Motors test questions
If a DC Generator were to deliver 180W to a load what would be the minimum HP required to drive the generator assuming it is 100% efficient.
0.241HP
For the LabVolt DC Motor module what is considered as full load?
10 lbf-in of torque
What minimum size transformer would be required to supply an induction motor that uses 50KW of real power operating at a lagging power factor of 0.7 and a synchronous motor that uses 50KW of real power operating at a leading power factor of 0.7?
100KVA
How many horsepower is a 208V three-phase 100% efficient motor delivering to a load if it is drawing 7500W and 30A from the power line?
10HP
If an induction motor nameplate indicated the motor was rated for 1145 RPM, what would the synchronous speed of the motor be?
1200 or 1400?
If a DC Generator were to deliver 100kW to a load what would be the minimum HP required to drive the generator assuming it is 100% efficient.
134HP
What minimum size transformer would be required to supply an induction motor that uses 100KW of real power operating at a lagging power factor of 0.7?
143KVA
If a given motor is delivering 44 ft-lb of torque at 1800 RPM to a load, how much horsepower is this motor developing? (use the traditional formula)
15 hp
For a Separately-Excited DC Generator what is the typical voltage regulation from no-load (IA = 0A) to full-load (IA = 1A)?
15%
What is the typical speed regulation for a shunt DC motor?
15%
The synchronous speed of the three-phase induction motor used in this lab is _____.
1800RPM
If a shunt DC motor is delivering 70 ft-lb of torque at 1500 RPM to a load, how much horsepower is this motor developing? (use standard formula)
20 HP
If an alternator were delivering 120 Watts and 240VA to a 208V three-phase grid, how many VARs will the alternator be creating?
208 VAR
If a 208V three phase induction motor has an efficiency of 93% and a PF of .70 how much current will it draw from the power line when it is delivering 25ft-lb of torque at 1670 RPM to a load?
25.3
If a DC Generator were to deliver 20kW to a load what would be the minimum HP required to drive the generator assuming it is 100% efficient.
26.8HP
If a series DC motor is delivering 63 lbf-in of torque at 3000 RPM to a load, how much horsepower is the motor developing? (use the lab formula)
3 hp
A prime mover driving an alternator is developing 500 HP. If the efficiency of the alternator is 92%, calculate the real power the alternator can deliver to the grid.
343 KW
What is a typical relationship between starting current and running current for a Series DC Motor?
4 to 1
The starting current of a three-phase squirrel cage induction motor is typically _____ times the normal running current.
4 to 7
What is a typical relationship between starting current and running current for a cumulative compound connected DC motor?
5 to 1
If a given motor is delivering 2 HP to a load while drawing 33 amps from a 90 VDC source, how efficient is the motor?
50%
What is the efficiency of a DC Generator that is being driven by a 500HP Prime mover while delivering 200kW to a load?
54%
If an induction motor nameplate indicated the motor was rated for 1145 RPM, how many pole motor would this be?
6
What is a typical relationship between starting current and running current for a shunt DC motor?
6 to 1
If a motor is delivering 20ft-lb of torque at 1750RPM to a given load how many horsepower is the motor delivering to the load?
6.7HP
If a shunt DC motor is delivering 10 HP to a load while drawing 52 amps from a 240 VDC source, how efficient is the motor?
60%
What is the efficiency of a DC Generator that is being driven by a 100HP Prime mover while delivering 50kW to a load?
67%
If a series DC motor is delivering 25 HP to a load while drawing 53 amps from a 500 VDC source, how efficient is the motor?
70%
What minimum size transformer would be required to supply a synchronous motor that uses 100KW of real power operating at a leading power factor of 0.7?
70KVA
For the Compound DC Generator if the terminal voltage were slightly lower than expected what should be done to increase the voltage?
Adjust the shunt field rheostat.
Which of the following does not contribute to efficiency loss in a DC motor?
Ambient heat
Which of the following represents the correct polarity relationship for a DC generator?
Apply negative to F1, turn the armature CCW ODE, and get a negative on A1. Apply negative to F1, turn the armature CW DE, and get a positive on A1. Apply positive to F1, turn the armature CCW ODE, and get a negative on A1
Which of the following represents the correct polarity relationship for a DC generator?
Apply negative to F1, turn the armature CW ODE, and get a positive on A1
Which of the following represents the correct polarity relationship for a DC generator?
Apply positive to F1, turn the armature CCW ODE, and get a positive on A1.
What limits the starting current for a Series DC Motor?
Armature resistance and series field resistance
What limits the starting current for a cumulative compound DC motor?
Armature resistance and series field resistance
For a Seperately-Excited Shunt DC Generator why does the terminal voltage (VT) drop off as the armature current (IA) increases?
Because of the armature resistance.
For a Seperately-Excited Shunt DC Generator why is there a small output voltage even when there is no current applied to the shunt field?
Because of the residual magnetism in the field poles.
For a Self-Excited DC Generator why does the output voltage (VT) as a function of load current (IA) drop off faster than it did for the Separately-Excited DC Generator?
Because the Self-Excited DC Generator experiences IR Drop and loss of field strength.
If a Self-Excited DC Generator had lost its residual magnetism how could it be made to build-up output voltage (VT)?
By flashing the fields with a battery.
For the Compound DC Generator used in the lab what is the purpose of the shunt field rheostat?
Self-excited long-shunt Separately-excited Differential compound
For a Compound DC Generator which of the following is not a typical connection.
Self-excited short-shunt
Which one of the following is not another name for an Alternator?
Synchronous inductor
Which of the following is true about a Self-Excited DC Generator?
The Self-Excited DC Generator can be used for cold start applications.
Why is the Shunt DC Motor used as the prime mover for this lab instead of one of the other DC motor configurations?
The Shunt DC Motor has the best speed regulation.
A 500HP 4160V 100% efficient three-phase synchronous motor operating at full load is drawing 64.7A from the power line. Which of the following is true?
The motor is drawing 279 kVAR from the power line.
A 500HP 4160V 100% efficient three-phase synchronous motor operating at full load is drawing 64.7A from the power line. Which of the following is true?
The motor is drawing 373 kW from the power line.
A 500HP 4160V 100% efficient three-phase synchronous motor operating at full load is drawing 64.7A from the power line. Which of the following is true?
The motor is drawing 466 kVA from the power line.
A 500HP 4160V 100% efficient three-phase synchronous motor operating at full load is drawing 64.7A from the power line at a leading power factor. Which of the following is true?
The motor is over excited.
Which of the following is true about a three-phase squirrel cage induction motor?
The narrower the air gap the more efficient the motor is
Which of the following does not contribute to efficiency loss within a DC motor?
The type of mechanical load the motor is connected to
Which of the following is true about a synchronous motor?
Their power factor can be changed
What is the significance of the speed rating stamped on the nameplate of a three-phase induction motor?
This is the speed the motor will turn when operating under full load
For the Alternator lab why is the switch in the alternator excitation circuit left open until the alternator is turning?
To keep the field from overheating and becoming damaged.
What is the primary purpose of the end rings in the squirrel cage rotor?
To provide a path for the current induced in the rotor bars
For the Shunt DC Motor lab what was the purpose of the Electro-Dynometer?
To provide a variable mechanical load.
What limits the starting current for a shunt DC motor?
Torque or Armature resistance
By increasing the shunt field strength of a DC motor you can increase the speed of the motor.
True
DC motors are among the most efficient of all electrical devices.
True
For a shunt DC motor the armature is connected in parallel with the shunt field.
True
The more efficient a motor is the cooler it will run under full load.
True
Which of the following is true about DC machines?
Typically a DC machine can be run as either a motor or a generator.
Which of the following represents the DC Generator terminal voltage equation.
VT = Vinduced - IARA
Which of the following represents the typical DC generator induced voltage equation.
Vind = kfn
Which of the following is the standard equation for electromagnetic induction?
Vinduced= BLv
When a synchronous motor is started the field windings should be connected to _____.
a DC supply
All electric generators require _____ in order to generate a voltage?
a conductor, a magnetic field, and relative motion
All electic generators require _____ to create an induced voltage.
a conductor, magnetic field, and relative motion
The squirrel cage rotor consists of _____.
a laminated iron core, solid aluminum bars, and aluminum end rings
All electric generators work on the principle of _____.
electromagnetic induction
The current that creates the magnetic field for electic machines is called the _____.
excitation current
A synchronous motor will only turn at synchronous speed when lightly loaded.
false
Because the rotor bars are typically made of aluminum they see very low currents.
false
Brushes associated with synchronous motors are easily adjustable to compensate for armature reaction.
false
Differential compound motors have a wide range of applications.
false
For a Series DC Motor the armature is connected in parallel with the series field.
false
For a compound DC motor the armature is connected in parallel with the series and shunt fields.
false
For a three-phase induction motor as slip decreases the frequency of the rotor current decreases which in turn decreases the inductive reactance and increases the torque.
false
For an alternator operating off the grid the field can be saturated by overloading the alternator.
false
For an alternator operating off the grid the voltage regulation is the worst when resistive loading is applied.
false
For an alternator operating off the grid the voltage regulation when connected to a resistive load is typically less than 1%.
false
For an alternator operating off the grid with no excitation current there will be absolutely no voltage generated at the output of the alternator.
false
For an alternator the conductors that receive the induced voltage are typically in the rotating portion of the machine.
false
For an alternator the magnetic field is typically created by an electromagnet in the stationary portion of the machine.
false
If an industrial customer connects a large single phase load to a three-phase power line it will not affect the other customers' voltage balance.
false
If the rotor of a synchronous motor "unlocks" from the stator rotating magnetic field you should leave it running until it comes back into synchronization.
false
It is always recommended to connect alternators to long transmission lines that have significant capacitance.
false
Series DC Motors must always be started under a no-load condition.
false
The Compound DC Motor has the best speed regulation of all the motors studied in this course.
false
The DC power must be applied to the rotor of a synchronous motor before starting the motor.
false
The Series DC Motor has the best speed regulation of all of the DC motors.
false
The Series DC Motor has the highest starting current of the DC motors studied in this course.
false
The Shunt DC Motor has the highest starting torque of all the DC motors studied in this course.
false
The alternator used in this lab the line to line output was 208 volts because the alternator phase windings are connected in a delta arrangement.
false
The power factor of a synchronous motor can only be adjusted under no-load conditions.
false
The speed of a Series DC Motor depends upon its field current only.
false
The speed regulation of the Cumulative Compound DC Motor is better than the shunt DC motor.
false
The starting torque of the typical three-phase induction motor is greater than the starting torque of a series DC motor of the same horsepower.
false
The stator windings of the synchronous motor used in the lab were connected in a delta configuration so maximum power could be delivered to the motor.
false
Three-phase Induction Generators are far more common than three-phase Synchronous Generators.
false
To change the direction of rotation for a synchronous motor simply change the polarity of the DC applied to the field.
false
To increase the real (true) power an alternator delivers to the grid you must increase the excitation current to the alternator.
false
When an alternator is connected to the grid the speed of the prime mover can be used to control the grid frequency.
false
When under full load conditions the torque developed by the synchronous motor is based soley upon the current induced in the rotor.
false
For the DC Generator labs we set the prime move to "CW CS Prime Mover/Brake" ____.
for standard CCW ODE rotation and constant speed throughout the experiment
For an alternator which of the following is an example of a prime mover?
gas turbine
If a synchronous motor is mechanically loaded and started with DC applied to the rotor the motor will _____.
growl, shake, and vibrate
A Compound DC Generator _____.
has the best speed regulation
When operating at synchronous speed the squirrel cage windings _____.
have no effect on the operation of the motor
For a DC generator the conductors that receive the induced voltage are _____.
in the armature
In order for an alternator to create leading VARs you must ____.
increase excitation current to the alternator
In order for an alternator to deliver power to the grid you must ____.
increase torque from the prime mover
When minimum rotor current is applied to a synchronous motor the reactive power to the stator is of a(n) _____ .
inductive lagging nature
The preferred method for changing the direction of rotation for a compound DC motor is to ____.
interchange the armature leads
A Compound DC Generator _____.
is typically connected cumulative compound
Before DC excitation is applied to a synchronous motor the motor will have a _____.
leading power factor
For an alternator which one of the following is required for electromagnetic induction?
magnetic field
For an alternator operating off the grid which of the following will affect the output frequency?
number of stator poles per phase
For an alternator operating off the grid which of the following will affect the output voltage?
number of turns of wire in the stator
What is a typical speed regulation for a Series DC Motor?
over 100%
When the rotor of a synchronous motor falls out of synchronous speed the motor is said to be _____.
overloaded
Which of the following is not a good application of a series DC motor?
paper machines
A resistor connected to the DC windings of a synchronous motor during start up will _____.
prevent high voltage build up and aid in the production of torque
For a Compound DC Generator the series field _____.
provides additional induced voltage to offset IR Drop
The force required to "unlock" the rotor from the stator rotating magnetic field of a synchronous motor is called the _____.
pull-out torque
Before opening the breaker to disconnect an alternator from the grid you must ____.
reduce the power applied to prime mover until 0W are being generated
For an alternator which one of the following is required for electromagnetic induction?
relative motion
The primary disadvantage of a Separately-Excited Shunt DC Generator is that it _____.
requires a separate DC source to supply the shunt field
For a Self-Excited DC Generator the self-excitation would not be possible without the _____.
residual magnetism in the field poles
For the synchronous motor used in the lab the voltage induced in the rotor decreased as the motor speed increased because the_____.
resistance of the DC windings decreases
When a synchronous motor is operating at unity power factor it looks like a _____ to the source.
resistor
To change the output voltage polarity of a Seperately-Excited Shunt DC Generator _____.
reverse the shunt field polarity or the direction of rotation
The commutator of a DC generator acts as a _____.
rotary rectifier
For an alternator operating off the grid under no load conditions the flux in the air gap is established by the ____.
rotor magnetic field strength
In order to synchronize an alternator to the power grid what four conditions are typically required?
same phase sequence, both in phase, same voltage, same frequency
When a DC generator's shunt field current is created by a DC power supply the generator is said to be ______.
seperately excited?
The rotor of a synchronous motor consists of _____ .
slip rings, an armature, and squirrel cage windings
For an alternator operating off the grid which of the following will affect the output voltage?
speed of the rotor
For a DC generator the magnetic field that causes induction comes from the _____.
stator
For an alternator which of the following is an example of a prime mover?
steam turbine
To change the output voltage (VT) polarity of a Separately-Excited Cumulative Compound DC Generator _____.
swap the shunt field leads
If it were necessary to reverse the direction of rotation of a prime mover what would also have to be done before synchronizing and alternator to the grid?
switch any two phase conductors
A synchronous motor is sometimes called a(n) _____.
synchronous capacitor
If the alternator synchronizing lamps flash rapidly on and off simultaneously ____.
the alternator and grid are operating at different frequencies
If the alternator synchronizing lamps flash in sequence, one at a time, it indicates that ____.
the alternator and grid phase sequences are not the same
Whenever the alternator synchronizing lamps are on brightly it is an indication that ____.
the alternator is out of phase with the grid
For an alternator operating off the grid under loaded conditions the flux in the air gap is determined by ____.
the ampere turns of the rotor windings, the ampere turns of the stator windings, and their phase relationship
For a Self-Excited DC Generator the shunt field receives its power from _____.
the armature
For an alternator operating off the grid which of the following will affect the output voltage?
the field strength
The synchronous speed of a motor is determined by _____.
the number of stator poles and the applied frequency
For an alternator operating off the grid which of the following will affect the output voltage?
the operating power factor
For a Self-Excited DC Generator if the direction of rotation were changed _____.
the output voltage (VT) would not build up
For a Self-Excited DC Generator if the shunt field leads were swapped _____.
the output voltage (VT) would not build up
For the DC Machines the brush neutral must be set before each lab otherwise _____.
the output voltage would be the wrong polarity or the brushes would arc significantly under load
The principle of operation of the three phase synchronous motor, like all three phase motors, is based on the rotating magnetic field.
true
The rotor of an alternator that is operating at its rated voltage output dissipates more heat with a lagging power factor load than it does with a leading power factor load.
true
The speed of a Series DC Motor will always drop off as load is added.
true
The speed of a cumulative compound DC motor will always drop off as load is added.
true
The speed of any DC motor can be decreased by lowering its armature voltage or increasing its field current.
true
The synchronous motor gets its name from the fact that the rotor turns at the same speed as the rotating magnet field even when load is added to the motor.
true
The three phase synchronous motor is not naturally self starting so it cannot be started with a standard across the line starter.
true
Three phase alternators require balanced loads in order to maintain balanced output voltages.
true
Three-phase Synchronous Generators are far more common than three-phase Induction Generators.
true
When a synchronous motor is overloaded the rotor and can "unlock" from the stator rotating magnetic field.
true
When starting a three-phase induction motor the rotor has a relatively large inductive reactance with respect to resistance.
true
In the electromagnetic induction equation the lower case "v" represents the _____.
velocity of the conductor relative to the magnetic field
If these two conditions are not met before connecting an alternator to the grid, the alternator or power distribution equipment can be seriously damaged.
voltage level and frequency
If the rotor of a synchronous motor "unlocks" from the stator rotating magnetic field the motor_____.
will continue to turn due to the amortisseur windings
For an alternator operating off the grid a resistive load ____.
will decrease the output voltage of the alternator
For an alternator operating off the grid a capacitive load ____.
will increase the output voltage of the alternator
For an alternator operating off the grid an inductive load ____.
will not change the output voltage of the alternator
What is the power factor of a 208V three-phase 100% efficient motor that is drawing 7500W and 30A from the power line?
.69
What value of voltage regulation can be expected from a Compound DC Generator?
0%
Which of the following is considered a safe lab procedure?
Deenergize the circuit before making or changing electrical connections.
For an alternator which of the following is an example of a prime mover?
Diesel engine
Adding a fan to the shaft of a DC motor to cool it will improve the motor efficiency.
False
The "infinite bus" is a term used to define a large resistive load.
False
The speed of a shunt DC motor will always stay constant as load is added.
False
The speed of any DC motor depends upon its armature voltage only.
False
For an alternator which of the following is an example of a prime mover?
Gasoline engine
For all DC Generators the voltage induced in the rotating armature conductors is _____.
a sine wave?
For an alternator to create 180 leading VARs and deliver no true power to a 208V three-phase system one must ____.
adjust the prime mover to deliver 0W and over excite the alternator to develop 0.50A of line current
For an alternator to deliver 200W of true power at a unity power factor to a 208V three-phase system one must ____.
adjust the prime mover to deliver 200W and excite the alternator to develop 0.55A of line current
The torque produced by any DC motor is determined by the _____.
all of the above
To change the direction of rotation of a synchronous motor you must _____.
all of the above
When three phase power is applied to the stator of a three-phase induction motor ;
all of the above
Which of the following is a lab rule?
all of the above
You would not want to operate an alternator near the knee of its saturation curve because ____.
all of the above
The speed of the Series DC Motor will drop off as load is added primarily because of ____.
an increase in field strength
The speed of the shunt DC motor will drop off as load is added because of ______.
armature reaction
For a Shunt DC Generator when the terminal voltage (Vt) is plotted as a function of shunt field current (IF) the curve is not a straight line _____.
because the field poles begin to saturate with lines of flux or because the brush drop increased significantly
The speed of the compound DC motor will drop off as load is added because ____.
both a and c
To change the output polarity for a Self-Excited DC Generator _____.
change the polarity of the residual magnetism
For the DC Machine Labs before you connect any wires or turn on any power you should _____.
check the brush neutral of the DC machine
For an alternator which one of the following is required for electromagnetic induction?
conductor
For a Self-Excited DC Generator the shunt field is _____.
connected in parallel with the armature
A Compound DC Generator _____.
contains a shunt field and a series field
The three phases of a typical alternator are ____.
created by the three poles in the rotor
If the rotor terminals are left open when a synchronous motor is started _____.
dangerous voltages will be developed across the rotor terminals
In order for an alternator to create lagging VARs you must ____.
decrease excitation current to the alternator
The output voltage magnitude of an alternator connected to the grid depends essentially upon ____.
the power factor
A DC generator will experience much higher short circuit currents than an alternator because ____.
the short circuit current of the DC generator is limited only by winding impedance
For a Shunt DC Generator the output voltage (VT) is typically controlled by changing the _____.
the shunt field strength
For an alternator operating off the grid which of the following will affect the output voltage?
the speed of the rotor
When a synchronous motor is operating at unity power factor _____.
the stator current will be at its minimum value
For an alternator operating off the grid the output voltage magnitude depends upon ____.
the total flux in the air gap
The output voltage magnitude of an alternator not connected to the grid depends essentially upon ____.
the total flux in the air gap
The simplest most widely used motor for industrial applications is the _____.
three-phase squirrel cage induction motor
Which of the following does not contribute to efficiency loss for a DC motor?
torque losses
A paper machine would typically use compound DC motors.
true
A synchronous generator can only deliver power to the grid if it operates at a higher frequency than the grid before being connected to the grid.
true
A synchronous motor should never be started with DC already applied to the rotor.
true
A synchronous motor when used on the same system as induction motors can improve the overall system power factor.
true
A three-phase alternator, driven by a water turbine, delivers 150,000 KVA at a lagging power factor of .90 to a 150kV system. If the alternator excitation current is increased slightly, the apparent power will decrease.
true
All of DC motor configurations used in the lab have about the same efficiency.
true
Alternators that feed into large distribution networks are controlled differently from those that stand alone.
true
An alternator will operate as a synchronous motor if the prime mover does not deliver enough torque to move the rotor ahead of its no-load position.
true
Another term used for a large power system is the "infinite bus."
true
As the rotor speed approaches synchronous speed the relative motion between the rotating stator field and rotor decreases thus decreasing the rotor induced voltage and motor torque.
true
Before connecting an alternator to the power grid the alternator must be synchronized with the grid.
true
Compound DC motors can be connected aiding or opposing.
true
DC motors are typically less efficient than most AC motors.
true
Due to the simplicity of the rotor the squirrel cage induction motor is far more reliable than a DC motor.
true
For an alternator operating off the grid if the output is shorted the current will be limited by a combination of winding resistance and winding reactance.
true
For an alternator operating off the grid when the field become saturated the alternator output voltage will no longer increase proportionately with an increase in excitation.
true
For an alternator the conductors that receive the induced voltage are typically in the stationary portion of the machine.
true
For an alternator the magnetic field is typically created by an electromagnet in the rotating portion of the machine.
true
If a compound DC motor is connected differential compound the motor will perform normally when lightly loaded but will likely flash over as load is increased.
true
Increasing the field strength of a DC motor will cause the speed of the motor to decrease.
true
Most alternators are connected to large distribution networks
true
Synchronizing an alternator to a power system is analogous to synchronizing gears in a transmission.
true
The Series DC Motor is popular because of its high starting torque.
true
The incoming alternator will have a constant flux in its air gap because of the fixed frequency and voltage of the "infinite bus."
true
