Wednesday, January 11, 2017
Ammeter Working Principle and Types of Ammeter
Introduction of Ammeter
As we know a word "meter"
associated with the measurement. Meter is an instrument which can measure a
particular quantity. we know, the unit of current is Ampere. Ammeter means
Ampere-meter which measures ampere value. Ampere is the unit of current so an
ammeter is a meter or an instrument which measures current.
Working Principle of Ammeter
The main principle of ammeter is that
it must have a very low resistance and also inductive reactance. Now, why do we
need this? can't we connect an ammeter in parallel? The answer to this question
is it has very low impedance because it must have very low amount of voltage
drop across it and must be connected in series connection because current is
same in the series circuit. Also due to very low impedence the power loss will
be low and if it is connected in parallel it becomes almost a short circuited
path and all the current will flow through ammeter as a result of high current
the instrument may burn. So due to this reason it must be connected in series.
For an ideal ammeter, it must have zero impedance so that it has zero voltage
drop across it so the power loss in the instrument is zero. But the ideal is
not achievable practically.
ammeter
Classification or Types of Ammeter
Depending on the constructing
principle, there are many types of ammeter we get, they are mainly –
· Permanent Magnet Moving Coil(PMMC) ammeter.
· Moving Iron(MI) Ammeter.
· Electrodynamometer type Ammeter.
· Rectifier type Ammeter.
Depending on this types of measurement
we do, we have-
DC Ammeter.
AC Ammeter.
DC Ammeter are mainly PMMC
instruments, MI can measure both AC and DC currents, also Electrodynamometer
type thermal instrument can measure DC and AC, induction meters are not
generally used for ammeter construction due to their higher cost, inaccuracy in
measurement.
Description of Different Types of
Ammeters
PMMC Ammeter
Principle PMMC Ammeter: When current
carrying conductor placed in a magnetic field, a mechanical force acts on the
conductor, if it is attached to a moving system, with the coil movement, the
pointer moves over the scale.
Explanation: As the name suggests it
has permanent magnets which are employed in this kind of measuring instruments.
It is particularly suited for DC measurement because here deflection is
proportional to the current and hence if current direction is reversed,
deflection of the pointer will also be reversed so it is used only for DC
measurement. This type of instrument is called D Arnsonval type instrument. It
has major advantage of having linear scale, low power consumption, high
accuracy. Major disadvantage of being measured only DC quantity, higher cost
etc.
Deflecting torque,
Where,
B = Flux density in Wb/m².
i = Current flowing through the
coil in Amp.
l = Length of the coil in m.
b = Breadth of the coil in m.
N = No of turns in the coil.
Extension of Range in a PMMC Ammeter:
Now it looks quite extraordinary that we can extend the range of measurement in
this type of instrument. Many of us will think that we must buy a new ammeter
to measure higher amount of current and also many of us may think we have to
change the constructional feature so that we can measure higher currents, but
there is nothing like that, we just have to connect a shunt resistance in
parallel and the range of that instrument can be extended, this is a simple
solution provided by the instrument.
pmmcIn the figure I = total
current flowing in the circuit in Amp.
Ish is the current through the shunt
resistor in Amp.
Rm is the ammeter resistance in Ohm.
MI Ammeter
It is a moving iron instrument, used
for both AC and DC, It can be used for both because the deflection θ
propotional square of the current so what ever is the direction of current, it
shows directional deflection, further they are classified in two more ways-
Attraction type.
Repulsion type.
Its torque equation is:
where I is the total current flowing
in the circuit in Amp.
L is the self inductance of the
coil in Henry.
θ is the deflection in Radian.
Attraction Type MI Instrument
Principle: When an unmagnetised soft iron is placed in the magnetic field, it
is attracted towards the coil, if a moving system attached and current is
passed through a coil, it creates a magnetic field which attracts iron piece
and creates deflecting torque as a result of which pointer moves over the
scale.
Repulsion Type MI Instrument
Principle: When two iron pieces are magnetized with same polarity by passing a
current than repulsion between them occurs and that repulsion produces a
deflecting torque due to which the pointer moves.
The advantages of MI instruments are
they can measure both AC and DC, cheap, low friction errors, robustness etc. It
is mainly used in AC measurement because in DC measurement error will be more due
to hysteresis.
Electrodynamometer Type Ammeter
This can be used to measure both i.e.
AC and DC currents. Now we see that we have PMMC and MI instrument for the
measurement of AC and DC currents , a question may arise - "why do we need
Electrodynamometer Ammeter? if we can measure current accurately by other instrument also?". The answer
is Electrodynamometer instruments have the same calibration for both AC and DC
i.e. if it is calibrated with DC , then also without calibrating we can measure
AC.
Principle Electrodynamometer Type
Ammeter: There we have two coils, namely fixed and moving coils. If a current
is passed through two coils it will stay in the zero position due to the
development of equal and opposite torque. If somehow, the direction of one torque
is reversed as the current in the coil reverses, an unidirectional torque is
produced.
For ammeter, the connection is a
series one and φ = 0
Where, φ is the phase angle.
Where, I is the amount of current
flowing in the circuit in Amp.
M = Mutual inductance of the coil.
They have no hysteresis error, used
for both AC and DC measurement, the main disadvantages are they have low
torque/weight ratio, high friction loss, expensive than other measuring
instruments etc.
Rectifier Ammeter
rectifier ammeter
Principle of Rectifier Ammeter: They
are used for AC measurement which is connected to secondary of a current
transformer, the secondary current is much less than primary and connected with
a bridge rectifier to moving coil ammeter.
Advantages:
It can be used in high frequency also.
Uniform scale for most of the ranges.
Disadvantages
being error due to
temperature decrease in sensitivity in AC operation.
