The measurement of a given electrical quantity is essentially an act or the result of
Comparison between the quantity (whose magnitude is unknown) and a predefined standard. Since two quantities are compared, the result is expressed in numerical values. A measuring Instrument is a piece of apparatus used to measure a quantity such as Voltage, Current, and Power ETC. It may indicate by a deflection, the quantity under measurement or give consumption of electrical energy during a specified period or produce a continuous record of the variations in a quantity. Over a period of time the process of measurement and the functioning of measuring instruments have under gone a revolutionary changes. This information sheet is aimed at providing some fundamental aspects of measuring instruments both traditional and the advanced.
Need for Measurements:
- To quantitatively express various electrical engineering variables observed during experimentation and operation of equipment.
- To carry out the design of various equipment and systems.
- To operate and maintain various equipment and systems effectively.
- To verify and confirm the validity of the various phenomenon and Relationship
Methods
Direct Method:
In these methods, the unknown quantity [measurand] is directly
compared against a standard. The result is expressed in numerical number and a unit. Eg. Measurement of length, Mass etc
Indirect Method:
It is not possible to use direct methods for all the measurements due to various reasons and hence an indirect methods are incorporated, in this the quantity to be measured is obtained using some form of energy conversions with the help of transducers.
Eg. Measurement of Current, Voltage etc.
Classification of Measuring Instruments:
Absolute Instruments:
Used only in standard Laboratories under standard atmospheric conditions and used for calibrating the secondary instruments.
Secondary Instruments:
These instruments are widely used in the field for all types of measurements.
Traditional Measuring Instruments:
various type of measuring instruments which were used before the advent of Digital Technology.
Important Characteristics of Measuring Instruments:
ERROR:
It is the difference between the MEASURE VALUE and the TRUE VALUE .True value is one which is obtained based on computations and the measured value is one which is obtained from the measurement.
Error = M.V.- T.V.
Adjustment = - ve (ERROR)
% ERROR = [M.V.- T.V./ T.V] *100
Accuracy:
It is the closeness with which an instrument reading approaches the true value of the quantity to be measured.
Sensitivity:
Ratio of the magnitude of the output quantity (signal) to the magnitude of the Input quantity (signal)
Dead Zone:
Largest change of the input quantity for which there is no output of the instrument
Fundamentals of Traditional Type of Instruments:
Basic Torques Required for deflecting type of Instruments: Following are the three different essential torques require for functioning of any deflecting type of Instruments:
Deflecting Torque:
This torque acts on the moving system of the instrument to give the required deflection. It exists as long as the instrument is connected to the supply. This torque is produced by any one effects of Electric current like magnetic, electromagnetic, induction, thermal etc. The deflection torque is proportional to the quantity to be measured.
Control Torque:
This torque oppose the deflection torque and increases with the deflection torque. This torque always exists in the instrument. The pointer comes to steady position when the deflecting torque is equal to controlling torque.
Damping Torque:
This torque is produced only when the instrument is in operation. This ensures that the moving system takes just the required time to reach to the final position. If the damping torque is not sufficient the pointer will be vibrating at its final position.
Working of Moving Iron Instrument:
The instrument in which the moving iron is used for measuring the flow of current or voltage is known as the moving iron instrument. It works on the principle that the iron place near the magnet attracts towards it. The force of attraction depends on the strength of the magnet field. The magnetic field induces by the electromagnet whose strength depends on the magnitude of the current passes through it. There are two types of Moving Iron Instruments Attraction Type and the Repulsion Type.
Construction of Moving Iron Instrument
The stationary coil of the attraction type instrument is flat and has a narrow opening. The moving element is the flat disc of the iron core. The current flow through the stationary coil produced the magnetic field which attracts the iron coil.
Working of the Moving Iron Instrument:
The iron vane deflects from the low magnetic field to the high magnetic field, and the strength of the deflection is directly proportional to the magnitude of the current flow through it. In short, we can say that the iron coil attracts towards in. The attraction type instruments use spring, which provided the controlling torque. The deflection of the coil is reduced by the aluminum piston which is attached to the moving coil. Application of Moving Iron Instruments: These are used to measure both A.C. &D.C. voltages and current.
Working of Moving Coil Instrument:
There two types of Moving Coil instruments: Permanent magnet type [PMMC] and the Dynamo meter type.
Construction of PMMC Instrument
A PMMC Equipment consists of two main parts; moving coil and a permanent magnet along with other parts. These parts are explained below:
Magnet System
The instrument consists of two high intensity, high coercive force magnets or a big Ushape magnet based on design. These magnets are made up of Alcomax and Alnico for higher coercive force and better field intensity. In many designs an additional soft iron cylinder is placed in between the magnetic poles to make the field uniform; while reducing air reluctance to increasing field strength.
Moving Coil
It is one of the main component of permanent magnet moving coil equipment; and is made up of copper coils wounded to a rectangular block in between the magnetic poles. Made up of Aluminum; the rectangular block is called Aluminum former pivoted to the jeweled bearing. It is what that allows the coil to rotate freely. When current is passed through these coils; it receives a deflection in the field which is then used to determine voltage / Current magnitude. Non-metallic former like that of aluminum are used for current measurement; while metallic former with high electromagnetic damping are used to measure voltage.
Control
Two spring made of phosphorus bronze acts as a control system for the permanent magnet moving coil. These springs are mounted on the jewel bearing of PMMC; providing the essential controlling torque. The controlling torque produced is mainly due to ribon suspension. They oppose the force of deflection; so as the electromagnetic force ( Of Moving Coil ) came in equilibrium with the spring tension. This helps in keeping the pointer at a fixed position after initial deflection. These control springs also serves the purpose of providing lead current path in and out of the system.
Damping System
Damping torque is produced in the PMMC equipment by the movement of aluminum core in the magnetic field. It keeps the pointer at rest after the initial deflection. This helps in proper measurement without fluctuations. Due to the movement of coil in the magnetic field; eddy current is produced in the aluminum former. This produce the damping force / Torque which oppose further motion of the coil. Slowly the pointer deflection reduce and finally stop’s at a fixed position.
Scale and Pointer
The pointer connected to the moving coil, moves over a marked scale. The pointer moves along with the coil deflection to show readings marked on the scale. Pointer is a simple construction with light weight design and twisted section to reduce parallax error. A Parallax error can be further reduced by proper alignment of pointer blades to the initial scale.
Working Principle of a PMMC Instrument
“When a current caring conductor is placed in a magnetic field; it experiences a force perpendicular to the field and the current.” (Fleming Left Hand Rule). This force tends to move the conductor. According to Fleming left hand rule; if your left hand thumb, fore finger and middle finger are at 90 degree to each other. Then the magnetic field would be along the fore finger, current across the middle while the force along the thumb. When current flows in the coil on the aluminum former; a magnetic field is produced in the coil in proportion to the current flow. This electromagnetic force along with static magnetic field from the permanent magnet produce the deflection force in the coil. The spring then produce the controlling force to oppose further deflection; thus helps in balancing the pointer. Then damping force is produced in the system by the movement of aluminum core in the magnetic field. It keeps the pointer fixed to a position after it reach equilibrium with the controlling and deflection torque; providing better precision in measurement.
Application:
This instruments can be used only for D.C. Volt meter and Ammeter and not for A.C. applications as the Permanent magnets are used. However the Dynamo meter type can be used for both A.C. &D.C
Induction Type of Energy Meter:
The meter which is used for measuring the energy utilizes by the electric load is known as the energy meter. The energy is the total power consumed and utilized by the load at a particular interval of time. It is used in domestic and industrial AC circuit for measuring the power consumption. The meter is less expensive and accurate.
Construction of Energy Meter
The construction of the single phase energy meter is shown in the figure below.
Has 4 parts
Driving System:
The electromagnet is the main component of the driving system. It is the temporary magnet which is excited by the current flow through their coil. The core of the
electromagnet is made up of silicon steel lamination. The driving system has two electromagnets. The upper one is called the shunt electromagnet, and the lower one is called series electromagnet. The series electromagnet is excited by the load current flow through the current coil. The coil of the shunt electromagnet is directly connected with the supply and hence carry the current proportional to the shunt voltage. This coil is called the pressure coil. The center limb of the magnet has the copper band. These bands are adjustable. The main function of the copper band is to align the flux produced by the shunt magnet in such a way that it is exactly perpendicular to the supplied voltage.
Moving System:
The moving system is the aluminum disc mounted on the shaft of the alloy. The disc is placed in the air gap of the two electromagnets. The eddy current is induced in the disc because of the change of the magnetic field. This eddy current is cut by the magnetic flux. The interaction of the flux and the disc induces the deflecting torque.
When the devices consume power, the aluminum disc starts rotating, and after some number of rotations, the disc displays the unit used by the load. The number of rotations
of the disc is counted at particular interval of time. The disc measured the power consumption in kilowatt hours.
Braking system:
The permanent magnet is used for reducing the rotation of the aluminum disc. The aluminum disc induces the eddy current because of their rotation. The eddy current cut
the magnetic flux of the permanent magnet and hence produces the braking torque. This braking torque opposes the movement of the disc, thus reduces their speed. The
permanent magnet is adjustable due to which the braking torque is also adjusted by shifting the magnet to the other radial position.
Registration (Counting Mechanism):
The main function of the registration or counting mechanism is to record the number of rotations of the aluminum disc. Their rotation is directly proportional to the energy
consumed by the loads in the kilowatt hour. The rotation of the disc is transmitted to the pointers of the different dial for recording the different readings. The reading in kWh is obtained by multiply the number of rotations of the disc with the meter constant.
Megger:
Construction and working principle of Megger for measurement of High resistances. We know that the ratio meter ohmmeters may be designed to cover a wide range of
resistances. The principle of ratio meter ohmmeters is particularly adapted to application in portable instruments measuring insulation resistance. This principle
forms the basis of insulation testing instrument known as Meggar.
Megger for measurement of high resistance
Construction and working of Megger:
The current coil is same as that of permanent magnet moving coil instrument. V1 and V2 these are the two potential or voltage coils. The voltage coil V1 embraces the annular magnetic core. As shown in figure voltage coil V1 is in weak magnetic field when the pointer is at infinity and hence this coil exerts very little torque.
The torque exerted by this voltage coil increases as it moves into a stronger field and this torque will be maximum when it is under the pole face and under this condition the pointer will be at its zero end of the resistance scale. In order to modify further the torque in the voltage circuit, another voltage coil V2 is used. This coil is also located in such a way it can move from infinity to zero position of the resistance scale.
The coil finally embraces the extension H of the pole piece. The combined action of the two voltage coils V1 and V2 may be considered as though the coils constituted a spring of variable stiffness ,being very stiff near the zero end of the scale where the current in the current coil is very small (on account of unknown resistance Rx is very large).
Thus this effect compresses the low resistance portion of the scale and opens up the high resistance of the scale. This is a great advantage since this instrument is meant to be used as “insulation tester” as the insulation resistances are quite high
Fundamentals of Modern Instruments:
Digital Instruments and the Virtual Instruments are the most widely used type of modern instruments in the industry.
Digital Instruments: The digital instrument indicates the value of the quantity to be measured in the form of a digital number.
Virtual Instruments: It is an application of a customized software and modular measurement hard ware to create user defined measurement system.
Advantages:
- Digital Instruments:
- indicates the reading directly in decimal number and hence there is no reading error.
- The reading may be carried to any value of the significant figure merely positioning the decimal point.
- Output can be fed to any type of digital storage device.
- Power Requirement is less.
- Relatively maintenance free.
Virtual Instruments:
- Performance is very good
- Multiple, measurements are possible with necessary software modifications.
- Measurement system is very flexible for operation.
- Cost reduction.
- Power consumption is less.
Block Diagram of Digital Volt Meter
We can see the working of digital voltmeter that it is nothing but an analog to digital converter which converts an analog signal into a train of pulses, the number of which is proportional to the input signal. So a digital voltmeter can be made by using any one of the A/D conversion methods.
- Ramp type digital voltmeter
- Integrating type voltmeter
- Potentiometric type digital voltmeters
- Successive approximation type digital voltmeter
- Continuous balance type digital voltmeter
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