# Magnetic Principles in Electrical Measurement Instruments

The article covers the application of magnetic principles in various electrical measurement instruments used by electricians. It provides an overview of moving coil meters, moving iron meters, dynamometer meter movements, voltage testers, and current testers, including multifunction clamp meters, highlighting their operational principles and practical applications in monitoring and measuring electrical parameters.

The skills and knowledge associated with testing and using test equipment are beneficial to apprentices and qualified electricians alike. The following information provides a foundational awareness of test instruments and equipment in the context of the application of magnetic principles.

Moving Coil Meters

Figure 1 is an exploded view of a moving coil movement. It can be seen that a coil that is free to rotate is suspended in the field of a permanent magnet. The coil ends are connected to a suspension system so that current can be passed through the coil.

### Figure 1. Exploded view of a moving coil movement

The current is governed by the value of the applied voltage. The coil sets up its own magnetic field. This field reacts with that of the permanent magnet and causes the coil to rotate. A pointer attached to the coil gives a voltage reading against a scale.

The meter movement can only work satisfactorily on direct current. If AC is applied to the movement, it tries to turn the coil rapidly in opposite directions, with the result that the coil effectively remains stationary.

The meter can only operate on AC if the AC is rectified to DC before it flows through the meter. Because of these factors, a moving coil meter always reads average values of current and voltage.

## Moving Iron Meters

Figure 2(a) is an exploded view of a moving iron meter to illustrate its operating principle. In practice, the construction is slightly different and is shown in Figure 2(b).

### Figure 2. Moving iron meter labeled diagram

There are two magnetically soft iron vanes in the movement. One vane is fixed, and the other is pivoted and free to rotate. A pointer attached to the moving vane moves across a scale as an indicator.

When an electric current is passed through the coil, both the fixed and the moving vanes are magnetized and have like poles at adjacent ends. As like poles repel each other, the movable vane moves away from the fixed vane. The attached pointer then indicates a value against a calibrated scale. A restraining spring provides opposing torque so that the vane movement can be stabilized. It should be noted that the scale of a moving iron meter is non-linear.

## Dynamometer Meter Movement

An exploded view of a dynamometer movement is shown in Figure 3(a). The meter has two circuits—one for voltage, the other for current. The model illustrated has a soft iron core, around which is wound a low-resistance coil to carry the circuit current. This coil produces a magnetic flux proportional to the current flowing in a circuit. Not all dynamometer-movement meters have an iron core; some are air cored.

### Figure 3. A dynamometer movement labeled diagram

The meter’s second circuit consists of a coil with a series resistance of high value. This is the voltage circuit, and it produces a magnetic field proportional to the applied voltage.

The direct multiplication of voltage and current values in an AC circuit to obtain a power value can at best be only an approximation. With some electrical components, the voltage and current can be out of step with each other. This type of meter construction, with its two magnetic fields, takes into account any displacement between voltage and current and gives a ‘true power’ reading.

## Voltage Testers

These testers can detect either ‘moving charges’ (magnetic field indicator) or electric fields associated with AC voltages. Figure 4 shows the electric field and magnetic field. There are two types of voltage tester, capacitive coupled and inductively coupled. The inductive coupled tester has a sensor winding in its tip. A voltage is induced within the winding in the presence of an electromagnetic field. This type of tester works for wires in operational conditions such as circuits under load. They will not detect voltage or live conductors when there is no current flow (‘moving charge’ is needed to produce a magnetic field).

Figure 4. Electric and magnetic fields testing diagram

Voltage is detected by a ‘capacitive coupled’ device that is sensitive to the electric fields produced by the circuit voltage. It is the basis of the ‘finder’ used to locate live conductors buried in walls up to a depth of 2.5 cm. Most units give both audible and visual signals.

## Supply Current and Voltage—Monitoring Instruments

Some meters for testing voltage and current are designed to make no electrical contact with the circuit under test. The majority of these (for AC) work on the basis of mutual induction. Several types are available, either as fixed monitoring instruments or as portable test equipment used by electricians. They essentially operate on a similar principle to transformers.

Instrument transformers are used in electrical installations for the safe monitoring of supply voltage and current. Voltage or potential transformers (VTs or PTs) step down the supply voltage to a lower, safer level for monitoring (typically 110 V). Current transformers (CTs) step down the current to either 1 amp or 5 amps. This transformation ratio is determined by the number of turns of the primary and secondary windings.

Current Testers

Current testers are marketed under various names, most of which are trade related—‘tong testers’, ‘clamp meters, ‘clip-on testers’, ‘link test meters’ and so on. Clamp action meters are generally used to measure current without having to interrupt the circuit being tested.

The jaws of the instrument are opened with a lever and then placed around the chosen conductor. They are then allowed to close. The magnetic field around the conductor enters the low-permeability path of the iron, and the meter movement responds according to the strength of that magnetic field.

Originally there were only two types of current tester. One worked on the repulsion principle of the moving iron meter while the other used a transformer combined with a switch to select the desired current ranges.

## Repulsion-Type Movement (AC And DC)

The operating principle was that of the moving iron meter. Plug-in modules catered for different current ranges. The magnetic field created by the current produced a repulsion between the meter elements and caused the moving section with the pointer attached to rotate. They were capable of use on both AC and DC.

## Transformer Operated (AC Only)

Different current ranges were catered for by using a transformer with tappings connected to a range switch. The transformer prevented it from being used on DC. The basic principle is shown in Figure 5.

### Figure 5. Internal circuit arrangement diagram of a current transformer

The indicating meter could be a DC-operated one if a rectifying unit was connected between the transformer output and the meter movement. With a DC meter, the scale then became linear (with a moving iron meter, the scale was non-linear).

## Multifunction Clamp Meters

The repulsion- and transformer-type current testers described above have been superseded by technological innovations and improvements incorporated into multifunction clamp meters. Three types that are now available are:

1. Current transformer clamp meters—only measure alternating current.
2. Flexible clamp meters (Rogowski coil)—only measure alternating current.
3. Hall-effect clamp meters—measure both alternating and direct current.

CTs, PTs and AC clamp meters work on a similar principle to that of transformers. A cable under test acts as the primary winding while the clamp meter jaws act as a secondary. The jaws are made of ferrite iron and are individually wrapped by coils of wire. The jaw tips are flush when closed (as shown in Figure 6(b)), but when open reveal bare metal core faces. The jaws form a magnetic core during measurements.

### Figure 6. Transformer principle for AC clamp meters labeled diagram

Flexible clamp meters which also only measure AC operating on the Rogowski coil principle do not have an iron core. They work on the principle of an air gap between coil windings for measurements.

Rogowski coils are sometimes called ‘air-cored coils’ or ‘flexible current probes’ because they use a wound, helix-shaped coil for measuring the rate of change of a conductor’s magnetic field.

## Hall-Effect Sensor

The Hall-effect sensor is a transducer which converts the changing magnetic field of the conductor to a voltage proportional to the magnetic flux in the jaw tip’s air gap. These meters have a Hall-effect semiconductor encased by a thin plastic molding where the jaws meet. This establishes an air gap which the magnetic flux field passes through, completing the magnetic circuit. This concept of an air gap and iron core construction is shown in Figure 7(a). The iron core jaws themselves permit the magnetic flux to pass through more easily than air. The air gap ensures that the core is not saturated by limiting the magnetic flux.

### Figure 7. Clamp meter jaw construction labeled diagram

Figure 7(b) shows the physical jaw construction for an AC clamp meter (on the left) and a DC clamp meter (on the right).

Hall-effect clamp meters are used for measuring DC current in electrical cables and conductors. Current flow in a conductor produces a magnetic field, and Hall elements respond to the field flux created by producing a voltage proportional to the current in the conductor under test. If the cable current is DC, the magnetic field will be steady and the Hall voltage constant. When the cable current is AC, the flux field will vary, and the Hall voltage will also vary to match the field changes.

Key Takeaways

Understanding the application of magnetic principles in measurement instruments is crucial for both apprentices and qualified electricians, as these instruments are fundamental tools for accurately monitoring and diagnosing electrical systems. Moving coil meters, moving iron meters, and dynamometer meters provide precise readings of voltage, current, and power, essential for ensuring the safety and efficiency of electrical installations. Voltage testers and current testers, including advanced multifunction clamp meters, offer reliable methods to detect and measure electrical parameters without direct contact, enhancing safety and convenience.