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Alternating Current (AC)

A current flow which increases to a maximum in one direction, decreases to zero, then reverses direction and increases to a maximum in the opposite direction; this process is repeated continuously. This is opposed to direct current (DC), which flows in one direction continuously.


A rotating machine for producing an alternating voltage and hence an alternating current (AC).

Ambient Temperature

The temperature of the environment of a given location.

  • Indoors, ambient temperature can be taken as room temperature.
  • In instrument work, Standard Temperature and Pressure (STP) is taken as 25°C.

An instrument for measuring the current (in amperes) flowing in a circuit.


A transformer with only one winding, in which part of the winding is common to both the primary and secondary.

Air-Core Inductor

An inductor wound in the form of a coil with air or vacuum as the core.


Circuit Diagram

A diagram in which lines are used to represent the circuit conductors, and standard symbols are used to represent the circuit components.


The transmission of energy through a conductor: electric current through an electrical conductor or heat energy through a thermal conductor.


An electrically operated switch by which one circuit can be opened or closed by the opening or closing of an independent circuit, the current in the controlling circuit usually being much smaller than the current in the controlled circuit. Contactors are relays designed to carry larger currents.


The action of removing excess heat from a machine or process.

Copper Losses

The power losses (as heat) in copper conductors due to current flow through their resistance. The copper loss is equal to I2R. Copper losses are also called I2R losses.

  1. The connection of two or more circuits so that energy may be transferred from one to another, e.g. magnetic or capacitive coupling.
  2. The connection between the shaft of a motor and the input shaft of a machine.


Direct Current (DC)

A current that flows in one direction only; it never changes its polarity.

Distribution Transformer

A transformer for stepping down high transmission voltages to voltages suitable for use with household electrical appliances and equipment, e.g. a transformer for stepping down from 11 kV to 400/230 V.


Eddy Current

A circulating current induced within the core of a conducting material by a varying magnetic field.

Electrical Intensity (EI)

The intensity of an electric current (measured in amperes) flowing in a circuit.

Electromagnetic Chuck

A chuck that uses the holding power of electromagnets to retain magnetic materials firmly in position during a machining process.

Electromagnetic Force (EMF)

The force that moves electrons: an electrostatic force resulting from a difference in potential that causes a flow of electrons from the negative potential to the positive potential.


The process of producing a magnetic field by passing a current through the windings of an electric machine.


Flux Leakage

The flux set up in a magnetic circuit which does not follow the desired path and returns to the source via the surrounding air.


The rate at which an instance is repeated, e.g. the number of cycles per second of an alternating waveform; 1 cycle per second is called 1 hertz (Hz).


A protective control device that breaks a circuit when the circuit current exceeds the value for which it is designed.



A machine or device that converts one form of energy to another, e.g. an electrical generator converts mechanical energy into electrical energy.



Lagging behind; the time delay between a change and the effect that it causes:

  1. The delay between the change to a system and the resulting output
  2. The amount by which the magnetic flux density lags behind the magnetizing force in a magnetic circuit or material.



The opposition to current flow in an AC circuit.


The setting up of an electric or magnetic field by proximity to another electric or magnetic field.

In Series

The linear connection of circuit components (one after the other) so that there is only a single path for the current through the circuit.

Insulation Resistance

The resistance between live conductors, or between live conductors and earth, of an electrical circuit or electrical equipment.

Iron Losses

The losses in the iron core of an electrical machine due to eddy currents and magnetic hysteresis.



A single sheet of steel used in layers to build up the laminated core of an electrical machine.

Lenz's Law

States that the direction of an induced EMF is such that the resulting current flow will produce a magnetic field that tends to oppose the original motion causing the induced EMF.


Magnetic Circuit

The path of the magnetic flux as it progresses from the north pole to the south pole of a magnetic source.

Magnetic Flux

The magnetic lines of force established by a magnetic circuit or magnet.

Magnetic Fringing

The tendency of magnetic flux to spread out when crossing an air gap, causing the flux density in the air gap to be less than in the magnetic material on either side of the air gap.

Magnetic Leakage

The flux set up in a magnetic circuit which does not follow the desired path and returns to the source via the surrounding air.

Magnetic Losses

The magnetic flux set up in a magnetic circuit which does not link with the desired elements in the circuit.


The Megger brand insulation resistance tester was originally manufactured by Evershed & Vignoles Limited in the UK. Because of its widespread use, the term ‘megger’ is taken to mean both:

  1. An insulation resistance tester
  2. To test the insulation resistance of an electrical circuit or appliance.



A meter for measuring the resistance of a circuit (in ohms).

Ohm's Law (V = IR)

Named after George Ohm, who first postulated the theory, Ohm’s Law states: The current flowing between any two points in an electric circuit is directly proportional to the potential difference between these points, and also inversely proportional to the resistance of the circuit between these points.

Open Circuit

An electric circuit in which the current is prevented from flowing by a break in the circuit.


Permanent Magnet

A magnet made of a material that enables it to maintain its magnetic properties over an extended period of time.

Phase Angle
  1. The angular displacement, in electrical degrees, between two waveforms of the same frequency.
  2. The angle, in electrical degrees, between two phasors on a phasor diagram.

A condition of having two opposing poles, such as north and south for a magnetic circuit or positive and negative for an electric circuit.

Power Factor
  1. The factor by which the apparent power in an AC circuit must be multiplied to obtain the true power consumed by the circuit.
  2. The ratio of the true power to the apparent power in an AC circuit.


Short Circuit

A fault of negligible impedance between two points in a circuit, resulting in excessive current flow between the two points; the excessive current usually causes damage to the circuit.

Single-phase Transformer

A transformer designed to operate from a single-phase supply.


An electromagnet, usually in coil form, with an armature or plunger that moves when the electromagnet is energized.


Three-Phase Motor

An electric motor designed to be connected to a three-phase AC supply.

Three-Phase Transformer

A transformer designed to be connected to a three-phase AC supply.


A force that tends to produce rotational motion. Torque is measured in newton meters (Nm).

  1. An electrical device for changing the value of an alternating voltage.
  2. A step-up transformer has a larger output voltage than the input voltage, while a step-down transformer has a lower output voltage than the input voltage.
Transformer Losses

The iron and copper losses in a transformer. The iron losses are due to eddy currents and magnetic hysteresis, while the copper losses are due to the heating effect of an electric current (H = I2Rt).



An autotransformer that has a toroidal winding and an adjustable carbon brush so that the value of the output voltage can be adjusted to any value between zero and the input voltage.

  1. The electrical force that causes current to flow in a circuit.
  2. The general term for electromotive force.

An instrument for measuring the value of electromotive force in volts.



An instrument for measuring the true power consumed by a circuit.

Wheatstone Bridge

A null-type measuring circuit in which a resistance is measured by comparison with a standard resistance value.


The losses in an electrical machine due to the energy consumed by the rotation of the cooling fans and other rotating components.


A number of turns of a conductor, wound around a core or former for the purpose of producing a magnetic field in an electrical machine such as a transformer, motor or alternator.

Transformer Applications



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