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Energy Efficiency Reference Guide Electric Motors

13. Glossary

Air Gap

The space between the rotating (rotor) and stationary (stator) member in an electric motor.


The atmospheric altitude (height above sea level) at which the motor will be operating; NEMA standards call for an altitude not exceeding 3,300 ft. (1,000 meters). As the altitude increases above 3,300 ft. and the air density decreases, the air’s ability to cool the motor decreases - for higher altitudes higher grades of insulation or a motor derating are required. DC motors require special brushes for high altitudes (Ref. 18).

Ambient Temperature 

The temperature of the surrounding cooling medium, such as gas or liquid, which comes into contact with the heated parts of the motor. The cooling medium is usually the air surrounding the motor. The standard NEMA rating for ambient temperature is not to exceed 40oC.

Anti-Friction Bearing

An anti-friction bearing is a bearing utilizing rolling elements between the stationary and rotating assemblies


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

Breakdown Torque

The maximum torque a motor will develop at rated voltage without a relatively abrupt drop or loss in speed.


A piece of current conducting material (usually carbon or graphite) which rides directly on the commutator of a commutated motor and conducts current from the power supply to the armature windings.


Capacitance is that property of a system of dielectrics and conductors that allows for the storage of electrically separated charges when a potential difference exists between the conductors.  A capacitor does not dissipate real energy (watts).


A cylindrical device mounted on the armature shaft and consisting of 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.

Constant Horsepower

Describes a load which decreases with increasing speed. Common applications are variable speed processes that are changing diameters such as lathes, winders, unwinders, and metal-cutting tools. With an initial large diameter work piece, maximum torque and slow speeds are required.

Constant Torque

Describes a load where the torque required is constant throughout the speed range e.g. a friction load. Friction loads require the same amount of torque at low speeds as at high speeds. The horsepower requirement, however, increases with speed. Common applications include general machinery, hoists, conveyors, etc.


Canadian Standards Association

Drip-Proof Motor

An open motor in which the 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.

Duty Cycle

The relationship between the operating and rest times or repeatable operation at different loads. A motor which can continue to operate within the temperature limits of its insulation system, after it has reached normal operating (equilibrium) temperature is considered to have a continuous duty (CONT.) rating. One which never reaches equilibrium temperature, but is permitted to cool down between operations is operating under intermittent duty (INT.) conditions such as a crane and hoist motor which are often rated 15 or 30 min. duty.

Eddy Currents

Localized currents induced in an iron core by alternating magnetic flux. These currents translate into losses (heat) and their minimization is an important factor in lamination design.


The efficiency of a motor is the ratio of mechanical output to electrical input. It represents the effectiveness with which the motor converts electrical energy into mechanical energy.

Fractional-Horsepower Motor

A motor usually built in a frame smaller than that having a continuous rating of one horsepower, open construction, at 1700 -1800 rpm.

Full-Load Current

The current flowing through the line when the motor is operating at full-load torque and full-load speed with rated frequency and voltage applied to the motor terminals.

Full-Load Torque

That torque of a motor necessary to produce its rated horsepower at full-load speed, sometimes referred to as running torque.

Hertz (Hz)

One cycle per second (as in 60 Hz. which is 60 cycles per second).


The measure of rate of work. One horsepower is equivalent to lifting 33,000 pounds to a height of one foot in one minute. The horsepower of a motor is expressed as a function of torque and rpm.
Equation 22
HP = horsepower,
T = torque (in. lb.ft.), and
RPM = revolutions per minute.


A lagging of the resulting magnetization in a ferromagnetic material caused by a changing magnetic field.

Hysterisis Loss

The resistance to becoming magnetized (magnetic orientation of molecular structure) offered by materials results in energy being expended and corresponding loss. Hysterisis loss in a magnetic circuit is the energy expended to magnetize and demagnetize the core.


International Electrotechnical Commission


Institute of  Electrical and Electronic Engineers


Propensity of a circuit or device to impede the flow of current.
The real part of impedance is the resistance, and the imaginary part is the reactance.


Represents the propensity of a conductor to store energy in an associated magnetic field.
Opposes the change of alternating current, but does not oppose the flow of steady current, such as direct current.
Can be thought of as electrical inertia.

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.

Inertial Load

A load (flywheel, fan, etc.) which tends to cause the motor shaft to continue to rotate after the power has been removed (stored kinetic energy). If this continued rotation cannot be tolerated, some mechanical or electrical braking is needed.  This application may require a special motor due to the energy required to accelerate the inertia.

Insulation Class

Since there are various ambient temperature conditions a motor might see and different temperature ranges within which motors run and insulation is sensitive to temperature; motor insulation is classified by the temperature ranges at which it can operate for a sustained period of time.


An electronic device that converts fixed frequency and voltage to variable frequency and voltage.


Japanese Electrotechnical Committee


The steel portion of the rotor and stator cores made up of a series of thin laminations (sheets) which are stacked and fastened together by cleats, rivets or welds. Laminations are used instead of a solid piece in order to reduce eddy-current losses.

Magnetic Field

The portion of space near a current-carrying body or a magnetic body in which a voltage can be induced in a second current carrying body when the state of changes or when the second current-carrying body moves in prescribed ways relative to the medium.

Magnetic Flux

The integral over a specified surface of the component of magnetic induction perpendicular to the surface.

Magnetic Pole

The portions of a magnet which appear to generate or absorb the flow of the external magnetic induction.


National Electrical Manufacturers Association is a non-profit organization organized and supported by manufacturers of electric equipment and supplies. NEMA has set standards on:  HP ratings, Speeds, frame sizes and dimensions, standard voltages and frequencies with allowable variations, service factors, torques, starting current & kVA, enclosures

Power Factor

The ration of total watts to the total root mean square (RMS) volt-Amperes
Equation 23Equation 24


Pulse Width Modulation – when applied to a variable speed drive, the inverter adjusts both the width of the output voltage pulses as well as the frequency to improve the sinusoidal shape of the output voltage waveform.


The opposition to the flow of alternating current by the inductance or capacitance of a component or circuit.
Reactance is inductive if the imaginary part of the impedance is positive.
Reactance is capacitive if the imaginary part of impedance is negative.


A rectifier is a device which may be used to convert alternating current to direct current (by conducting current easily in one direction and negligibly in the opposite direction).


A physical property of a circuit that impedes the flow of current.  For AC circuits, resistance limits the current which is in phase with the voltage. When current flows through a resistance, a voltage drop develops across the resistance (Ohm’s Law).
It is the real part of impedance and usually represents the conversion of electrical energy into heat.


The capacity to retain magnetism after the magnetizing action has ceased.

Service Factor

When used on a motor nameplate, a number which indicates how much above the nameplate rating a motor can be loaded without causing serious degradation, (i.e., a 1.15 S-F can produce 15% greater torque than the 1.0 S-F rating of the same motor).


The ration of the difference between the synchronous speed and the actual speed of the rotor to the synchronous speed of the rotor.

Slip Ring

Continuous conducting rings on the rotor from which brushes conduct current into or out of the motor.


ANSI - American National Standards Institute
BASEEFA - British Approval Service for Electrical Equipment in Flammable Atmospheres
CE - Compliance to European Standards
CSA - Canadian Standards Association
EPACT - 1997 U.S. Energy Policy Act
IEC - International Electrotechnical Commission
IEEE - Institute of Electrical and Electronics Engineers
ISO - International Standards organization
MIL - Military Specifications
MSHA - U.S. Mining, Safety, Health Administration
NAFTA - North American Free Trade Agreement
NEC - National Electric Code
NEMA - National Electrical Manufacturers Association
UL - Underwriter's Laboratories

Synchronous Speed

The speed of the rotation of the magnetic flux produced by the stator windings.
The lines of force which represent magnetic induction.

Temperature Rise

Each NEMA temperature code has an associated temperature rise which when added to the ambient and hot spot should not exceed the temperature handing of the insulation system


Underwriter’s Laboratories (the UL approval mark with a subscripted letter “C” indicates the device has been tested by UL to Canadian Standards).

Variable Torque

Variable torque loads increase torque with speed and are usually associated with centrifugal fan and pump loads, where, in theory, the horsepower requirement varies as the cube of the speed change. Variable torque loads provide the greatest savings potential for VFD’s.

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