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Glossary of Electromechanical Terms



Adjustable Speed Drive (ASD) and Variable Speed Drive (VSD): Equipment used to control the speed of machinery. Where process conditions call for adjustment of flow from a fan or a pump, varying the speed of the drive may reduce energy consumption compared with other techniques for flow control. ASD allows speeds to be selected from several different pre-set ranges, whereas VSD allows the output speed to be changed without steps over a range. Both types can be purely mechanical, electromechanical, hydraulic, or electronic.


Actuator: A device that creates mechanical motion by converting various forms of energy to rotating or linear mechanical energy.


Affinity Laws: They predict how capacity, head, and horsepower are affected by changes in the centrifugal pump impeller diameter or shaft speed.


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): An electric current that changes its direction very frequently at regular intervals.


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.


A.N.S.I. Standard: American National Standards Institute. A set of specifications for centrifugal pumps manufactured in the U.S.


Armature: The rotating part of a brush type direct current (DC) motor. In an induction motor, the rotating part is called a rotor.


Base Plate: The unit on which the pump or motor are mounted. 

Bearings: Supports the rotating shaft and allows it to turn with a minimum amount of friction. There are two common types:

  1) Sleeve: Common in home appliance motors. Normally used in blower applications where low noise levels are important.

  2) Ball: Used when high shaft load (radial or axial thrust load) capacity is required. Ball bearings are usually used in industrial and agricultural motors. Occasionally, roller bearings may be used on larger horsepower motors for maximum radial load capacity.


B.E.P.: The best efficiency point. It is the point where the power coming out of the pump (water horsepower) is the closest to the power coming into the pump (break horsepower) from the driver. This is also the point where there is no radial deflection of the shaft caused by unequal hydraulic forces acting on the impeller.


B.H.P.: Break horsepower. The actual amount of horsepower being consumed by the pump as measured on a pony brake or dynamometer.


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.


Capacitor (also referred to as a condenser): 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 Pump: Moves liquid with centrifugal force.


CHP: Combined heat and power.


Closed Control System (CCS): A system used to regulate a process using feedback control. This system responds to actual system conditions with a range of responses. 


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.


Coupling: Used to connect the pump to the driver. It transmits torque and compensates for axial growth, but not for radial misalignment.


Direct Current (DC): A current that flows in one direction only and is substantially constant in value.


Direct Torque Control: A drive system that controls the speed of an electric motor, and hence the torque it can produce on a rotating shaft.


Disaster Bushing: Used in A.P.I. glands to support the shaft in the event of a bearing failure, or to prevent a product from rushing to atmosphere after a seal failure.


Duty Cycle: The relationship between operating time and the resting time of an electric motor.

   * Continuous Duty: The operation of loads for over one hour.

   * Intermittent Duty: The operation during alternate periods of load and rest. Usually expressed as 5 minutes, 30 minutes, or one hour.


Dynamic Balancing: Identifies and corrects mass distribution minimizing vibration.


EASA: Electrical Apparatus Service Association, an international trade organization of more than 1,900 electromechanical sales and service firms in 62 countries. Through its many engineering and educational programs, EASA provides members with a means of keeping up to date on materials, equipment, and state-of-the-art technology.


Efficiency: The ration of the useful work performed and the energy expended in producing it. 


Enclosure: Term used to describe the motor housing.

   * ODP: Open Drip Proof, housing has openings in end shields and shell to allow air to cool the motor. Normally used in “clean” applications.

   * TEFC: Totally Enclosed Fan Cooled, housing has no openings. Motor is cooled by an external fan on the non-drive end of the motor shaft (motor is not air tight or waterproof). Normally used in dirty, oily, or damp applications.

   * TENV: Totally Enclosed Non-Ventilated. Not equipped with an external cooling fan, depends on convection air for cooling.

   * TEAO: Totally Enclosed Air Over. Air flow from the driven or external device provides cooling air flow of the motor.


Encoder: A device, circuit, transducer, software program or algorithm that converts information from one format or code to another.


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.


Explosion Proof: The enclosure’s ability to prevent an internal spark or explosion from causing a much larger blast.


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.


Frett (or fretting): Damage or grooving caused by the removal of the protective oxide that is formed on most corrosion resistant metals.


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.


Gearless Mill Drive (GMD): A system consisting of a ringmotor and its associated equipment such as transformers and control systems.


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.


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.


HVACR: The technology of indoor and vehicular environmental comfort. HVACR system design is a sub discipline of mechanical engineering based on the principals of thermodynamics, fluid mechanics, and heat transfer.


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.


Impeller: Attaches to the end of the shaft to impart energy to the fluid being pumped.


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: Class A=105C, Class B=130C, Class F=155C and Class H=180C.


Integral: Of, pertaining to, or belonging as a part of the whole.


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. 


Inverter: An electrical device for converting DC into AC.


Jacket: Usually refers to the heating/ cooling jacket surrounding the stuffing box on some pumps.


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.


Metric: A decimal system of measurement.


Motor Enclosures: There are many types of motor enclosures, each designed for specific applications. Here are brief descriptions of some of the more popular and common motor enclosures:

  • DP / ODP: DRIP-PROOF MOTOR:   A drip-proof motor is an open motor in which the ventilating openings are so constructed that successful operation is not interfered with when drops of liquid or solid particles strike or enter the enclosure at any angle from 0 to 15 degrees downward from the vertical.
  • OAO / DPAO / OPAO: OPEN AIR MOTOR:  An open motor is one having ventilating openings which permit passage of external cooling air over and around the windings of the motor. The term “open” designates a motor having no restriction to ventilation other than that necessitated by mechanical construction.
  • TE: TOTALLY-ENCLOSED MOTORA totally enclosed motor is one so enclosed as to prevent the free exchange of air between the inside and outside of the case but not sufficiently enclosed to be termed air-tight.
  • XP: EXPLOSION-PROOF MOTOR:  An explosion proof motor’s enclosure is designed and constructed to withstand an explosion of a specified gas or vapor which may occur within it and to prevent the ignition of the specified gas or vapor surrounding the motor by sparks, flashes or explosions of the specified gas or vapor which may occur within the motor casing.


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.


O.E.M.: Original equipment manufacturer.


Ohm: Unit of electrical resistance.


Open Circuit: A break in an electrical circuit that prevents normal current flow.


OSHA: Occupational and Safety Health Act.


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: 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.


Parallel: Electrical components that are connected in a way so that the flow of electricity can take multiple, or parallel, paths through the circuit.


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.


Phase-Shifting Transformer (quadratic booster): A specialized type of transformer used on 3-phase power grids (AC) to balance the active (real) and reactive power in the system and thereby preventing the loss of lines through physical overloading.


Phasor Measurement Units (PMUs): Monitoring devices installed at critical nodes in a power network to collect data on power flow.


Photovotaic Cells (PV): Device that converts radiation from the sun directly into DC electricity.


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.


Power Capacity: In terms of generation, the capacity of a power plant is the maximum power that installation is capable of producing.


Power Factor: A measure of an electrical circuit’s effectiveness on a scale of zero (lowest) to 1 (highest).


Pulley: A wheel on an axle that is designed to support movement and change of direction of a cable or belt along its circumference.


Rectifier: An electrical device used to convert AC into DC.


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).


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.


Rewind: AC and DC motors employ an insulated, current-carrying coil essential to their operation. The method of motor rewinding is traditionally done by hand by a highly skilled technician that involves removing the old coil, winding the new coil, and varnishing. 


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 theshorted conductors.


RPM: Revolutions per minute.


SIL (Safety Integrity Level): A rating given of a system to indicate the level of risk associated with it. It is a measure of its ability to perform safely and, in the event of failure, to fail safely.


Series Wound: Noting a commutator motor in which the field current and armature circuit are connected in a series.


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.


Sheave: A grooved wheel or pulley.


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.


Shunt Wound: Noting a motor or generator that has the field circuit connected in parallel with the armature winding.


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, etc.


Split-Phase Start-Capacitor Run: (Single Phase)


Stator: The fixed part of an AC motor, consisting of copper windings within steel laminations.


Surge: To suddenly increase as an electric current or voltage. 


Temperature Rise: The amount by which a motor, operating under rated conditions, is hotter than its surrounding ambient temperature.


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.


Three-Phase Power: A form of electricity used to supply heavy loads.


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 (sometimes called locked rotor torque): Force produced by a motor as it begins to turn from standstill and accelerate.

   * Full-Load Torque: The force produced by a motor running at rated full-load speed at rated horsepower.

   * Breakdown Torque (sometimes called pull-out torque): The maximum torque a motor will develop under increasing load conditions without an abrupt drop in speed and power.

   * 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.


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


Vector Control (also called field oriented control- FOC): A variable frequency drive (VFD) control method which controls 3-phase AC electric motor output by means of two controllable VFD inverter output variables.


Vibration Analysis: A test conducted to identify trouble spots that could cause problems (such as resonance and fatigue). 


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.