I - Terms
In most instances, the following information will help identify a motor:
1. Frame designation (actual frame size in which the motor is built).
2. Horsepower, speed, design and enclosure.
3. Voltage, frequency and number of phases of power supply.
4. Class of insulation and time rating.
The characteristic of an electric circuit by which varying current in it produces a varying magnetic field which causes voltages in the same circuit or in a nearby circuit.
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.
A load (flywheel, fan, etc.) which tends to cause the motor shaft to continue to rotate after the power has been removed (stored kinetic energy). If this continued rotation cannot be tolerated, some mechanical or electrical braking means must normally be applied . This application may require a special motor due to the energy required to accelerate the inertia. Inertia is measured in either lb.ft.2 or OZ.jn.2
Inertia reflected to the shaft of the motor = (Load RPM) 2/Motor RPM
A material which tends to resist the flow of electric current (paper, glass, etc.) In a motor the insulation serves two basic functions:
1. Separates the various electrical components from one another.
2. It protects itself and the electrical components from attack of contaminants and other destructive forces.
Five specialized elements are used, which together constitute the motor's INSULATION SYSTEM. The following are typical in an AC motor:
1. TURN-TO-TURN INSULATION between separate wires in each coil. (Usually enamel on random wound coils of smaller motors - tape on "form wound" coils of larger motors.)
2. PHASE-TO-PHASE INSULATION between adjacent coils in different phase groups. (A separate sheet material on smaller motors - not required on form wound coils because the tape also performs this function.)
3. PHASE-TO-GROUND INSULATION between windings as a whole and the "ground" or metal part of the motor. (A sheet material, such as the liner used in stator slots, provides both di-electric and mechanical protection.)
4. SLOT WEDGE to hold conductors firmly in the slot.
5. IMPREGNATION to bind all the other components together and fill in the air spaces. (A total impregnation, applied in a fluid form and hardened, provides protection against contaminants.
Since there are various ambient temperature conditions a motor might see 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:
|Class||AC Motor DC. Motor W/1.00 S.F.
Max. Total Temperature Range
(Including Ambient and Temperature
110f Hot Spot) Range
|DC Motor Total
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; or (3) load, no load and rest; such alternate 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 vary the speed of an AC motor.
Losses due to current flowing in a conductor caused by resistance (equals the current squared times the resistance.)