Deflecting Forces in Electrical Instruments — Effects of Current Used for Deflection
In any indicating electrical instrument — ammeter, voltmeter, or wattmeter — the pointer must move from its rest position to show the measured value on the calibrated scale. The force responsible for this movement is called the deflecting force (or deflecting torque). Without it, the pointer would remain at zero regardless of the current or voltage applied.
This article explains the deflecting force in detail, covers all six effects of current used to produce it, and compares the instruments based on each effect.
Forces in Indicating Instruments
Every indicating instrument requires three essential forces (torques) in its moving system:
- Deflecting Force (Td) — moves the pointer proportional to the measured quantity
- Controlling Force (Tc) — opposes deflection and brings the pointer to a definite position
- Damping Force — prevents oscillation and brings the pointer to rest quickly
At steady-state deflection: Td = Tc. The damping force acts only during motion and becomes zero at rest.
What is Deflecting Force?
The deflecting force is produced by the current or voltage being measured. It causes the moving system (pointer + coil or iron piece) to rotate from its zero position. The magnitude of deflection is proportional to the quantity being measured.
Td ∝ V (for voltmeters)
Td ∝ V × I × cos φ (for wattmeters)
The deflecting torque is generated using various effects of electric current. Each effect leads to a different type of instrument with unique characteristics.
1. Magnetic Effect
When current flows through a conductor, it produces a magnetic field around it. This magnetic effect is the most widely used method for producing deflecting torque. Three sub-types exist:
a) PMMC Principle (Permanent Magnet Moving Coil)
A current-carrying coil placed between the poles of a permanent magnet experiences a force (Lorentz force). The coil is mounted on a spindle with a pointer attached. When current flows, the coil rotates and the pointer deflects proportionally.
- Used in: PMMC instruments (D'Arsonval galvanometer)
- Measures: DC only (uniform, linear scale)
- Accuracy: Very high (±0.5% or better)
b) Attraction Type Moving Iron
A soft iron piece is placed near a current-carrying coil. The iron is attracted into the coil due to the magnetic field. A pointer attached to the iron piece gives the reading.
c) Repulsion Type Moving Iron
Two soft iron pieces (vanes) are placed inside a coil. When current flows, both get magnetized with the same polarity and repel each other. One vane is fixed, the other is movable with a pointer.
- Used in: Moving Iron instruments
- Measures: Both AC and DC
- Scale: Non-uniform (square law)
2. Electrodynamic Effect
Two current-carrying coils — one fixed and one movable — interact magnetically. The fixed coil produces the field, and the movable coil (carrying current) experiences a torque. The deflecting torque is proportional to the product of currents in both coils.
- Used in: Electrodynamometer instruments (wattmeters, ammeters, voltmeters)
- Measures: Both AC and DC with high accuracy
- Application: Standard wattmeters for power measurement
3. Thermal Effect
Current flowing through a resistance wire produces heat (I²R). This heat is either:
- Used to expand a wire that deflects the pointer (hot wire instruments)
- Converted to EMF via a thermocouple, which drives a PMMC meter
- Used in: Hot wire instruments, thermocouple meters
- Measures: AC (true RMS value, independent of waveform)
- Frequency range: Up to several MHz
4. Electrostatic Effect
Force of attraction exists between two oppositely charged plates. One plate is fixed and the other is movable with a pointer. The deflecting force is proportional to the square of the voltage.
An Electrostatic Instrument
- Used in: Electrostatic voltmeters
- Measures: Both AC and DC voltage (only voltmeters possible)
- Advantage: Draws negligible current, suitable for high-voltage measurement
5. Induction Effect
An aluminium disc placed in a rotating or alternating magnetic field has eddy currents induced in it. The interaction between eddy currents and the magnetic field produces a torque that rotates the disc. A pointer (or counting mechanism) is attached to the disc.
- Used in: Induction type energy meters (kWh meters)
- Measures: AC only (requires alternating flux for induction)
- Cannot measure DC quantities
- Application: Domestic and industrial energy metering
6. Chemical Effect
When current passes through an electrolyte, chemical decomposition occurs (electrolysis). The amount of substance deposited is proportional to the charge passed (Faraday's law). This principle is used in ampere-hour meters to measure battery capacity.
- Used in: Ampere-hour meters
- Measures: DC only (total charge over time)
- Application: Battery capacity measurement
Comparison of Effects Used in Instruments
Deflecting Torque Formula
The general expression for deflecting torque depends on the instrument type:
Moving Iron: Td = ½ × I² × dL/dθ (L = inductance of coil)
Electrodynamometer: Td = I₁ × I₂ × dM/dθ (M = mutual inductance)
Electrostatic: Td = ½ × V² × dC/dθ (C = capacitance between plates)
At equilibrium, the deflecting torque equals the controlling torque (spring constant × angle): Td = K × θ, giving the deflection angle θ proportional to the measured quantity.
Frequently Asked Questions
Q1: What is the purpose of deflecting torque in an instrument?
The deflecting torque moves the pointer from its zero position to a value on the scale that corresponds to the measured current or voltage. Without deflecting torque, no reading is possible.
Q2: Which effect is most commonly used for deflecting torque?
The magnetic effect is most commonly used. PMMC instruments (DC) and Moving Iron instruments (AC/DC) together cover the majority of industrial and laboratory measurements.
Q3: Can induction-type instruments measure DC?
No. Induction instruments require a time-varying (alternating) magnetic field to induce eddy currents in the disc. A steady DC field produces no induction, so these instruments work only on AC.
Q4: Why are electrostatic instruments used only as voltmeters?
Electrostatic instruments work on the force between charged plates, which depends on voltage (not current). They draw negligible current, making them ideal for high-voltage measurement but unsuitable as ammeters.
Q5: What happens if there is no controlling torque?
Without controlling torque, the pointer would swing to the maximum position regardless of the measured value. The controlling torque (spring or gravity) opposes deflection and ensures the pointer stops at a position proportional to the measured quantity.