HORSE POWER (E * I * EFF) / 746 (E * I * EFF) / 746 (1.732 * E * I * EFF * PF) / 746
CURRENT (746 * HP) / (E * EFF) (746 * HP) / (E * EFF * PF) (746 * HP) / (1.732 * E * EFF * PF)
EFFICIENCY (746 * HP) / (E * I) (746 * HP) / (E * I * PF) (746 * HP) / (1.732 * E * I * PF)
POWER FACTOR —- (Input Watts) / E * I (Input Watts) / (1.73 * E * I)

E = Volts
EFF = Efficiency (decimal)
HP = Horsepower
I = Amperes
PF = Power factor (decimal)

AMPERES Watts / Volts (Watts) / (Volts * power factor) (Watts) / (1.732 * Volts * power factor)
VOLT- AMPERES ————– Volts * Amperes 1.732 * Volts * Amperes
WATTS Volts * Amperes Volts * Amperes * Power factor 1.732 * Volts * Amperes * power factor
RC = RH X (K+TC) / (K+TH)

RH = RC X (K+TH) / (K+TC)

Material               K
Aluminum         225
Copper            234.5
RC = Resistance at temperature TC (Ohms)

RH = Resistance at temperature TH (Ohms)

TC = Temperature of cold winding (OC)

TC = Temperature of hot winding (OC)


Horsepower = (Torque (lb.ft) * RPM) / 5250 Kilowatts = (Torque (N.m) * RPM ) / 9550
Torque (lb.ft) = (Horsepower * 5250) / RPM Torque (N.m) = (Kilowatts * 9550) / RPM


Synchronous RPM = (120 x Frequency) / Number of Poles
Percent slip = (Synchronous RPM – Full-load RPM) / Synchronous RPM


Alternator A synchronous machine used to convert mechanical power into alternating current electric power.
Ambient Temperature The temperature of the surrounding cooling medium. Commonly known as room temperature when the air is the cooling medium in contact with the equipment.
Base Line A vibration reading taken when a machine is in good operating condition used as a reference for monitoring and analysis.
Breakdown Torque The maximum torque an A/C motor will develop with rated voltage applied at rated frequency without an abrupt drop in speed. Also termed pullout torque or maximum torque.
Code Letter A letter, which appears on the nameplates of A/C motors, to show their locked-rotor kilovolt amperes per horsepower at rated voltage and frequency.
Constant Horsepower A term used to describe a multi-speed motor in which the rated horsepower is the same for all operating speeds. When applied to a solid-state drive unit, the term refers to the ability to deliver constant horsepower over a predetermined speed range.
Constant Torque Motor A multi-speed motor for which the rated horsepower varies in direct ratio to the synchronous speeds. The output torque is essentially the same at all speeds.
Delta Connection A three–phase winding connection in which the phases are connected in series to form a closed circuit.
Design NEMA design letters A, B, C, D, and E define certain starting and running characteristics of polyphase squirrel-care induction motors. These characteristics include locked-rotor torque, locked-rotor current, pull-up torque, breakdown torque, slip at the rated load, and the ability to withstand full-voltage starting.
Duty A continuous or short-time rating of a machine. Continuous-duty machines reach an equilibrium temperature within the temperature limits of the insulation system. Machines, which do not or cannot reach an equilibrium temperature have short-time ratings of usually one hour or less for motors.
Efficiency The ratio between useful work performed and the energy expended in producing it. It is the ratio of output power divided by the input power.
Foot-Pound The amount of work, in the English system, required to raise a 1 pound weight a distance of 1 foot.
Frequency The number of cycles in a time period  usually one second). Alternating current frequency is expressed in cycles per second, termed Hertz (Hz).
Full – Load Current The current required for any electrical machine to produce its rated output or perform its rated function.
Full – Load Speed The speed at which any rotating machine produces its rated output.
Full – Load Torque The torque required to produce rated power at full-load speed.
Harmonic A multiple of the fundamental electrical frequency. Harmonics are present whenever the electrical power waveforms (voltage and current) are not pure sine waves.
Hertz ( Hz ) The preferred terminology for cycles per second (frequency).
Horsepower A unit for measuring the power of motors or the rate of doing work. one horsepower equals 33,000 foot –pounds of work per minute (550 ft-lbs per second ) or 746 watts.
IEC International Electrotechnical Commission
IEEE Institute of Electrical and Electronics-Engineers
Insulation Non-conducting materials separating the current-carrying parts of an electric machine from each other or from adjacent conducting material at a different potential.
Insulation Class A letter or number that designates temperature rating of an insulation material or system with respect to thermal endurance.
Kilowatt A unit of electrical power. Also refers to the output rating of motors manufactured and used off the North American continent.
Locked-Rotor Current Steady-state current taken from the line with the rotor of a motor at standstill and at rated voltage and frequency.
Locked-Rotor Torque The minimum torque a motor will develop at standstill for all angular positions of the rotor with rated voltage and frequency.
MegOhmmeter An instrument for measuring insulation resistance.
Motor A rotating machine that converts electrical power, either alternating current or direct current, into mechanical power.
NEC National Electrical Code
NEMA National Electrical Manufacturers Association
Newton-Meter Unit of torque in the metric system that is a force of 1 Newton applied at a radius of 1 meter and in a direction perpendicular to the radius arm.
Part-Winding Starting A polyphase motor arranged for starting by first energizing part of its primary winding and, subsequently, energizing the remainder of the primary winding. the leads are normally numbered 1, 2, 3 (starting) and 7, 8, 9 (remaining).
Poles The magnetic poles set up inside an electric machine by the placement and connection of the windings.
Pound-Foot Unit of torque, in the English system, that is a force of 1 pound applied at a radius of 1 foot and in a direction perpendicular to the radius arm.
Power Factor The ratio of watts to volt-amperes of an ac electric circuit.
Rated Temperature Rise The permissible rise in temperature above ambient for an electric machine operating under load.
Resistance Temperature Detectors A device used for temperature sensing consisting of a wire coil or deposited film of pure metal for which the change in resistance is known functional of temperature. the most common type is nickel, with other types being copper, platinum, and nickel-iron.
Rotor The rotating element of any motor or generator.
Service Factor A multiplier which, when applied to rated power, indicates a permissible power loading that may be carried under the conditions specified for the service factor.
Slip The difference between synchronous and operating speeds, compared to synchronous expressed as a percentage. Also refers to the difference between synchronous and operating speeds, expressed in RPM.
Starting Torque The torque produced by a motor at rest when power is applied. for an A/C machine, this is the locked-rotor torque.
Stator The stationery part of a rotating electric machine. The term is commonly used to describe the station any part of an A/C machine that contains the power windings.
Synchronous The speed of the rotating machine element of an A/C motor that matches speed of the rotating magnetic field created by the armature winding.

Synchronous speed = (Frequency x 120) / (Number of Poles)

Thermistor A resistive device used for temperature sensing that is composed of metal oxides formed into a bead and encapsulated in epoxy or glass. A typical thermistor has a positive temperature coefficient; that is, resistance increases dramatically and non-linearly with temperature. Though less common, there are negative temperature coefficient thermistors.
Torque The rotating force produced by a motor. The units of torque may be expressed as pound-foot, pound-inch (English system), or newton meter (metric system).
Trending Analysis of the change in measured data over at least three data measurement intervals.
Variable Torque Motor A multi-speed motor in which the rated horsepower varies as the square of the synchronous speeds.
Wye Connection A three-phase winding connection formed by joining one end of each phase to make a “Y” point. The other ends of each phase are connected to the line. Also termed a star connection.
Wye-Delta Starting Wye-Delta is a connection used to reduce the inrush current and torque of a polyphase motor. A Wye (star) start, delta run motor is one arranged for starting by connecting to the line with the winding initially connected Wye (star). The winding is then reconnected to run in Delta after a predetermined time. The lead numbers for a single run voltage are normally 1, 2, 3, 4, 5, and 6.
Frequently Asked Questions


  • NEMA motors standards
  • Motor enclosures
  • Nameplate data
  • Standardized NEMA color code for identifying leads for a single-phase motors

NEMA Motor Standards

Many motors are manufactured according to standards set by the National Electrical Manufactures Association (NEMA). A standard motor made by one manufacturer can be replaced by the same standard motor made by another manufacturer. The motor ratings, operating characteristics, and mechanical construction and dimensions are identical for all motors manufactured according to NEMA standards. Standard motors are identified by frame number. All motors identified by the Frame Number will have same physical dimensions. These standards are part of technical motor information Buna provides.

Motor Enclosures

There are two general classifications of motor enclosures – the open motor and totally enclosed motor.
Open motors have openings to allow air to pass through and cool the windings. The openings are usually located in the end plates.
Open motors are further classified into the following categories.

  • Drip-proof
  • Splash-proof
  • Semi-guarded or guarded
  • Open pipe-ventilated
  • Open pipe-ventilated
  • Weather-protected

Totally enclosed motors are enclosed tightly enough to prevent air from entering the enclosing. However, the motors are not sealed tightly enough to be considered “airtight.” The classifications are important technical motor information. They include:

  • Totally enclosed non-ventilated
  • Totally enclosed fan-cooled
  • Explosion-proof
  • Dust ignition-proof
  • Waterproof
  • Totally enclosed pipe-ventilated
  • Totally enclosed water-cooled

Nameplate Data

A typical motor nameplate has following information:

  • NEMA Designation: Indicates the torque and current characteristics.
  • Phase: Indicates the kind of power for which the motor is designed.
  • Hertz: Indicates the frequency of the A/C power required to run the motor properly.
  • Serial: Manufacturers code number.
  • RPM: Speed of the motor at rated power. This is basic technical motor information.
  • Frame: Size defined by NEMA.
  • Time Hours: How long the motor may be operated at one time without overheating.
  • Insulation Class: The class of insulation used in motors.
  • KVA Code: Locked-rotor power input (measured in kilowatts) per horsepower of rated output.
  • Model: NEMA letter code.
  • Catalog number: Used by the manufacturer or user to identify the motor.
  • Motor Style: Number is the manufacturer’s specification.
  • Amps: Normal current drawn at the motor’s rated load, potential difference and frequency.
  • Volts: Potential difference of the power supply for the motor.
  • Ambient Temperature: The temperature immediate location of the motor.
  • Service Factor: How much the motor may be overloaded when operating at its rated potential difference and frequency.
  • Horsepower: The amount of power the motor can produce at its rated speed. This is basic technical motor information.

Standardized NEMA color code for identifying leads for a single-phase motors

  • T1 – Blue
  • T2 – White
  • T3 – Orange
  • T4 – Yellow
  • T5 – Black
  • T8 – Red
  • P1 – No color assigned
  • P2 – Brown
Maintenance for Electric Motor Bearings

Maintenance Tips for Electric Motor Bearings

Good maintenance practices yield excellent service from your electric motors. Up to 80% of all electric motors, regardless of size, are rebuilt because of bearing failures. It is a sad statistic because few of these failures are the result of true bearing steel fatigue. The majority are caused by improper bearing use or inadequate maintenance. To keep equipment running, it’s important to know basic technical motor information.. The following tips will help you provide trouble-free service.

1.  Create the best possible operating environment

The perfect operating environment for bearings is contaminant-free, low-humidity, have moderate fluctuating temperature, and no vibration. In a perfect environment, bearings would be in constant use with no static storage or downtime. Loads would be perfectly balanced, never exceeding specifications, and bearings would be lubricated continuously to eliminate friction wear. Theoretically, bearings could last indefinitely under such conditions.

In reality, the best you can do is strive to create the perfect environment. Some of the biggest contributors to bearing failure are in areas frequently overlooked in maintenance. Knowing basic technical motor information helps to create the ideal environment for bearings.

2.  Manage moisture to extend motor life

Although it is not always possible to control moisture, especially in humid environments, it can always be managed. When motors are up and running, humidity is rarely harmful, but when they are turned off and cool, condensation builds up inside, often collecting around a bearing’s rolling elements. You cannot stop condensation, but you can guard against its harmful effects. One way is to use grease fortified with rust inhibitors in bearing assemblies wherever you suspect condensation. Another way is to rotate the shafts if idle motors frequently to reestablish the protective lubricant film that should always be present between the rolling elements and the raceways.

3.  Protect idle motors from vibration

A common misconception is that bearing wear occurs only when motors are running. In reality, some of the worst wear occurs when motors are idle yet subject to shock and vibration. A secondary motor may be mounted near a primary motor or on some other assembly that is subject to vibration. When the motor is not running, the bearings vibrate in place, creating depressions in the raceway called false brinelling. It may seem like too much technical motor information, but false brinelling is important.
False brinelling occurs when the rolling elements break through the thin lubricating film separating them from raceway. The inevitable metal-to-metal contact wears depressions in the raceways. When the motor is operated, the rolling elements roll over these depressions, generating noise and shortening the bearing’s working life. Whatever steps are necessary should be taken to protect static motors from shock and vibration, including motors and vibration and motors in transit. One of the best ways to protect a motor being transported is to firmly secure the rotor shaft before transit.

4.  Align motor shafts carefully during installation

One of the greatest causes of premature bearing failure is misalignment, however minor, between the electric motor shaft and the shaft of the driven equipment. Such misalignment introduces excessive vibration and loads.
Although couplings used to join shafts are typically flexible and can accommodate misalignment, it is a mistake to take advantage of that flexibility. Any time shafts are misaligned, the working life of the electric motor is shortened. A coupling has only one desired operating position, the position it takes when at rest, like when it is on a work table free of connections. When a coupling operates in any other position, it creates unnecessary stress for the motor bearings.
Equipment and motor shafts must be properly aligned so the coupling will not bend or twist during operation. The same alignment principles hold for pulleys and belts that join shafts. Over-tightening belts also introduces unnecessary loads. Only enough tension to prevent belt slippage should be used. The best shaft alignment procedure is to secure the driven equipment first and then install the coupling to the equipment. Only after the coupling is attached to the equipment should the motor be moved into proper alignment and joined to the coupling. The final step is to secure the motor. This is important basic technical motor information.

5)  Know the ins & outs of lubrication

Everyone understands the need for lubrication in bearings, but not everyone understands how easy it is to overdo it. Proper lubrication includes the right quantity, the right interval, the right type of lubricant, and the right application method.

(i) Quantity
A new electric motor arrives with its bearings properly lubricated for the dimensions of the bearing envelope, the cavity that encompasses the rolling elements. An excess of lubricant would be potentially harmful to the bearing. As soon as a bearing’s rolling elements begin to move, they must push through whatever grease is in the cavity. If there is too much grease, that pushing requires more energy and places a greater burden on the motor.

Over-lubricating (pushing excessive grease into cavity) can actually cause undesirable heat buildup as the rolling elements continuously try to push the extra grease out of the way. Heat buildup represents friction and wear, and reduces grease life. So recommendations that come with the electric motor will ensure that it is not over-lubricated.

(ii) Interval
There is no rule of thumb for identifying correct electric motor lubrication interval. Bearing manufacturers make recommendations based on bearing size, bearing type, speed of operation, general operating environment and type of electric motor. Guidance on lubrication interval is usually provided with a motor ; If not the information can be obtained from the bearing supplier.

(iii) Type of lubricant
Not all greases are compatible. Many deep groove ball bearings come lubricated with a polyurea-based grease, a high-temperature all purpose lubricant. Polyurea lubricants are sensitive to other lubricants, particularly lithium-based greases. Before lubricating a bearing , the technician must know what grease or a compatible product. Compatibility charts are available from lubricant manufacturers.

(iv) Application Method
Most large electric motors come with a grease fitting and a drain plug, the key ports that allow lubricants in and out of the bearing cavity. The proper bearing lubrication method is to pump new grease into the bearing through the appropriate fitting and allow the old grease to exit through drain plug. However, if the drain plug clogs or if the technician forgets to open it, too much grease will be pumped into the bearing. The result, once again, is heat buildup and friction wear.
After injecting the amount of grease recommended by the motor manufacturer or the bearing supplier, stop greasing and begin or continue running the motor with the drain plug open. Run the motor with the drain plug open. Run the motor long enough to allow the bearings to purge the excess grease. When grease stops exiting the drain plug, cap the plug. At that point, the motor has the proper amount of lubrication.

6.  Consider all requirements when rebuilding

The following tips can be used as a guide or as discussion points with the rebuilder to ensure that new bearings are properly installed:

  • Use a puller to remove old bearings and take care not to damage the motor shaft.
  • Use hydraulic-assisted removal techniques if bearings are repeatedly removed.
  • Put in what you pull out. The original bearings in the motor were carefully selected for compatibility with the motor’s functional capacity.
  • Heat new bearings uniformly before fitting them over shaft. Use an induction heater or a hotplate, never a blow torch. Bearings should be heated to a maximum of 230 F. Higher temperatures are likely to a alter the microstructure of the steel, causing changes in bearing shape or hardness.
  • Decide between seals and shields for bearing protection. The decision requires some technical motor information. Seals offer the best protection from contaminant-laden operating environments because they make positive contact with the rotating inner ring of the bearing and lock out contaminants. However, seals may not be practical for all motors, particularly larger motors, because contact creates drag and heat that limit motor operating speed. In such a case, shielded bearing may be more appropriate. Shields are more practical approach for most motors because they extend close to the inner ring but do not make direct contact.

7.  Keep a motor vibration history

One of the first steps after a new motor is installed is to take a vibration reading. Regularly scheduled readings will provide a historical perspective on motor wear.

References for this technical motor information:

  • Maintenance Technology Magazine – November 1992
  • EASA standards


The technical motor information posted here came from the following sources. If you would like further technical motor information, please visit the following websites.

How to Get the Most From Your Electric Motors

Understanding Energy Efficient Motors

Outdoor Electrical Safety

Control of Hazardous Energy

OSHA fact Sheet

OSHA Occupational Safety and Health Administration

OSHA Respiratory Protection


Industrial Service Solutions is your source for many industrial services, products, and information. Take a moment
to explore the other companies in the ISS family that are eager to serve you with unparalleled variety, expertise, and value.

Choose a service below to learn more about our capabilities and offerings. If you do not find what you’re looking for,
feel free to give us a call at 239.288.5230 or email We’re always here to help.

Copyright © 2017. Buna Electric Motor Service. All rights reserved.