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Electric Motor Terminology

AC Motor A motor (see Motor definition) operating on AC current that flows in either direction (AC current). There are two general types: induction and synchronous.Air Gap Opening between the stator and rotor.Air Over Air Over - Motors designed for fan or blower service and cooled by the air stream from the driven fan or blower.Altitude General purpose motors are suitable for operation up to 3300 feet. Class F insulation is suitable to 9900 feet. Ambient The temperature of the space around the motor. Most motors are designed to operate in an ambient not over 104F.Amperes (amps) or A Current flow at a specific load condition.AFBMA Anti-Friction Bearing Manufacturers Association - an organization of most bearing manufacturers that establishes standards for bearings. Armature The portion of the magnetic structure of a DC or universal motor which rotates.Adapter Base or Conversion Base: Same as conversion base. An adapter to convert current Nema "T" frame motors (which are smaller) to older Nema "U" frame motor mounting dimensions.Automatic Reset Overload After the motor cools, this line-interrupting protector automatically restores power. It should not be used where unexpected restarting would be hazardous.Bearing Housing Same as end bell. Houses the bearing of motor and supports the rotor.Bearings 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.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.Bearing Life 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.Brakes 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.Breakaway Torque Same as Starting Torque or Lock Rotir Torque -- The torque developed by the motor when starting or when stalled (rotor blocked).Breakdown Torque (BDT) Same as Pull Out Torque or Maximum Run Torque. Usually is the maximum value of torque that a motor will develop without a sudden decrease in speed(breakdown).Breather or Breather Drain Plug type device to provide drainage of condensation or water from motor.Brush 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."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.Capacitor Capacitors are used on single-phase induction motors except shaded-pole, split-phase and polyphase. Start capacitors are designed to stay in circuit a very short time (3-5 seconds), while run capacitance are permanently in circuit. Capacitors are rated by capacitance and voltage. Never use a capacitor with lower capacitance or voltage ratings for replacement. A higher voltage is acceptable.Capacitor Motor 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: 1) capacitor start, in which the capacitor phase is in the circuit only during starting; 2) permanent-split capacitor, which has the same capacitor and capacitor phase in the circuit for both starting and running; 3) two-value capacitor motor, in which there are different values of capacitance for starting and running.Capacitor Start 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.CSA Canadian Standards Association sets standards and approves motor for use in Canada.Commutator 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.Conduit or Terminal Box Contains the motor leads or terminals for connection to power source.Conversion Base Same as adapter base. An adapter to convert current Nema "T" frame motors (which are smaller) to older Nema "U" frame motor mounting dimensions.Core 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.DC Motor A motor using either generated or rectified DC power (See "Motor"). A DC motor is often used when variable-speed operation is required.Design A, B, C, D - For AC Motors NEMA has standard motor designs with various torque characteristics to meet specific requirements posed by different application loads. The design "B" is the most common designDimensions 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.Drip-proof Same as Open Drip-proof (ODP. Ventilation openings in bearing housings and some yokes placed so drops of liquid falling within an angle of 15 from vertical will not affect performance. Normally used indoors in fairly clean, dry locations.Dynamometer 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.End Bell Same as bearing housing. Houses the bearing of motor and supports the rotor.Explosion-proof Motor designed to withstand an internal explosion of gas or vapor and not allow flame or explosion to escape. Generally TEFC but also built TENV in smaller horsepower ratings. Motors are labeled to meet UL and NEC requirements.Design or Design Letter Letter assigned by NEMA to denote standard performance characteristics relating to torque, starting current and slip.Drip Cover Umbrella type cover used to keep water out of motor.Duty Cycle Standard is continuous duty, suitable for 24 hour per day operation. Some special motors may be rated for intermittent use (15 min., 30 min., etc.).Efficiency (Motor) A motor's efficiency is a measurement of useful work produced by the motor versus the energy that it consumes (heat and friction). An 84% efficient motor with a total watt draw of 400W produces 336 watts of useful energy (400 x .84 = .336W). The 64 watts lost (400 - 336 = 64W) becomes heat. Electric motors, and everything else involved with power production or conversion, have an efficiency. The efficiency of a motor is the ratio of useful mechanical power at the pulley to the electrical power input. A perfect motor would have an efficiency of 1.0 or 100%, meaning that all the electrical power input would appear as mechanical power. All electrical power that does not contribute to mechanical power is loss. The loss is from several sources, including heat and friction. Full-load motor efficiencies usually range from 50% to 95%. Fractional horsepower motors usually have efficiencies under 75%. Standard 1-10 HP motors have efficiencies between approximately 75% and 85%. Note that efficiency decreases as the load decreases from maximum, and drops severely for less than 50% of full load. Note that the rated horsepower takes this into account; you do not derate the nameplate rating by the efficiency. What the efficiency tells you is how much electrical power you are wasting. Spending more money on a higher efficiency motor will reduce your electric bill. For example, a 1 HP 75% efficient motor would require 746W/0.75 or 995W of electrical power. An 85% efficient motor would require 878W. This would save about 1.5 cents per hour of operation at Philadelphia area residential electric rates. If the motor runs continuously, the savings would be about $131 per year.Electrical Unbalance In a three-phase supply, where the voltages of the three different phases are not exactly the same. Measured as a percent of unbalance.Enclosure (ENCL) Term used to describe motor housing. Common types are:Flange or Face Specially machined drive end bearing housing with flat surface and bolt holes to provide easy mounting to driven equipment. Used extensively on pumps and gear reducers, NEMA flanges are designated by C, D or P and the letter will appear on the nameplate in the frame space, i.e. 256TC, etc.Frame or Frame Size Generally refers to the NEMA Standardized dimensioning system. Also used to refer to the yoke or supporting structure for the stator parts.Frequency - Hertz (HZ) Frequency in cycles per second of AC power; usually 60 Hz in U.S. and 50 Hz is common overseas.Full-Load Amps (F.L.A.) Current (Amps) drawn by motor operating at rated horsepower and voltage. Important for wire and control selection and is on the motor nameplate.Horsepower The output power rating of the motor shown on the nameplate. One horsepower is equivalent to lifting 33,000 pounds to a height of one foot in one minute. Exactly 746 watts of electrical power will produce 1 HP if a motor could operate at 100% efficiency, but of course no motor is 100% efficient. A 1 HP motor operating at 84% efficiency will have a total watt consumption of 888 watts. This amounts to 746 watts of usable power and 142 watts loss due to heat, friction, etc. (888 x .84 = 746 = 1 HP).Induction Motor An induction motor is an alternating current motor in which the primary winding on one member (usually the stator) is connected to the power source and a secondary winding or a squirrel-cage secondary winding on the other member (usually the rotor) carries the induced current. There is no physical electrical connection to the secondary winding, its current is induced.Inrush Current See Locked Rotor Amps.Insulation Generally refers to the maximum allowable operating temperature of the motor. Class A -105, C, B - 130C, F - 155C, H - 180C. The motor rise plus the ambient temperature should be equal to or less than the maximum allowable temperature for the insulation class.Insulation Class Insulations have been standardized and graded by their resistance to thermal aging and failure. Four insulation classes are in common use. For simplicity, they have been designated by the letters A, B, F, and H. The temperature capabilities of these classes are separated from each other by 25 C increments. The temperature capabilities of each insulation class is defined as being the maximum temperature at which the insulation can be operated to yield an average life of 20,000 hours.Insulation Temperature Rating The insulation class temperature rating for 20,000 hours of average insulation life is : Class A 105 C (221 F) ; Class B 130 C (266 F); Class F 155 C (311 F); Class H 180 C (356 F)KVA Code Designated by a letter on the motor nameplate and indicates a range for values for locked rotor kva per horsepower.Laminations Slotted stampings or punchings of thin (0.018"-0.026") electrical grade steels, stacked and joined together that contain the motor windings and form the magnetic "circuit" of a motor.Locked Rotor Amps (L.R.A.) Line current drawn by a motor at starting or when nameplate voltage is applied and the rotor is not rotating (locked).Locked Rotor Time or Stall Time Time in seconds that a motor can withstand locked rotor (stalled) current without damage.Locked Rotor Torque (L.R.T.) Starting Torque or Breakaway Torque -- The torque developed by the motor when starting or when stalled (rotor blocked).Manual Reset Overload This line-interrupting protector has an external button that must be pushed to restore power to the motor. Use where unexpected restarting would be hazardous, as on saws, conveyors, compressors and other machinery.Magnetic Polarity It is a fundamental principle of a winding that adjacent poles must be wound to give opposite magnetic polarity. This does not mean that the coils actually have to be wound in this direction before being placed into the stator. It does mean that the winding must be connected so that, if the current proceeds through one pole in a clockwise direction, it must proceed through the next pole in a counterclockwise direction. This principle is used to determine the correctness of connection diagrams.Maximum Run Torque See Breakdown Torque.Meggar Test A measure of an insulation system's resistance. This is usually measured in megohms and tested by passing a high voltage at low current through the motor windings and measuring the resistance of the various insulation systems.NEMA National Electrical Manufacturers Association - an organization that develops voluntary standards of performance.Maximum Pull Out Torque Same as Breakdown Torque (BDT) or Pull Out Torque. Usually is the maximum value of torque that a motor will develop without a sudden decrease in speed(breakdown).Motor Electrical Rating In addition to horsepower, a motor's nameplate will specify the operating voltage and the current draw at rated horsepower. You have to operate the motor at close to its specified voltage, but the current (amps) will vary with the actual load. A lightly loaded motor will draw less current. However, since the power factor and efficiency (both described below) decrease with decreasing load, the current will not drop as much as expected. Since all types of power (mechanical, electrical, etc.) are equivalent, these electrical ratings can be related to the horsepower rating. Most generally, electrical power in watts is equal to the voltage in volts times the current in amps. 1 horsepower (mechanical power) is approximately equal to 746 watts (electrical power.) Thus a perfect motor (the type used in physics classes, which do not exist in the real world) rated at 120 volts and 1 HP would draw about 6.2 amps ([1 HP = 746 watts]/120 volts gives 6.2 amps.) However, in the real world things are not quite so simple, and corrections must be applied.Open Drip-proof (ODP) Same Drip Proof. Ventilation openings in bearing housings and some yokes placed so drops of liquid falling within an angle of 15 from vertical will not affect performance. Normally used indoors in fairly clean, dry locations.Phase 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).Poles 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).Polyphase Motor 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.Power Factor Motors typically do not have a rating in watts. Instead you get amps at rated voltage and full load. To relate the voltage and amperage ratings to the horsepower, they must be converted to watts. As stated above, watts is volts times amps. However, this is not true for AC (alternating current) systems as we typically describe them. Typical "AC voltages" are only averages, known as RMS or Root-Mean-Square averages. Thus "120 Volts" really means that the average voltage, when computed by this particular method, is 120. The actual voltage is 170sin(21600t), where t is in seconds and the sin is of degrees. The current is Ipeaksin(21600t + a), where Ipeak is the peak current in amps, t is in seconds, and a is the phase angle in degrees. Clearly we want to avoid multiplying these two and instead figure out how to use the averages. This is done with the power factor, which is the cosine of the phase angle. power (RMS Watts) = Voltage (RMS volts) x Current (RMS amps) x Power-Factor RMS power is what your power meter measures and what converts to horsepower, so that is the figure you want. For resistive loads such as light bulbs and toasters the power factor is 1.0. Thus a light bulb rated at 120 VAC and drawing 1 amp will draw a power of (120 volts)(1 amp)(1.0) or 120 watts. For reactive loads, which include motors, the power factor is always less than 1. Motor power factors typically range from 0.5 to 0.95. For motors from 1 to 10 HP the power factor would typically increase from 0.75 to 0.85 for single phase induction motors. Like efficiency, the power factor is only valid at full load. It drops significantly for a partial load. As an example, consider a motor rated at 15 amps for 120 VAC with an efficiency of 0.75 and a power factor of 0.7. The net power in watts would be (15 A)(120 V)(0.75)(0.7) or 945 watts. At 746 watts per horsepower the proper rating would be about 1 1/4 HP. Smaller motors usually will not have either the efficiency nor the power factor on the nameplate. You can usually get their product and hence if one is given you can get the other. For example, consider a motor rated at 1 1/2 HP, 18 amps at 120VAC with a 63% efficiency. Its power output is (1.5 HP)(746 watts/HP) or 1119 watts. Based on the efficiency alone we would expect to get (18 A)(120 V)(0.63) or 1361 watts. Hence the power factor is (1119 W) / (1361 W) or 0.82. Note that power factors do not affect residential electric bills; you only pay for the wattage. However, if you size a circuit breaker for a low power factor motor based on wattage rather than volt-amps, it might turn out to be too small. And if you are an industrial customer, you can expect to pay more for low power factor machines (since it costs the utility more to transmit the extra current.)Power Factor The power factor of an AC electrical power system is defined as the ratio of the real power flowing to the load to the apparent power in the circuit, and is a dimensionless number between 0 and 1. Real power is the capacity of the circuit for performing work in a particular time. Apparent power is the product of the current and voltage of the circuit. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power will be greater than the real power.Pull Out Torque Pull Out Torque Same as Breakdown Torque (BDT) or Maximum Run Torque. Usually is the maximum value of torque that a motor will develop without a sudden decrease in speed (breakdown).Random Wound The standard type of stator winding used in motors under 1,000 volts. The coils are random wound with round wire as opposed to flat form wound coils.Rated Horsepower The most important power rating of a motor is the "rated horsepower," which is the continuously available net shaft power. This is the power available at the pulley for doing useful work elsewhere. It is typically measured by a dynamometer. These are braking devices attached to the shaft used to measure the net available power. A motor can always produce more than its rated power, but continuously overloading it will reduce its life. Excessive overload will cause it to shut down, or catch fire. An overload will also cause the motor to draw more than its rated current, which risks tripping the circuit breaker.Resilient Mounting A suspension system or cushioned mounting designed to reduce the transmission of normal motor noise and vibration to the mounting surface. This type of mounting is typically used in fractional horsepower motors for fans and blowers.Roller Bearing A special bearing system with cylindrical rollers capable of handling belted applications too large for standard ball bearings.Rotor The rotating member of an induction motor made up of stacked laminations. A shaft running through the center and a squirrel cage made in most cases of aluminum, which holds the laminations together, and act as a conductor for the induced magnetic field. The squirrel cage is made by casting molten aluminum into the slots cut into each laminationRPM (Revolutions Per Minute) The number of times per minute the shaft of the motor (machine) rotates. This is a function of design and the power supply.Service Factor The service factor (SF) is a measure of continuous overload capacity at which a motor can operate without overload or damage, provided the other design parameters such as rated voltage, frequency and ambient temperature are within norms. Example: a 3/4 HP motor with a 1.15 SF can operate at .86 HP, (.75 HP x 1.15 = 862 HP) without overheating or otherwise damaging the motor if rated voltage and frequency are supplied at the motor's leads. Some motors , have higher service factors than the NEMA standard. It is not uncommon for the original equipment manufacturer (OEM) to load the motor to its maximum load capability (service factor). For this reason, do not replace a motor with one of the same nameplate horsepower but with a lower service factor. Always make certain that the replacement motor has a maximum HP rating (rated HP x SF) equal to or higher than that which it replaces. Multiply the horsepower by the service factor for determining maximum potential loading.Sleeve Bearings A type of bearing with no rolling elements, where the motor shaft rides on a film of oil.Slide base An adjustable frame on which the motor sets. Used for belt drives to adjust beltSpace Heaters Small resistance heater units mounted in a motor that are energized during motor shutdown to prevent condensation of moisture in the motor windings.Split Phase Start Motors, which employ a main winding and an auxiliary winding, called the starting winding. The windings are unlike and thereby "split" the single phase of the power supply by causing a phase displacement between the currents of the two windings thus producing a rotating field. After the motor has attained approximately 75% of rated speed, the starting winding is automatically disconnected by means of a centrifugal switch or by a relay. The motor then continues to run on a single oscillating field, which in conjunction with the rotation of the rotor, results in a rotating field effect. Since there is no rotating field, after the starting winding is deenergized, the rotation cannot be changed until the motor has come to rest or at least slowed down to the speed at which the automatic switch closes. Special starting switches are available as well as special reversing switches which have a means for shunting the open contracts of the automatic switch while the motor is running and thus permits the split phase motor to be reversed while rotating. This type of starting is found typically on single phase fractional motors.Surface Temperature The temperature of a motor to the "touch". The surface temperature of continuously (and correctly) operating general purpose industrial electric motor will easily be 80 C (176 F) and perhaps as high as 100 C (212 F). You cannot keep your hand on a surface that hot long enough to discern differences, and if you try, you could get a nasty burn. There are no published standards regarding surface temperatures of general purpose motors, though UL does set such standards for explosion-proof motors.Synchronous Speed Motor 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"T" Frame 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.Thermal Protector A thermal protector, automatic or manual, mounted in the end frame or on a winding, is designed to prevent a motor from getting too hot, causing possible fire or damage to the motor. Protectors are generally current and temperature sensitive. Some motors have no inherent protector, but they should have protection provided in the overall system's design for safety. Never bypass a protector because of nuisance tripping. This is generally an indication of some other problem, such as overloading or lack of proper ventilation. Never replace nor choose an automatic-reset thermal overload protected motor for an application where the driven load could cause personal injury if the motor should restart unexpectedly. Only manual-reset thermal overloads should be used in such applications.Thrust Bearing Special bearings used to handle higher than normal axial forces exerted on the shaft of the motors as is the case with some fan or pump blade mountings.Torque The turning effort or force applied to a shaft, usually expressed in inch-pounds or inch-ounces for fractional or sub-fractional HP motors. The rated horsepower is what is important for continuous service. Additional ratings may be important if the motor must start against a large load, or may sometimes be significantly overloaded. Since the primary problem with an overload is thermal, a properly designed motor can tolerate an overload if it is short enough and the motor then has sufficient time to cool down before the next one. These ratings are specified as torques instead of power, were torque is rotational force. The start-up torque and pull-up torque are related to the motors ability to overcome inertia when starting loads. The break-down torque tells how much overload can be tolerated before the motor stalls. Note that the break-down torque only applies to a transient load; the continuous load still may not exceed the rated power times the service factor. And if the break-down torque is used, the current will be quite high for that time.Totally Enclosed Fan Cooled (TEFC) Has an external fan to move cooling air over the motor. Suitable for outdoor and dirty locations.Totally Enclosed Non-Ventilated (TENV) Does not have external cooling fan but is dependent on radiation and convection for cooling.Totally Enclosed Air Over (TEAO) Special motor used to drive a fan blade. Has no external fan and is dependent on air stream of driven fan for cooling.Transformer A device which converts electrical power (alternating current) to electrical power of a different voltage. In this device, both primary and secondary windings are usually stationary and are wound on a common magnetic core.U" Frame 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.Voltage Drop 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.Watt 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.Wye-Delta Starting A method for starting a motor at rated voltage but drawing locked rotor current and producing reduced locked rotor torque to provide lower starting torque than a straight delta connection. Once the load and motor have been started, the wiring will switch from the wye connection to a delta connection in which mode it must run and deliver full torque.

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