Alternating Current (AC) - The commonly available electric power supplied by an AC generator and is distributed in single or three phase forms. AC current changes its direction of the flow (cycles)

AC Motor - A motor operating on AC current that flows in either induction or synchronous direction (AC current).

Alternator - a synchronous machine used to convert mechanical power into alternating current electric power.

Armature - The portion of the magnetic structure of a DC or universal motor which rotates.

Armature Resistance, Ohms - Armature resistance is measured in ohms at 25 degree Celsius.


Base Line - A vibration reading taken when a machine is in good operating condition that is used as a reference for monitoring and analysis.

Bearings - Are used to reduce friction and wear while supporting rotating elements. For a motor it must provide a relatively rigid support for the output shaft. The bearing acts as the connection point between the rotating and stationary elements of a motor.

Brakes - An external device or accessory that brings a running motor to a standstill and/or holds a load. Can be added to a motor or incorporated.

Braking Torque - The torque required to bring a motor down to a standstill. The term is also used to describe the torque developed by a motor during dynamic braking conditions.

Breakdown Torque - The maximum torque that an AC 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 piece of current conducting material (usually carbon or graphite) which rides directly on the commentator of a committed motor and conducts current from the power supply to the armature windings.


Capacitor - A device which, when connected in an alternating current in an alternating-current circuit, causes the current to lead the voltage in time phase. The peak of the current wave is reached ahead of the peak of the voltage wave. This is the result of the successive storage and discharge of electric energy used in 1 phase motors to start or in 3 phase for power factor correction.

Code Letter - A letter which appears on the nameplates of AC motors to show their locked-rotor kilovolt-amperes per horsepower at rated voltage and frequency.

Conductor - A material, such as cooper or aluminum, which offers low resistance or opposition to the flow of electric current.

Conduit Box - The metal container usually on the side of the motor where the stator (winding) leads are attached to leads going to the power supply.

Coil (Stator or Armature) - The electrical conductors wound into the core slot, electrically insulated from the iron core. These coils are connected into circuits or windings which carry independent current. It is these coils that carry and produce the magnetic field when the current passes through them.

Commutator - A cylindrical device mounted on the armature shaft and consisting for a number of wedge-shaped copper segments arranged around the shaft (insulated from it and each other). The motor brushes ride on the periphery of the commutator and electrically connect and switch the armature coils to the power source.

Corrosion - Corrosion is electromechanical in nature. Common steel will corrode and form rust when it is exposed to moisture. Other metals will corrode in varying degrees upon contact with dissimilar metals under conditions that encourage such disintegration.

Couplings - The mechanical connector joining the motor shaft to the equipment to be driven.

Current - The time rate of flow of electrical charge and is measured in amps (amperes).


Delta Connection - A three-phase winding connection in which the phases are connected in series to form a closed circuit.

Direct Current (DC) - A current that flows only in one direction in an electric circuit. It may be continuous or discontinuous and it may be constant or varying.

DC Motor - A motor using either generated or rectified DC power. A DC motor is usually used when variable speed operation is required.

Drip-Proof Guarded - A drip-proof machine with ventilating openings are so constructed that drops of liquid or solid particles falling on it, at any angle not greater than 15 degrees from the vertical, cannot enter either directly or by striking and running along a horizontal or inwardly inclined surface.


Efficiency - The ratio between useful work performed and the energy expended in producing it. It is the ratio of output power divided by the input power.

Encapsulated Winding - A motor which has its winding structure completely coated with structure completely coated with an insulating resin (such as epoxy). This construction type is designed for exposure to more severe atmospheric conditions than the normal varnished winding.

Enclosures - The housing frame, of the motor of which there are two broad classifications; open and totally closed.

End shield - The part of the motor housing which supports the bearing and acts as a protective guard to the electrical and rotating parts inside the motor. This part is frequently called the "end bracket" or "end bell."

Explosion-Proof Enclosure - A totally enclosed which is constructed to withstand and explosion occur,the enclosure will prevent the ignition or explosion of the gas or vapor which may surround the motor enclosure.

Extrusion - The pushing of metal, usually at high temperature, through a die to form various shapes.


Fatigue - The tendency for a metal to fail structurally due to repeated cyclic stress at considerably less than its yield strength.

Field - A term commonly used to describe the stationary (stator) member of a d-c motor. The field provides the magnetic field with which the mechanically rotating (armature or rotor) member interacts.

Frame - The supporting structure for the stator parts of an a-c motor; in a d-c motors frame usually forms a part of the magnetic coil. The frame also determines mounting dimensions

Frequency - The number of cycles in a time period (usually one second). Alternating current frequency is expressed in cycles per second, termed Hertz (Hz).

Full-Load Current - The current required for any electrical machine to produce its rated output or perform its rated function.

Full-Load Speed - The speed at which any rotating machine produces its rated output.

Full-Load Torque - The torque required to produce rated power at full-load speed.



Hertz (Cycles per Second) - The preferred terminology for cycles per second (frequency). One complete reverse of flow of alternating current per rate of time (a measure of frequency.) 60 Hz (cycles per second) AC power is common throughout the U.S. and 50 Hz is more common in some foreign countries.

Horsepower - A unit for measuring the power of motors or the rate of doing work. One horsepower equals 33,000 foot-pounds of work per minute (550 ft-lbs per second) or 746 watts.

Hot Forging - The shaping of any metal, while hot, by the blow of a hammer.


IEEE - Institute of Electrical and Electronics Engineers

Induction Motor - An induction motor is an alternating current motor in which the primary winding on one member (usually the stator is connected to the power source and a secondary winding or a squirrel-cage secondary winding on the other member (usually the rotor) carries the induced current. There is no physical electrical connection to the secondary winding, its current is induced.

Insulation - Non-conducting materials separating the current-carrying parts of an electric machine from each other or from adjacent conducting material at a different potential.

Insulation Class - A letter or number that designates the temperature rating of an insulation material or system with respect to thermal endurance.



Kilowatt - A unit of electrical power. Also, the output rating of motors manufactured and used off the North American continent.


Load - The burden imposed on a motor by the driven machine. It is often stated as the torque required to overcome the resistance of the machine it drives. Sometimes "load" is synonymous with"required power."

Locked-Rotor Current - Steady-state current taken from the line with the rotor of a motor at standstill and at rated voltage and frequency.

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

Losses - A motor converts electrical energy into a mechanical energy and in so doing, encounters losses. These losses are all the energy that is put into a motor and not transformed to usable power but are converted into heat causing the temperature of the windings and other motor parts to rise.

Lubrication - In order to reduce wear and avoid overheating certain motor components require lubricating (application of an oil or grease). The bearings are the major motor components requiring lubrication (as per manufacturer's instructions).


Megohmmeter - An instrument for measuring insulation resistance.

Megger Test - A measure of an insulation system's resistance. This is usually measured in megohms and tested by passing a high voltage at low current through the motor windings and measuring the resistance of the various insulation systems.

Motor - A rotating machine that converts electrical power (either alternating current or direct current)into mechanical power.


Nameplate - The plate on the outside of the motor describing the motor, hp, voltage, rpm's, efficiency,design, enclosure, etc.

NEMA - National Electrical Manufacturers Association



Part-Winding Starting - A part-winding start three-phase motor is one arranged for starting by first energizing part of its primary winding. The leads are normally numbered 1, 2, 3 (starting) and 7, 8, 9(remaining).

Phase - Indicates the space relationships of winding and changing values of the recurring cycles of A.C. Voltage and currents. Due to the positioning (or the phase relationship) of the windings, the various voltages and currents will not be similar in all aspects at any given instant. Each winding will lead or lag another voltage, in time. Each current will lead or lag another current, in time. The most common power supplies are either single (1) or three phase (with 120 electrical degrees between the 3 phase).

Pound-Foot - Unit of torque, in the English system, that is a force of one pound, applied at a radius of one foot, and in a direction perpendicular to the radius arm.

Power Factor - The ratio of watts to volt-amperes of an AC electric circuit.



Rated Temperature Rise - The permissible rise in temperature above ambient for an electric machine operating under load.

Residual Stresses - Stresses that are set up within a metal as a result of deformation; caused by cold working or drastic temperature gradients.

Resistance - The degree of obstacle presented by a material to the flow of electric current is known as resistance and is measured in ohms.

Resistance Temperature Detector (RTD) - A device used for temperature sensing consisting of a wire coil or deposited film of pure metal for which the change in resistance is a known function of temperature. The most common type is nickel, with other types being copper, platinum, and nickeliron.

Revolutions Per Minute (RPM) - The number of times per minute the shaft of the motor (machine)rotates. This is a function of design and the power supply.

Rotor - The rotating element of any motor or generator.


Season Cracking - Spontaneous failure of some metals by cracking under the combined action of corrosion and residual stresses over time.

Service Factor - A multiplier which, when applied to rated power, indicates a permissible power loading that may be carried under the conditions specified for the service factor.

Shaft - The rotating member of the motor which protrudes past the bearings for attachment to the driven apparatus.

Shear Strength - The maximum shearing stress that a material can develop. In practice, it is considered to be the maximum average stress computed by dividing the ultimate load in the plane of shear by the original area that is subject to shear.

Short- Circuit - A defect in a winding which causes part of the normal electrical circuit to be bypassed.This frequently results in reducing the resistance or impedance to such an extent as to cause overheating of the winding, and subsequent burnout.

Starting Torque - The torque produced by a motor at rest when power is applied. For an AC machine, this is the locked-rotor torque.

Stator - The stationary part of a rotating electric machine. Commonly used to describe the stationary part of an AC machine that contains the primary windings.

Strain - Deformation caused by stress.

Stress - The intensity of force within a body that resists a change in shape. It is measured in pounds per square inch or kilograms per square meter.

Synchronous Motor - A motor which operates at a constant speed up to full load. The rotor speed is equal to the speed. Of the rotating magnetic field of the stator; there is no slip. A synchronous motors is often used where the exact speed of a motor must be maintained.

Synchronous Speed - The speed of rotating magnetic field created by the primary winding of a rotating electric machine. When the speed of the rotating element matches the speed of the rotating magnetic field, it is said to be rotating at synchronous speed. Frequency x 120 Synchronous Speed = Number of Poles


Thermal Conductivity - The capability of conducting heat; measured by the quantity of heat that passes in unit time through a unit area of plate whose thickness is unity, when its opposite faces differ in temperature by one degree.

Tolerance - The amount by which any characteristics may vary from that specified.

Torque - The rotating force produced by a motor. The units of torque may be expressed as poundfoot, pound-inch (English system) or Newton-meter (metric system).

Transformer - A device which converts electrical power (alternating current) to electrical power of a different voltage. In this device both primary and secondary windings are usually stationary, and are wound on a common magnetic core.



Variable Torque Motor - A multispeed motor in which the rated horsepower varies as the square of the synchronous speeds.

Voltage - The force that causes a current to flow in an electrical circuit. Analogous to pressure in hydraulics, voltage is often referred to as electrical pressure. The voltage of a motor is usually determined by the supply to which it is being attached.


Watt - The amount of power required to maintain a current of one ampere at a pressure of one volt.Most motors are rated in kilowatt (Kw) equal to 1,000 watts. One horsepower is equal to 746 watts.

Welding - The process of producing localized coalescence (growing together into one body) of metal by heating to suitable temperatures, with or without the application of pressure, and with or without use of filler (above 840 degrees F or 450 degrees C) material.

Wye Connection - A three-phased winding connection formed by joining one end of each phase to make a "Y" point. The other ends of each phase are connected to the line. Also termed a star connection.

Wye-Delta Starting - Wye-delta is a connection which is used to reduce the inrush current and torque of a three-phase motor. A wye (star) start, delta run motor is one arranged for starting by connecting to the line with the winding initially connected wye (start). The winding is then reconnected to run in delta after a predetermined time. The lead numbers for a single run voltage are normally 1, 2, 3, 4, 5 and 6.



Yield Strength - In many materials a point reached on the stress-strain diagram at which there is marked increase in strain or elongation without an increase in stress or load. The point at which this occurs is termed the yield point. It is usually quite noticeable in ductile materials but may be scarcely perceptible or possibly not present at all in certain hard-drawn materials such as hard drawn copper.