Ammeter Working Principle, Types, Advantages, disadvantages
Table of Contents
The word “meter” is, as we all know, connected to the measurement system. An object that can measure a specific quantity is a meter. Ampere is used to measure current, as we all know. Ampere-meters, also known as ammeters, measure ampere values. A meter or instrument that measures current is called an ammeter because an ampere is the unit of current.
Working Principle of Ammeter
The fundamental requirement for an ammeter is that it have extremely low resistance as well as inductive reactance. Why do we need this, exactly? An ammeter can’t be connected in parallel, right? The answer to this query is that it has a very low impedance because it must have a very small voltage drop across it and must be connected in series because current flows in the same direction in a series circuit. Additionally, due to the extremely low impedance, there will be little power loss. If it is connected in parallel, it almost creates a short circuited path where all the current flows through the ammeter, risking instrument burn due to the high current.
It must therefore be connected in series for this reason. An ideal ammeter would have zero impedance, which would result in zero voltage drop across the device and zero power loss. The ideal, however, is not practically attainable.
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.
Induction meters are not frequently used for ammeter construction due to their higher cost and measurement inaccuracy. DC ammeter are primarily PMMC instruments, MI can measure both AC and DC currents, as can Electrodynamometer type thermal instruments.
Description of Different Types of Ammeters
If a current-carrying conductor is placed in a magnetic field, a mechanical force is applied to it. If the conductor is connected to a moving system, the pointer moves over the scale as the coil moves.
Explanation: It contains permanent magnets, as the name would imply, which are used in this type of measuring equipment. Due to the fact that the pointer’s deflection is proportional to the current in this case and will therefore change if the current direction changes, it is only used for DC measurement. D Arnsonval type instrument is the name given to this kind of instrument. Its linear scale, low power consumption, and high accuracy are its three main advantages. Being measured only in DC quantity has significant drawbacks, including higher costs.
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 appears quite extraordinary that we can increase the instrument’s measurement range. Many of us will believe that we need to purchase a new ammeter in order to measure higher currents, and many of us may also believe that we need to alter the instrument’s design. However, there is no need for either of these things; all we need to do is connect a shunt resistance in parallel to the instrument in order to increase its range.
In 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.
It is a moving iron instrument that can be used for both AC and DC because the directional deflection is proportional to the square of the current, regardless of the direction of the current. They are also divided into two additional categories:
- Attraction type.
- Repulsion type.
Its torque equation is:
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:
If a moving system is attached and current is passed through a coil, it creates a magnetic field that attracts an iron piece and produces deflecting torque, which causes the pointer to move over the scale when the unmagnetized soft iron is placed in the magnetic field.
- Repulsion Type MI Instrument Principle:
The pointer moves because of the deflecting torque produced when two iron pieces that have the same polarity are magnetized by a current repel one another. The benefits of MI instruments include their affordability, low friction errors, robustness, and ability to measure both AC and DC. It is primarily used for AC measurements because hysteresis will cause more error in DC measurements.
Electrodynamometer Type Ammeter
Both AC and DC currents can be measured using this. Why do we need an electrodynamometer ammeter when we have PMMC and MI instruments for measuring AC and DC currents, one may wonder. If we can accurately measure current using other instruments too?” The answer is that electrodynamometer instruments have the same calibration for both AC and DC, meaning that if they are calibrated with DC, we can measure AC without further calibration.
Principle Electrodynamometer Type Ammeter:
There are two coils present: a fixed coil and a moving coil. Because equal and opposite torque develops when a current is passed through two coils, the current will remain in the zero position. A unidirectional torque is created if the direction of one torque somehow changes as the coil’s current changes.
The connection for an ammeter is a series one, and φ = 0.
Where, φ is the phase angle.
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.
Principle of Rectifier Ammeter:
They are connected to a current transformer’s secondary, where the secondary current is much lower than the primary, and are used to measure AC current. So A moving coil ammeter is connected to the secondary through a bridge rectifier.
- It can be used in high frequency also.
- Uniform scale for most of the ranges.
disadvantages include errors in AC operation caused by a drop in temperature sensitivity.