Induction Motors

PART ONEA BRIEF EXPLANATION of some alternating-current formulasand definitions; and a review of induction motor characteristics,insulations, and constructions.Alternating current (AC)A-C Voltage alternates regularly in value and in direction.A single-phase wave of a-c voltage is illustrated inFig. 1. One complete set of alterations, positive and negative,is one cycle or 360 electrical degrees.Inductance (L)—Any device with iron in the magneticcircuit has what amounts to magnetic inertia. This inertia iseffective on any change in current. Since the voltage, andtherefore the current, is always changing in value on alternatingcurrent, the inertia characteristic of opposing anychange in current causes the current change to lag behindthe voltage change. It also serves to limit the current value.This effect, known as inductance, is measured in henries.Inductive Resistance (X 1 )—The effect of inductancevaries with the frequency, and the term Inductive Reactanceis used to express this effect.Frequency (f) is the number of such alterations per second.Perfect alternating current is a sine wave with no ripplesor distortions.Phase—If the electrical power is supplied over two circuits,in one of which the voltage reaches zero and othercorresponding values 90 degrees later than in the other circuit,the service is Two Phase (Fig. 2). If the power is providedin what amounts to three circuits which reach correspondingvalues at 120 deg. intervals, the service is ThreePhase (Fig. 3).Impedance (Z)—On alternating-current circuits the flowof current is limited by both resistance and inductive reactance.The combination of these two elements is known asimpedance. Mathematically, when resistance and inductivereactance are present:Volts (E)—The Volt is the common measure of electricalpressure. Effective Voltage, also called RMS voltage (rootmean-squarevoltage), is .707 of the maximum value of thevoltage wave as shown in Fig. 1. Effective voltage is thevoltage measured by the usual a-c voltmeter.Amperes (I)—The Ampere is the unit rate of flow ofelectric current. As commonly used it is the effective(RMS) value of alternating current and is the value indicatedby a common a-c ammeter.Resistance (R)—Resistance is the measure of hindranceor impedance to current flow on direct current. Resistanceis measured in ohms. Fundamental Ohms law is:Actually in most a-c circuits the value of X 1 is so high comparedto the value of R that for a first approximation it isassumed:Power (W)— Electrical power is measured in watts (W),kilowatts (Kw+W/1000) or megawatts (Mw=W/1,000,000).A watt is the product of ampere effective current flowing ata pressure one effective volt. In alternating current a furthercondition is that the voltage and current must be in phase,that is they must both reach zero, maximum positive andmaximum negative values at the same instant.5