Glossary of Motor Terms
This motor terminology glossary is guide to explain and define a variety of terms and characteristics that apply to industrial AC and DC electric motors.Reliance Motor Product Names
Duty Master® - Includes the complete line of Reliance AC NEMA frame motors from 1/8 through 500 horsepower.
E-Master® - Energy-efficient AC motors from 1/4 through 300 horsepower that meet U.S. Energy Policy Act regulations.
Easy Clean® - Three-phase washdown duty AC motors with ratings from 3 to 20 horsepower.
Easy Clean® Plus - Washdown duty AC motors available in single-phase designs from 1/2 to 11/2 horsepower, and three-phase designs from 1/2 to 3 horsepower.
841 XL - Extra tough, extra life and energy efficient AC motors that meet IEEE 841 standards. They are available with horsepower ratings from 1-1/2 through 250.
Enduraseal® - Premium Class F motor insulation system. See document B-2583.
Enduralok - Value-added Class F motor insulation rewind system. See document H-2000.
Enviroseal - Non-CFC refrigerant compatible motor rewind system. See document H-2027.
Enviroseal - VOC (Volatile Organic Compound) free motor insulation system.
EMT - E-Master extra tough single-phase and three- phase AC motors from 1/4 through 2 horsepower.
Excel - Large constant and variable-speed AC motors with ratings from 450 through 25,000 horsepower. See document B-2710.
G50 - Fin-framed AC motors with ratings from 150 through 3500 horsepower. See document B-
IEC - Complete lines of Reliance AC and DC motors that meet IEC electrical and mechanical standards.
M-2000 - AC motors that meet IEC dimensional and mechanical standards and NEMA electrical standards.
Omega® XL - Critical duty AC motors with a 5-year warranty. Available in horsepower ratings from 5 to 300. See document B-2717.
PreAlert - Reliance motors with continuous condition monitoring capability. See document B2779.
PLS® - Positive Lubrication System for motor bearings assures optimum lubrication/relubrication directly into and through the bearing track in any mounting position. See document B-2633.
Q5800 - Large AC motors up to 2000 horsepower or 7000 volts.
RPM III - Laminated frame variable-speed DC motors with ratings from 1/8 through 400 horsepower. See document C-
RPM XL - DC motors with ratings from 1/8 through 2 horsepower. See document C-2551 and C-2552.
Super Clean - Stainless steel frame washdown duty AC motors with ratings from 1/2 to 1 horsepower. See document B-2761.
Super RPM - Custom DC motors from 200 to 3500 horsepower. See document C-2519.
SXE Plus - Small premium efficient three-phase AC motors with ratings form 1/4 through 2 horsepower. See document B-2755.
SXT - Small extra tough single-phase AC motors from 1/3 to 2 horsepower. See document B2762.
VSM - Integrated motor and variable-speed controller. See documents B-2786, VSM100: B-2780-1 , B-2786, VSM200: C-2555-2, C-2559 , VSM500:
VXS - Variable-speed AC motors in NEMA frame sizes 1807 to 4497 and with horsepower ratings from 3 to 300 horsepower.
XE - Premium efficient three-phase AC motors with horsepower ratings from 3 to 500 horsepower. See document B-2639.
XEX - Premium efficient extra tough AC motors with ratings from 1 through 350 horsepower.
XP - Explosion-proof AC motors.
XT - Extra tough AC motors. See document B-2634.
AC (ALTERNATING CURRENT)
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 flow (cycles).
A motor (see Motor definition) operating on AC current that flows in either direction (AC current). There are two general types: induction and synchronous.
The amount of steel (iron) in the stator and rotor of a motor. Usually the amount of active iron is increased or decreased by lengthening or shortening the rotor and stator (they are generally the same length).
The space between the rotating (rotor) and stationary (stator) members in an electric motor.
AIR PRESSURE SWITCH
Used on motors with blowers to measure the difference in pressure across the filter to detect a clogged filter.
AIR TEMPERATURE SWITCH
A device used with an air hood motor to detect the temperature of the exhausted air. When used in this manner an air temperature switch will detect blockage in the cooling air system or long-term motor overload.
The atmospheric altitude (height above sea level) at which the motor will be operating; NEMA standards call for an altitude not exceeding 3,300 feet (1,000 meters). As the altitude increases above 3,300 feet and the air density decreases, the air's ability to cool the motor decreases. For higher altitudes, higher grades of insulation or motor derating are required. DC motors require special brushes for operation at high altitudes.
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 40 degrees C.
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.
ARMATURE CURRENT, AMPS
Rated full load armature circuit current.
ARMATURE INDUCTANCE, MH
Armature inductance in milli-henries (saturated).
The current that flows in the armature winding of a DC motor tends to produce magnetic flux in addition to that produced by the field current. This effect, which reduces the torque capacity, is called armature reaction and can effect the commutation and the magnitude of the motor's generated voltage.
ARMATURE RESISTANCE, OHMS
Armature resistance is measured in ohms at 25 degrees C (cold).
The force or loads that are applied to the motor shaft in a direction parallel to the axis of the shaft (Such as from a fan or pump.)
BACK END OF A MOTOR
The back end of a normal motor is the end that carries the coupling or driving pulley (NEMA). This is sometimes called the drive end (D.E.), pulley end (P.E.) etc. (See "Reliance Motor Product Names" )
BASE SPEED, RPM
The speed in revolutions per minute (RPM) which a DC motor develops at rated armature and field voltage with rated load applied.
Bearings reduce friction and wear while supporting rotating elements. When used in a motor, they must provide a relatively rigid support for the output shaft. (See "Reliance Motor Product Names")
Bearings act as the connection point between the rotating and stationary elements of a motor. There are various types such as roller, ball, sleeve (journal) and needle.
Ball bearings are used in virtually all types and sizes of electric motors. They exhibit low friction loss, are suited for high- speed operation and are compatible with a wide range of temperatures. There are various types of ball bearings such as open, single shielded and sealed. Reliance offers a unique PLS? bearing system. (See "PLS")
Rating life, L10 (B10), is the life in hours or revolutions in which 90% of the bearings selected will obtain or exceed. Median life (average life), L50 (B50), is the life in hours or revolutions in which 50% of the bearings selected will obtain or exceed.
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 as part of it.
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.
BREAK AWAY TORQUE
(See "Locked Rotor Torque")
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.
This designation shows that a product such as a motor or control meets European Standards for safety and environmental protection. A CE mark is required for products used in most European countries.
CIV (Corona Inception Voltage)
The minimum voltage amount that begins the process of ionization (corona) of motor windings.
Canadian Standards Association like U.L., sets specific standards for products used in Canada.
"C" FLANGE OR C-FACE
A type of flange used with close-coupled pumps, speed reducers and similar equipment where the mounting holes in the flange are threaded to receive bolts. Normally the "C" flange is used where a pump or similar item is to be connected on the motor. The "C" type flange is a NEMA standard design and available with or without feet.
CANOPY (DRIP COVER)
A protective cover placed on the top of a motor being mounted vertically to protect it from liquids or solids that might drop onto the motor (functions as an umbrella for the motor).
A device which, when connected 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 single- phase motors to start, or in three-phase motors for power factor correction.
A single-phase induction motor with a main winding arranged for direct connection to the power source, and an auxiliary winding connected in series with a capacitor. There are three types of capacitor motors: capacitor start, in which the capacitor phase is in the circuit only during starting; permanent-split capacitor, which has the same capacitor and capacitor phase in the circuit for both starting and running; two-value capacitor motor, in which there are different values of capacitance for starting and running.
The capacitor start single-phase motor is basically the same as the split phase start, except that it has a capacitor in series with the starting winding. The addition of the capacitor provides better phase relation and results in greater starting torque with much less power input. As in the case of the split phase motor, this type can be reversed at rest, but not while running unless special starting and reversing switches are used. When properly equipped for reversing while running, the motor is much more suitable for this service than the split phase start since it provides greater reversing ability at less watts input.
CENTRIFUGAL CUTOUT SWITCH
A centrifugally operated automatic mechanism used in conjunction with split phase and other types of single-phase induction motors. Centrifugal cutout switches will open or disconnect the starting winding when the rotor has reached a predetermined speed and reconnect it when the motor speed falls below it. Without such a device, the starting winding would be susceptible to rapid overheating and subsequent burnout.
A mechanical device for engaging and disengaging a motor. It is often used when many starts and stops are required.
A term used to describe non-uniform angular velocity. It refers to rotation occurring in jerks or increments rather than smooth motion. When an armature coil enters the magnetic field produced by the field coils, it tends to speed up and slow down when leaving it. This effect becomes apparent at low speeds. The fewer the number of coils, the more noticeable it can be.
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. There are two major types: "Mush" or "random" wound, round wire found in smaller and medium motors where coils are randomly laid in slot of stator core; and formed coils of square wire individually laid in, one on top of the other, to give an evenly stacked layered appearance. (See "Reliance Motor Product Names")
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.
COMPOUND WOUND DC MOTORS
Designed with both a series and shunt field winding, the compound motor is used where the primary load requirement is heavy starting torque and variable speed is not required (See "Paralleling") Also used for parallel operation. The load must tolerate a speed variation from full load to no-load.
Industrial machine applications include large planers, boring mills, punch presses, elevators and small hoists.
A material such as copper or aluminum which offers low resistance or opposition to the flow of electric current.
The metal container usually on the side of the motor where the stator (winding) leads are attached to leads going to the power supply. (See "Reliance Motor Product Names")
A DC motor which changes speed only slightly from a no-load to a full-load condition. For AC motors, these are synchronous motors.
The iron portion of the stator and rotor made up of cylindrical laminated electric steel. The stator and rotor cores are concentric and separated by an air gap, with the rotor core being the smaller of the two and inside to the stator core.
This is the electrical discharge breakdown of a winding through the application of excessive voltage.
COUNTER ELECTROMOTIVE FORCE (CEMF)
The induced voltage in a motor armature caused by conductors moving through or "cutting" field magnetic flux. This induced voltage opposes the armature current and tends to reduce it.
The mechanical connector joining the motor shaft to the equipment to be driven.
This time rate of flow of electrical charge and is measured in amps (amperes).
CYCLES PER SECOND (HERTZ)
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 US and 50 Hz is common in many foreign countries.
A special end shield with untapped holes for through bolts in the flange. It is primarily used for mounting the motor to gear boxes or bulkheads. They are available in frame sizes 143T through 445T.
DC (DIRECT CURRENT)
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.
A motor using either generated or rectified DC power (See "Motor"). A DC motor is often used when variable-speed operation is required.
DEFINITE PURPOSE MOTOR
A definite purpose motor is any motor design listed and offered in standard ratings with standard operating characteristics but with special mechanical features for use under service conditions other than usual or for use on a particular type of application (NEMA).
Some motors can operate on two different voltages, depending upon how it is built and connected. The voltages are either multiples of two or the the square root of 3 of one another.
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. A motor which never reaches equilibrium temperature but is permitted to cool down between operations, is operating under intermittent (INT) duty. Conditions such as a crane and hoist motor are often rated 15 or 30 minute intermittent duty.
A device which places a load on the motor to accurately measure its output torque and speed by providing a calibrated dynamic load. Helpful in testing motors for nameplate information and an effective device in measuring efficiency.
Design A,B,C,D - For AC Motors
NEMA has standard frame sizes and dimensions designating the height of the shaft, the distance between mounting bolt holes and various other measurements. Integral AC motor NEMA sizes run from 143T-445T, and the center of the shaft height in inches can be figured by taking the first two digits of the frame number and dividing it by 4.
Fractional horsepower motors, for which NEMA spells out dimensions, utilize 42, 48 and 56 frames. The shaft height in inches can be established by dividing the frame number by 16.
A drip-proof machine with ventilating openings guarded (with screens) as in a guarded 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.
A dual speed motor with torque values that vary with speed (as the speed changes the horsepower remains constant).
Electrical current generated (and dissipated) in a conductive material (often a rotor shaft) when it intercepts the electromagnetic field of a displacement or proximity probe. 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 electrical input to mechanical output. It represents the effectiveness with which the motor converts electrical energy into mechanical energy. NEMA has set up codes, which correlate to specific nominal efficiencies. A decrease in losses (the elements keeping the motor from being 100% efficient) of 10% constitutes an upward improvement of the motor of one code on the NEMA table. Each nominal efficiency has a corresponding minimum efficiency number.
A unit of measurement of time as applied to alternating current. One complete cycle equals 360 electrical degrees. One cycle in a rotating electrical machine is accomplished when the rotating field moves from one pole to the next pole of the same polarity. There are 360 electrical degrees in this time period. Therefore, in a two pole machine there are 360 degrees in one revolution, and the electrical and mechanical degrees are equal. In a machine with more than two poles, the number of electrical degrees per revolution is obtained by multiplying the number of pairs of poles by 360.
ELECTRICAL TIME CONSTANT (FOR DC MOTORS)
The ratio of electrical inductance to armature resistance. Electrical time constant in seconds defined as Electrical
TC = (La x 1a)/Hot IR voltage drop
Where La is the armature circuit inductance in henries and 1a is the rated full load armature current.
In a three-phase supply, where the voltages of the three different phases are not exactly the same. Measured as a percent of unbalance.
ELECTROMOTIVE FORCE (EMF)
A synonym for voltage, usually restricted to generated voltage.
A motor which has its winding 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.
The housing or frame of the motor.
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" (See "Reliance Motor Product Names")
A totally enclosed enclosure, which is constructed to withstand an explosion of a specified gas, vapor or dust which, may occur within it. Should such an explosion occur, the enclosure would prevent the ignition or explosion of the gas or vapor which may surround the motor enclosure. These motors are listed with Underwriter's Laboratories.
DIVISION I - Locations in which ignitable concentrations of flammable or combustible material exist and come in contact with the motor.
DIVISION II - Locations in which ignitable concentrations of flammable or combustible material exist but are contained within closed systems or containers and normally would not come in contact with the motor.
CLASS I - Those in which flammable gasses or vapors are or may be present in the air in quantities sufficient to product explosive or ignitable mixtures.
Group C - Atmospheres containing ethyl or ether vapors.
Group D - Atmospheres containing gasoline, hexane, benzine, butane, propane, alcohol, acetone, benzol, lacquer solvent vapors, natural gas, etc.
CLASS II - Those which are hazardous because of the presence of combustible dust.
Group E - Atmospheres containing metal dust, including aluminum, magnesium, or their commercial alloys.
Group F - Atmospheres containing carbon black, charcoal, coal or coke dust.
Group G - Atmospheres containing flour, starch, grain or combustible plastics or chemical dusts.
A motor using an external cooling system. This is required in applications where the motor's own fan will not provide sufficient cooling. These cooling systems are used in certain duty cycle applications, with slow speed motors, or in environments with extreme dirt. Often a duct with an external blower is used to bring clean air into the motor's air-intake.
A term commonly used to describe the stationary (stator) member of a DC motor. The field provides the magnetic field with which the mechanically rotating (armature or rotor) member interacts.
The introduction of resistance in series with the shunt wound field of DC motor to reduce the voltage and current which weakens the strength of the magnetic field and thereby increases the motor speed.
Mounting endshield with special rabbets and bolt holes for mounting such equipment as pumps and gear boxes to the motor or for overhanging the motor on the driven machine. (See "C" flange and "D" flange )
The magnetic field which is established around an energized conductor or permanent magnet. The field is represented by flux lines creating a flux pattern between opposite poles. The density of the flux lines is a measure of the strength of the magnetic field.
A figure of merit which indicates how much rectified current departs from pure (non-pulsating) DC. A large departure from unity form factor (pure DC) increases the heating effect of the motor and reduces brush life. Mathematically, form factor is the ratio of the root-mean square (rms) value of the current to the average (av) current or Irms/lav.
A type of coil in which each winding is individually formed and placed into the stator slot. A cross sectional view of the winding would be rectangular. Usually form winding is used on high voltage (2300 volts and above) and large motors (449T and above). Form winding allows for better insulation on high voltage than does random (mush) winding.
FRACTIONAL-HORSEPOWER MOTOR (FHP)
A motor usually built in a frame smaller than that having a continuous rating of one horsepower, open construction, at 1700 - 1800 rpm. Within NEMA frame sizes, FHP encompasses the 42, 48 and 56 frames. (In some cases the motor rating does exceed one horsepower, but the frame size categorizes the motor as a fractional.) The height in inches from the center of the shaft to the bottom of the base can be calculated by dividing the frame size by 16.
The supporting structure for the stator parts of an AC motor. In a DC motor, the frame usually forms a part of the magnetic coil. The frame also determines mounting dimensions (See "Frame Size")
Refers to a set of physical dimensions of motors as established by NEMA. These dimensions include critical mounting dimensions. NEMA 48 and 56 frame motors are considered fractional horsepower sizes even though they can exceed one horsepower. NEMA 143T to 449T is considered integral horsepower AC motors and 5000 series and above are called large motors. (For definition of letters following frame number, see "Suffixes")
The rate at which alternating current makes a complete cycle of reversals. It is expressed in cycles per second. In the U.S., 60 cycles (Hz) is the standard while in other countries 50 Hz (cycles) is common. The frequency of the AC current will affect the speed of a motor (See "Speed")
FRONT END OF A MOTOR
The front end of a normal motor is the end opposite the coupling or driving pulley (NEMA). This is sometimes called the opposite pulley end (O.P.E.) or commutator end (C.E.) (See "Reliance Motor Product Names")
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.
That torque of a motor necessary to produce its rated horsepower at full-load speed, sometimes referred to as running torque.
The portion of a gearmotor, which contains the actual gearing which, converts the basic motor speed to the rated output speed.
A gearhead and motor combination to reduce the speed of the motor to obtain the desired speed or torque.
GENERAL PURPOSE MOTOR
A general-purpose motor is any motor having a NEMA "B" design, listed and offered in standard ratings, with standard operating characteristics and mechanical construction for use under usual service conditions without restriction to a particular application or type of application (NEMA).
A motor with an electrical connection between the motor frame and ground.
An open motor in which all openings giving direct access to live or rotating parts (except smooth shafts) are limited in size by the design of the structural parts or by screens, grills, expanded metal, etc., to prevent accidental contact with such parts. Such openings shall not permit the passage of a cylindrical rod 1/2-inch in diameter.
In most instances, the following information will help identify a motor:
A letter used to indicate the "Design Code Rating" on the nameplate is referred to as the "Code Letter." The Code letter of the motor is an indication of the locked rotor KVA per horsepower. It is a function of the motor's design. Code letter ratings indicate the starting current motor will draw. Code letters below F indicates a low starting current; beyond F indicates a high starting current.
The motor's Code Letter is helpful in determining the maximum rating of the motor's electrical circuit protection. A replacement motor should have the same rating as its predecessor.
Five specialized elements are used, which together constitute the motor"s INSULATION SYSTEM. The following are typical in an AC motor:
Since there are various ambient temperature conditions a motor might encounter 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.
There are four common classes:.
*(Including Ambient and 110 degree Hot Spot)
When a motor insulation class is labeled on the nameplate, the total insulation system is capable of sustained operation at the above temperature.
A requirement of service that demands operation for alternate intervals of (1) load and no load; or (2) load and rest; (3) load, no load and rest; such alternative intervals being definitely specified.
An auxiliary set of field poles carrying armature current to reduce the field flux caused by armature reaction in a DC motor.
An electronic device that converts fixed frequency and fixed voltages to variable frequency and voltage. Enables the user to electrically adjust the speed of an AC motor.
Losses due to current flowing in a conductor caused by resistance (equals the current squared times the resistance).
J SECONDS (DC MOTORS)
J is the per unit moment of inertia. It is defined as the time in seconds to accelerate the motor armature to rated base speed using rated full load torque.
J = ((WR)squared x Base RPM (seconds))/(308 x Rated Torque)
A device used for leveling the positioning of a motor. These devices are adjustable screws that fit on the base or motor frame. Also a device for removing endshields from a motor assembly.
The steel portion of the rotor and stator cores make up 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. (See "Reliance Motor Product Names" )
Usually refers to AC motors with 5000 series frames and above or DC motors with 500 series frames and larger.
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 at standstill (at rated voltage and frequency). This is the current seen when starting the motor and load.
LOCKED ROTOR TORQUE
The minimum torque that a motor will develop at rest for all angular positions of the rotor (with rated voltage applied at rated frequency).
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.
In order to reduce wear and avoid overheating certain motor components lubrication is required (application of an oil or grease). The bearings are the major motor component requiring lubrication (as per manufacturer's instructions). Excess lubrication can however damage the windings and internal switches, etc. (See "PLS")
Due to the nature of motors there are two types of currents developed, one being torque producing current (load). The other is magnetizing current; this is the amount of current it takes to make the motor rotate under no load conditions.
Magnetizing current can be as much as 30% of a motor full load ampere rating.
This means that even though there may be a small torque producing load on the motor, there is always approximately 30% current flowing in motor that is operating. Current flowing through wire creates heat. Excessive heat can cause motor winding insulation to break down and fail.
No load amps are the magnetizing current in an inverter duty motor. It is shown as Im in the spec sheet.
MOTOR SURFACE TEMPERATURES
The plate on the outside of the motor describing the motor horsepower, voltage, speed efficiency, design, enclosure, etc.
NAVY SERVICE "A"
Motors designed to meet requirements of MIL M-17059 or MIL M-17060 for high shock and service and are essential to the combat effectiveness of a ship. These motors are usually made of nodular iron.
N.E.C. TEMPERATURE CODE ("T" CODE)
A National Electrical Code index for describing maximum allowable "skin" (surface) temperature of a motor under any normal or abnormal operating conditions. The "T" codes are applicable to U.L. listed explosion-proof motors. The skin temperature shall not exceed the minimum ignition temperature of the substances to be found in the hazardous location. The "T" code designations apply to motors and other types of electrical equipment subject to hazardous location classification.
The National Electrical Manufacturers Association is a non- profit organization organized and supported by manufacturers of electric equipment and supplies. NEMA has set standards for:
Reliance is one of the founding members of NEMA.
(See "Weather Protected Machine, Type I")
(See "Weather Protected Machine, Type II")
NODULAR IRON (DUCTILE IRON)
Special cast iron with a crystalline formation, which makes it capable of handling high shock.
OIL MIST LUBRICATION-DRY SUMP
A method for lubricating anti-friction bearings, which utilizes oil, dispersed on an air stream. The mist is exhausted from the bearing housing so as not to permit oil to accumulate.
OIL MIST LUBRICATION-WET SUMP
Similar to Oil Mist Lubrication - Dry Sump, except that a pool of oil is developed in the bearing chamber. This oil pool will continue to supply oil to the bearing in the event that the oil mist is interrupted and is fed from a source outside the bearing housing such as a constant level oiler.
A ball bearing that does not have a shield, seal or guard on either of the two sides of the bearing casing.
OPEN EXTERNALLY-VENTILATED MACHINE
A machine which is ventilated with external air by means of a separate motor-driven blower mounted on machine enclosure.
OPEN PIPE-VENTILATED MACHINE
An open machine except that openings for admission of ventilating air are so arranged that inlet ducts or pipes can be connected to them. Air may be circulated by means integral with machine or by means external to and not a part of the machine. In the latter case, this machine is sometimes known as a separately- or force-ventilated machine.
OPEN (PROTECTED) MOTOR
A motor having ventilating openings which permit passage of external cooling air over and around the windings. The term "open machine," when applied to large apparatus without qualification, designates a machine having no restriction to ventilation other than that necessitated by mechanical construction.
A special mounting similar to "D" flange except with a machined fit tenon recessed instead of protruding. Usually found on pumps.
When two or more DC motors are required to operate in parallel - that is, to drive a common load while sharing the load equally among all motors - they should have speed-torque characteristics which are identical. The greater the speed droop with load, the easier it becomes to parallel motors successfully. It follows that series motors will operate in parallel easier than any other type. Compound motors, which also have drooping speed characteristics (high regulation), will generally parallel without special circuits or equalization. It may be difficult to operate shunt or stabilized-shunt motors in parallel because of their nearly constant speed characteristics. Modifications to the motor control must sometimes be made before these motors will parallel within satisfactory limits.
PART WINDING START MOTOR
Is arranged for starting by first energizing part of the primary winding and subsequently energizing the remainder of this winding in one or more steps. The purpose is to reduce the initial value of the starting current drawn or the starting torque developed by the motor. A standard part winding start induction motor is arranged so that one-half of its primary winding can be energized initially and subsequently the remaining half can be energized, both halves then carrying the same current. Part winding starters can only be used with part winding motors. We do not make these motors.
PERMANENT MAGNET SYNCHRONOUS (PMR) (HYSTERESIS SYNCHRONOUS)
A motor with magnets embedded into the rotor assembly, which enable the rotor to align itself with the rotating magnetic field of the stator. These motors have zero slip (constant speed with load) and provide higher torque, efficiency and draw less current than comparable reluctance synchronous motors.
Indicates the space relationships of windings and changing values of the recurring cycles of AC voltages 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 in position. Each voltage 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- or three-phase (with 120 electrical degrees between the three- phases).
Reconnecting a motor's winding in reverse to apply a reverse braking torque to its normal direction of rotation while running. Although it is an effective dynamic braking means in many applications, plugging produces more heat than other methods and should be used with caution.
A ratio of one-minute meggar test (see "Meggar Test") to ten- minute meggar test. Used to detect contaminants in winding insulation done typically on high voltage V.P.I. motors, which are tested by water immersion.
In an AC motor, refers to the number of magnetic poles in the stator winding. The number of poles determines the motor's speed. (See "Synchronous Speed")
In a DC motor, refers to the number of magnetic poles in the motor. They create the magnetic field in which the armature operates (speed is not determined by the number of poles).
Two- or three-phase induction motors have their windings, one for each phase, evenly divided by the same number of electrical degrees. Reversal of the two-phase motor is accomplished by reversing the current through either winding. Reversal of a three-phase motor is accomplished by interchanging any two of its connections to the line. Polyphase motors are used where a polyphase (three-phase) power supply is available and is limited primarily to industrial applications.
Starting and reversing torque characteristics of polyphase motors are exceptionally good. This is due to the fact that the different windings are identical and, unlike the capacitor motor, the currents are balanced. They have an ideal phase relation, which results in a true rotating field over the full range of operation from locked rotor to full speed.
Identifies the type of power supply providing power to a DC motor. Frequency, voltage, and type of rectifier configuration.
A measurement of the time phase difference between the voltage and current in an AC circuit. It is represented by the cosine of the angle of this phase difference. For an angle of 0 degrees, the power factor is 100% and the volt/amperes of the circuit are equal to the watts (this is the ideal and an unrealistic situation). Power factor is the ration of Real Power-KW to total KVA or the ratio of actual power (watts) to apparent power (volt amperes).
The winding of a motor, transformer or other electrical device which is connected to the power source.
The principal function of a relay is to protect service from interruption, or to prevent or limit damage to apparatus.
The maximum constant torque, which a synchronous motor will accelerate into synchronism at, rated voltage and frequency.
The minimum torque developed by an AC motor during the period of acceleration from zero to the speed at which breakdown occurs. For motors, which do not have a definite breakdown torque, the pull-up torque is the minimum torque developed during the process of achieving rated speed.
Are protection which can be placed over openings in the fan cover on a fan-cooled motor or ventilation openings of a protected motor to help keep out large particles and/or animals, but not block ventilation.
Winding which is not connected to the power source, but which carries current induced in it through its magnetic linkage with the primary winding.
SERIES DC MOTORS
Where high starting torques are required for a DC motor, the series motor is used. The load must be solidly connected to the motor and never decrease to zero to prevent excessive motor speeds.
The load must tolerate wide speed variations from full load to light load. Typical areas of application are industrial trucks, hoists, cranes and traction duty.
SERVICE FACTOR (SF)
INTEGRAL HORSEPOWER MOTORS
A capacitor device usually mounted in the conduit box to flatten the voltage surges that may occur as a result of lighting or a power supply surge (short-period peak). These surges can result in more than twice the rated voltage going to the windings and in turn cause winding damage.
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. There are two major synchronous motor types: reluctance and permanent magnet. A synchronous motor is often used where the exact speed of a motor must be maintained.
The speed of the rotating magnetic field set up by the stator winding of an induction motor. In a synchronous motor, the rotor locks into step with the rotating magnetic field and the motor is said to run at synchronous speed. Approximately the speed of the motor with no load on it.
This is equal to 120 x Frequency = RPM (revolutions per minute) No. Poles
Current NEMA designation identifying AC induction motor frames. (NEMA has dimension tables which offer standard frame measurements.) Replaced the previous standard "U" frame in 1965.
A small generator normally used as a rotational speed sensing device. Tachometers are typically attached to the output shaft of DC or AC variable-speed motors requiring close speed regulation. The tachometer feeds its signal to a control which adjusts its output to the DC or AC motor accordingly (called "closed loop feedback" control).
Has direct effect on motor life when considering life expectancy. The following application considerations that affect a motor's operating temperature should be taken into account:
A previously used NEMA designation indicating frame size and dimension (prior to 1965 the standard frame sizes per horsepower rating).
U.L. (UNDERWRITER'S LABORATORY)
An independent testing organization, which examines and tests devices, systems and materials with particular reference to life, fire and casualty hazards. It develops standards for motors and controls used in hazardous locations through cooperation with manufacturers. U.L. has standards and tests for explosion- proof and dust ignition-proof motors, which must be met and passed before application of the U.L. label.
VACUUM DEGASSED BEARINGS
Vacuum degassing is a process used in the purifying of steel for ball bearings ensuring a very dense and consistent bearing surface. This results in a longer lasting superior bearing. All Reliance ball bearings are vacuum degassed bearings.
A multi-speed motor used on loads with torque requirements, which vary with speed as with some centrifugal pumps and blowers. The horsepower varies as the square of the speed.
A motor being mounted vertically (shaft up or down) as in many pump applications.
VERTICAL "P" BASE MOTOR
A vertical motor with a special mounting face conforming to NEMA design "P" and with a ring groove on the shaft.
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 attached. NEMA requires that motor be able to carry its rated horsepower at nameplate voltage plus or minus 10% although not necessarily at the rated temperature rise.
Loss encountered across a circuit impedance from power source to applicable point (motor) caused by the resistance in conductor. Voltage drop across a resistor takes the form of heat released into the air at the point of resistance.
Wk2 (MOMENT OF INERTIA)
The moment of inertia is expressed as Wk2 in terms of pound- feet squared. It is the product, the weight of the object in pounds and the square of the radius of gyration in feet.
If the application is such that the motor is driving through a pulley or gear so that the driven equipment is operating at a higher or lower speed than the rotor, it is necessary to calculate the inertia reflected to the motor shaft. This is an equivalent Wk2 (reflected to motor shaft) = Wk2 based on the speed (rpm) of the motor.
Wk2 (reflected to motor shaft) = Wk2 (driven equipment) x ((driven equipment rpm)2/ (motor rpm)2)
The amount of power required to maintain a current of one ampere at a pressure of one volt. Most motors are rated in Kwatt equal to 1,000 watts. One horsepower is equal to 746 watts.
Type I (WPI) weather-protected machine is an open machine with its ventilating passages so constructed as to minimize the entrance of rain, snow and airborne particles to the electric parts. Its ventilating openings are constructed to prevent the passage of a cylindrical rod 3/4 inch in diameter.
Type II (WPII) have, in addition to the enclosure defined for a Type I weather-protected machine, its ventilating passages at both intake and discharge so arranged that high velocity air and airborne particles blown into the machine by storms or high winds can be discharged without entering the internal ventilating passages leading directly to the electric parts of the machine itself. The normal path of the ventilating air which enters the electric parts of the machines are arranged by baffling or through a separate housing to provide at least three abrupt changes in direction, none of which shall be less than 90 degrees. In addition, an area of low velocity not exceeding 600 feet per minute shall be provided in the intake air path to minimize the possibility of moisture or dirt being carried into the electric parts of the machine.
WOUND ROTOR INDUCTION MOTOR
A wound rotor induction motor is an induction motor in which the secondary circuit consists of polyphase winding or coils with terminals that are either short circuited or closed through suitable circuits. A wound rotor motor is sometimes used when high breakdown torque and soft start or variable-speed operation are required.
starters can only be used on wye-delta motors which have six leads that allow for motor winding to be connected in either a wye or delta configuration. During start up, the windings are connected in the wye resulting in 58% of line voltage applied across two windings. This reduces both inrush and starting torque to 33% of the delta connected values. After a set time delay, the motor leads are switched to the delta connection.