Direct Current (dc) Machines Flashcards
Which three components exist in every generator?
Permanent magnets or electromagnets;
An armature winding, which is rotated;
A commutator and brush set.
What are the five main properties of magnetic flux lines?
Oriented N to S in the external circuit;
Form complete loops;
Repel each other, never cross;
Become as short as possible;
Follow the path of least reluctance.
What is magnetic reluctance?
The opposition to flux in a magnetic circuit.
What is a “Weber” used to measure?
Magnetic flux.
How is the left hand coil rule used?
The thump points towards the north pole,
The fingers point in the direction of current flow through the coil.
{Oriented as if the pinky finger follows the first wrap of the coil}
How is flux density of a coil affected?
Current through the coil;
Number of turns in the coil;
Composition of core material.
Where does the “knee” occur in an air core transformer?
Never, only magnetic materials can saturate.
What is magnetomotive force?
The measure of a coil’s ability to produce flux.
{measured in amp-turns}
When is flux proportional to current in an iron core coil?
Up to just before saturation (at the knee).
Why is a commutator important for a DC machine?
To keep the polarity consistent with respect to the armature winding spinning in the field.
What is commutation?
A mechanical process which converts induced AC voltage in an armature into a “bumpy” DC voltage.
How is severe arcing at the brushes minimized?
By adding interpoles and compensating windings to shift the neutral plane back in line with the brushes.
Why does the neutral plane shift in a generator under load?
Which way does it shift?
The flux is distorted by current flowing through the armature.
In the direction of rotation.
Why are interpole and commutating windings constructed of heavy guage wire?
To minimize volt drop as they will share armature current.
What is the force which pushes a current carrying conductor at right angles to a magnetic field?
The Lorrentz force.
How is polarity of induced voltage determined in a conductor passing through a magnetic field?
Flemming’s left hand rule for generators.
What is an “electrical balance”?
As a motor speeds up, more cemf is generated. This limits current to be just sufficient enough to balance the torque and counter torque of a load.
How is starting current reduced in a DC motor?
By placing a resistor in series while starting.
Reducing the voltage at start and gradually increasing it.
Which direction does the neutral plane shift to in a Dc motor?
opposite the direction of rotation.
Which three factors affect the speed of a DC motor
Amount of load;
Intensity of the magnetic field;
Amount of armature voltage.
Why is runaway dangerous?
Because the motor can tear itself apart through centrifugal forces.
Why must a shunt motor be started with a resistor in series with the armature?
To prevent runaway.
Why must a series motor always have a load?
It can accelerate to dangerous speeds (runaway).
Which Dc motor has the best speed regulation?
A shunt motor.
