AEMT - Association of Electrical and Mechanical Trades

Glossary of Engineering Terms

Glossary Index

Aa | Bb | Cc | Dd | Ee | Ff | Gg | Hh | Ii | Ll | Mm | Nn | Pp | Rr | Ss | Tt | Uu | Vv | Ww | Yy


Actuator: A device that creates mechanical motion by converting various forms of energy to rotating or linear mechanical energy.

Active power (P): Real power in an AC circuit measured in Watts (W).

Adjustable Speed Drive: Output speed of drive can be selected from several preset ranges.

Aging: The reduction of insulation life due to thermal, voltage and other stress.

Air Gap: Short gap in the magnetic material forming a magnetic circuit.

Alternator: Electromechanical device that converts mechanical energy to electrical energy in the form of alternating current.

Ambient Temperature: Temperature of the air in the immediate vicinity of the apparatus or component.

Amortisseur Winding: Embedded windings in the field poles of synchronous machines to aid starting and / or dampen rotor oscillations.

Ampere (A): SI base unit of electric current.

Apparent Power (S): Measured in volt-amperes (VA) in an AC circuit.

Armature: The part of a machine which receives or delivers active power to the external electrical system.

Asynchronous Machine: An AC machine in which the speed on load and the frequency of the system to which it is connected are not in a constant ratio.

American Wire Gauge: Standardised wire gauge system used predominantly in the USA.



Brush: A sliding electrical contact, usually rotating.



Capacitance: The amount of electrical energy stored for a given electric potential.

Chord or Pitch Factor (kp): The factor by which short pitching a coil reduces the back e.m.f.

Cogging Torque: Magnetic interaction between stator and rotor resulting in undesired machine operation such as jerkiness.

Coil: Set of series connected turns.

Coil Pitch or Span: The number of tooth pitches which separate the slots in which the two sides of a coil are placed.

Commutation: Change-over switching in a periodic and automatic manner without interruption of the electric current.

Commutator: An assembly of insulated conducting segments connected to the rotating armature winding.

Concentric Winding: A distributed winding in which the individual coils of each phase group per pole are concentric and have different coil spans.

Conductor: A material which contains movable electrical charges.

Core Loss: The electrical losses in a machine caused by the magnetisation of the core iron.

Critical Speed: The rotating speed at which resonance occurs.



Delta Connection: The 3 phases are connected in series to form a closed circuit.

Direct Current Machine: A machine incorporating an armature winding connected via a commutator to a direct current system.

Distribution Factor (kd): Ratio of the actual voltage of a phase band to the arithmetic sum of the individual coil voltages.

Dhalander Connection: Pole-changing winding giving a rotational speed ratio of 2:1.

Duplex winding: An armature winding in which the coil ends are connected to alternate commutator segments.

Duty Cycle: Specified sequence of operating conditions.



Eddy Currents: Localised magnetically induced currents in an iron core.

Efficiency: Ratio of output power to input power of a device.

Electromotive Force: The force induced in a conductor when it is moved across a magnetic field. (e.m.f.)

Exciter: A source of the power to the field winding of an electrical machine.



Foot-Pound (energy): The energy transferred on applying a force of 1 pound-force (lbf) through a displacement of 1 foot.

Foot-Pound (torque): The torque created by one pond-force (lbf) acting at a perpendicular distance of one foot from a pivot point.

Form Wound Coil: Coil of square or rectangular insulated magnet wire preformed to fit into winding slots.

Form Factor: Ratio of the root-mean-square value of an alternating quantity to its rectified value.



Generator: Rotating electric machine used to transform mechanical energy into electric energy.



Harmonic Content: Difference between alternating quantity and its fundamental component.

Hermetic Machine: Air-tight machine protected from the environment.

Hertz (Hz): SI derived unit for frequency defined as the number of cycles per second of a periodic function.

Horsepower (hp): Unit of power. 1hp = 746W



Impedance: The ratio of the voltage to the electric current, a measure of opposition to time-varying electric current in an electric circuit.

Inductance: Property in an electric circuit where a change in the current induces an electromotive force that opposes the change in current.

Induction Motor: An AC motor where power is supplied to the rotor by magnetic induction.

Induction Generator: An induction machine, connected to a reactive power source, working as a generator.

Insulation: The materials and parts used to insulate conductive elements of a machine.

Insulation Class: Temperature capability of insulation materials and systems.

Inverter: Converter that changes direct electric current to single-phase or poly-phase alternating currents.



Lap Winding: Armature winding with each coil of 2 conductors placed in slots 90° apart, connections made in adjacent segments of a commutator.

Leakage Current: Electric current in an unwanted conductive path.

Loop Test: Test to detect hot spots due to shorted electrical laminations.



Magnetic Flux Density: Strength of a magnetic field.

Magnetomotive Force (m.m.f.): Physical force that produces magnetic flux.

Motor: A machine which converts electrical energy into mechanical energy.

Mush Coil: A wound coil where the conductors occupy random positions in the slot.



Newton: Force required to accelerate a mass of one kilogram at a rate of one meter per second per second.



PAM winding: Pole Amplitude Modulation winding with connections giving speed combinations other than 2:1.

Part Winding Start: Starting with only part of the winding energised to reduce starting current.

Phase Voltage: Voltage between the two terminals of a phase.

Poles: Magnetic poles created in a machine by windings.

Pole Pitch: The distance between two consecutive poles; usually expressed as a number of tooth pitches.

Power: The rate at which work is performed.

Power Factor: Ratio of the absolute value of the active power P to the apparent power S.



Random Wound Coil: The conductors occupy random positions in the slot.

Rated Temperature Rise: The permitted rise in operating temperature above ambient when running on load.

Reactance: The resistance to AC current due to capacitance or inductance.

Reactive Power (Q): Measured in var in an AC circuit.

Regeneration: Operation of a motor in a generator mode to feed energy back into the power supply.

Regulation: Amount of speed or voltage change due to load variations.

Reluctance: The ‘resistance’ of a magnetic circuit.

Resistance: Defined as the ratio of voltage to current.

Rotor: The rotating part of an electrical machine.



Salient Pole: A field pole which projects from the yoke or hub towards the air-gap.

Secondary Winding: A winding in an electrical machine which is not connected to the external electrical system.

Series Excitation: DC machine with field coils connected in series with the armature.

Service Factor: The factor by which the rated power can be increased under specified conditions of service.

Shaft Currents: Induced circulating currents due to machine asymmetries or supply voltage effects.

Shunt Excitation: DC machine with field coils connected in parallel with the armature.

Skew: Non parallel stator and rotor slot alignment.

Slip : The difference between the synchronous speed and the actual speed of a rotor expressed as per unit or as a percentage of the synchronous speed.

Slot: A recess in a core in which the conductors of a winding may be placed.

Squirrel Cage: Rotor winding consisting of a number of conducting bars with their ends connected by conducting rings.

Space Factor: The ratio of the area of bare insulated copper conductors to the available slot area.

Star Delta Starting: A delta wound motor connected in star during starting to reduce inrush current.

Synchronous Machine: An AC machine in which the frequency of the generated voltages and the speed of the machine are in a constant ratio.

Full load Speed: Machine speed at rated load.

Synchronous Speed: The speed of magnetic field rotation determined by the supply frequency and the number of poles.

Stator: The stationary portion of a machine.



Tolerance: Permitted deviation between the declared value of a quantity and the measured value.

Torque: The rotational force produced by a motor.

  • Breakdown Torque: Maximum torque an AC motor will develop.
  • Full Load Torque: The torque the rated load demands on the shaft end of a motor.
  • Starting Torque The torque produced by a motor at initial switch on.


Unbalanced Magnetic Pull: Unequal radial pull on a rotor due to magnetic eccentricities in the air gap.



Variable Speed Drive (VSD): System for altering the rotational speed of an AC motor by controlling the frequency of the electrical supply.

Volt (V): Derived SI unit for electromotive force (e.m.f.)



Watt (W): SI unit of power.

Wave Winding: Armature winding with each coil of 2 conductors placed in slots 90° apart, connections made in segments approximately 180° apart on the commutator.

Winding Factor: The product of the Pitch Factor and the Distribution Factor. (kp x kd)



Yoke: Part of a device, composed of magnetic material and intended to complete a magnetic circuit.