Theory Test 2

What is a Motor?

Converts electric energy into mechanical energy

What do DC Motors operate on?

They operate on principle of attraction and repulsion of magnetism

Factors that determine torque

• Strength of magnetic field of pole pieces

• The Armature

What is Torque

It is proportional to armature current if field excitation current remains constant

Factors that determine CEMF produced

• Strength of magnetic field of pole pieces

• Number of turns of wire in the armature

• Speed of the armature

Basic types of DC motors

• Shunt motors

• Series motors

• Compound motors

Direction of rotation of a DC motor

Can be changed by reversing the connection of either the armature or the field leads

What is the Shunt field relay

Used to disconnect power to the armature if shunt field current drops below a certain leve

What is a Brushless DC motor

• Do not contain a wound armature, commutator, or brushes

• Require a converter to change DC into multiphase AC

• Require less maintenance and have less downtim

Effective Value is?

RMS

Can AC and DC be transformed?

AC can be transformed and DC cannot

What does AC do to direction

Reverses direction of flow at periodic intervals

What is the most common AC waveform?

Sine wave

What are Sine waves produced by

rotating machines

Instantaneous voltage on a Sine Wave

is equal to the peak, or maximum, voltage times the sine of the angle of rotation

What is Peak-Peak Voltage?

It is the amount of voltage measured from the positive-most peak to the negative-most peak

What is Peak Value?

Peak value is the maximum amount of voltage attained by the waveform

What is RMS value?

RMS value of voltage will produce as much power as a like amount of DC voltage

What is Average Value?

Average value of voltage is used when an AC sine wave is changed into DC

What is a Pure resistive circuit?

When current and voltage are in phase

What is True power, or watts?

Produced only when current and voltage are both positive or both negative

Resistance in AC circuits

the resistive part will produce heat

What are the three basic types of AC loads?

• Resistive,

• Inductive

• Capacitive

What do Electrons in an AC circuit do?

They are forced toward the outside of the conductor by eddy current induction in the conductor itself

What is Induced voltage proportional to?

• Proportional to the rate of change of current

What is Induced Voltage always opposite to?

• Always opposite in polarity to the applied voltage

What is Inductive reactance (XL)

• Countervoltage that limits current flow

• Measured in ohms

What is Inductive reactance (XL) proportional to?

• Proportional to coil inductance and line frequency

Inductance (L) is measured in?

Measured in henrys (H)

When Inductors are connected in Series

the Total inductance is equal to sum of inductors

Inductors that are connected in parallel?

Reciprocal of the total inductance is equal to sum of reciprocals of inductors

What do Inductors have?

Current lags with applied voltage by 90 degrees

What do Inductors contain?

Contain some amount of resistance

Reactive power is measured in?

Volts-amperes-reactive (VARs)

Q: ratio of inductive reactance to resistance is considered?

To be “pure” inductors

What do Pure Inductive circuits contain?

Contains no true power

Shunt motors are?

constant-speed motors

Series motors have?

Extremely high starting torque

Compound motors?

• contain series and shunt field windings

How does a megger work?

Works by generating a high DC voltage and measuring the tiny amount of leakage current that flows through or across electrical insulation to calculate resistance using Ohm's Law