Final Review Flashcards
Define armature reaction
Distortion of the main field flux caused by current flowing from the armature to the conductors
What is flat compound?
Where the terminal voltage is flat or constant with respect to the load
What is over compound?
The rise in voltage at the terminals of the generator compensates for the line drop at the load
The condition under which the magnetic flux of the series and shunt fields oppose each other is called __________
Differential compound
What application is differential compound most commonly used in?
DC arc welding
Heat produced by current flowing through the armature and field coils is called ________________
Copper losses
Interpole winds consist of _______________
Small number of turns of large-gauge wire
The effects of armature reaction can be minimized by placing windings called __________________ in the main pole faces
Compensating windings
In practice, speed control of series motors is accomplished using _____________
Solid-state drives
The speed at which a DC motor operates without speed control devices is called __________
Base speed
Define torque
Torque is a twisting or turning force that is capable of producing rotation about an axis
Define the term slip speed
Slip speed is the difference between synchronous speed and rotor speed.
How is the direction of rotation reversed in a squirrel-cage induction motor?
By reversing the direction of the rotating field. This is accomplished by interchanging any two of the stator line connections to the source
How is the direction of rotation reversed in a split-phase motor?
By reversing the direction of current through the auxiliary winding
How is the direction of rotation reversed in a series motor?
By interchanging the two armature leads, or the two series field leads
How is the direction of rotation of a shaded-pole face motor reversed?
By removing the stator and turning it end-for-end before replacing it over the rotor
Define the term breakdown torque
The breakdown torque of a motor is the maximum torque the motor will produce
Why does a standard split-phase motor (without a starting capacitor) have a low starting torque compared to a capacitor-start, split-phase motor?
The standard split-phase motor has a phase shift between the main and auxiliary winding of 30–50 electrical degrees. The capacitor-start, split-phase motor has a phase shift of 90 electrical degrees, which corresponds better with the physical displacement of the poles in the motor
Which type of split-phase motor combines good starting torque with good overload capacity?
The capacitor-start, capacitor-run split-phase motor
What are the two types of windings in the stator of a shaded-pole motor?
Exciter winding, shading coil
In what direction does the field move across the pole face in a shaded-pole motor?
From the unshaded portion to the shaded portion
Is the starting torque of a shaded-pole motor high or low compared to a split-phase motor?
Low
What is the purpose of the tertiary winding included in some wye-connected autotransformer banks?
It permits the flow of third-harmonic currents. This eliminates the third-harmonic voltages in the wye-connected windings
What advantage does the extended-delta autotransformer have over the standard-delta autotransformer?
It permits transformations in ratios higher than 2:1
Which autotransformer connection would likely be used to achieve a desired phase displacement between the primary and secondary systems?
The closed-delta connection
What is the “form factor” commonly used for the three-phase half-wave rectifier
1.17 × AC phase (rms) value
What is the “form factor” commonly used for the three-phase full-wave rectifier
1.35 × AC line (rms) value
What advantage is offered by using SCRs instead of diodes in three-phase rectifier circuits?
SCRs can be phase-controlled to offer a variable DC output.
How is an SCR normally turned on?
By applying a small amount of power to the gate terminal
How does the power-handling ability of the SCR compare to that of a transistor?
SCRs can handle much more power.
What happens if the SCR’s reverse-blocking voltage is exceeded?
The SCR will conduct in the reverse direction and will be permanently damaged.
What happens if the SCR’s forward-blocking voltage is exceeded?
The SCR will be turned on without a gate signal.
If resistor triggering is used, the SCR can be fired between ________ degrees up to ________ degrees in the positive half-cycle
0 to 90
If resistor-capacitor triggering is used, the SCR can be fired from ________ degrees up to ________ degrees in the positive half-cycle
0 to 180
Whereas the SCR is a unidirectional device, a triac is a ________ device
Bidirectional
The action of a triac could be likened to two ________ connected back-to-back
SCR’s
Triacs are available in rms current ratings up to a maximum of about ________ A
45
What is the difference between a reduced-voltage starter with an open transition and a reduced-voltage starter with a closed transition?
For open-transition starters, there is a momentary loss of voltage (power) to the motor as the control transfers from the starting to the running stage.
But for closed-transition operation, there is no interruption of voltage
What is anti-plugging protection?
Anti-plugging uses a zero-speed switch to prevent a motor from reversing until it has come to a complete stop
Explain what is meant by the term plugging
Plugging is a control function that provides braking by reversing the motor line voltage polarity or phase sequence so that the motor develops a counter-torque that exerts a retarding force
What conditions must be met in order that alternators may be paralleled?
Same: voltage, frequency, phase rotation. ( then phase opposition for sync. point.)
What happens when a coil is rotated between a pair of opposite polarity field magnetic poles?
AC voltage is induced into the coil
Brushes are positioned to short out a coil when it is ___________
Crossing the neutral plane
Increasing the firing angle for an SCR from 0 75 degrees would result in _________
Less power supplied to the load
The output of a revolving field 3 phase AC machine is taken from _____________
Slip rings and brushes
Eddy currents and hysteresis losses are referred to as __________
Core losses
The variable resistance placed into the rotor circuit is mainly for the purpose of __________
Varying the torque
The maximum value of torque in a SCIM will ______
Occur when the rotor has XL = Resistance
When a holding contact is wired into the control circuit, the motor is considered to have ________
Low voltage protection
