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WorksheetsDC and AC Machines Objective Questions
Total questions: 138
Worksheet time: 1hrs 9mins
The rotating part of a DC motor?
Rotor
Armature
Stator
Yoke
An electric current is defined as the flow of ? through a conductor.
electrons
protons
nuclei
ohms
The unit of electrical potential is the ? , which is represented by the letter ? .
ampere/A
resistance/R
volt/V
watt/W
What is the unit of measurement for resistance?
Voltage
Ampere
Watts
Ohms
This allows energy to flow.
conductor
insulator
This does not allow energy to flow.
insulator
conductor
Which is NOT a conductor
Which is NOT an insulator
When the electric current in wire XY is in the direction shown, there is an upward force on the wire. If the north and south poles of the magnet exchange positions, in which direction will the force on the wire act?
downwards
upwards
to the left
to the right
A student carries out an experiment to see the effect of a magnetic field on a wire carrying a current. The wire moves upwards as shown.
What should the student do to make the wire move downwards?
change the direction of the current
move the poles of the magnet closer together
send a smaller current through the wire
use a stronger magnet
A current-carrying coil in a magnetic field experiences a turning effect.
How can the turning effect be increased?
increase the number of turns on the coil
reduce the size of the current
reverse the direction of the magnetic field
use thinner wire for the coil
The diagram shows a simple DC electric motor which is rotating.
Which change will make the motor rotate more quickly?
reversing the polarity of the magnets
reversing the battery
removing the magnets
using more powerful magnets
A simple DC electric motor is fitted with a coil that rotates in a magnetic field. A commutator connects the power supply to the coil.
What is the purpose of the commutator?
It converts AC into DC in the coil.
It prevents the current from becoming too great, because the coil has a low resistance
It reverses the direction of the current in the coil after every 180° rotation of the coil
It switches the current off momentarily after every 90° rotation of the coil
What does not alter the size of the turning effect on the coil of an electric motor?
the strength of the magnetic field
the direction of the current in the coil
the number of turns in the coil
the size of the current in the coil
A current-carrying wire lies between the poles of two magnets, as shown.
What is the direction of the force on the wire?
into the plane of the paper
out of the plane of the paper
towards the left
towards the right
The diagram shows a current-carrying wire placed between two magnetic poles. The current is in the direction shown. What is the direction of the force on the wire?
towards the bottom of the page
towards the top of the page
towards the left-hand side of the page
towards the right-hand side of the page
In a _______ a spinning turbine moves magnets which generates an electric current.
Motor
Generator
What is the transfer of energy in when you plug in a motor and it spins?
chemical to mechanical
electrical to mechanical
mechanical to electrical
physical to potential
What energy does a motor need in order to produce a mechanical energy?
Electrical energy
Heat energy
Chemical energy
Mechanical Energy
Which one of these best describes function of the alternator?
Electric generator that uses mechanical (engine) power to produce electricity
Ammeter, voltmeter, or warning light that warns the driver about charge condition
Wiring that connects the parts of the system.
Large device that stores electricity for use later on
True or False: An alternator produces a Alternating Current (AC).
True
False
What is the name of an electrical device that only allows current flow in one direction. Otherwise known as a electrical check valve?
Fuse
Diode
Photocell
Multi-meter
What is the purpose of the diode in the charging system?
Transform the AC voltage from the alternator into DC voltage
Transform the DC voltage from the alternator into AC voltage
The alternator rotor spins inside the alternator stator.
True
False
The alternator pulley spins what part inside the alternator?
The stator
The rotor
The voltage regulator
The voltage rectifier
Which factors not affecting the speed of dc motors?
Flux
Current
Applied voltage.
Voltage across the armature
What type of DC motor is shown?
Short Shunt Cumulative Compound Motor
Short Shunt Differential Compound Motor
Long Shunt Cumulative Compound Motor
Long Shunt Differential Compound Motor
What type of DC motor is shown?
Short Shunt Cumulative Compound Motor
Short Shunt Differential Compound Motor
Long Shunt Cumulative Compound Motor
Long Shunt Differential Compound Motor
What type of DC motor is shown?
Short Shunt Cumulative Compound Motor
Short Shunt Differential Compound Motor
Long Shunt Cumulative Compound Motor
Long Shunt Differential Compound Motor
Split-phase method
Capacitance start method
Resistance start method
Inductance start method
A Universal Motor
All have zero starting torque.
IA Shaded Pole Motor
A Capacitor Start Motor
Capacitance
Induction
Conversion
Flux
Electric Fan is an example of what single phase motor?
Shaded pole
Capacitor run motor
Capacitor Start motor
Repulsion motor
Grinder is an example of what single phase motor?
Shaded pole
Capacitor run motor
Capacitor Start motor
Repulsion motor
An AC motor that has zero slip.
Shaded pole motor
Induction motor
Synchronous motor
Split phase motor
AC generator converts:
Mechanical energy to electrical energy
Electrical energy to mechanical energy
Heat energy to mechanical energy
Chemical energy to electrical energy
In an AC generator, the contacts are made from a soft but highly conductive material, usually carbon, and are called....
Coils
Armatures
Slip Rings
Brushes
The electricity generated from the power plant is ……
Alternating Current
Direct Current
Delayed Current
Eddy Current
A brushless motor is more efficient than a brushed motor because...
There is less energy lost inside the motor due to friction and heat.
There is less power used to spin the motor.
There is less noise created in spinning the motor.
This is an incorrect statement. Brushed motors are more efficient.
The part of an electric motor that doesn't spin is called the...
armature
commutator
stator
bearings
It is basically a series DC motor which is specially designed to operate on a single-phase AC as well as DC supplies.
Universal motor
Induction motor
Synchronous motor
Split phase motor
What is true of AC GENERATOR?
It works on the principle of electromagnetic induction
The mechanical energy of the rotation is converted into electrical energy
A pair of slip rings are used in the AC generator
All the above
Which of the following DC Motor is best suited for speed control applications such as in rolling mills and paper mills:
DC shunt motor
DC series motor
DC compound motor
DC differential compound motor
The main advantage of dc motors compared to ac motors are:
high starting torque
frequency control
less maintenance
low cost
Which type of power supply is given to Generator Field winding:
pulsating DC supply
AC supply
DC supply
none of these
The induced EMF of DC Shunt Generator can be calculated by using which formula?
E= (NP∅Z)/(120 A)
E= (N∅Z)/(2p A)
E= (NP∅Z)/(60 A)
E= (N∅Z)/(p A)
Type of a DC Generator that is used as a voltage booster
Shunt
Series
Separately Excited
Cumulative
A DC motor receives mechanical energy as input power and produces electrical energy as output power
True
False
DC Machines are classified into motors and generators
True
False
A DC motor can be operated as a generator and vise versa.
True
False
DC motors and generators are known as commutator machines
True
False
In DC machines, the stator part is either a permanent magnet or electromagnet
True
False
In DC machines, the rotor part is either a permanent magnet or electromagnet
True
False
The rotating coil inside the stator is called:
Magnet
Pole
Armature
Commutator
Which part rotates together with the armature
brushes
commutator
wire
Electromechanical energy conversion device can convert
Electrical energy into mechanical energy only
Mechanical energy into electrical energy only
Either electrical energy into mechanical energy or mechanical energy into electrical energy
Only increase the voltage level of electrical energy
DC generator can convert
DC Electrical energy into mechanical energy only
Mechanical energy into dc electrical energy only
Either dc electrical energy into mechanical energy or mechanical energy into dc electrical energy
Only increase the voltage level of electrical energy
DC motor can convert
Only increase the voltage level of electrical energy
Either dc electrical energy into mechanical energy or mechanical energy into dc electrical energy
DC Electrical energy into mechanical energy only
Mechanical energy into dc electrical energy only
The basic law for the working of dc motor is based on
Right hand palm rule
Faraday law
Biot Savert law
Fleming left hand rule
The relation that depicts the basic law for dc generator is
Which one is not a part of dc machine?
Stator and rotor
Brushes
Commutator
Slip rings
Which one is not a part of the field system of dc machines?
Pole shoe and pole core
Yoke or frame
Exciting winding
Armature
In a dc machine, if number of parallel path is equal to number of poles this implies
The armature winding is wave connected.
The armature winding is open.
The armature winding is lap connected.
The armature winding is short circuited.
In a dc machine, if number of parallel path is equal to 2 this implies
The armature winding is lap connected.
The armature winding is open.
The armature winding is wave connected.
The armature winding is short circuited.
The emf induced in the dc machine is given by,
DC Shunt generator
DC Series generator
DC Shunt motor
DC Series motor
When the conductors are rotated in the magnetic field an emf is induced in it. This law is applicable for
DC Generator only
DC and AC Generator, both
DC and AC Motor, both
DC motor only
A single-phase induction motor is rated 5 hp, 75% power factor and 220 volts. What is the full-load current? Assume 100% efficiency.
22.6
20.5
16.9
18.5
A 1/20-HP 1,550 rpm shaded-pole motor takes 120 watts when operating at full-load . What is its efficiency?
39.71%
23.98%
31.08%
93.20%
A ½ -hp 1,750-rpm capacitor motor develops 350 percent starting torque. What is the starting torque in pound-feet?
6.36
7.23
5.25
8.46
How many poles is a motor wound that operates at 1,130 rpm from a 60-Hz source?
6
4
8
2
A 3-phase motor is rated 750 hp at 480 volts line to line, 0.85 pf lagging and 0.95 efficiency. Solve for the rated current.
869.64 A
702.23 A
405.43 A
917.32 A
A 40-hp, 3-phase , 4-pole, 230-volt , 60-Hz induction motor operating at 0.90 efficiency, 0.85 power factor and 3 percent slip, drives a water pump. Determine the current at which the motor is operating.
94.531 A
96.234 A
97.915 A
93.351 A
Calculate the speed of a 60-Hz , 14-pole motor is the slip is 5%.
514.28
488.57
456.32
429.63
The rotor speed of a six-pole, 50-Hz induction motor is 960 rpm. Calculate the percent slip.
5%
2%
4%
3%
A 50-hp three-phase motor rated 220 V, 60 Hz , 75% power factor has 6 poles . The slip at full load is 3%. What is the frequency of the rotor current at full load?
1.8 Hz
60 Hz
5.4 Hz
58.2 Hz
A 150-HP, 3-phase 6-pole, 460-V, 60-Hz induction motor drives 195 A line current at full load speed of 1170 rpm and 0.85 pf. Solve for the percent slip at full load.
2.5%
3.0%
2.8%
2.0%
3-Phase Induction motor will have ____ power factor during no load condition:
High
Negative
Low
Moderate
Which of the following method cannot be used for starting of 3-Phase Squirrel Induction Motor:
DOL Starter
Rotor Resistance Starter
Line Resistance Starter
Auto Transformer Starter
Star Delta Starter
The rotor slots of 3-Phase Induction Motor are kept inclined inorder to reduce noise, vibrations and synchronous cusps. This phenomenon in known as:
Plugging
Cogging
Crawling
Skewing
3-Phase Induction Motor will have ____ Starting Torque and _____ Starting Current
Low, Low
High, High
Low, High
High, Low
What happens when Induction motor runs at synchronous speed, motor will:
Delivers real Power
Stops
Burn
Delivers Reactive Power
3-Phase Induction Motor has the advantage of Low Cost, Less Maintenance and Rugged Construction:
True
False
Crawling occurs in Induction Motor due to which of the following:
Harmonics
Low Voltage
High Voltage
Over Load
A single Phase Induction motor is self starting
True
False
In large synchronous machine, field winding is placed on ______ and ac supply on ______
Rotor, Stator
Armature, Slots
Stator, Rotor
pole shoes, stator
Which is called optimistic Method?
EMF Method
ZPF Method
ASA Method
MMF method
Which alternator is used for high speed applications?
Saleint pole
Slip Ring
Squirrel Cage
Cylindrical pole
Which factor is not considered for synchronization?
Voltage
Current
Frequency
Phase Sequence
Which of the following supply is given to the rotor winding of 3-Phase Induction Motor:
DC
No Supply but by induction
pulsating supply
AC
In Case of Synchronous Generator, when the load is thrown-off what happens to terminal voltage:
Decreases
Increases
Remains same
A Synchronous Machine having 4 poles and supplied with 60 Hz supply will rotate at ___:
1500
3000
3600
1800
No load losses in three phase induction motor includes
eddy current loss
hysteresis loss
rotational loss
all the above
The No load test conduct under
Rated current
Rated output power
Rated Voltage
Rated input power
The circle diagram is the locus of
Voltage
current
flux
frequency
For a constant load if torque is reduced then
speed increases
speed decreases
speed remains constant
Voltage reduces
As the load of induction motor varies then
copper loss varies
core loss varies
copper loss and core loss remains constant
both the loss varies
When the voltage is zero then the value of core loss is
Eddy current loss
hysteresis loss
friction and windage loss
copper loss
under no load test the the watt meter gives constant loss only due to
low power factor
low voltage
low value of flux
low value of supply current
The graph plotted between watt meter reading and square of voltage has a shape of
hyper bola
straight line
ellipse
parabola
The equivalent resistance of induction motor is obtained from
No load test
blocked rotor test
find rotor frequency if supply frequency is 50 hz and slip of 0.03
50
1.5
2
16
The starters are used to
reduce starting torque
reduce starting voltage
reduce starting current
smooth starting of machine
Find synchronous speed of 4 pole 50 HZ three phase induction motor
1000 rpm
1500 rpm
2000 rpm
3000 rpm
how may single phase transformers required for a three phase induction motor
1
2
3
4
In a star connected three phase transformer the voltage induced in secondary is 240 V then output line voltage is.
415 V
240 V
138 V
0 V
If 415 V is applied to a star delta starting type three phase induction motor then the voltage obtained across the phase under starting condition is
415 V
138 V
240 V
0 V
The rotor resistor speed control is possible only in
Slip ring induction motor
squirrel cage induction motor
Which of the following are the starting methods of three phase induction motor
DOL
Star delta
auto transformer
stator resistance
In distribution end the three phase transformer used is
Star star
Delta delta
Delta star
star delta
Three phase induction motors are self starting motors due to
production of split phase
production of rotating magnetic field
constant frequency supply
best starting methods
The core material of transformer has a property of
high current conductivity
high reluctance
high magnetic conductivity
rigid solid structure
