Ammeter Working Principle and Types of Ammeter

Introduction of Ammeter

An ammeter measures the electric current through a circuit branch. It displays the result in amperes, the SI unit of current. Both the expected current and whether it is AC or DC determine which ammeter and range are suitable.

Working Principle of Ammeter

The basic principle of an ammeter is to place the meter in series so that the branch current passes through its measuring element. An ammeter has low, but not zero, input resistance. Its internal resistance produces a small voltage drop called burden voltage, which can disturb the circuit if the wrong meter or range is used.

Never connect an ammeter across a voltage source or circuit component while its leads are in current terminals. That parallel connection can draw a dangerously high current, blow the meter fuse, damage equipment or cause injury. De-energize the circuit where required, open the branch, connect the correctly rated meter in series, and start with a safe range. An ideal ammeter would have zero impedance, but every practical instrument introduces some burden.

Classification or Types of Ammeter

Ammeters can be classified by the mechanism that converts current into an indication. Common analogue types include:

1. Permanent Magnet Moving Coil(PMMC) ammeter.
2. Moving Iron (MI) Ammeter.
3. Electrodynamometer type Ammeter.
4. Rectifier type Ammeter.

They can also be classified by the current they are designed to measure:

1. DC Ammeter.
2. AC Ammeter.

DC Ammeter designs commonly use PMMC instruments. Moving-iron and electrodynamometer instruments can measure both AC and DC currents. Rectifier instruments combine an AC-to-DC rectifier with a DC movement. Traditional induction meters operate only on AC and are not normally used as general-purpose ammeters.

Description of Different Types of Ammeters

PMMC Ammeter

Principle PMMC Ammeter:
A current-carrying coil in a permanent magnetic field experiences a force. The resulting torque turns the coil and its pointer against a control spring.
Explanation: In this class of measuring instruments, deflecting torque is proportional to coil current, so the scale is nearly linear. Reversing DC reverses the pointer torque. An unrectified PMMC movement therefore measures DC; an alternating current would produce torque that reverses each half-cycle and has an average near zero. The mechanism is also called a D’Arsonval movement. It offers good sensitivity and low power use, but overload and polarity must be controlled.
The deflecting-torque expression is:

Where:
B = flux density in Wb/m².
i = current through the coil in amperes.
l = active coil length in metres.
b = coil breadth in metres.
N = number of coil turns.
Extension of Range in a PMMC Ammeter:
A calibrated shunt can extend the current range of this type of instrument. The low-value shunt resistance in parallel with the movement carries most of the total current. Its resistance and power rating must match the movement current and voltage drop; an arbitrary resistor can give a wrong reading or overheat.

In the figure, I is the total circuit current in amperes.
I_sh is the shunt current in amperes.
R_m is the ammeter-movement resistance in ohms.

MI Ammeter

A moving iron instrument (MI) ammeter can measure AC or DC because its deflecting torque is approximately proportional to the square of current. The torque therefore keeps the same direction when current reverses. MI ammeters use either attraction or repulsion.

1. Attraction type.
2. Repulsion type.

A common torque expression is:
Where:
I is the current through the instrument in amperes.
L is the coil self inductance in henries.
θ is the angular deflection in radians.

1. Attraction Type MI Instrument Principle:
   Current in the fixed coil creates a magnetic field that pulls a soft-iron vane into the region of stronger flux. The vane is linked to the pointer, so this motion produces the indication.
2. Repulsion Type MI Instrument Principle:
   The coil magnetizes two iron vanes with the same polarity. Their repulsion turns the moving vane and pointer.
   MI instruments are robust and work on AC or DC, but their scale is nonlinear. Hysteresis affects DC readings, while coil inductance and frequency can affect AC readings.

Electrodynamometer Type Ammeter

An electrodynamometer ammeter can measure AC and DC. An Electrodynamometer instruments movement contains fixed and moving air-cored coils, so it avoids iron hysteresis. A DC calibration can serve as an AC transfer reference only within the instrument’s specified frequency, waveform, range and accuracy limits. It is not automatically error-free on every AC signal.

Principle Electrodynamometer Type Ammeter:
The current passes through the fixed and moving coils in series. Their interacting magnetic fields produce a deflecting torque proportional to the product of the coil currents. Both currents reverse together on AC, so the torque direction does not reverse.
For an ammeter series connection, the two coil currents are in phase and φ = 0.
Here, φ is the phase angle between the coil currents.

Where:
I is the measured current in amperes.
M is the Mutual inductance between the coils.
The air-cored movement has no iron hysteresis error and can indicate AC or DC. Its disadvantages include low torque-to-weight ratio, greater sensitivity to friction and stray magnetic fields, and higher cost than common moving-iron instruments.

Rectifier Ammeter

Principle of Rectifier Ammeter:
A rectifier ammeter converts AC to a unidirectional current that a PMMC movement can indicate. Some high-current systems feed the rectifier from the secondary of a rated current transformer; lower-current instruments may use an internal series network instead. The scale is often calibrated in RMS units for a sine wave, so waveform distortion, diode drop and frequency can introduce error.

Advantages:

1. It can cover AC ranges that a bare PMMC movement cannot measure.
2. The PMMC movement provides a nearly uniform scale.

Disadvantages include rectifier temperature error and diode nonlinearity at low current. Readings can also vary with waveform and frequency. Use the instrument only within its stated input category and ratings.