Working Principle of Voltmeter and Types of Voltmeter

What is Voltmeter?

A Voltmeter measures the potential difference between two circuit nodes. It is a measuring instrument with the SI unit volt, V. A reading is meaningful only when the meter range, input mode, bandwidth and reference node suit the signal.

Working Principle of Voltmeter

The working principle of a voltmeter requires connection across the two nodes whose voltage is required. A practical voltmeter has finite input resistance or impedance. It draws some current and can alter the circuit; a higher input impedance generally reduces this loading error for a given source impedance.

Connecting the meter in parallel places its input impedance across the measured circuit. The source resistance and meter input can form a voltage divider, so the displayed value may be lower than the undisturbed node voltage. An ideal voltmeter has infinite input resistance and draws no current, but real meters specify finite resistance and capacitance.
Before live measurement, select the voltage function and a safe range. Confirm that the meter, leads and probes have suitable voltage and measurement-category ratings, and inspect them for damage. Hazardous live work belongs to qualified people using the required isolation and protective procedures.

Classification or Types of Voltmeter

Voltmeters can be grouped by their sensing and display principle. Common historical and modern types of voltmeters include:

1. Parmanent Magnet Moving coil (PMMC) Voltmeter.
2. Moving Iron (MI) Voltmeter.
3. Electro Dynamometer Type Voltmeter.
4. Rectifier Type Voltmeter
5. Induction Type Voltmeter.
6. Electrostatic Type Voltmeter.
7. Digital Voltmeter (DVM).

By input quantity, voltmeters are also classified as:

1. DC Voltmeter.
2. AC Voltmeter.

Direct-reading DC voltmeters can use PMMC instruments. A MI instrument or electrodynamometer type can respond to AC and DC, subject to its frequency and waveform specifications. Thermal instruments can also measure both by heating effect. Induction meters work only with AC and are uncommon as general-purpose voltmeters. Rectifier and digital meters may provide both modes, but their AC accuracy depends on detector type, bandwidth, crest factor and waveform.

PMMC Voltmeter

A moving coil inside a magnetic field carries current in each active conductor. The resulting forces produce torque and turn a pointer across the scale.
A PMMC instrument uses a permanent magnet. With fixed total circuit resistance, coil current and deflection are proportional to applied DC voltage. Reversing polarity reverses torque; unrectified AC gives nearly zero average torque, so a conventional PMMC movement measures DC. This D’Arsonval movement has a linear scale, low power demand and good sensitivity.
Limitations include polarity sensitivity, moving-part wear and higher cost than simple moving-iron instruments.
It directly measures DC; AC requires a rectifier or another converter.

Where,
B = flux density in Wb/m².
i = V/R, where R is the total resistance of the meter movement and its series multiplier, not the measured load.
l = active coil length in m.
b = active coil breadth in m.
N = number of coil turns.

Extension of Range in a PMMC Voltmeter

A PMMC voltage range is extended with a series multiplier resistor that limits movement current at full-scale voltage. The resistor needs the required resistance, power, voltage and temperature-coefficient ratings.

Let,
V be the required full-scale voltage in volts.
R_v be the voltmeter movement resistance in ohms.
R be the external series multiplier resistance in ohms.
V₁ be the full-scale voltage across the movement alone.
The required external resistance is then given by

MI Voltmeter

An MI voltmeter uses a moving iron instrument. Its deflecting torque is proportional to i² times the rate of change of coil inductance with position. If meter impedance remains constant, current is proportional to applied voltage, so average torque follows V² and keeps the same direction for either polarity. MI movements can therefore indicate DC and the RMS value of AC within their calibrated frequency and waveform range. The two main forms are:

1. Attraction type.
2. Repulsion type.

Here, I is the current in the meter coil in amperes; I = V/Z.
V is measured voltage and Z is meter impedance, not load impedance.
L is the coil’s self inductance in henries.
θ is deflection in radians.

Attraction type MI Instrument Principle

Current in the fixed coil creates a magnetic field that attracts a soft-iron vane toward the position of higher inductance. The attached spindle and pointer move until deflecting torque balances the spring control torque.

Repulsion type MI Instrument Principle

Coil current magnetises two soft-iron vanes with the same instantaneous polarity. Their repulsion produces deflecting torque and moves the pointer.
MI instruments are robust, economical and usable on AC or DC. Their scale is nonlinear and they draw more power than PMMC instruments. Hysteresis, frequency, waveform, temperature and stray magnetic fields can cause error. AC indication is their common use.

Electrodynamometer Type Voltmeter

Electrodynamometer instruments use air-cored fixed and moving coils. They can serve as AC-DC transfer instruments because the same torque law applies on DC and on low-frequency AC, but AC use still has frequency, phase and waveform limits.

Electrodynamometer Type Voltmeter Principle

The fixed and moving coils carry the same meter current because they are connected in series. Interaction between their fields creates torque proportional to i² and to the change of mutual inductance with angle. Reversing voltage reverses both coil currents, so average torque keeps the same direction.
For voltage measurement, the coil circuit includes a non-inductive series multiplier and the complete instrument connects across the measured nodes.
For DC, or low-frequency AC with a mainly resistive meter circuit, the phase angle φ is approximately zero.

Here, I is meter current in amperes and I = V/Z.
V is applied voltage and Z is the impedance of the complete meter circuit.
M is the coils’ Mutual inductance.
Electrodynamometer voltmeters avoid iron hysteresis and can compare AC with DC. Limitations include low torque-to-weight ratio, friction, higher power demand and cost, plus frequency and stray-field errors.

Rectifier Voltmeter

Rectifier Voltmeter Principle

A rectifier voltmeter extends a PMMC meter to AC. A diode or bridge rectifier converts the input to pulsating DC, and the movement responds to its average value. The scale is commonly calibrated in RMS volts for a sine wave, so a non-sinusoidal waveform can give error unless the instrument specifies true-RMS response. DC normally uses a direct PMMC path rather than requiring rectification.

Advantages of Rectifier Voltmeter

1. It provides AC indication with a sensitive PMMC movement over its specified frequency range.
2. Its scale is approximately uniform above the region affected by diode threshold.

Disadvantages include diode nonlinearity at low voltage, temperature-dependent forward voltage, reduced AC sensitivity and errors from waveform and frequency outside the calibrated range.

Digital Voltmeters (DVM)

Digital Voltmeters (DVM) Principle

A Digital Voltmeter conditions the input through protection, attenuation or amplification, samples it and uses an analogue-to-digital converter to produce a numeric display. The display removes pointer parallax and can support automatic ranging and data logging through an interface. Accuracy still depends on range, input impedance, bandwidth, resolution, gain, linearity, noise, temperature, calibration and the meter’s AC conversion method; a larger digit count alone does not guarantee a more accurate result.

Electrostatic Instruments

Electrostatic Instruments Principle

An electric field between charged conductors produces mechanical force. An electrostatic voltmeter makes one electrode movable, so the force changes electrode position and produces a reading. Ideally, current flows only while capacitance charges or discharges, apart from insulation leakage. The movement can use:

1. Two oppositely charged electrodes, one fixed and one movable.
2. Electrode force that produces linear or rotary motion against a restoring spring or balance.

Here, V is measured voltage, C is electrode capacitance in farads and θ is deflection in radians.
An electrostatic meter draws little steady power, works on AC or DC and has no magnetic hysteresis loss. Its limitations include a nonlinear scale, low operating force and poor low-voltage sensitivity, plus comparatively large size, cost and fragile construction in traditional designs.