Glossary of Terms


A device that creates mechanical motion by converting various forms of energy to rotating or linear mechanical energy.
Adjustable Speed Drive:
A mechanical, fluid or electrical device that variably changes an input speed to an output speed matching operating requirements.
AGMA (American Gear Manufacturers Association):
Standards setting organization composed of gear products manufacturers and users. AGMA standards help bring uniformity to the design and application of gear products.
Air-Over (AO):
Motors for fan or blower service that are cooled by the air stream from the fan or blower.
Alternating Current (AC):
The standard power supply available from electric utilities.
Ambient Temperature:
The temperature of the air which, when coming into contact with the heated parts of a motor, carries off its heat. Ambient temperature is commonly known as room temperature.
Ampere (Amp):
The standard unit of electric current. The current produced by a pressure of one volt in a circuit having a resistance of one ohm.
  • The rotating part of a brush-type direct current motor.
  • In an induction motor, the squirrel cage rotor.
Axial Movement:
Often called “endplay.” The endwise movement of motor or gear shafts. Usually expressed in thousandths of an inch.


Back Driving:
Driving the output shaft of a gear reducer – using it to increase speed rather than reduce speed. Worm gear reducers are not suitable for service as speed increasers.
Rotational movement of a gear reducer’s output shaft clockwise and counter clockwise, while holding the input shaft stationary. Usually expressed in thousandths of an inch and measure at a specific radius at the output shaft.


Counter Electromotive Force:
Voltage that opposes line voltage caused by induced magnetic field in a motor armature or rotor.
Current, AC:
The power supply usually available from the electric utility company or alternators.
Current, DC:
The power supply available from batteries, generators (not alternators), or a rectified source used for special applications.


Duty Cycle:
The relationship between the operating time and the resting time of an electric motor. Motor ratings according to duty are:
  • Continuous duty, the operation of loads for over one hour.
  • Intermittent duty, the operation during alternate periods of load and rest.
Intermittent duty is usually expressed as 5 minutes, 30 minutes or one hour.


A ratio of the input power compared to the output, usually expressed as a percentage.
The term used to describe the motor housing. The most common industrial types are: Open Drip Proof (ODP), Totally Enclosed Fan Cooled (TEFC), Totally Enclosed Non-Ventilated (TENV), Totally Enclosed Air Over (TEAO). (See Chapter IV for additional information).
The part of a motor that houses the bearing supporting the rotor and acts as a protective guard to the internal parts of the motor; sometimes called endbell, endplate or end bracket.
The act of creating magnetic lines of force from a motor winding by applying voltage.
Explosion-Proof Motors:
These motors meet Underwriters Laboratories and Canadian Standards Association standards for use in hazardous (explosive) locations, as indicated by the UL label affixed to the motor. Locations are considered hazardous because the atmosphere does or may contain gas, vapor, or dust in explosive quantities.


The stationary part of a DC motor, commonly consisting of permanent magnets. Sometimes used also to describe the stator of an AC motor.
Flanged Reducer:
Usually used to refer to a gear reducer having provisions for close coupling of a motor either via a hollow (quill) shaft or flexible coupling. Most often a NEMA C face motor is used.


Frequency, in cycles per second, of AC power; usually 60 hz in North America, 50 hz in the rest of the world. Named after H. R. Hertz, the German scientist who discovered electrical oscillations.
High Voltage Test:
Application of a voltage greater than the working voltage to test the adequacy of motor insulation; often referred to as high potential test or “hi-pot.”
A measure of the rate of work. 33,000 pounds lifted one foot in one minute, or 550 pounds lifted one foot in one second. Exactly 746 watts of electrical power equals one horsepower. Torque and RPM may be used in relating to the horsepower of a motor. For fractional horsepower motors, the following formula may be used.
HP = T (in.-oz) x 9.917 x N x 107 where, HP = horsepower T = Torque N = revolutions per minute
The lagging of magnetism in a magnetic metal, behind the magnetizing flux which produces it.


IEC (International Electrotechnical Commission):
The worldwide organization that promotes international unification of standards or norms. Its formal decisions on technical matters express, as nearly as possible, an international consensus.
Stands for isolated gate bipolar transistor. The most common and fastest-acting semiconductor switch used in pulse width modulated (PWM) AC drives.
The total opposition in an electric circuit to the flow of an alternating current. Expressed in ohms.
Induction Motor:
The simplest and most rugged electric motor, it consists of a wound stator and a rotor assembly. The AC induction motor is named because the electric current flowing in its secondary member (the rotor) is induced by the alternating current flowing in its primary member (the stator). The power supply is connected only to the stator. The combined electromagnetic effects of the two currents produce the force to create rotation.
Input Horsepower:
The power applied to the input shaft of a gear reducer. The input horsepower rating of a reducer is the maximum horsepower the reducer can safely handle.
In motors, classified by maximum allowable operating temperature. NEMA classifications include: Class A = 105°C, Class B = 130°C, Class F = 155°C and Class H = 180°C.
Integral Horsepower Motor:
A motor rated one horsepower or larger at 1800 RPM. By NEMA definitions, this is any motor having a three digit frame number, for example, 143T.
An electronic device that changes direct current to alternating current; in common usage, an AC drive.


A unit of power equal to 1000 watts and approximately equal to 1.34 horsepower.


The work required of a motor to drive attached equipment. Expressed in horsepower or torque at a certain motor speed.
Locked Rotor Current:
Measured current with the rotor locked and with rated voltage and frequency applied to the motor.
Locked Rotor Torque:
Measured torque with the rotor locked and with rated voltage and frequency applied to the motor.


Magnetic Polarity:
Distinguishes the location of north and south poles of a magnet. Magnetic lines of force emanate from the north pole of a magnet and terminate at the south pole.
Mechanical Rating:
The maximum power or torque a gear reducer can transmit. Many industrial reducers have a safety margin equal to 200% or more of their mechanical rating, allowing momentary overloads during start-up or other transient overloads.
Motor Types:
Classified by operating characteristics and/or type of power required. The AC induction motor is the most common. There are several kinds of AC (alternating current) induction motors, including, for single-phase operation: shaded pole, permanent split capacitor (PSC), split phase, capacitor start/induction run and capacitor start/capacitor run.Polyphase or three-phase motors are used in larger applications. Direct current (DC) motors are also common in industry as are gearmotors, brakemotors and other types. (See Chapter III for additional details).
The most common motor mounts include: rigid base, resilient base C face or D flange, and extended through bolts. (See Chapter IV for additional details). Gear reducers are similarly base-mounted, flangemounted, or shaft-mounted.


National Electric Code (NEC):
A safety code regarding the use of electricity. The NEC is sponsored by the National Fire Protection Institute. It is also used by insurance inspectors and by many government bodies regulating building codes.
NEMA (National Electrical Manufacturers Association):
A nonprofit trade organization, supported by manufacturers of electrical apparatus and supplies in the United States. Its standards alleviate misunderstanding and help buyers select the proper products. NEMA standards for motors cover frame sizes and dimensions, horsepower ratings, service factors, temperature rises and various performance characteristics.


Open Circuit:
A break in an electrical circuit that prevents normal current flow.
Output Horsepower:
The amount of horsepower available at the output shaft of a gear reducer. Output horsepower is always less than the input horsepower due to the efficiency of the reducer.
Output Shaft:
The shaft of a speed reducer assembly that is connected to the load. This may also be called the drive shaft or the slow speed shaft.
Overhung Load:
A force applied at right angles to a shaft beyond the shaft’s outermost bearing. This shaft-bending load must be supported by the bearing.


The number of individual voltages applied to an AC motor. A single-phase motor has one voltage in the shape of a sine wave applied to it. A three-phase motor has three individual voltages applied to it. The three phases are at 120 degrees with respect to each other so that peaks of voltage occur at even time intervals to balance the power received and delivered by the motor throughout its 360 degrees of rotation.
A method of braking a motor that involves applying partial or full voltage in reverse to bring the motor to zero speed.
As applied to electric circuits, polarity indicates which terminal is positive and which is negative. As applied to magnets, it indicates which pole is north and which pole is south.
Magnetic devices set up inside the motor by the placement and connection of the windings. Divide the number of poles into 7200 to determine the motor’s normal speed. For example, 7200 divided by 2 poles equals 3600 RPM.
Power Factor:
The ratio of “apparent power” (expressed in kVA) and true or “real power” (expressed in kW).
Power Factor = Real Power/ Apparent Power
Apparent power is calculated by a formula involving the “real power,” that which is supplied by the power system to actually turn the motor, and “reactive power,” which is used strictly to develop a magnetic field within the motor. Electric utilities prefer power factors as close to 100% as possible, and sometimes charge penalties for power factors below 90%. Power factor is often improved or “corrected” using capacitors. Power factor does not necessarily relate to motor efficiency, but is a component of total energy consumption.
Prime Mover:
In industry, the prime mover is most often an electric motor. Occasionally engines, hydraulic or air motors are used. Special application considerations are called for when other than an electric motor is the prime mover.
Pull Out Torque:
Also called breakdown torque or maximum torque, this is the maximum torque a motor can deliver without stalling.
Pull Up Torque:
The minimum torque delivered by a motor between zero and the rated RPM, equal to the maximum load a motor can accelerate to rated RPM.
Pulse Width Modulation:
Abbreviated PWM, the most common frequency synthesizing system in AC drives; also used in some DC drives for voltage control.


The opposition to a flow of current other than pure resistance. Inductive reactance is the opposition to change of current in an inductance (coil of wire). Capacitive reactance is the opposition to change of voltage in a capacitor.
A device or circuit for changing alternating current (AC) to direct current (DC).
Regenerative Drive:
A drive that allows a motor to provide both motoring and braking torque. Most common with DC drives.
A device having two separate circuits, it is constructed so that a small current in one of the circuits controls a large current in the other circuit. A motor starting relay opens or closes the starting circuit under predetermined electrical conditions in the main circuit (run winding).
The characteristics of a magnetic field which resist the flow of magnetic lines of force through it.
A device that resists the flow of electrical current for the purpose of operation, protection or control. There are two types of resistors – fixed and variable. A fixed resistor has a fixed value of ohms while a variable resistor is adjustable.
The direction in which a shaft turns is either clockwise (CW) or counter clockwise (CCW). When specifying rotation, also state if viewed from the shaft or opposite shaft end of motor.
The rotating component of an induction AC motor. It is typically constructed of a laminated, cylindrical iron core with slots for cast-aluminum conductors. Short-circuiting end rings complete the “squirrel cage,” which rotates when the moving magnetic field induces a current in the shorted conductors.


SCR Drive:
Named after the silicon controlled rectifiers that are at the heart of these controls, an SCR drive is the most common type of generalpurpose drive for direct current motors.
The inability of a gear reducer to be driven backwards by its load. Most general purpose reducers are not self-locking.
Service Factor for Gearing:
A method of adjusting a reducer’s load carrying characteristics to reflect the application’s load characteristics. AGMA (American Gear Manufacturers Association) has established standardized service factor information.


Temperature Rise:
The amount by which a motor, operating under rated conditions, is hotter than its surrounding ambient temperature.
Temperature Tests:
These determine the temperature of certain parts of a motor, above the ambient temperature, while operating under specific environmental conditions.
Thermal Protector:
A device, sensitive to current and heat, which protects the motor against overheating due to overload or failure to start. Basic types include automatic rest, manual reset and resistance temperature detectors.
Thermal Rating:
The power or torque a gear reducer can transmit continuously. This rating is based upon the reducer’s ability to dissipate the heat caused by friction.
Are conductive ceramic materials, whose resistance remains relatively constant over a broad temperature range, then changes abruptly at a design threshold point, creating essentially a solid-state thermal switch. Attached control modules register this abrupt resistance change and produce an amplified output signal, usually a contact closure or fault trip annunciation. Thermistors are more accurate and faster responding than thermostats.
A pair of dissimilar conductors joined to produce a thermoelectric effect and used to accurately determine temperature. Thermocouples are used in laboratory testing of motors to determine the internal temperature of the motor winding.
A protector, which is temperature-sensing only, that is mounted on the stator winding. Two leads from the device must be connected to a control circuit, which initiates corrective action. The customer must specify if the thermostats are to be normally closed or normally open.
Thrust Load:
Force imposed on a shaft parallel to a shaft’s axis. Thrust loads are often induced by the driven machine. Be sure the thrust load rating of a gear reducer is sufficient so that its shafts and bearings can absorb the load without premature failure.
The turning effort or force applied to a shaft, usually expressed in inch-pounds or inch-ounces for fractional and sub-fractional HP motors.
  • Starting Torque: Force produced by a motor as it begins to turn from standstill and accelerate (sometimes called locked rotor torque).
  • Full-Load Torque: The force produced by a motor running at rated fullload speed at rated horsepower.
  • Breakdown Torque: The maximum torque a motor will develop under increasing load conditions without an abrupt drop in speed and power. Sometimes called pull-out torque.
  • Pull-Up Torque: The minimum torque delivered by a motor between zero and the rated RPM, equal to the maximum load a motor can accelerate to rated RPM.
Used to isolate line voltage from a circuit or to change voltage and current to lower or higher values. Constructed of primary and secondary windings around a common magnetic core.


Underwriters Laboratories (UL):
Independent United States testing organization that sets safety standards for motors and other electrical equipment.


Vector Drive:
An AC drive with enhanced processing capability that provides positioning accuracy and fast response to speed and torque changes. Often used with feedback devices in a closed-loop system.
A unit of electromotive force that, when applied to conductors, will produce current in the conductors.


The amount of power required to maintain a current of 1 ampere at a pressure of one volt when the two are in phase with each other. One horsepower is equal to 746 watts.
Typically refers to the process of wrapping coils of copper wire around a core. In an AC induction motor, the primary winding is a stator consisting of wire coils inserted into slots within steel laminations. The secondary winding of an AC induction motor is usually not a winding at all, but rather a cast rotor assembly. In a permanent magnet DC motor, the winding is the rotating armature.