NEETS MODULE 5; CHAPTER 1,2,3,4; Flashcards
Which is cheaper AC motors or DC motors?
AC motors are cheaper
What are the three basic types of ac motors?
series, synchronous, and
induction motors
Series motors are generally used to operate what type of equipment?
To power small appliances.
Why are series motors sometimes called universal motors?
They operate on either ac or dc.
characteristics of a series ac motor?
It has low speeds for large loads and high speeds for light loads. It is a
varying-speed machine.
synchronous and induction types of motors rely on rotating _______ in their
stators to cause their rotors to turn.
magnetic fields
What determines the number of field poles required to establish a rotating magnetic
field in a multiphase motor stator?
The number of phases in the applied voltage.
What is the angular displacement between field poles in a two-phase motor stator?
90º.
What is the major difference between a two-phase and a three-phase stator?
Number and location of field poles
Synchronous motors have the characteristic of?
constant speed between no load and full load. capable of correcting the low
power factor of an inductive circuit
disadvantages of a synchronous
motor is that it cannot be started from a standstill by applying ______ ac power to the stator.
three-phase
The induction motor is the most commonly used type of?
AC motor
. Why is the ac induction motor used more often than other types?
They are simple and inexpensive to make.
examples of induction motors?
washing machines, refrigerator
compressors, bench grinders, and table saws.
The difference between
the speed of the rotating stator field and the rotor speed is called?
the slip
What determines the amount of slip in an induction motor?
Load
the most common type of winding in a induction motor?
The most common is
the squirrel-cage winding.
This other type of winding contains actual coils placed in the
rotor slots.
wound rotor
. What type of ac motor is most widely used?
Single-phase induction motor.
Split-phase motors are designed to use inductance, __________, or
resistance to develop a starting torque.
capacitance
How do split-phase induction motors become self-starting?
By using combinations of inductance and capacitance to apply out-of phase
currents in starting windings.
Why are shaded-pole motors used to drive only very small devices?
They have very weak starting torques.
SYNCHRONOUS SPEED?
is the speed of stator field rotation. It is determined by the
number of poles and the frequency of the input voltage
Magnetic induction occurs when there is relative motion between what two
elements?
A conductor and a magnetic field.
A rotating armature requires _______ and ______ to conduct the current from the
armature to the load.
slip rings and brushes
What is the part of an alternator in which the output voltage is generated?
Armature
What are the two basic types of alternators?
Rotating armature and rotating field.
What is the main advantage of the rotating field alternator?
. Output voltage is taken directly from the armature
. Most large alternators have a small dc generator built into them. What is its
purpose?
To provide dc current for the rotating field
Prime movers are divided into two classes?
generators-high-speed and low-speed.
There are two types of rotors used in rotating-field alternators?
turbine-driven and salient-pole rotors.
How are alternators usually rated?
Kilovolt-amperes (volt amperes).
Salient-pole rotors may be used in alternators driven by what types of prime movers?
Internal combustion engines, water force and electric motors.
What does the term single phase indicate?
One voltage (one output).
in single-phase alternators, in order for the voltages induced in all the armature
windings to add together for a single output, how must the windings be connected?
In series.
. What determines the phase relationship between the voltages in a two-phase ac
generator
Placement of armature coils.
How many voltage outputs are available from a two-phase three-wire alternator?
Three.
. What are the two methods of connecting the outputs from a three-phase alternator
to the load?
Wye and Delta.
What generator characteristics must be considered when alternators are
synchronized for parallel operation?
. Output voltage, frequency, and phase relationships
How is output voltage controlled in practical alternators?
By varying the voltage applied to the field windings.
What factors determine the direction of rotation in a dc motor?
. Direction of armature current, and direction of magnetic flux in field
The right-hand rule for motors is used to find the relationship between what motor
characteristics?
Direction of conductor movement (rotation), direction of flux, and the direction of
current flow.
. What causes counter emf in a dc motor?
Generator action.
What motor characteristic is affected by counter emf?
Speed.
What is the load on a dc motor?
The device to be driven by the motor.
What is the main disadvantage of a dc series motor?
. It must have a load connected to avoid damage from excess speed.
What is the main advantage of a dc series motor?
High torque (turning force) at low speed.
What advantage does a shunt motor have over a series motor? (DC)
It maintains a constant speed under varying loads.
As with dc generators, dc motors can be constructed using one of two types of armatures?
Drum-wound and gramme-ring
Why is the Gramme-ring armature not more widely used?
Only outside of coils cut flux (inefficient).
How is the disadvantage of the Gramme-ring armature overcome in the drum-wound armature?
By winding the armature in a way that places the entire coil where it is exposed to
maximum flux.
In a dc motor that must be able to rotate in both directions, how is the direction
changed?
By reversing either field or armature connections.
What is the effect on motor speed if the field current is increased?
Motor will slow down.
Armature reaction in a dc motor causes a shift of the neutral plane in which
direction?
Opposite the rotation.
What current flows in the interpole windings?
Armature current.
What is the purpose of starting resistors?
To limit armature current until counter emf builds up
A generator is a machine that converts mechanical energy into?
electrical energy.
Generators convert mechanical motion to electrical energy using what principle?
Magnetic induction.
What rule should you use to determine the direction of induced emf in a coil?
The left-hand rule for generators.
What is the purpose of the slip rings?
To conduct the currents induced in the armature to an external load.
Why is no emf induced in a rotating coil when it passes through the neutral plane?
No flux lines are cut.
What component causes a generator to produce dc voltage rather than ac voltage at
its output terminals?
A commutator
At what point should brush contact change from one commutator segment to the
next?
The point at which the voltage is zero across the two segments.
An elementary, single coil, dc generator will have an output voltage with how many
pulsations per revolution?
Two.
How many commutator segments are required in a two-coil generator?
Four
How can field strength be varied in a practical dc generator?
By varying the input voltage to the field coils.
What causes sparking between the brushes and the commutator?
Improper commutation
What is armature reaction?
Distortion of the main field due to the effects of armature current.
What is the purpose of interpoles?
To counter act armature reaction.
What is the effect of motor reaction in a dc generator?
A force which causes opposition to applied turning force.
How can eddy current be reduced?
By laminating the core material.
Why are drum-type armatures preferred over the Gramme-ring armature in modern
dc generators?
Drum-type armatures are more efficient, because flux lines are cut by both sides of
each coil.
Lap windings are used in generators designed for what type of application?
Higher currents
What are the three classifications of dc generators?
Series-wound, shunt-wound, and compound-wound.
What is the main disadvantage of series generators?
Output voltage varies as the load varies.
What term applies to the voltage variation from no-load to full-load conditions and
is expressed as a percentage?
Voltage regulation.
What term applies to the use of two or more generators to supply a common load?
Parallel operation.
What is the purpose of a dc generator that has been modified to function as an
amplidyne?
It can serve as a power amplifier.
What is the formula used to determine the gain of an amplifying device?
Gain = output ÷ input.
What are the two inputs to an amplidyne?
The mechanical force applied to turn the amplidyne, and the electrical input signal
Laminations in a small generator armature may be as thin as?
1/64 inch
