Glossary of Electric Motor Terms
Actuator: A device that creates mechanical motion by converting various forms of energy to rotating or linear mechanical energy.
Air-Over (AO): Motors for fan or blower service that are cooled by the air stream from the fan or blower. Motor is located in the air stream to cool the motor.
Alternating Current (AC): The standard power supply available from local electric utility companies.
Ambient Temperature (AMB): The temperature of the space (air) around the motor. Most motors are designed to operate in an ambient not to exceed 40C (104F).
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.
Armature: The rotating part of a brush type direct current (DC) motor. In an induction motor, the rotating part is called a rotor.
Bearings: Sleeve: Common in home appliance motors.
Normally used in blower applications where low noise levels are
Brush: Current conducting material in a DC motor, usually graphite, or a combination of graphite and other materials. The brush rides on the commutator of a motor and forms an electrical connection between the armature and the power source.
Canadian Standards Association (CSA): The agency that sets safety standards for motors and other electric equipment used in Canada.
Capacitance: As the measure of electrical storage potential of a capacitor, the unit of capacitance is the farad, but typical values are expressed in microfarads (MFD).
Capacitor: A device that stores electrical energy. Used on single phase motors.
Capacitor Start Motor: Or more specifically, Capacitor-Start, induction-run. Provides high starting and break-down torque, medium starting current. Used on hard starting applications such as compressors, positive displacement pumps, farm equipment, etc.
Capacitor-Start, capacitor-run: Similar to capacitor-start, except have higher efficiency. Generally used in higher HP single phase ratings.
Centrifugal Start Switch: A mechanism that disconnects the starting circuit (start winding) when the rotor reaches approximately 75% of operating speed (usually in 2 or 3 seconds).
Commutator: The part of a DC motor armature that causes the electrical current to be switched to various armature windings. Properly sequenced switching creates the motor torque. The commutator also provides the means to transmit the electrical current to the moving armature through the brushes that ride on the commutator.
DC Current: The power supply available from batteries, generators (not alternators), or a rectified source used for special purpose applications.
Duty Cycle: The relationship between operating time and the resting
time of an electric motor.
Efficiency: The ration of the useful work performed and the energy expended in producing it.
Enclosure: Term used to describe the motor housing.
Endshield: Also referred to as "End Bell". The part of the motor that houses the bearing supporting the rotor and acts as a protective guard to the internal parts of the motor.
Excitation: The act of creating magnetic lines of force from a motor winding by applying voltage.
Field: The stationary part of a DC motor, commonly consisting of permanent magnets. Sometimes used also to describe the stator of an AC motor.
Frame: Standardized motor mounting and shaft dimensions as established by NEMA or IEC.
Frequency: An expression of how often a complete cycle occurs. Cycles per second describe how many complete cycles occur in a given time increment. Hertz (hz) has been adopted to describe cycles per second so that time as well as number of cycles is specified. The standard power supply in North America is 60hz. Most of the rest of the world has 50hz power.
Full Load Amperes (FLA): Line current (amperage) drawn by a motor when operating at rated load and voltage on motor nameplate. Important for proper wire size selection, and motor starter or drive selection. Also called full load current.
Full Load Torque: The torque a motor produces at its rated horsepower and full-load speed.
Fuse: A piece of metal, connected in the circuit to be protected, that melts and interrupts the circuit when excess current flows.
Generator: Any machine that converts mechanical energy into electrical energy.
Hertz: Frequency, in cycles per second, of AC power. 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".
Horsepower (HP): 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.
International Electrotechnical Commission (IEC): 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.
Impedance: 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 (stator). the power supply is connected only to the stator. The combined electromagnetic effects of the two currents produce the force to create rotation.
Insulation: In motors, classified by maximum allowable operating
temperature. NEMA Classifications include:
Integral Horsepower Motor: A motor rated one horsepower or larger at 1800RPM. By NEMA definitions, this is any motor having a three digit frame, for example 143T.
Kilowatt: A unit of power equal to 1000 watts and approximately equal to 1.34 horsepower.
Load: 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.
Mounting, Basic Types: The most common motor mounts include: rigid base, resilient base C face or D flange, and extended through bolts.
Mush Coil: A coil made with round wire.
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 Manufactures Association): A non-profit trade organization, supported by manufacturers of electrical apparatus and supplies in the United States. Its standards alleviate misunderstandings and help buyers select the proper products. NEMA standards for motors cover frame sizes and dimensions, horsepower ratings, service factors, temperature rises and performance characteristics.
Open Circuit: A break in an electrical circuit that prevents normal current flow.
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: Is the perpendicular force pushing against the side of an output shaft. This force is either from a weight hanging on the output shaft or from a sprocket, pulley or gear being used on the shaft.
Permanent Split Capacitor (PSC): (Single Phase) Performance and applications similar to shaded pole motors, but more efficient, with lower line current and higher horsepower capabilities.
Phase: 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.
Plugging: A method of braking a motor that involves applying partial or full voltage in reverse in order to bring the motor to zero speed.
Polarity: 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.
Poles: 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 3600RPM.
Relay: A device have 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).
Reluctance: The characteristics of a magnetic field which resists the flow of magnetic lines of force through it.
Resistor: 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.
Rotation: The direction in which a shaft turns is either clockwise (CW) or counterclockwise (CCW). When specifying rotation, also state if viewed from the shaft end or the opposite shaft end of the motor.
Rotor: The rotating component of an induction AC motor. It is typically constructed of a laminated, cylindrical iron core with slots of cast-aluminum conductors. Short-circuiting end rings complete the "squirrel cage," which rotates when the moving magnetic field induces current in the shorted conductors.
Service Factor: A measure of the overload capacity built into a motor. A 1.15 SF means the motor can deliver 15% more than the rated horsepower without injurious overheating. A 1.10 SF motor should not be loaded beyond its rated horsepower. Service factors will vary for different horsepower motors and for different speeds.
Shaded Pole Motor: (Single Phase) Motor has low starting torque, low cost. Usually used in direct-drive fans and small blowers, and in small gearmotors.
Short Circuit: A fault or defect in a winding causing part of the normal electrical circuit to be bypassed, frequently resulting in overheating of the winding and burnout.
Split Phase (or more specifically Split-Phase start-induction run): (Single Phase) Motor has moderate starting torque, high breakdown torque. Used on easy-starting equipment, such as belt-driven fans and blowers, grinders, centrifugal pumps, gearmotors, ect.
Split-Phase Start-Capacitor Run: (Single Phase)
Stator: The fixed part of an AC motor, consisting of copper windings within steel laminations.
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.
Thermostat: A protector, which is temperature-sensing only, that is mounted on the stator winding. Two leads from the device must be connected to control circuit, which initiates corrective action. The customer must specify if the thermostats are to be normally closed or normally open.
Thermocouple: 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.
Torque: 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
Transformer: 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.
Voltage: A unit of electromotive force that, when applied to conductors, will produce current in the conductors.
Watt: 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.
Winding: Typically refers to the process of wrapping coils of copper wire around a core, usually of steel. 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.
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