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Induction Motor Reference Guide

Induction Motor Reference Guide

Enclosures & Frames

There are two broad enclosure classifications: OPEN and TOTALLY ENCLOSED. Each is divided into a number of specific types. The primary types are as follows:

Drip ProofNon Ventilated
Splash ProofFan Cooled
Weather ProtectedExplosion Proof
Externally Ventilated
Heat Exchanger

Only those most frequently used will be discussed here.

Drip Proof

The NEMA definition for this type of enclosure is “a drip-proof machine is an open machine in which the ventilating openings are so constructed that successful operation is not interfered with when drops of liquid or solid particles strike or enter the enclosure at any angle from 0 to 15 degrees downward from the vertical.” In the drip-proof motor, the cooling air is drawn into the motor from the outside and comes into direct contact with the winding, rotor, and all other internal parts of the motor. Therefore, the internal motor parts must be able to withstand any contaminants present in the atmosphere in which the motor is located.

The open drip-proof motor is designed to be an economical choice for most indoor applications where reasonably clean environments exist. As the name implies, this motor is adequately protected so that water dripping from overhead will not be drawn into the motor. The open drip-proof (ODP) should not, however, be used on applications where washdown with hoses is apt to occur or when driving rain, fog or snow will occur.

Totally Enclosed

A totally enclosed motor is designed to prevent the free exchange of air between the inside and the outside of the motor housing, but not sufficiently tight to be termed airtight. When the motor is equipped with an integral fan that blows air over the exterior surface of the frame, the motor is classed as being TOTALLY ENCLOSED FAN-COOLED (TEFC). Without an external cooling fan, the motor is classed as being TOTALLY ENCLOSED NON-VENTILATED (TENV).

Totally Enclosed Fan Cooled

Cooling of TEFC motors takes place due to a continuous flow of air over the outside surface of the motor. The motor’s surface is usually ribbed to obtain additional surface area for heat dissipation. The airflow is produced by a fan that is mounted on the shaft at the front of the motor. In order to provide power to drive this fan, the motor shaft is extended through the front of the motor and a suitable fan is mounted on this stub shaft. For protection and direction control of the airflow, a fan cover is installed over the fan. Fan covers are either fabricated with pressed steel or for severe duty applications constructed of cast-iron.

Breather / Drains

To prevent against damaging amounts of water, every standard TEFC motor is supplied with one or two drain holes in the lowest point of the motor frame. This hole can easily be closed with a tapped pipe plug. Advantage Plus and Optim® TEXP motors are supplied with automatic Breather/Drains as standard equipment. Such breathers allow the motor to drain any condensed moisture without allowing contaminants to be sucked into the motor during operation. Breather/Drains can be installed in any TECO-Westinghouse motor on frame sizes 140T or larger. Breather drains can also be installed or relocated to accommodate vertical mounting on any TECO-Westinghouse motor with the exception of Optim® TEXP due to restrictions with the certifications.

Space Heaters

Space heaters can also be installed inside the motor enclosure to heat the inside air and maintain an internal temperature above the ambient dew point. This assures that cooling air will not condense and produce moisture inside the motor. Space heaters available for motors are the wrap-around type. Space heaters are rated for operation on 120V or 240V, 60 Hz, single phase power and are installed in the stator winding end-turns. Space heater leads can be terminated in the main conduit box or auxiliary box located off the main conduit box.

Rotating Shaft Seals

Special protection is provided at the point where the motor shaft passes through the bearing housing in the form of an external flinger. This flinger keeps water, dust and other foreign matter out of the motor that might enter the motor along the shaft. All motors are furnished with a rotating neoprene flinger or machined metal labyrinth seal. INPRO/SEALS™ are optional for Advantage Plus Motors. TEXP motors are furnished with shaft seals approved for hazardous locations as defined by CSA.

Sealed Leads

Additional protection against moisture is provided by a neoprene seal through which each motor lead wire is brought out of the motor frame to the terminal box. This is a standard feature for all TECO-Westinghouse motors.

Explosion Proof

The Standard explosion proof motor is to be used in all hazardous atmospheres listed under the following classifications:

  • Class I, Group D, gasoline, naptha, alcohols, acetone, lacquer solvent vapors and natural gas
  • Class II, Group E, metal dust including magnesium and aluminum or their commercial alloys
  • Class II, Group F, carbon black, coal or coke dust
  • Class II, Group G, flour, starch or grain dust

Note: Under Class 1 only, there are two divisions which allow some latitude on motor selection:

Generally, Class I, Division 1, locations are those in which the atmosphere is or may be hazardous under normal operating conditions. Included are the locations which can become hazardous during normal maintenance. An explosion-proof motor is mandatory for Division 1 locations.

Class I, Division 2, refers to locations where the atmosphere may become hazardous only under abnormal or unusual conditions (breaking of a pipe, for example). In general, motors in standard enclosures can be installed in Division 2 locations if the motor has no normally sparking parts. Thus, open or standard totally enclosed squirrel cage motors are acceptable, but motors with open slip rings or commutators (wound rotor, synchronous or DC) are not allowed unless the commutator or slip rings are in an explosion-proof enclosure. Optim® TEFC and Advantage Plus motors are approved for Class I, Division 2, Group B, C and D environments.

Explosion proof motors bear a label that is certified by Underwriters’ Laboratories and CSA. This label can be applied only by a motor manufacturer, such as TECO-Westinghouse, whose facilities are under the constant supervision of Underwriters’ Laboratories and CSA. Once a motor is disassembled in the field, the label no longer applies, unless the work is performed in a plant approved by Underwriters’ Laboratories or CSA.

A Word of Caution

It is the user’s responsibility to specify the atmosphere in the plant in which motors are to be applied. The user normally receives a classification from their insurance company as to the type of motor enclosure to be used where a hazardous atmosphere exists. If the user asks for recommendations, ask him to get a classification from his insurance company.

Externally Ventilated

Occasionally, an external cooling system is used. This is necessary in case the motor’s own fan will not provide sufficient cooling; such as on certain duty-cycle applications, planer drives, certain slow speed motors, etc. In other cases, the motor may be located in an extremely dirty atmosphere where it is desirable to connect the motor’s air intake to a source of clean air by means of a duct. In either case, the air discharge can either be into the room directly from the motor, or it can be carried away from the motor through a pipe.

Motors with motor-mounted, separately driven blowers are usually called “force ventilated.”

Motors ventilated from an external source are called “pipe ventilated.” In this case, the air pressure can be supplied by an external blower, or an oversize fan can be used in the motor itself to draw air through the intake duct and force it through the motor.

Frame Size

Motor frame dimensions have been standardized with a uniform frame size numbering system developed by NEMA. The current standardized frames for integral horsepower induction motors range from 143T to 449T. These standards cover most motors in the range of 1 through 250 Horsepower.

The numbers used to designate frame sizes have specific meanings based on the physical size of the motor. The first two digits are related to the motor shaft height and the remaining digit or digits relate to the length of the motor.

The shaft height of horizontal motors can be calculated by dividing the first two digits of frame size by 4.

For example, a 256T frame motor would have a shaft height (“D”) dimension of 25/4 = 6.25 inches. Similarly, a 444T frame motor would have a shaft height of 44/4 = 11 inches.

The third digit of the frame size is related to the length of the motor. There is no general rule of thumb that can be easily applied to determine the length of the motor.

It is important to note that when standard foot mounted motors have frame sizes that differ only in the third digit, the shaft diameters, shaft lengths and distance from the end of the shaft to the bolt holes in the feet on the shaft end of the motor will be the same. It is the motor length or distance from the drive end bolt-down holes to the “front” bolt-down holes that will vary.

Frame Suffix Letters

When a motor is provided with special mounting dimensions, flanges or special shaft features, this is indicated by adding a suffix letter to its frame number.

FOR EXAMPLE: A 254TS frame motor indicates that the motor has a shaft extension length and diameter different from that of a 254T frame motor.

The standard suffix letters and their meanings are as follows:

  • C = C flange mounting
  • D = D flange mounting
  • P or PH = Vertical P or PH flange mounting
  • T = Standard shaft for belted service
  • TS = Standard short shaft for coupled service
  • Y = Special mounting dimensions
  • Z = Special shaft extension

Mechanical Considerations

Motor Mounting

Installation Requirements

The factors to consider in connection with the mounting requirements of the specific installation are:

First, how is the motor to be mounted?

There are two main categories:

  • Some type of base that will accommodate standard motor feet.
  • Installation in such a way that a special mounting flange is required on the motor.

Second, in what position is the motor to be mounted?

The possibilities include:

  • Horizontal
  • Vertical
  • Anglular

Third, is other equipment to be mounted on the motor?

This may include:

  • Pumps
  • Brakes
  • Gears
  • Zero-speed switches
  • Tachometers

Fourth, where is the conduit box to be located on the motor frame to best meet the needs of the installation’s wiring arrangement?

Two points must be checked:

  • The position of the conduit box on the motor frame.
  • The position of the conduit opening.

Conduit Box Location

The standard location for the conduit box is on the right-hand side when facing the “front” of the motor. This is referred to as F1.

Definition of “Front”

There is still some confusion regarding which end of a motor is its “front.” Since this is a frequently used term, it is important that everyone is consistent.

NEMA defines the “front” of a motor as the end opposite the shaft extension. (“When a motor has a flange, the flanged end is always considered to be the “rear”).

The reason for this is that most motors are mounted with the shaft extension more or less hidden from view. Thus, the “front” is the end you normally see.

Foot Mounting

Most of the mounting problems encountered can be solved with one of the assembly combinations of a standard foot mounted motor. A thorough understanding of the basic components and how they can be positioned will result in a greater use of standard motors.

The Frame includes the mounting feet and forms the foundation for the complete assembly. It can be positioned with the feet in any plane . . . top, bottom or side.

The Conduit Box can be located on either side of the frame.

  • F-1 = Conduit box on right-hand side of frame when facing “front” of motor.
  • F-2 = Conduit box on left-hand side of frame when facing “front” of motor.

The conduit opening can be placed in any one of four positions by rotating the conduit box on its axis in 90º steps.

A standard motor can be adapted to any of these standard mounting methods. TECO-Westinghouse will provide motors configured as ordered, either F-1 or F-2. Price adders may apply.

Angle Mounting

Where the application requires the shaft to be at an angle to the horizontal, special attention must be paid to the motor bearings.

Grease-lubricated ball bearings are suitable for operation at any angle. However, motors equipped with sleeve bearings or slipring induction motors are limited to angle mounting that does not exceed 10º from the horizontal.

Flange Mounting

There are three basic types of flange mountings; each are designed to meet different installation requirements.


This type of flange is used with close-coupled pumps and similar applications where the mounting holes in the flange are threaded to receive bolts from the pump. Normally the “C” flange is used where a pump or similar item is to be overhung on the motor, which then must have feet.


When the motor is to be built-in as part of the machine, such as on machine tools, the “D” flange is usually selected. The mounting holes in this flange are not threaded. Bolts protrude through the flange from motor side. Normally, “D” flange motors are supplied without feet, since the “D” flange is generally used to mount the motor on the driven machine. The motor is lined up with the driven machine by tightening the bolts to press the machined surface and ring of the flange against corresponding surfaces.


The primary application of this flange is on such equipment as pumps and agitators where the motor is usually mounted in a vertical position on top of the equipment.

Vertical Mounting

When a horizontal motor is to be mounted in the vertical position special considerations must be made to ensure proper operation and motor life. The considerations are dependent on the motor shaft orientation.

  1. Shaft Down:
    • Drain holes/breathers must be relocated to the lowest part of the motor and the original holes plugged. (Except TEXP Motors)
    • Drip cover should be installed for outdoor applications, or if motor is exposed to overhead/spraying moisture
    • *NDE bearing must be locked with a shaft locknut 5000 frames and larger or if a Roller Bearing is installed on the DE.
    • C-Flange motors to be mounted on a pump must have the DE BRG locked. The only axial movement will be the internal clearance of the bearing.

* Motor shall not be subject to additional axial loading from driven equipment.

  1. Shaft Up:
    • Drain holes/breathers must be relocated to the lowest part of the motor and the original holes plugged. (Except TEXP Motors)
    • If an inpro/seal is to be used it should be suitable for vertical use with no expulsion port.

Motor Designs

Power Supply

Motor Efficiency

Starting Methods

VFD Driven Motor Application


Field Service


Useful Formulas