AD/DC Machines: Motors and Generators
Because brushes are made from a material that is softer than the copper bars of the commutator, this permits the brushes to wear instead of the commutator.
True
In a generator, the _____ permits part of the current that normally flows through the series field to flow through the resistor.
series field shunt rheostat
Which of the following are used to operate tape drives on computers?
servomotors
The following formula can be used to determine the speed of the rotating magnetic field when the frequency and number of stator poles per phase are known: S = _____.
(120 x f) / P
When using a converter, turning the _________ on or off in the proper sequence means that current can be routed through the stator windings.
Transistor
Which type of generator must be self-excited?
series
There may be occasions when one DC generator cannot supply enough current to operate the connected load. When this occurs, another generator is connected in series with the first.
False
A square wave voltage varies between 0 V and 12 V. The positive pulses are 25 μs wide and there is a 75 μs gap between them. What is the DC average of the waveform?
3V
A square wave voltage varies between 0 V and 12 V. The positive pulses are 50 μs wide and there is a 50 μs gap between them. What is the DC average of the waveform?
6V
A square wave voltage varies between 0 V and 12 V. The positive pulses are 75 μs wide and there is a 25 μs gap between them. What is the DC average of the waveform?
9V
_____ is the twisting or bending of the magnetic lines of flux of the pole pieces.
Armature reaction
The _____ ride against the commutator segments and are used to connect the armature to the external circuit of the DC machine.
Brushes
What type of DC motor uses a rotor shaped like a hollow cylinder or cup?
Brushless
What type of generator is most used by large DC machines?
Compound
Brushes are made from a material that is softer than the __________ bars of the commutator.
Copper
A motor is a device used to convert electrical energy into potential energy.
False
A series motor may be operated with no load connected.
False
In a DC motor, terminals A1 and A2 are connected to the shunt fields.
False
Many large DC compound motors intended to operate in an overspeed condition will actually contain three separate shunt fields.
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 simplest method of setting the brushes to the neutral plane position is to connect a DC voltmeter across the series field leads.
False
The voltage produced in all rotating commutators is alternating voltage.
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
_____ left-hand generator rule can be used to determine the relationship of the motion of the conductor in a magnetic field to the direction of the induced current.
Fleming's
Armatures are wound to provide high voltage, high current, or some specific combination of voltage and current. Which type of winding provides low voltage and high current?
Lap wound
In a compound generator, which connection has the shunt field connected in parallel with both the armature and series field?
Long shunt
A generator is a device that converts _____ energy into electrical energy.
Mechanical
The ServoDisc motor is also often called a _____ motor.
Printed circuit
The speed of the disc servomotor can be varied by changing the amount of voltage supplied to the armature. The voltage is generally varied using _____.
Pulse width
During a 360° rotation of the loop, the polarity of voltage remained the same in both halves and waveform.
Rectified DC voltage
The series generator has only a series field, which is connected in _____ with the armature.
Series
Which type of generator's amount of output voltage produced is proportional to the number of turns of wire in the armature, the strength of the magnetic field of the pole pieces, and the speed of rotation?
Series
When the field winding of a DC motor is connected in parallel with the armature, the motor is called a _____ motor.
Shunt
_____ motors are often referred to as constant speed motors.
Shunt
When using the right-hand motor rule, what indicates the direction of thrust of the armature?
Thumb
Compound motors can be connected as a short shunt or long shunt just as compound generators can. They can also be cumulative-compounded motors or differential-compounded motors.
True
Converters are used to change the DC into multiphase AC to produce a rotating magnetic field in the stationary armature or stator of the brushless DC motor.
True
DC motors and DC generators use the same terminal identification.
True
Flashing the field results in polarity of the output voltage being reversed when the machine is restarted as a generator.
True
Most DC machines are designed in such a manner that the position of the brushes on the commutator can be set or adjusted.
True
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
The efficiency of a motor can be found by using the following formula: (power out / power in) x 100.
True
The speed regulation of a direct current motor is proportional to the resistance of the armature.
True
The terminal leads of DC machines are labeled so they can be identified when they are brought outside the motor housing to the terminal box.
True
The voltage produced in all rotating armatures is alternating voltage
True
What is a device that converts mechanical energy into DC electrical energy?
a generator
The speed regulation of a DC motor is proportional to the _____.
armature resistance
The relationship of the strengths of the two fields in a generator determines the amount of _____.
compounding
Because the fields of PM motors are permanent magnets, the field flux remains ________ at all times.
constant
_____ is a measure of the useful electric energy produced by the generator.
counter torque
A compound generator has both series and shunt field windings. When those windings are connected so they aid each other in the production of magnetism, that connection is called _____.
cumulative-compound
In the left-hand rule, the center finger shows the direction if the _____.
current induced into the armature
A(n) _____ compound generator has its fields connected in such a manner that they oppose each other in the production of magnetism.
differential
The compound motor connection that should generally be avoided is the _____.
differential
Armature reaction can be corrected by rotating the brushes an amount _____ the shift of the neutral plane.
equal to
What type of connection is used to connect the series fields in parallel with each other?
equalizer
Field _____ current is the DC current that flows through the shunt field winding.
excitation
One advantage of interpoles is that they restore the field outside their immediate area and are able to overcome all the field distortion
false
Some DC generators use electromagnets to provide the magnetic field instead of permanent magnets.
false
The armature of a ServoDisc motor does not contain any copper.
false
The three basic types of armature windings are the lap, wipple, and dogleg.
false
Series field windings are connected in series with the armature. They are wound with _____ turns of fairly _____ wire.
few; large
Most large compound direct current motors have a protective device connected in series with the shunt field called the _____ relay.
field loss
Shunt field windings are connected in parallel with the armature. They are wound with _____ turns of fairly _____ wire.
many; small
What kind of magnets does the armature or rotor of a brushless DC motor contain?
permanent
What provide(s) the magnetic field necessary for the operation of a DC machine?
pole pieces
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
Some converters that supply power to brushless motors produce sine waves, but most produce a _____ waveform.
trapezoidal AC
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
There may be occasions when one direct current generator cannot supply enough current to operate the connected load. In such a case, another generator is connected in parallel with the first. Direct current generators should never be connected in parallel without an equalizer connection.
true