Saturday, June 28, 2014

Voltmeter and Ammeter working principle; Moving iron type instruments working principle

Basic principle and building process of Voltmeter and Ammeter devices

Ammeter and Voltmeter as everyone knows are devices which are typically used to measure alternating currents and voltages. In moving-iron devices the movable system includes a number of pieces of specially-molded soft iron, as to be acted upon by the magnetic field created by the current which are thus pivoted.

Basic circuit by which Voltmeter and Ammeter are connected is as shown below:-

You will find two general kinds of proceeding-iron devices specifically:
1. Attraction (or single-iron)
2. Repulsion (or double iron)

The simple description of distinct parts of a moving-iron device is given below:

1.  Moving component
   a little piece in the shape of pole or a vane.
2.  Coil: 
   to magnetize the iron pieces and to create the magnetic field as a result of current flowing through it.

In repulsion kind, stick or a set vane can also be used and magnetized with the exact same polarity.

Control torque is supplied by spring or weight (gravitation).

Damping torque is usually pneumatic, the damping apparatus comprising a moving vane and an air chamber attached to the instrument spindle.

A motion is produced by deflecting torque on an aluminum pointer over a scale that is calibrated.

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Moving Iron Type Instruments Operating Principle:-
Moving iron type instruments working principle is illustrated below:-
A thin disk is pivoted before a coil. This iron will go inward that's from poorer magnetic field to more powerful magnetic field. In attraction going device gravity control was used formerly but now spring control in comparatively contemporary device replaces gravitation control system. Correcting equilibrium weight null deflection reached. The damping force that was required is supplied in this device by air friction. The figure reveals an average kind of damping system supplied in the device, where damping is achieved by a moving piston within an air syringe.

Attraction Type Instruments working principle:-

It consist of fixed coil C and moving iron piece D. The coil moving coil is a flat disc which is eccentrically mounted on the spindle.
èThe spindle is supported between the jewel bearings .
èThe spindle carries pointer which moves over a graduated scale.
èThe no. of turns of the fixed coil are dependent on the range of the instrument.
èFor passing large current through the coil only few turns are required.
The controlling torque is provided by the springs but the gravity control may be used for vertically mounted panel type instrument.
èThe damping torque is provided by the air friction . A light aluminum piston is attached  to the moving system .
èIt 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.
èThe operating magnetic field is very weak. Hence eddy current damping  is not used since it requires a permanent magnet which would affect or distort the operating field

Repulsion type instruments working principle:-

Basic working principle:
“ There is repulsion between two iron pieces as they are magnetized with the same polarity. ”
A hollow circular coil is used and one of the iron pieces is attached inside the hollow coil.
èThe other iron piece is attached to the spindle.
èThe current to be measured is passed through the coil, coil produced the magnetic field.
èAs both the iron pieces are placed in the same magnetic field, they are magnetized with same polarity.
Hence the deflecting torque is produced due to the force of repulsion between two iron pieces.
èIndependent of the direction of current, the deflection is produced in the same direction.
èthe force of repulsion between iron  pieces is directly proportional to square of current hence scale is non-uniform.
èControlling torque is produced by either spring or gravity control.
èDamping is produced by the air friction damping.

Below diagrams are of repulsion and attraction type moving iron instruments.

Repulsion Type Instruments

Just load current is carried by the fixed coil, to be able to resist high transient current and it's built.
Moving iron devices having scales which can be not linear and packed in the lower range.

Repulsion moving iron-device
Measurement of Current and Electrical Voltage
Moving iron devices are used as Ammeter and Voltmeter.
Both can work on DC in addition to on AC.

Device used to measure current.
Constantly linked in line and carries the current to be quantified.
This current flowing through the coil creates the wanted deflecting torque.

Constantly joined in parallel.
Current flowing through the meter's operating coil creates deflecting torque.
So there is of sequence a high resistance connected in line with the coil of the device.

Ranges of Voltmeter and Ammeter
For a specified moving-iron device the ampere turns essential to generate full scale deflection are continuous.
You can change ammeters' range by supplying the moving coil to a shunt coil.
Voltmeter range may be changed joining a resistance in series. Thus exactly the same coil may be used for several ranges.

The devices are ideal for use in DC circuits and AC.
The devices are robust, owing to the straightforward building.
The standing portions of the devices can also be not complex.
Device is low cost compared to going coil device.
Torque/weight ratio is high, so less frictional error.

Malfunctions or Errors
Malfunction or Errors as a result of variation.
Error is rather modest -weight ratio is not low in going coil devices.
Scattered fields cause comparatively low values of magnetizing force generated by the coil. Magnetic screening that is efficient is necessary to reduce this effect.
Error as a result of variation also alters the eddy currents caused in neighboring metal and causes change of reactance.

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