Electrical Measuring Instruments
Instruments used for measuring the physical as well as electrical quantities is termed as measuring instruments. Electrical measuring instruments may be classified according to their functions as
- Indicating Instruments
- Integrating Instruments
- Recording Instruments
Those instruments which directly indicate the value of the electrical quantity at the time when it is being measured are called indicating instruments. e.g ammeters ,voltmeters and wattmeter’s. In such instruments , a pointer moving over a graduated scale directly gives the value of the electrical quantity being measured. For example, when a ammeter is connected in the circuit, the pointer of the meter directly indicates the value of the current flowing in the circuit at that time.
Those instruments which measure the total quantity of electricity(in ampere-hours) or electrical energy(in watt-hours) in a given time are called integrating instruments. e.g ampere-hour meter and watt-hour meter. In such instruments, there are sets of dials and pointers which register the total quantity of electricity or electrical energy supplied to the load.
Those instruments which give a continuous record of the variations of the electrical quantity to be measured are called recording instruments. A recording instruments is similar to an indicating instruments with a pen attached to its pointer. The pen rests lightly on a chart wrapped over a drum moving with a slow uniform speed. The motion of the drum is in a direction perpendicular to the direction of the pointer. The path traced out by the pen indicates the manner in which the quantity, being measured, has varied during the time of the record. Recording voltmeters are used in supply stations to record the voltage of the supply mains during the day. Recording ammeters are employed in supply stations for registering the current taken form the batteries.
Principle of Operation of electrical Instruments
An electrical instruments essentially consists of a movable element and a scale to indicate or register the electrical quantity being measured. The movable element is supported on jewelled bearings and carries a pointer or sets of dials. The movement of the movable element is carried by utilizing one or more of the following effects of current or voltage,
- Magnetic effect ….Moving-iron instruments
- Electrodynamic effect …(i) permanent-magnet moving coil (ii) Dynamometer type
- Electromagnetic-Induction …Induction type instruments
- Thermal effect …Hot-wire instruments
- Chemical effect……..Electrolytic instruments
- Electrostatic effect …..Electrostatic instruments
|1||Moving-Iron||Magnetic effect||d.c and a.c||Ammeter, voltmeter|
|2||Permanent-magnet moving coil||Electrodynamic effect||d.c only||Ammeter, voltmeter|
|3||Dynamometer type||Electrodynamic effect||d.c and a.c||Ammeter, voltmeter, wattmeter|
|4||Induction type||Electro-magnetic induction effect||a.c only||Ammeter, voltmeter, wattmeter, energy-meter|
|5||Hot-wire||Thermal effect||d.c and a.c||Ammeter, voltmeter|
|6||Electrostatic type||Electrostatic effect||d.c and a.c||Voltmeter only|
Operation of Indicating Instruments
An indicating instruments essentially consists of moving system pivoted in jewel bearings .A pointer is attached to the moving system which indicates on a graduated scale, the value of the electrical quantity being measured. In order to ensure proper operation of indicating instruments, the following three torques are required:
- Deflecting torque(operating torque)
- Controlling torque(restoring torque)
- Damping torque
In indicating instruments the arrangement for producing deflecting or operating torque(Td) is most important when the instrument is connected in the circuit to measure the given electrical quantity. The deflecting torque causes the moving system to move from Zero position to indicate on a graduated scale the value of electrical quantity being measured. The actual method of producing the deflecting torque depends upon the type of instrument.
If deflecting torque were acting alone, the pointer would continue to move indefinitely and would swing over to the maximum deflected position irrespective of the magnitude of current to be measured. This controlling torque should oppose the deflecting torque and should increase with the deflection of the moving system. The pointer will be brought to rest at a position where the two opposing torques are equal i.e Td=Tc.
Functions of controlling torque
- It increases with the deflection of the moving system so that the final position of the pointer on the scale will be according to the magnitude of current to be measured.
- It brings the pointer back to Zero position when the deflecting torque is removed. If it were not provided, the pointer once deflected would not return to Zero position on removing the deflecting torque.
controlling torque in indicating instruments may be provided by one of the following two methods:
- By one or more springs(spring control)
- By weight of moving parts(gravity control)
The most common method of providing controlling torque is by the use of one or more springs. One or two spiral hair springs made of some nonmagnetic material are attached to the moving system of the instrument. With the deflection of the pointer, the spring is twisted in the opposite direction. This twist in the spring provides the controlling torque.
In the gravity control, a small adjustable weight W is attached to the moving system. In the deflected position, only the component W sinθ provides the controlling torque.
If the moving system is acted upon by deflecting and controlling torques alone, then pointer ,due to inertia, will oscillate about its final deflected position for quite some time before coming to rest. This is often undesirable because it makes difficult to obtain quick and accurate readings. In order to avoid these oscillations of the pointer and to bring it quickly to its final deflected position ,a damping torque is provided in the indicating instruments.
Damping torque acts only when the pointer is in motion and always opposes the motion. The damping torque in indicating instruments can be provided by
- Air friction
- Fluid friction and
- Eddy currents
In air friction damping one or two light aluminum vanes are attached to the same spindle that carries the pointer. The vanes are permitted to swing in a sector-shaped closed box that is just large enough to accommodate the vanes. As the pointer moves, the vanes swing in the box, compressing the air in front of them .The pressure of compressed air in the vanes provides the necessary damping torque.
In fluid friction damping discs or vanes attached to the spindle of the moving system are kept immersed in a pot containing oil of high viscosity. As the pointer moves ,the friction between the oil and vanes opposes motion of the pointer and thus necessary damping torque is provided.
In eddy currents damping a thin aluminum or copper discs attached to the moving system is allowed to pass between the poles of a permanent magnet. As the pointer moves, the disc cuts across the magnetic field and eddy currents are produced in the disc. Eddy currents react with the field of the magnet to produce a force which opposes motion (Lenz’s law) thus eddy currents provide damping torque to reduce the oscillations of the pointer.
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