Constanton is used for the construction of shunt in AC ammeter.
Effect of Temperature Change in Ammeter Reading
As temperature increases the resistance of copper increases and this result into a change of reading of the instrument. To reduce the effect of temperature a resistance having very small temperature coefficient made up of Magnin is connected in series with the coil and this is known as swamping resistance.
The combination of a millivoltmeter and shunt employed as an ammeter is readily adaptable to multirange construction either by separate, in interchangeable, single-range shunts or multirange shunts.
By Using Separate Shunts
The circuits have three shunts which can be placed in parallel with the meter movement to give three different ranges I1, I2 and I3.
By Using Universal or Ayrton Shunt
The universal shunt or ayrton shunt is shown in the figure is also used for multi-range ammeters. The advantage of this is that it eliminates the possibility of the meter being in the circuit without shunt.
Moving Iron Instrument Working Operation
The moving iron instruments are classified as:
- Moving iron attraction type instruments
- Moving iron repulsion type instruments
Attraction Type Instrument Working Operation
Moving Iron Instrument Working Principle: The basic working principle of these instruments is very simple that a soft iron piece if brought near magnet gets attracted by the magnet.
The construction of the attraction type moving iron instrument is shown in the below figure.
- lt consists of a fixed coil C and moving iron piece D. The oil is flat and has narrow slot like opening.
- The moving iron is a flat disc which is eccentrically mounted on the spindle.The spindle is supported between the jewel bearings.The spindle carries a pointer which moves over a graduated scale.
- The number of turns of the fixed coil are dependent on the range of the instrument. For passing the large current through the coil only a few turns are required.
- The Controlling Torque is provided by the springs but gravity control may also be used for vertically mounted panel type instruments.
- The Damping Torque is provided by the air friction.
- A light aluminum piston is attached to the moving system. it moves in a fixed chamber. The chamber is closed at one end. it can also be provided with the help of vane attached to the moving system.
Torque in Moving Iron Instrument
Deflecting torque Newton meter
as θ ∞ I2, the scale is non-linear in moving iron type.
For linear scale
For scale to be linear constant
Where, θ = Deflection of the pointer
L = Inductance of the coil
I = RMS value of current in the coil
The curve between dL/dθ and θ is a rectangular hyperbola.
Rectifier Type Instrument
It measures the alternating voltage and current with the help of rectifying elements and permanent magnet moving coil type of instruments. However, the primary function of rectifier type of instruments work as voltmeter. Now one question must arises in our mind why we use rectifier type of instruments widely in the industrial world though we have various other AC voltmeter like electrodynamometer type instruments, thermocouple type instruments etc.
- Cost of electrodynamometer type of instruments is quite high than rectifier type of instruments. However rectifier type of instruments as much accurate as electrodynamometer type of instruments. So rectifier type of instruments are preferred over electrodynamometer type instruments.
- However, thermocouple type of instruments is more widely used at very high frequencies.
- Before we look at the construction principle and working of rectifier type instruments, there is need to discuss in detail about the voltage-current characteristics of ideal and practical rectifier element called diode.
- Now, what is an ideal rectifying element? A rectifying element is one which offers zero resistance if it is forward biased and offers infinite resistance if it is reversed biased.
Factors affecting the Performance of Rectifier type instruments:
- Rectifier type of instruments is calibrated in terms of root mean square values of the sinusoidal wave of voltages and current. The problem is that the input waveform may or may not have same form factor on which the scale of these meter is calibrated.
- There may be some error due to the rectifier circuit as we are not included the resistance of the rectifier bridge circuits in both the case. The nonlinear characteristics of a bridge may distort the current and voltage waveform.
- There may variation in the temperature due to which the electrical resistance of the bridge changes hence in order to compensate this kind of errors we should apply multiplier resistor with a high temperature coefficient.
- Effect of the capacitance of the bridge rectifier: Bridge rectifier has imperfect capacitance thus due to this it by passes the high-frequency currents. Hence there is a decrement in the reading.
The sensitivity of Rectifier type instruments is low in case of ac input voltage.
Advantages of Rectifier Type Instruments
Following are the advantages of the rectifier type of instruments:
- The accuracy of rectifier type instrument is about 5 percent under normal operating condition.
- The frequency range of operation can be extended to high value.
- They have uniform scale on the meter.
- They have low operating value of current and voltages.
The loading effect of an ac rectifier voltmeter in both the cases (i.e. half wave diode rectifier and full wave diode rectifier) is high as compared to the loading effects of DC voltmeters as the sensitivity of the voltmeter either using in half wave or full wave rectification is less than the sensitivity of DC voltmeters.
The Seebeck effect is fairly linear; that is, the voltage produced by a heated junction of two wires is directly proportional to the temperature. This means that the temperature of the metal wire junction can be determined by measuring the voltage produced. Thus, the Seebeck effect provides for us an electric method of temperature measurement.
When a pair of dissimilar metals are joined together for the purpose of measuring temperature, the device formed is called a thermocouple. Thermocouples made for instrumentation use metals of high purity for an accurate temperature/voltage relationship.
- The thermocouple instruments are more delicate than the rectifier type of instruments.
- Seebeck voltages are quite small, in the tens of millivolts for most temperature ranges. This makes them somewhat difficult to measure accurately. Also, the fact that any junction between dissimilar metals will produce temperature-dependent voltage creates a problem when we try to connect the thermocouple to a voltmeter
- Two thermocouple junctions can be connected in opposition to each other to generate a voltage signal proportional to differential temperature between the two junctions. A collection of junctions so connected for the purpose of generating electricity is called a thermopile.
- When electrons flow through the junctions of a thermopile, heat energy is transferred from one set of junctions to the other. This is known as the Peltier Effect.
- Multiple thermocouple junctions can be connected in parallel with each other to generate a voltage signal representing the average temperature between the junctions. “Swamping” resistors may be connected in series with each thermocouple, to maintain equality between the junctions, so the resultant voltage will be more representative of a true average temperature.
You can avail of BYJU’S Exam Prep Online classroom program for all AE & JE Exams:
You can avail of BYJU’S Exam Prep Test series specially designed for all AE & JE Exams:
Team BYJU’S Exam Prep
Download BYJU’S Exam Prep APP, for the best Exam Preparation, Free Mock tests, Live Classes.