Measuring instruments are an important part of electrical engineering and they are essential for analyzing the electrical circuits. Some of the important instruments which are used occasionally are ammeter, voltmeter, wattmeter, energy meter etc. But in this article we are about to discuss the two most fundamental of all - Ammeter and Voltmeter. Ammeters and voltmeters are generally classed together because of the similarities in their operating principles, with some exceptions of course!
WHAT IS AN AMMETER?
An ammeter
is an electrical measurement instrument which is used to measure current
through a point or wire in a circuit. It is connected in series with the
circuit whose current is to be measured. Therefore, they should have a low
electrical resistance. This is essential in order that they cause a small
voltage drop and consequently absorb small power.
Ammeter can
also be made from a galvanometer by placing a shunt resistance in parallel with
the galvanometer.
WHAT IS A VOLTMETER?
A voltmeter is an electrical measurement instrument which is used to measure voltage across two given points in a circuit. It is connected in parallel with the circuit or the part of the circuit whose voltage is to be measured. They should have high electrical resistance so that the current drawn by them is small and ultimately the power absorbed is small.
voltmeter can also be made from a galvanometer by placing a high resistance in series with the galvanometer.
Ammeters and
voltmeters can be of different types based on their construction and working.
TYPES OF AMMETERS AND VOLTMETERS –
1. Moving iron type (both for AC/DC)
(a) Attraction type
(b) Repulsion type
2. Moving coil type
(a) Permanent magnet type (for DC only)
(b) Electrodynamic or dynamometer type
(for DC/AC)
3. Hot wire type (both for DC/AC)
4. Induction type (for AC only)
(a) Split phase type
(b) Shaded pole type
5. Electrostatic type (for voltmeters
only) – Both DC/AC
MOVING IRON TYPE –
In moving
iron type, there are two basic types, which are:
(a) Attraction type
(b) Repulsion type
For both
type of these instruments, the necessary magnetic field is produced by the
ampere turns of a current carrying coil. As we are talking about ammeter; the
coil has comparatively fewer turns of thick wire so that the ammeter has low
resistance because it is to be connected in series with the circuit.
ATTRACTION TYPE INSTRUMENT –
The figure
below shows the constructional details of an attraction type moving iron
instrument.
The coil is flat disc or a sector eccentrically mounted. When the
current flows through the coil, a magnetic field is set up and the moving iron
moves from a region of weaker field to region of stronger magnetic field,
thereby deflecting the pointer over a calibrated scale.
 |
| ATTRACTION TYPE MOVING IRON INSTRUMENT |
The
controlling torque is provided by springs but can also be gravity controlled.
Damping is provided by air friction usually by a vane moving in a sector shaped
chamber.
REPULSION TYPE
INSTRUMENT –
REPULSION TYPE MOVING IRON ONSTRUMENT
In repulsion
type, there are two iron vanes inside the coil; one fixed and other movable.
When the current flows through the coil, these vanes get similarly magnetized
and there is force of repulsion between the two vanes resulting in the movement
of moving vane, and therefore the pointer.
Why moving iron can be used in both AC and DC?
The function
of moving iron instruments depends on the attraction or repulsion of iron
vanes. Thus, they are unpolarised i.e. they are independent of the direction in
which the current flows. Therefore, they can be used both in AC as well as DC.
DEFLECTION PRODUCED –
The
deflection in a moving iron instrument is given by
\[\theta
=\frac{1}{2}\frac{{{I}^{2}}}{K}\frac{dl}{d\theta }\]
Thus, $\theta
\propto {{I}^{2}}$
As, the
deflection is proportional to the square of the current, it is evident that the
scale of such an instrument is non-uniform.
If there is
no saturation, the change of inductance with the angle of deflection is uniform
i.e. $\frac{dl}{d\theta }$ is constant. Thus, the scale can be easily laid as
the measured quantity is proportional to the square root of deflection.
SOURCES OF ERROR –
(a) Errors with both AC and DC work –
(i) Errors due to hysteresis -
Because of hysteresis in
the iron parts of the moving system, readings are higher for descending values
and lower for ascending values. This can be completely eliminated by using Mu
metal or Perm-alloy, which has negligible hysteresis loss.
(ii) Errors due to stray fields –
Unless shielding is not
done from external fields, the results obtained may be wrong. Thus, the
instrument is shielded with cast iron.
(b) Errors with AC work –
Changes of frequency produce change in the impedance of the coil and
change in the magnitude of AC currents.
ADVANTAGES -
- Cheap and robust.
- Can be used both in AC and DC.
DISADVANTAGE –
- They cannot be calibrated with DC because of the effect of hysteresis in the iron vanes. Hence, they are usually calibrated by comparison with AC standard.
