Electro mechanical engineering glossary


A range of key electro mechanical engineering terms are defined in our electro mechanical engineering glossary of terms and phrases.


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. Commonly known as room temperature.


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.



Baseline: A measurement taken when a motor is in good operating condition that is used as a reference for monitoring and analysis.

Breakdown torque: The maximum torque that a motor will develop with rated voltage applied at rated frequency without an abrupt drop in speed; also termed pull-out torque or maximum torque.

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.




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


Efficiency: The ratio between useful work performed and the energy expended in producing it. 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.


Full load speed: The speed at which any motor produces its rated output.


Full load torque: The torque required to produce rated power at full load speed.



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): The preferred terminology for cycles per second (frequency)


Horsepower (hp): Unit of power. 1hp = 746W. Also, the output rating of motors manufactured off the North American continent.



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: Nonconducting materials that separate the current carrying parts of a motor from each other or from adjacent conducting material at a different potential.


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.


Locked-rotor current: Steady-state current taken from the line with the rotor at a standstill and at a rated voltage and frequency.


Locked-rotor torque: The minimum torque that a motor will develop at a standstill for all angular positions on the rotor with rated voltage applied at rated frequency.


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




Magnetic Flux Density: Strength of a magnetic field.


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.



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 set up inside a motor by the placement and connection of the 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.


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. Commonly used to describe the stationary part of a motor that contains the power windings.



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


Torque: The rotational force produced by a motor. The units of torque are expressed as pound-foot, pound-inch or newton-meter.



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.