Electrical measuring instruments:
A measuring instrument is intended to measure an electrical and non-electrical quantity using electrical means.
A measuring instrument displays at any time the value of measured quantity without recording it.
An instrument which operates by the attraction between a movable piece of ‘soft ‘magnetic material and the field due to a current in a fixed coil or by repulsion (and attraction) between one or more fixed pieces of soft magnetic material and a movable piece of soft magnetic material , both (all magnetised by a current in a fixed coil).
The movement consists of a diametrically oriented cylindrical permanent magnet and a coil wound on aluminium coil former which is suspended on non-magnetic stainless steel pivots between two springs loaded sapphire jewel bearings.
When an electric current is passed thru the coil, a magnetic field is produced. This field interacts with the magnetic field of permanent magnet. The torque produced deflects the coil former along with the attached pointer, showing the reading on dial.
A shield is provided to protect the movement against external magnetic field due to and to provide a low reluctance path for magnetic flux.
Moving iron instrument:
These instruments are normally used for measurement of AC current and voltage. A movement having spindle with moving iron, fixed iron and compensating iron is situated inside a copper coil. When current follows through the coil, it produces the magnetic field. The irons inside the coil core gets repelled away from fixed iron.
A pointer fixed on the spindle, on which a moving iron is fixed, gets deflected showing reading on the dial.
Controlling torque is provided by spiral phosphorus bronze hair spring.
Pointer comes to rest at equilibrium where deflecting torque and controlling torque becomes equal and opposite.
Damping is achieved by the use of jewel bearing with silicon coil.
Since the deflecting torque is directly proportional to the squire of current flowing through the coil, the nature of the scale is highly non linear in the conventional design of the moving iron meter.
The scale can be made linear by using specially designed movement, introduction of third iron, called compensation iron and specially designed shield.
Following types of meters are used in panel-
3. Frequency meters
4. Power factor meters
5. Maximum demand ammeter
6. KW meters
7. KVAR meters
Power factor meter
This instrument is used to measure power factor (cosine of angle between current and voltage vectors) of the supply. These are available in two types:
· Electrodynamometer type
· Electronic transducer type
These meters are used for measurement of supply frequency.
These are available in two types:
· Pointer type
· Vibrating reed type
Pointer type frequency meters are available as electronic transducer which may be built in the meter.
Maximum demand ammeter
These meters have a bi-metal movement which is fully compensated against variations in ambient temperature. When current follows through the spring, due to unequal expansion of the two different bi-metals, a torque is created by the spring which is directly proportional to RMS current following through the spring. This torque in turn produces rotation off a spindle on which a pointer is fixed. Pointer indicates the value of RMS current on a printed scale.
The bi-metal is so designed that the full deflection is obtained after either 08 or 15 minutes of current that follows through the bi-metal spring depending on meters selected. The trailing red pointer which gets pushed by measuring pointer when the current increase and remains in the same position unless it is rest to zero by external zero knob which can be locked /sealed.
This meter indicates maximum current drawn by an electrical installation over a period of time. Following are the applications such as:
a) For electricity board to know whether a premise has exceeded the load sanctioned to him.
b) In a high rise building, the electrical cabling is done for specified load however over a period of time, the load in the building increases, which when exceeds the specified limit, hits of the cabling.
This is a measure reason of a short circuit and fire in high rise building. If this instrument is installed, it will always indicate to the regulating authorities the maximum current drawn. If it exceeds this specified limit the corrective action can be taken before hand and fire hazards can be prevented.
c) In a motor control circuit, the use of such instrument will indicate to the company if the operator is continuously overloading the motor.
d) For a generating set, such instrument will indicate to the supplier company what has been the load on which generating set has been operated. This is very much useful for maintenance.
Active/reactive power meters:
Watt meters/VAR are used for active /reactive power measurement respectively for different types of circuit network and nature of load. These meters are of two types-
· Electronic transducer type
· Electrodynamometer type
Accuracy classification is as per IS:1248 1983.It is indicated on the scale.
Required Mounting position is indicate on the scale in accordance with IS 1248 whereby a deviation of +/- 50 from the position is permissible.; positional error(( in addition to the error on the indication ) must not exceed the error appropriate to the meter classification for frequency.