The measurement of a given quantity is essentially an act or the result of comparison between the quantity and a predefined standard.
Two basic requirements for meaningful measurement
- 1.The standard used for comparison purpose must be accurately defined and should be commonly accepted.
- 2.The apparatus used and the method adopted must be provable.
An instrument may be defined as a device for determining the value or magnitude of a quantity or variable.
Application of Electrical Measurements
- Monitoring of processes and operations.
- Control of processes and operations
- Experimental engineering analysis.
- Static characteristics of Measurement: Accuracy,sensitivity , Reproducibility, Drift, static error and Dead Zone
- Null Type Instrument: In a null type of instrument ,a zero or null indication leads to determination of the magnitude of measured quantity.
- Two method of measurement: Direct method, Indirect method.
Electrical Measurements And Measuring Instruments
This instrument very commonly used in various methods of resistance measurement and also in d.c potentiometer work.
Different parts are moving coil, Damping, suspension, indication and zero setting.
Ammeters are connected in series in the circuit whose current is to be measured . The power loss in an ammeter is I^2R .
Where I is the current to be measured and Ra is the resistance of ammeter . Therefore ammeters should have a low electrical resistance so that they cause a small voltage drop and consequently absorb small power.
Voltmeter connected in parallel with the circuit whose voltage is to be measured . The power in voltmeter is V^2/Rv Where V is the voltage to be measured and Rv is the resistance of voltmeter.
Therefore voltmeter should have a high electrical resistance , in order that the current drawn by then is small and consequently the power consumed is small.
PMMC is the most accurate type for d.c measurements. The working principle of these instrument is the same as that of d’Arsonval type of galvanometer .The difference being that a direct reading instrument is provided with a pointer and scale.
Errors in PMMC
- Weakening of permanent magnet due to ageing at temperature effects.
- Weakening of spring due to ageing and temperature effects.
- Change of resistance of the moving coil with temperature
Two winding transformers are used o step-down the voltage , whereas instrument transformers are used the range of instruments for the measuring purposes.
The primary, which consists of only one or a few turns connected in series with the load circuit and so carries the load current to be measured . The secondary winding consists of many more turns than the primary and have the instrument( an ammeter or current coil of wattmeter) directly across it . The connections of an ammeter where used in conjunction with a C.T for the measurement of current .
The current transformer in operation slightly differs from that of power transformer. In case of current transformer, as the load impedence is very small ,the C.T operates on short circuit conditions. Also the current in the secondary winding is not governed by the load impedance on secondary but depends upon the current flowing in the primary winding . The important precaution to be taken while causing a C.T. is it secondary should not be open circuited any condition. Under open circuit condition(secondary) very high voltage will induced in the secondary.
Characteristics of C.T
- Effect of P.F of secondary Winding Burden on Errors.
- Effect of change of primary winding current
- Effect of change frequency.
Errors in C.T
- Transformer input must have a component which supplies the copper loss and iron loss
- The flux density in the core is not a linear function.
- There is always magnetic leakage and consequently the primary flux linkage not equal to secondary flux linkage
Potential transformer are used to operate voltmeter,the potential of wattmeter and relays from high voltage lines . The primary winding of the transformer is connected across the line carrying the voltage to be measured and he voltage circuit is connected across the secondary winding.
Fuses are used in the secondary side to protect the PT against\faulty switching and effective earthing
The accurate device which is used to determine the difference in frequency and phase of voltages of incoming machine and busbar is called synchroscope.
The process of switching of an alternator with a common busbar without any interruption Is called synchronization.
Effective Synchronization certain conditions are
- 1.The terminal voltage of incoming machine must be same as that busbar.
- 2.The frequency of incoming machine must be same as that of busbar.
- 3.The phase of the voltage of incoming machine must be same as the phase of the busbar voltage.
Two types of synchroscopes
- 1.Electrodynamometeror Weston type synchroscope
- 2.Moving iron type synchroscope.
Energy meter is an instrument which measures electrical energy. It is also known as watt-hour (Wh) meter. It is an integrating device.
A single phase induction type energy meter
A single phase induction type energy meter consists of driving system, moving system, braking system and registering system.
When the disc rotates in the filed of series magnetic filed under load conditions , it cuts the series flux and dynamically , emf is induced in the disc. This produces eddy currents in the disc which interacts with series magnet flux to produce breaking torque. This self breaking torque to produces measurement errors. At high loads, magnetic shunt saturates and diverts some of the series magnetic flux. This compensates for the self breaking torque
When the supply voltages varies, the energy meter can cause errors due to
- Nonlinear magnetic characteristics of shunt magnetic core.
- The breaking torque which is proportional to square of the supply voltage.
As temperature increases , the resistance of the copper and aluminium parts increases. This has following effects,
- Small reduction in shunt magnetic flux.
- Reduction in torque produced by all shading bands.
- Increases in eddy current resistance path.
Advantage of induction type energy meter.
- It construction simple and strong.
- It is cheap in cost.
- It has more accuracy.
- It requires less maintenance.
- It can be used only for a.c circuits.
- The creeping cause error.
- Lack of symmetry in magnetic circuit causes errors.
- Creep: In some meters a slow but continues rotation is obtained even when there is no current flowing through the current coil and only pressure coil energized is called creeping. In order to prevent creeping two diametrically opposite holes are drilled in the disc.
- Maximum Demand:It is the highest demand of the load on the power station for a given period.
- Demand factor: The ratio of maximum demand to the connected load is called the demand factor.
- Average load: It is defined as the average of loads occurring on the power station at a specified period.
- Load factor: It is the ratio of average load to the maximum demand during the a specified period.
The resistance is the ratio of voltage and current. Thus measurement of voltage and current separately is sufficient for the resistance measurement.
Measurement of point of view resistance are classified as
- Low resistance:1 ohm or less
- Medium resistance 1ohm on wards up to 0.1Mohm
- High resistance 0.1Mohm and higher.
Ohmmeter , Ohmmeter are used for measurement of resistance , Voltmeter-Ammeter , Megohm bridge, Megger (Ressitance 0.1 Mohm and above)
- A course in Electrical and Electronic Measurements and Instrumentation – A.K.Sawhney
- Electrical Measurements By K.A.Bakshi A.V.Bakshi U.A.Bakshi
- Energy Meter
- Electrical Instruments and Measurements
- Electrical Measurements and Measuring Instruments