Measurement & Electrical Quantities - Electrical and Electronic Measurements

Electrical Instruments and Measurement

Instruments used to measure electrical quantities such as current, voltage, power, energy, flux and frequency are collectively called electrical measuring instruments. These instruments convert an electrical quantity into a readable indication and are designed to give measurements with required accuracy for laboratory, industrial or field use.

Classification of Electrical Instruments

Electrical instruments may be classified on several bases. The following classification covers commonly used categories and examples.

• Absolute instrument: An instrument that measures a quantity in terms of fundamental physical laws and dimensions, so its readings can be calculated from first principles. Examples: tangent galvanometer, Rayleigh's current balance.
• Secondary instrument: An instrument that gives a direct reading of the quantity under measurement, often calibrated against a standard. Examples: voltmeter, thermometer, pressure gauge.
• Analog instrument: An instrument whose output (pointer position, deflection) varies continuously with the input quantity while maintaining a fixed relationship. Subtypes include:
  • Deflecting instrument: Indicates the measured quantity by deflection of a pointer away from zero. Example: galvanometer, PMMC instruments used as ammeters and voltmeters.
  • Indicating instrument: Gives the instantaneous value of the electrical parameter. Examples: ammeter, voltmeter, wattmeter, galvanometer.
  • Integrating instrument: Measures the total quantity accumulated over time. Example: energy meter.
  • Recording instrument: Records variations of the measured quantity over time. Example: recording voltmeter.
• Null-deflection instrument: Indicates balance by a zero deflection. Examples: bridge circuits, DC potentiometer. These instruments are highly accurate because at balance the operating power absorbed by the measurement device is zero.

Key points

• PMMC (permanent magnet moving coil) instruments are used only for DC voltage and current measurement.
• Induction type instruments are suitable only for AC measurement.
• Electrodynamometer type instruments can be used for both AC and DC measurements.

Principles of Operation of Analog Instruments

The principal physical effects used in electrical instruments are:

• Electromagnetic effect - moving-iron, moving-coil (PMMC), electrodynamometer instruments.
• Induction effect - induction energy meters.
• Heating effect - hot-wire instruments, thermocouple and bolometer types.
• Electrostatic effect - electrostatic voltmeters and instruments for high voltage.
• Hall effect - used in fluxmeters and some wattmeter designs.

Essentials of an Indicating Instrument

An indicating instrument that uses a moving system (pointer and coil or vane) requires the following torques or forces to function correctly:

• Deflecting torque or force: Produced by the electrical action (for example a current interacting with a magnetic field). Deflecting torque is proportional to the measured quantity and causes the pointer to move from zero.
• Controlling torque or force (T_C): Restoring torque that opposes deflection and brings the pointer to a steady reading. It returns the pointer to zero when the measured quantity is removed. If no control torque exists the pointer would deflect beyond the scale limits.
• Damping torque or force (T_D): Reduces oscillations of the moving system so the pointer settles quickly to its steady value.
• Moving system supports and construction: The moving system produces the deflecting torque. Supports include suspension strips, taut suspension, and pivot-and-jewel bearings. The design should provide a high torque-to-weight ratio for sensitivity; a typical desirable torque-to-weight ratio for sensitive instruments is about 0.1.

Support and Suspension

• Suspension is commonly used in galvanometer-class instruments and in vertically mounted instruments.
• Taut suspension is used in PMMC and other sensitive instruments where low friction and high sensitivity are required.
• Pivot and jewel bearings are used where the weight of the moving system is a design factor for sensitivity.

Control Systems

Two common controlling mechanisms provide the restoring torque for the moving system: spring control and gravity control.

Spring Control Mechanism

In spring control, the control torque is proportional to the angular deflection θ and is given by

T_CS = k θ

where k is the spring constant and θ is the angle of deflection. At steady state the deflecting torque equals the control torque. Spring control provides an approximately uniform scale.

For a rectangular spring strip: l = length of strip, b = width, t = thickness, E = Young's modulus. The spring's maximum fibre stress can be calculated from beam theory (illustrations often accompany practical instrument design references).

Gravity Control Mechanism

In gravity control the restoring torque varies with the sine of the deflection angle:

T_cg = K sin θ

At steady state the deflecting torque equals the gravity control torque. Gravity control instruments typically have a non-uniform scale (initially compressed near zero).

Gravity Control Mechanism

Note: If damping torque T_D is absent the pointer will oscillate around the mean position. Spring fatigue is prevented by annealing and ageing treatments of the metal springs.

Damping Systems

Damping allows the moving system to reach its final steady position rapidly without excessive oscillation. Common damping methods are:

• Eddy current damping: Used where there is a strong magnetic field, for example in PMMC instruments. A conductor moving in a magnetic field produces eddy currents which oppose motion and provide damping.
• Air friction damping: Used when the instrument's operating field is weak, typical for moving-iron and electrodynamometer types.
• Fluid friction damping: Used for some high-voltage or vertically mounted instruments (for instance in some electrostatic instruments).

Measuring Current - Ammeters

An ammeter measures current. To measure current at a point in a circuit the circuit must be opened there and the ammeter inserted so that it is in series with the circuit element whose current is to be measured. For accurate measurement the ammeter must have a very low internal resistance so it does not significantly change the circuit current.

Ammeters have a very low internal resistance, and must always be connected in series in a circuit.

Because ammeters have small internal resistance they must never be connected in parallel across a source or component; this would create a short-circuit through the instrument and could cause damage.

Measuring Voltage - Voltmeters

To measure potential difference (voltage) between two points, a voltmeter is connected across those two points; that is, in parallel with the part of the circuit under test. A voltmeter must draw as little current as possible to avoid the loading effect and so must have high internal resistance (typically many megohms for sensitive instruments).

Voltmeters must always be connected in parallel in a circuit, and have a very high internal resistance.

Types of Instruments Used as Ammeters and Voltmeters

• PMMC (Permanent Magnet Moving Coil) - only for DC measurement.
• Moving-iron type - suitable for both AC and DC measurement.
• Electrodynamometer type - can be used for both AC and DC.
• Electro-thermic type - includes hot-wire, thermocouple and bolometer types; used for both AC and DC.
• Induction type - used only for AC measurement (for example induction instruments and some watt-hour meters).
• Electrostatic type - used for both AC and DC high-voltage measurement.
• Rectifier type - converts AC to DC for measurement and may be used for both AC and DC with appropriate design.

Permanent Magnet Moving Coil (PMMC) Instruments

PMMC instruments are widely used for DC measurement. Typical permanent magnet materials are Alnico (Alloy of aluminium, nickel and cobalt) and Alcomax. The field strength in PMMC instruments typically ranges from about 0.1 Wb/m² to 1 Wb/m².

Instrument for PMMC

• Concentric magnetic construction is used to obtain long angular movement of the pointer and a linear scale.
• Due to the strong operating magnetic field, eddy current damping is commonly used in PMMC instruments.
• The control torque is provided by spiral-shaped hair springs made of phosphor-bronze; these springs also serve as the electrical leads to the moving coil.
• Deflecting torque in a PMMC is proportional to the product of coil turns, flux density, coil area and current and is given by:

  T_d = N B A I

  Controlling torque: T_C = k θ

  At balance: N B A I = k θ so θ ∝ I and the scale is uniform.

In these expressions: N = number of turns of the moving coil; B = flux density of the permanent magnet in Wb/m²; A = area of the coil in m²; I = current in amp; G = NBA is the displacement constant of the galvanometer or movement.

Key points for PMMC

• The control springs serve dual functions: they provide the controlling torque and also carry current into the moving coil.
• Strong operating magnetic field minimises the effect of external magnetic fields; magnetic shielding is normally not required.

Advantages and Disadvantages

• Advantages: High torque-to-weight ratio, high accuracy and sensitivity, low power consumption (typically 25-200 μW), and good linearity.
• Disadvantages: Relatively high cost, suitable only for DC measurement, and limited current-carrying capacity (practically up to about 100 mA for direct movements without external shunts).

Sources of Error in PMMC Instruments

• Ageing of the permanent magnet (may be mitigated by pre-ageing or magnetic stabilisation techniques).
• Ageing or creep of the control spring.
• Temperature effects on the coil resistance and on spring properties.

Applications of PMMC and Shunt Arrangements

To extend the range of a PMMC movement (which is intended for small currents), a shunt resistance is placed in parallel with the movement so that most of the current bypasses the movement. The basic relation for currents and resistances in an ammeter with shunt is:

I_sh R_sh = I_m R_m

where I is the total current, I_m is the full-scale current through the movement, R_m is the movement internal resistance and R_sh is the shunt resistance.

Basic ammeter circuit

If m = I / I_m is the multiplying factor, the shunt resistance can be written in the commonly used form:

R_sh = R_m / (m - 1)

where m > 1. The values of R_sh are chosen so that the movement sees only the full-scale current I_m while the required higher current I flows through the instrument as a whole.

Key points for shunt design

• Shunt material should have a small and constant temperature coefficient so that its resistance does not vary significantly with temperature.
• Magnin alloy is often used for shunts paired with copper to give low thermal emf; Constantan is commonly used in AC meter shunts.

Temperature's effect in ammeter reading

Effect of Temperature Change on Ammeter Reading

As temperature increases the resistance of copper (and the movement coil) increases, causing a change in instrument readings. To reduce temperature effects a low temperature coefficient resistance called a swamping resistance (made of Magnin or similar alloy) is connected in series with the coil to stabilise the effective resistance seen by the circuit.

Multirange Ammeters

Multirange ammeters allow a single movement to measure several ranges of current by using different shunt arrangements.

• Using separate shunts: Several fixed shunts are provided and switched in parallel with the movement to obtain different measurement ranges I₁, I₂, I₃, etc. The appropriate shunt is placed in circuit to select the desired range.

Circuit for three shunt

• Using a universal (Ayrton) shunt: The Ayrton or universal shunt arrangement provides multiple ranges without ever leaving the movement unshunted. Its advantage is that the movement cannot be accidentally placed in circuit without a shunt, improving safety and protecting the instrument.

Aytron shunt circuit

Final Notes and Practical Considerations

• Instrument selection must match the type of quantity (AC or DC), expected magnitude, required accuracy and environmental conditions (temperature, magnetic fields, vibration).
• Regular calibration and maintenance (inspection of springs, bearings, magnet condition and electrical connections) are required to preserve accuracy.
• Design features such as damping, proper suspension, spring treatment and choice of shunt material are critical to performance and long-term stability.

Basic ammeter circuit

Temperature's effect in ammeter reading

Circuit for three shunt

Aytron shunt circuit