BASIC ELECTRIC MOTOR PRINCIPLES
Electric motors are machines that change electrical energy into mechan ical energy. They are rated in horsepower. The attraction and repulsion of the magnetic poles produced by sending current through the armature and field windings cause the armature to rotate. The armature rotation produces a twisting power called torque.
FLEMING’S LEFT-HAND RULE FOR MOTORS. Place the thumb, first finger, and remaining fingers at right angles to each other. Point the first finger in the direction of the field flux and the remaining fingers in the direction of the armature current, and the thumb will indicate the direction of rotation.
The direction of rotation can be reversed on any de motor by revers ing either the armature or field leads but not both. It is standard practice to reverse the armature leads to reverse the direction of rotation.
The amount of torque developed by a motor is proportional to the strength of the armature and field poles. Increasing the current in the armature or field winding will increase the torque of any motor.
The armature conductors rotating through the field flux have a volt age generated in them that opposes the applied voltage. This opposing voltage is called counterelectromotive force (CEMF) and serves as a governor for the de motor. After a motor attains normal speed, the current through the armature will be governed by the CEMF generated in
the armature winding. This value will always be in proportion to the me chanical load on the motor.
The applied voltage is the line voltage. The effective voltage is the voltage used to force the current through the resistance of the armature
winding. This value can be determined by multiplying the resistance of the armature by the current flow through it. To find the resistance of the armature, measure the voltage drop across the armature and the current flow through it and use the Ohm’s law formula: R equals E over I.
The hp rating of a motor refers to the rate of doing work. The amount of hp output is proportional to the speed and torque developed by the motor.
BASIC MOTOR TERMS AND NAMEPLATE INFORMATION
Before covering electric motor applications and details of their mainte nance and repair, the reader should be thoroughly familiar with certain motor terms. Some of the more basic ones follow and are provided cour tesy of Westinghouse Electric Corp.
Style number: Identifies that particular Westinghouse motor from any other. Other manufacturers also provide style numbers on the motor nameplate and in the written specifications.
Serial data code: The first letter is a manufacturing code used at the factory. The second letter identifies the month, and the last two num bers identify the year of manufacture (D78 is April 78).
Frame (fr): Specifies the shaft height and motor mounting dimensions and provides recommendations for standard shaft diameters and usable shaft extension lengths.
Service factor: A service factor is a multiplier which, when applied to the rated horsepower, indicates a permissible horsepower loading which may be carried continuously when the voltage and frequency are maintained at the value specified on the nameplate, although the motor will operate at an increased temperature rise.
NEMA service factors: Open motors only.
Phase: Indicates whether the motor has been designed for single or three phase. It is determined by the electrical power source.
Thermoguard® motors: Incorporates an overload protector, which
is a heat-sensing device either attached to the motor winding or mounted in the end bell. The two most common types of Thermoguard motors are the following:
1. Type A: This is an automatic thermal device approved by UL. It will stop the motor when it overheats and will automatically restart the motor when it has cooled to a safe operating tempera ture.
2. Type M: This is a manual reset overload device which is ap proved by UL. It will also stop the motor when it overheats but will not start unless the thermal protector button is manually reset.
Degree C ambient: The air temperature immediately surrounding the motor. Forty degrees centigrade is the NEMA maximum ambient temperature.
Insulation class: The insulation system is chosen to ensure the motor will perform at the rated horsepower and service factor load.
Horsepower: Defines the rated output capacity of the motor. It is based on breakdown torque, which is the maximum torque a motor will develop without an abrupt drop in speed.
rpm: Revolutions per minute … speed. The rpm reading on motors is the approximate full-load speed.
The speed of the motor is determined by the number of poles in the winding.
A two-pole motor runs at an approximate speed of 3450 rpm. A four pole motor runs at an approximate speed of 1725 rpm. A six-pole motor runs at an approximate speed of 1140 rpm.
Amps: Gives the amperes of current the motor draws at full load. When two values are shown on the nameplate, the motor usually has a dual voltage rating. Volts and amps are inversely proportional; the higher the voltage, the lower the amperes, and vice versa. The higher amp value corresponds to the lower voltage rating on the nameplate. Two speed motors will also show two ampere readings.
Hertz (cycles per second): Just about everything in this country is serviced by 60-Hz alternating current. Therefore, most applications will be for 60-Hz operations.
Volts: Volts is the electrical potential “pressure” for which the motor is designed. Sometimes two voltages are listed on the nameplate,
such as 115/230. In this case the motor is intended for use on either a 115 or 230 circuit. Special instructions are furnished for connecting the motor for each of the different voltages.
k VA code: This code letter is defined by NEMA standards to designate the locked rotor kVA per horsepower of a motor. It relates to starting current and selection of fuse or circuit breaker size.
Housing: Designates the type of motor enclosure. The most common types are open and enclosed: Open drip-proof has ventilating open ings 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° downward from the vertical. Open guarded has all open ings giving direct access to live metal or hazardous rotating parts so sized or shielded as to prevent accidental contact as defined by probes illustrated in the NEMA standard. Totally enclosed motors are so con structed to prevent the free exchange of air between the inside and out side of the motor casing. Totally enclosed fan-cooled motors are equipped for external cooling by means of a fan that is integral with the motor. Air over motors must be mounted in the airstream to obtain their nameplate rating without overheating. An air-over motor may be either open or enclosed.
Explosion-proof motors: These are totally enclosed designs built to withstand an explosion of gas or vapor within the motor and to prevent ignition of the gas or vapor surrounding the motor by sparks or explo sions which may occur within the motor casing.
Hours: Designates the duty cycle of a motor. Most fractional horsepower motors are marked continuous for around-the-clock operation at the nameplate rating in the rated ambient. Motors marked “one half’ are for Y2-h ratings, and those marked “one” are for 1-h ratings.
The following sections consist of terms not found on the nameplate, but they are important considerations for proper motor selection.
BEARINGS TERMS
Sleeve bearings: Sleeve bearings are generally recommended for axial thrust loads of 210 lb or less and are designed to operate in any mounting position as long as the belt pull is not against the bearing win dow. On light-duty applications, sleeve bearings can be expected to per form a minimum of 25,000 h without relubrication.
Ball bearings: These are recommended where axial thrust exceeds 20 lb. They too can be mounted in any position. Standard and general purpose ball bearing motors are factory-lubricated and under normal con ditions will require no additional lubrication for many years.
MOUNTING TERMS
Rigid mounting: A rectangular steel mounting plate which is welded to the motor frame or cast integral with the frame; the most com mon type of mounting.
Resilient mounting: A mounting base which is isolated from motor vibration by means of rubber rings secured to the end bells.
Flange mounting: A special end bell with a machined flange which
has two or more holes through which bolts are secured. Flange mount ings are commonly used on such applications as jet pumps and oil burners.
Rotation: For single-phase motors, the standard rotation, unless
otherwise noted, is counterclockwise facing the lead or opposite shaft end. All motors can be reconnected at the terminal board for opposite rotation unless otherwise indicated.