Line Y-3 Flashcards
Define torque
Torque is a twisting or turning force that is capable of producing rotation about an axis
Define the motor effect principle of magnetism
Whenever a conductor carries current in the presence of a magnetic field, a force will act on the conductor at right angles to the field
Fleming’s right-hand motor rule is used to determine _____________________
The direction of force acting on a conductor
How does the operation of the armature differ between a DC generator and a DC motor?
In a generator, the armature is a source of emf. In a motor, the armature is a load
State two ways in which the direction of rotation of a DC motor can be reversed
Reverse the connection of the armature leads.
Reverse the connection of both the series and shunt field leads
What is torque directly proportional to?
The product of flux and armature current
The motion of armature conductors cutting a field results in an induced emf that ____________________ the applied emf
Opposes
The emf induced in armature conductors as they cut the magnetic field is called _____________
Counter emf
What is the counter emf of a motor proportional to?
Flux and speed
What is effective armature voltage the product of?
Armature current and armature resistance
The difference between applied voltage and counter emf is called ___________
Effective voltage
At standstill, the counter emf of a motor is _______________
Zero
At start-up, the current drawn by a DC motor is limited only by ____________
Armature resistance
How is the starting current limited to a safe value in large DC motors?
Resistance is placed in series with the armature
A locked rotor usually means that the motor will ________________
Fail to start when voltage is applied
Why is a locked rotor condition undesirable?
The persistence of high starting current may damage the motor
An increase in mechanical load on the shaft of a motor causes motor speed to ___________
Decrease
When the speed of a motor decreases due to an increase in mechanical load, the counter emf ___________
Decreases
A reduction in counter emf causes armature current to ______________
Increase
What would happen if an increase in mechanical load exceeded the ability of the motor to develop additional torque?
The motor would stall
In a motor, power is proportional to ________________ and ________________
torque and speed
The variation in motor speed with load is called ___________
Speed regulation
The speed at which a DC motor operates without speed control devices is called __________
Base speed
Why does increasing the resistance in the field circuit of a DC motor cause the speed to increase?
The increase in armature current (due to a decrease in counter emf ) is greater than the decrease in excitation
To operate a motor below base speed, it is necessary to decrease __________
Armature current
A practical method of operating below base speed involves controlling ______________
The voltage applied to the armature
List two adverse effects of armature reaction if it remains uncorrected
Weakening of the main field
Excessive brush sparking
The most common solution for minimizing the effects of armature reaction involves the use of __________________
Interpoles
Why are interpoles connected in series with the armature?
So that the strength of the interpole matches the strength of the armature field
Shunt field windings consist of a:
Large number of turns of small-gauge wire
T/F
Excitation of a shunt motor is provided by field coils connected in series with the armature
False
When using NEMA subscripts, the convention of A1 positive indicates (CCW or CW) rotation?
CCW
T/F
When a DC generator and a DC motor are operated with the same line voltage, polarity and direction of rotation, the respective armature currents will be in opposite directions
True
State two ways to reverse the direction of the force acting on the armature conductors of a shunt motor.
Reverse the direction of current in the armature
Reverse the direction of current in the shunt field windings
What is the most practical method of reversing the direction of rotation of a shunt motor?
Reverse the armature leads
What happens to the direction of rotation if the polarity of the line terminals is reversed?
The rotation remains the same
A shunt motor is commonly called a _____________________ motor
Constant speed
Why must the field circuit of a shunt motor never be allowed to become open while the motor is in operation?
The motor can accelerate to unsafe speeds
To prevent runaway in a shunt motor, an _______________ is used
Over-speed sensor (or loss of field relay)
List three typical uses for a shunt motor
Drives for paper machines
Printing presses
Drill presses
Series field windings consist of a:
Small number of turns of large-gauge wire
What is the main difference between series and shunt excitation?
The magnetic strength of series field coils varies with load current
State two ways to reverse the direction of rotation of a series motor
Reverse the connection of the armature leads
Reverse the connection of the series field leads
Will changing the polarity of the line leads change the direction of rotation of a series motor? Why?
No. Reversing line polarity reverses both armature current and field polarity, maintaining torque in the same direction as before
Why is the series motor unsuitable for constant-speed applications?
Since the torque varies with the square of the armature current, changes in mechanical load are accompanied by large changes in operating speed
In practice, speed control of series motors is accomplished using _____________
Solid-state drives
T/F
The main advantage of the series motor is its very high starting torque
True
Why should a series motor always be mechanically coupled to the driven load?
The sudden removal of load may cause the motor to accelerate to an unsafe speed
The main characteristics of the series motor are _______________ and _______________
High starting torque and poor speed regulation
List three common uses for series motors
Cranes, hoists, locomotives
A compound motor has both ___________________ and ___________________ field coils
Shunt and series
In most compound motors, the motor characteristics are predominantly like the ___________________ motor
Shunt
When the flux of the series field aids the flux of the shunt field, the connection is called a ___________________ compound
Cumulative
When the flux of the series field opposes the flux of the shunt field, the connection is called a ___________________ compound
Differential
Explain the difference between long shunt and short shunt connections
In the long shunt connection, the shunt field is connected across the line (armature and series field). In the short shunt connection, the shunt field is connected directly across the armature
State two ways of reversing the direction of rotation of a compound motor
Reverse the connection of the armature leads
Reverse the connection of both the series and shunt field leads
T/F
The speed of a compound motor can be increased by increasing resistance in the shunt field circuit
True
Is it necessary to take precautions to prevent runaway of a compound motor?
Yes. The degree of compounding may not be sufficient to prevent dangerous over-speed in the event of an open in the shunt field circuit
Is it acceptable to have belt-driven mechanical loads with a compound motor?
Yes. The no-load speed hazard of the series motor is not present
When selecting a compound motor, the _______________ and _______________
characteristics must be matched to the specific application
Torque and speed
Compound motors are more suitable than shunt motors for handling ______________
Sudden increases in mechanical load
List three applications for compound motors
Presses, shears, compressors
