DC motor and generator theory Flashcards
What is the construction of a DC generator stator?
The stator consists of:
A steel ring (yoke).
Two main poles made of sheet metal with pole core and pole shoe.
Excitation windings around each pole.
What is the construction of a DC generator armature?
The armature, also called the rotor, has three parts:
The shaft, which is fixed to the laminated core of the rotor.
The windings embedded in grooves in the armature.
The commutator, which is fixed on the shaft.
Why are the armature cores laminated?
To minimize eddie currents.
When on load in dc motors and generators, the neutral zone is advanced or retarded how?
In motors it is retarded,
In generators it is advanced
When is coil induction the highest?
Electromagnetic induction is highest under the poles and zero between them.
What is reactive sparking?
As the load on the motor and the armature reaction increase, the neutral zone moves. This enables the coils to meet the brushes outside the neutral zone, where there can be some induction in the coils, causing sparking at the commutator. This is known as “reactive sparking”.
How can you reduce reactive sparking when the neutral zone is advanced or retarded to reduce brush wear?
The use of interpoles can cancel out the displacement of the neutral zone.
Where are interpoles located?
Interpoles are small poles located between the main poles. They are connected in series with the armature and, therefore, oppose the armature cross field with an interpole field of the same size.
What can be used along side interpoles to improve the coverage of the cross field?
Compensating windings.
How are interpoles and compensating windings connected?
Interpoles and compensating windings are connected in series. Both are supplied with current from the armature. This means that as the load on a motor increases, or the current generated by a generator increases, so does the strength of the interpoles and compensating windings. By cancelling out the armature cross field, the brush adjustment in the neutral zone is not necessary.
A DC generators output depends on what factors?
DC generators produce an output voltage which is dependent on three main factors:
Number of turns on the armature windings
Rotational speed
Excitation current in the stator windings
Increasing any of these will also increase the output voltage.
Explain a DC generators typical outputs characteristics?
DC generators usually have a stated rotational speed at which they produce a particular output voltage. The current then depends on the external load. Generally, the more current that is drawn from the generator the lower the output voltage becomes.
How can a DC generators output be controlled?
The output voltage of a DC generator can be regulated by varying excitation current in the stator windings, preventing changes in output voltage due to changes in the load current or rotational speed.
What is a dc motors rated power output made up off?
The rated Power Output is the product of the motor’s rated torque and rotational speed
What is a dc motors power output define as?
Horsepower
