Every electrician and engineer relies on ammeters, voltmeters, wattmeters, energy meters and meggers. Questions such as “Why does a PMMC meter not work on AC?”, “What is the controlling torque?”, “How do you extend the range of an ammeter?” and “Which instrument reads RMS?” appear constantly in PGCB, BPDB, BREB/PBS, DPDC, DESCO, NESCO, polytechnic and BCS (EEE) exams. This post covers the fundamentals with an animation, comparison tables, range-extension problems and MCQs.
⚡ Key takeaways
- Analog indicating instruments need three torques: deflecting, controlling (spring/gravity) and damping (air friction, eddy current, fluid).
- PMMC: DC only, uniform scale, high accuracy, reads average value.
- Moving iron: AC and DC, non-uniform (square-law) scale, reads RMS.
- Electrodynamometer: AC/DC, used as wattmeter and transfer instrument. Induction type: AC only, used in energy meters.
- Ammeter range ↑ with a shunt Rsh = RmIm/(I − Im); voltmeter range ↑ with a series multiplier Rs = V/Im − Rm.
📑 Contents
1. Classification of Instruments
- Absolute instruments give the value in terms of physical constants (e.g. tangent galvanometer) — used in labs for calibration. Secondary instruments are calibrated against absolute ones and read directly (ammeters, voltmeters).
- By function: indicating (pointer shows instantaneous value), recording (continuous chart record), integrating (totalise over time, e.g. energy meter).
- By type of output: analog and digital.
- By principle: magnetic effect, heating effect, electrostatic effect, induction effect, Hall effect.
2. Three Essential Torques
| Torque | Purpose | Produced by |
|---|---|---|
| Deflecting torque | Moves the pointer from zero in proportion to the measured quantity | Magnetic, heating, electrostatic or induction effect |
| Controlling torque | Opposes deflection so pointer settles at a definite position; returns it to zero | Spring control (most common) or gravity control |
| Damping torque | Prevents oscillation so pointer settles quickly | Air-friction, eddy-current (best, used in PMMC), fluid friction |
3. PMMC vs Moving Iron on AC and DC (Animation)
A PMMC meter’s torque is proportional to the instantaneous current, and the heavy moving system responds to its average. On DC it reads correctly; on sinusoidal AC the average is zero, so the pointer just vibrates at zero. A moving-iron meter’s torque is proportional to I², which is always positive, so it reads the true RMS value on both AC and DC.
4. Types of Analog Instruments
| Instrument | Principle | Use | Scale | Notes |
|---|---|---|---|---|
| PMMC (permanent-magnet moving-coil) | Force on a current-carrying coil in a permanent magnet field; T ∝ I | DC only (AC with rectifier) | Uniform | High accuracy, low power, eddy-current damping; reverses with wrong polarity |
| Moving iron (attraction / repulsion) | Soft iron pulled into a coil’s field; T ∝ I² | AC & DC | Non-uniform (cramped at start) | Cheap, robust; errors from hysteresis and frequency |
| Electrodynamometer | Fixed and moving coils; T ∝ I₁I₂ cos φ | AC & DC; wattmeters | Non-uniform (ammeter/voltmeter), uniform (wattmeter) | Transfer instrument for AC calibration; low sensitivity |
| Induction type | Eddy currents in a rotating aluminium disc | AC only | — | Electromechanical energy meters (kWh) |
| Electrostatic | Attraction between charged plates | High-voltage AC/DC voltmeters | Non-uniform | Draws no current on DC |
| Thermocouple / hot-wire | Heating effect | RF current | Square-law | Reads true RMS at high frequency |
| Rectifier type | PMMC + diode rectifier | AC (multimeters) | Uniform | Calibrated for sine wave (form factor 1.11) |
5. Extending Range: Shunt and Multiplier
A basic meter movement has a small full-scale current Im and internal resistance Rm.
- Ammeter — a low resistance shunt in parallel bypasses most of the current: Rsh = Rm Im / (I − Im) = Rm/(m − 1), where m = I/Im is the multiplying power. An ammeter has very low resistance and is connected in series.
- Voltmeter — a high resistance multiplier in series: Rs = V/Im − Rm. A voltmeter has very high resistance and is connected in parallel. Sensitivity = 1/Im (Ω/V).
- For large AC currents and voltages, CTs and PTs are used instead.
6. Other Common Instruments
- Digital multimeter (DMM): ADC-based; high input impedance (≈ 10 MΩ); “true-RMS” models measure distorted waveforms correctly.
- Clamp meter: measures current without breaking the circuit (CT for AC, Hall sensor for DC).
- Energy meter: induction type (older) or electronic/smart meters; prepaid smart meters are now widely installed by Bangladeshi distribution utilities.
- Megger (insulation tester): applies 500 V – 5 kV DC to measure insulation resistance in MΩ/GΩ.
- Earth tester: measures earth electrode resistance (fall-of-potential method).
- Wattmeter, power-factor meter, frequency meter, synchroscope, and the oscilloscope for waveforms.
7. Errors and Accuracy
- Gross errors — human mistakes in reading or connecting.
- Systematic errors — instrumental (friction, ageing), environmental (temperature, stray fields), observational (parallax).
- Random errors — unpredictable variations; reduced by repeated readings.
- Accuracy class (e.g. class 0.5) = maximum error as a percentage of full-scale reading. Readings near the lower end of the scale have a larger percentage error, so choose a range where the pointer is in the upper part of the scale.
- Loading effect: a voltmeter with low resistance draws current and reads low; an ammeter with high resistance changes the circuit current.
8. Solved Problems
Problem 1. A PMMC movement has Rm = 5 Ω and full-scale current 10 mA. Convert it to (a) a 0–1 A ammeter and (b) a 0–100 V voltmeter.
- (a) Rsh = 5 × 0.01/(1 − 0.01) ≈ 0.0505 Ω in parallel.
- (b) Rs = 100/0.01 − 5 = 9995 Ω in series. Sensitivity = 1/0.01 = 100 Ω/V.
Problem 2. A class 1.0 voltmeter with 0–300 V range reads 60 V. Find the possible percentage error. Max error = 1 % × 300 = ±3 V ⇒ at 60 V: ±3/60 = ±5 % — so use a lower range for small readings.
Problem 3. A moving-iron ammeter and a PMMC ammeter are connected in series in a half-wave rectified circuit with peak current 10 A. Find their readings. PMMC reads average = Im/π ≈ 3.18 A; moving iron reads RMS = Im/2 = 5 A.
9. Practice MCQs (click to see answer)
1. PMMC instruments can be used for: (a) AC only (b) DC only (c) Both AC and DC (d) RF only
✔ (b) DC only (without rectifier).
2. Moving-iron instruments read the: (a) Average value (b) Peak value (c) RMS value (d) Peak-to-peak value
✔ (c) RMS value.
3. Damping in PMMC instruments is provided by: (a) Air friction (b) Eddy currents in the aluminium former (c) Fluid friction (d) Gravity
✔ (b) Eddy-current damping.
4. Controlling torque in most indicating instruments is provided by: (a) Gravity (b) Spring (c) Eddy current (d) Magnet
✔ (b) Spring control.
5. To extend the range of an ammeter, a ______ is connected. (a) High resistance in series (b) Low resistance in parallel (c) Capacitor in series (d) Inductor in parallel
✔ (b) Low-resistance shunt in parallel.
6. Energy meters (electromechanical) are of the ______ type. (a) PMMC (b) Induction (c) Electrostatic (d) Hot-wire
✔ (b) Induction type.
7. A megger is used to measure: (a) Earth current (b) Insulation resistance (c) Power factor (d) Frequency
✔ (b) Insulation resistance.
8. The scale of a PMMC instrument is: (a) Uniform (b) Square-law (c) Logarithmic (d) Cramped at the end
✔ (a) Uniform (linear).
9. An ideal voltmeter should have: (a) Zero resistance (b) Infinite resistance (c) 1 Ω resistance (d) Resistance equal to the load
✔ (b) Infinite resistance.
10. The instrument used as a transfer instrument for AC/DC calibration is the: (a) PMMC (b) Electrodynamometer (c) Induction type (d) Moving iron repulsion
✔ (b) Electrodynamometer.
10. বাংলায় দ্রুত রিভিশন
📝 পরীক্ষার আগে এক নজরে
- অ্যানালগ মিটারে তিন টর্ক: ডিফ্লেক্টিং, কন্ট্রোলিং (স্প্রিং/গ্র্যাভিটি) ও ড্যাম্পিং (এয়ার ফ্রিকশন, এডি কারেন্ট, ফ্লুইড)।
- PMMC: শুধু DC, সমান স্কেল, গড় মান পড়ে; AC-তে শূন্য দেখায়।
- মুভিং আয়রন: AC ও DC, অসম (বর্গ) স্কেল, RMS মান পড়ে।
- ইলেক্ট্রোডায়নামোমিটার: ওয়াটমিটার ও ট্রান্সফার যন্ত্র; ইন্ডাকশন টাইপ: শুধু AC, এনার্জি মিটার।
- অ্যামিটারের রেঞ্জ বাড়াতে সমান্তরালে শান্ট Rsh = RmIm/(I − Im); ভোল্টমিটারে সিরিজে মাল্টিপ্লায়ার Rs = V/Im − Rm।
- মেগার দিয়ে ইনসুলেশন রেজিস্ট্যান্স, আর্থ টেস্টার দিয়ে আর্থ রেজিস্ট্যান্স মাপা হয়।
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- 220 Most Repeated Power Sector MCQs
References: A. K. Sawhney, Electrical and Electronic Measurements and Instrumentation; J. B. Gupta, Electronic and Electrical Measurements; Helfrick & Cooper, Modern Electronic Instrumentation. Images and animations: original work by Totalecer.
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