Review of Alternators, Generators and Motors

Commutating field is normally marked:

C1/C2

Brushless DC motors contain a wound armature and commutator but do not contain brushes.

False

Permanent magnet direct current motors with iron armatures have a problem with cogging at high speeds.

False

Compound motors can be connected as short shunt or long shunt just as compound generators can. They can also be cumulative-compounded motors or differential-compounded motors.

True

The speed regulation of a direct current motor is proportional to the resistance of the armature.

True

There are several ways of controlling the speed of direct current motors. The method employed is generally dictated by the requirements of the load.

True

If the series motor is marked to ANSI standards, then which of the following statements are true:

V+ applied to A1, V- applied to S2, S1/A2 tied together will produce counterclockwise rotation

The most common item to fail on a DC motor is/are:

brushes

A ___ exciter is an AC generator and rectifier mounted on a motor shaft.

brushless

A(n) ____ compound generator has its fields connected in such a manner that they oppose each other in the production of magnetism.

differential

The simplest starting method for a synchronous motor is ___ starting, where the motor is wired directly across the line conductors.

direct online (DOL)

(Generator): Wave wound armatures are used in machines designed for ____.

high voltage and low current

Alternators are rated in ___ at a standard power factor of 80%.

kilowatts (kW)

As the rotor begins to move, the frequency of the induced current in the rotor decreases, until the rotor approaches synchronous speed, where the rotor frequency approaches ___ Hz.

0

Shaded-pole motors are commonly ___ HP or less and have low starting torque.

1/20

(Motors): How many watts (W) is one horsepower (HP)?

746

When the motor reaches approximately ___% of full speed, the centrifugal switch opens, disconnecting the start winding from the circuit.

75

A synchronous motor has a three-part rotor.

False

The direction of rotation of 3-phase motors cannot be reversed by interchanging any of the 3-phase power lines to the motor.

False

The three basic types of armature windings are the lap, wipple, and dogleg.

False

When a load is connected to the output of a compound generator, the initial voltage produced by the residual magnetism of the pole pieces produces a current flow through the load.

False

____ motors are often referred to as constant speed motors.

Shunt

The voltage output of an AC alternator must be kept constant.

True

To reverse the direction of most single-phase motors, all that is necessary is to switch the two start leads in the circuit.

True

Typical applications of capacitor motors include refrigerators, air conditioners, air compressors, and some power tools.

True

When starting a synchronous motor, the final step is to simultaneously remove the discharge resistor from the circuit and apply the DC voltage to the field windings on the rotor.

True

While almost all 3-phase motors have very similar stator designs, the main difference between different types of 3-phase motors is the design of the rotors and windings.

True

The motor ___ indicates whether a motor is rated to operate at either 50 Hz, 60 Hz, or both.

nameplate

In many cases, a single-phase motor is constructed with both a start and a ___ winding in the stator.

run

For a shaded-pole motor, the stator has two or more ___ poles that are divided into parts.

salient

Small permanent magnet motors are used as ____.

servomotors

The function of the ___ is to delay the magnetic flux in the area of the pole that is shaded.

shaded pole

There are three basic types of direct current generators: the series, ____, and compound.

shunt

A ___ alternator consists of a stationary magnet and a single winding that is rotated in the field.

single-phase AC

Although a capacitor-start motor develops less starting torque than a split-phase motor, both motor types are the same when running because the capacitor and start windings are removed from the circuit when the motor comes up to speed.

False

Each phase must have an unequal number of coils, so the total number of coils must be divisible by the number 3.

False

Single-phase, 120 V/240 V power has four conductors, with a potential of 240 V between conductors A and B.

False

The amount of compounding in a DC machine is controlled by connecting a high-value variable resistor in parallel with the series field.

False

The voltage produced in all rotating commutators is alternating voltage.

False

Three types of capacitor motors are the capacitor shut-down motor, capacitor-run motor, and the capacitor start-and-run motor.

False

Voltage is a twisting force that is used to rotate a motor shaft.

False

____________________ torque is the accelerating torque required to bring a load up to the correct speed. Incorrect answer: (no answer)

Pull-up, Pull up

___ control of shaded-pole motors is accomplished by adding windings in series with the main winding.

Speed

___ torque is provided by the interaction of the stator field and the rotor field windings.

Synchronous

____________________ is typically measured in foot-pounds (ft-lb), pound-feet (lb-ft), and inch-ounces (in.-oz).

Torque

A shaded-pole motor is an AC motor that uses a shaded stator pole for starting.

True

All the alternators must be connected in parallel and the power from each alternator must be at exactly the right phase, frequency, and voltage before it can be connected to the busbars.

True

In an induction motor, the shorted secondary in the rotor carries the induced secondary current.

True

Lap wound armatures are generally constructed with large wire because of high current.

True

Locked-rotor torque is also referred to as breakaway or starting torque.

True

Synchronous motors are similar to induction motors in that both have stator coils that produce a rotating magnetic field.

True

The amortisseur winding of a synchronous motor is generally smaller than the squirrel-cage winding of a comparable induction motor because the winding is only used for starting and is not expected to dissipate heat from continuous operation.

True

The amount of output voltage produced by a generator is proportional to three factors, including the number of turns of wire in the armature, the strength of the magnetic field of the pole pieces, and the speed of the cutting action (speed of rotation).

True

The main difference between different types of 3-phase motors is the design of the rotors and windings.

True

The purpose in making motors for two voltages is to enable the same motor to be used with two different power line voltages.

True

The rotor of a single-phase motor is the same as in an induction squirrel-cage motor.

True

The stator is enclosed within a housing made of cast iron, rolled steel, or cast aluminum.

True

There are no physical electrical connections between the stator and the rotor.

True

(Generators): Field ____ current is the DC current that flows through the shunt field winding.

excitation

A(n) ___ motor is an electric motor that uses the principles of mutual induction to develop current and torque in the rotor.

induction

A(n) ____________________ is the rotating, moving part of a motor, consisting of a core and windings, which convert the rotating magnetic field of the stator into the torque that rotates the shaft.

rotor

A ___ can be used to help match the frequency and phase of an alternator to the busbar, or to match two alternators

synchroscope

A ___ motor is commonly wound to run on either six or eight poles.

two-speed, split-phase

A(n) ___ is used to control the main alternator voltage.

voltage regulator

The stator ___ consist of coils of copper wire placed in slots 120 electrical degrees apart.

windings

How many horsepower is a motor that has the following information: Efficiency: 75% Input Voltage: 90VDC Amp Draw at Full Load: 2.76A Note: Pick the HP rating closest to the value you compute.

1/4 HP

A factor controlling the output voltage of an alternator is ___.

a. the speed of the rotor b. the strength of the magnetic field c. the length of wire

Series motors have the ability to develop extremely high starting torques. An average of about ____% of full torque is common.

450

(Generators): ____ is characterized by the fact that the output voltage at full load will be greater than the output voltage at no load.

Over-compounding

___ is the lack of rotation when the stator's field and the rotor's field are parallel to one another.

Locked in step

A capacitor-start motor operates much the same as a split-phase motor, in that it uses a centrifugal switch that opens at approximately ___% to ___% of full-load speed.

60; 80

To increase the amount of output voltage, it is common practice to increase the number of turns of wire for each loop. If a loop contains 20 turns of wire, the induced voltage will be 40 times greater than that for a single-loop conductor.

False

___ is the opposition to the flow of AC in a circuit due to inductance.

Inductive reactance

____________________ torque is the torque a motor produces when the rotor is stationary and full power is applied to the motor.

Locked-rotor, Locked rotor

The common types of torque related to motors are locked-rotor torque, and ___ torque.

a. breakdown b. pull-up c. full-load

Alternators are placed in parallel to ___

a. provide power during maintenance of other alternators b. carry added load when needed c. enhance reliability by having multiple alternators in operation

A typical dual-voltage, 3-phase motor rating is ___.

all of the above

A ___ motor is a single-phase motor with a capacitor connected in series with the start windings to produce phase displacement in the start winding.

capacitor

___ power is the product of the total current and voltage in a circuit.

Apparent

A capacitor-run motor has the same starting torque as a capacitor-start motor.

False

A single-phase motor does not have a method to create a rotating magnetic field.

False

In a typical two-speed, split-phase motor, one run winding and two start windings are used to develop two separate speeds.

False

In order to start automatically, some single-phase motors use a capacitor winding.

False

One advantage of interpoles is that they restore the field outside their immediate area and are able to overcome all the field distortion.

False

(Motor): The amount of counter-EMF produced in the armature is proportional to three factors: the number of turns of wire in the armature, the strength of the magnetic field of the pole pieces, and the speed of the armature.

True

A rotor always rotates at exactly the correct speed to induce the current required to develop the torque necessary to drive the load.

True

At approximately 95% of synchronous speed, DC excitation is applied to the wound rotor section.

True

Countertorque is a measure of the useful electrical energy produced by a generator.

True

Each rotation in a single-phase AC alternator produces one phase of electricity.

True

If the synchronous motor is overloaded to the point where the rotor is pulled out of step with the rotating magnetic field, no torque is developed and the OSR takes the motor off-line.

True

In a delta-connected, 3-phase motor, the current starts to change as the sine wave rotates through the cycle and the amount of current flowing through each coil changes.

True

The capacitors used to start motors are electrolytic capacitors.

True

A ___ motor has one capacitor in series with the start winding and one capacitor in series with the run winding.

capacitor start-and-run

A shorting ring, or ___ ring, is a metal ring used to electrically connect the bars of a squirrel-cage rotor at the end of the cage frame.

conducting

A series DC motor, when disconnected from the load while running, may:

continue to speed up past its operational limits

When a(n) ____________________ motor is connected for low voltage, L1 to T1, L2 to T2, and L3 to T3 are connected at the starter motor.

dual-voltage, delta-connected, 3-phase

The ___ typically is a shunt- or compound-wound DC generator with the armature in parallel with the shunt field and its shunt field rheostat.

exciter generator

A generator is a device that converts ____ energy into electrical energy.

mechanical

The speed at which a split-phase motor runs is determined by the number of ___ in the motor and the frequency of the supply voltage.

poles

A ___ is the power source used to create the relative motion between the coil and the magnetic field.

prime mover

Permanent magnet DC motors can act as a generator when:

rotational force (torque) is applied to the motor shaft by another device

Two factors that determine the amount of torque produced by a direct current motor are the strength of the magnetic field of the pole pieces and the ____.

strength of the magnetic field of the armature

You have a DC motor with the following markings: A1, A2, F1, F2. What do they mean?

this is a shunt motor, armature is A1/A2 and field is F1/F2

Induced secondary current interacts with the magnetic field and generates the ___ that turns the rotor and shaft.

torque

A single-___ motor is a motor that operates at only one voltage level.

voltage