Voltmeter and Ammeter working principle; Moving iron type instruments working principle

By -

Basic Principle and Construction of Voltmeter and Ammeter

Introduction

Ammeters and Voltmeters are essential instruments used to measure current and voltage in electrical circuits. One of the most common types is the moving-iron (MI) instrument, which works on the principle of the magnetic effect of current. These devices can measure both AC and DC quantities, making them versatile and widely used in power systems.



Principle of Moving-Iron Instruments

  • When a current flows through a coil, it produces a magnetic field.

  • A soft iron piece placed in this field experiences a force of attraction or repulsion, depending on the instrument type.

  • The force causes a deflection of the pointer, which is directly related to the magnitude of the current (in ammeters) or voltage (in voltmeters).

  • The deflection is proportional to the square of the current (i²), so the scale is non-linear, compressed at the lower end.

Types of Moving-Iron Instruments

There are two kinds of MI instruments:

  1. Attraction Type

    • A single piece of soft iron is attracted towards the coil when current flows.

    • Pointer moves over a scale due to this attraction.



  2. Repulsion Type

    • Two iron pieces are magnetized with the same polarity.

    • They repel each other, causing the pointer to deflect.

    • Deflection occurs irrespective of current direction (works for both AC & DC).



Construction of Moving-Iron Instruments

The key components are:

  1. Moving Element

    • A soft iron vane or piece that moves when subjected to the magnetic field.

  2. Coil

    • A hollow cylindrical coil through which the current flows, creating a magnetic field.

  3. Control System

    • Provides controlling torque.

    • Spring control (modern instruments) or gravity control (older instruments).

  4. Damping System

    • Prevents oscillations of the pointer.

    • Air friction damping is used (a piston moving inside an air chamber).

    • Eddy current damping is avoided because permanent magnets would distort the weak magnetic field.

  5. Pointer & Scale

    • Lightweight aluminum pointer attached to spindle.

    • Scale is non-uniform, crowded at the lower end.

Working of Ammeter and Voltmeter

Ammeter

  • Connected in series with the circuit.

  • Entire current to be measured passes through the coil.

  • To extend range, a shunt is connected so that only part of the current flows through the meter.

Voltmeter

  • Connected in parallel with the circuit.

  • A high resistance is connected in series with the coil to limit current.

  • This ensures correct proportional deflection without damaging the instrument.

Advantages

  • Can measure both AC and DC quantities.

  • Rugged and simple construction.

  • Low cost compared to moving-coil instruments.

  • High torque-to-weight ratio → less friction error.

Limitations / Errors

  • Non-linear scale (not suitable for precision measurements).

  • Errors due to frequency variation (in AC circuits).

  • Sensitive to stray magnetic fields → shielding required.

  • Temperature variation affects resistance and accuracy.

Application

  • Used as panel meters in laboratories and power stations.

  • Suitable where approximate readings are sufficient.

  • Common in AC measurement systems.


Comments

Post a Comment

Popular posts from this blog

A2XWY; A2XFY Cables meaning; XLPE cables advantages over PVC cables

By -
Image
🏗 ️ Technical Specification of Aluminium Cables (IS 7098: Part 1 – 1988) When specifying aluminium cables, you will often come across codes like A2XWY . This code represents the construction, insulation, armouring, and sheath of the cable. 🔑 Cable Nomenclature Codes Code Meaning A Aluminium Conductor 2X XLPE (Cross-linked Polyethylene) Insulation Y PVC Outer Sheath W Steel Round Wire Armour F Steel Strip Armour WW Double Steel Round Wire Armour FF Double Steel Strip Armour Wa Non-Magnetic Round Wire Armour Fa Non-Magnetic Strip Armour AW Aluminium Wire Armour 👉 Example: A2XWY = Aluminium conductor + XLPE insulation + Steel round wire armour + PVC outer sheath. 📌 Note: For Copper Cables , no pref...
Read more

YWY, AYFY, AYY, AYCY, A2XCY Cables Meanings

By -
Image
📘 Cable Nomenclature Explained (PVC & XLPE Cables) Cable designations like YWY, AYFY, A2XCY might look confusing at first glance. But once you understand the logic behind each letter , reading technical specs becomes straightforward. 1. Basic Conductor Material ·          A → Aluminium conductor ·          (If “A” is absent, it means Copper conductor) 2. Insulation Material ·          Y → TROPODUR = PVC insulation ·          2X → TROPOTHEN-X = XLPE insulation 3. Armouring Types ·          W → Round Steel Wire Armouring ·          WW → Double Round Steel Wire Armouring ·          F → Flat (formed) Steel Strip Armouring ·       ...
Read more

Cable size and current carrying capacity

By -
Image
Cable size and current carrying capacity for XLPE insulated cable, non-armored, Twin or multicore cable is as below:- Conductor operating temperature is 90 degree Celsius Ambient temperature: 30 degree Celsius. Cable size and current carrying capacity for XLPE insulated, Armored, Twin or multicore cable is as below:- Conductor operating temperature is 90 degree Celsius Ambient temperature: 30 degree Celsius. For air and 20 degree Celsius for ground for installations require to comply with BS: 7671 Correction factors:- For Ambient temperature: Ambient temperature in degree Celsius Correction Factor 25 1.02 30 1.00 35 0.96 40 0.91 45 0.87 50 0.82 55 0.76 60 0.65 65 0.58 70 0.50 75 0.41 80 Ground Temperature Ambient temperature in degree Celsius Correction F...
Read more