Theory Flashcards
Math Applications
Inductance in a circuit opposes a change of current by introducing a _____ force into the circuit.
Counter-electromotive
In a series phasor diagram, what value is used as the horizontal reference?
Current
In a series phasor diagram, what is in phase with the current?
Volt drop across resistance
In a series phasor diagram, current (leads or lags?) Inductive voltage drops by what angle?
Lags, 90 degrees
In a series phasor diagram, current (leads, lags?) Capacitive voltage drop by what angle?
Leads, 90 degrees
In a series phasor diagram, current (leads, lags?) The voltage drop across the coil with both R and XL by what angle?
Lags, phase angle of coil
In RLC circuits, total current is the ____ of the currents that flow through _____ of the parallel circuit.
Phasor sum, each branch
A dynamo can function as a DC _____ or _____
Generator, motor
A generator is rated by _____.
Kilowatt (or megawatt) of power that can be delivered at rated voltage.
An alternator is also called a _____.
Synchronous generator.
Alternators are rated by _____.
VA (or kVA, or mVA)
3 examples of a prime mover are _____.
1: turbines powered by falling water, steam, wind
2: diesel engines
3: natural gas engines
Motors are rated by the power in _____.
KW or HP at rated V or r/min
What’s the difference between the field winding on an AC versus a DC machine? What do they create?
On a DC, usually on stator. On AC, usually on the rotor.
They create magnetic flux.
What develops in the armature of a DC generator? And what in a DC motor?
Generator: voltage
Motor: torque
Where is the armature windings on an AC machine?
On the stator
Where are slip rings mounted?
Shaft of an AC machine. Will rotate with coil and shaft.
What is another name for commutator?
Split ring
What is the purpose of the commutator in a DC motor?
Convert applied DC voltage to AC
What is the purpose of a commutator in a generator?
Convert induced AC voltage to DC.
Commutation is the process of __________ of current in the armature coil
Reversing the direction
Neutral plane is _____.
Where the armature cuts the fewest lines of flux
Brushes on a DC machine _____
Form the sliding electrical connection between armature windings and an external circuit
Brushes in an AC machine _____
Are an electrical connection between field windings and an external circuit.
Arcing is reduced during commutation by _____.
Having armature conductors cut fewest lines of force.
What is the equation for electron flow?
6.242 x 10(exp 18) electrons per second
What are brushes made of?
Carbon
The field winding on a DC machine is located on the _____.
Stator
Energy input to a machine is supplied by the _____ _____.
Prime mover
A DC motor converts ______ into _____.
Electrical energy to mechanical energy
Are the field current and generated voltage after saturation point directly proportional?
No
The field current and generated voltage are almost _____ until the _____ is reached.
Linear
Knee
What is the “Knee” in relation to generated voltage and field current?
It is the abrupt change in the angle of the curve, where the core begins to reach saturation point.
At which angles will the current to the load be at it’s maximum in an AC generator?
90 and 270
What does it mean when induced voltages are series-adding?
Current in both halves of the armature loop is flowing in the same direction
What action does a DC commutator provide?
Switching/rectifying which allows current to flow in only one direction
What is the output voltage at any instant of a multiple coil winding that is connected in series?
Sum of the voltages in each winding
How do you produce a smoother output waveform in a DC generator?
Increasing number of armature coils and commutator bars.
Adding more coils to the armature will produce a waveform with less _____.
Ripple
Most generators are designed to operate at a relatively ______ speed.
Constant
What does a rheostat do?
Controls generated voltage by limiting the amount of current in the winding
If there is no field current, will there also be no voltage? Why?
No, there will be a small amount.
Residual magnetism
Generators are rated by their _____ Kilo or Mega Watt capacity.
Output
An IEC rated generator would be rated in _____.
Kilowatts
A Nema rated generator would be rated in _____.
Horsepower
Temperature rise rating is min __ and max __ degrees C
40, 50
If the ambient temperature exceeds a specified value, th allowable temperature rise must be _____.
Reduced
Rotational frequency is measured in _____ and sometimes also called _____.
R/min
RPM
Examples of DC generator speed ratings are: (4 items)
1800, 1200, 900, 720 r/min
Examples of DC motor speed ratings are: (5 items)
2500, 1750, 1150, 850, 500 r/min
Three phase alternators typically range from _____ V to ______ V.
208 - 13800
Typical DC voltage ratings for generators are: (3 items)
125, 250, 500
Typical DC voltage ratings for motors are: (6 items)
90, 120, 180, 240, 500, 550
Duty (ie continuous, standby) is determined based on the _____ rating.
Temperature rise
What is X’d SAT?
X prime d saturation.
=Reactance value indicating ability of generator to absorb sudden loads without having unacceptable levels of voltage drop.
What is X’‘d SAT?
X double prime d saturation.
=Reactance value indicating ability of generator to provie short circuit current.
FLC rating means the max current while operating at rated voltage and speed, with full load. T or F
True
The polarity of induced emf in a DC generator depends on what?
Direction of the relative motion between the conductor and the magnetic field.
What type of waveform is produced by a rotating loop of wire connected to slip rings?
Sine wave
Magnitude of induced voltage depends on which three factors?
1: Length of the conductor in the field
2: Velocity
3: Flux density
What type of waveform is produced by a single armature loops connected to a split ring?
Pulsating DC
