Electric Machines (5)

Question 1

What are the features of the most basic form of a DC generator? Explain how it work.

Answer 1

Features:
* Fixed magnetic poles
* Armature (made of laminates)
* Rotating coils in the Armature that rotate with it in the magnetic field
* Commutator (basically a slip ring cut up so it only provides current in one direction)
* Brushes

Explaination of how it works:
* Like an AC generator an AC voltage is produced from the rotating armature with the conductors (coils inside).
* This current is sent through the commutator rather than a slip ring, which is cut up so that the brushes only brush on the commutator to output the voltage in the positive direction only. As shown on the diagram below.

Question 2

How do we create a more constant DC voltage from a DC generator?

Answer 2

Have more segments to the commutator with many different phase conductors (coils) which result in smaller peaks that provide a smoother DC voltage out. As shown below.

Question 3

What is the equation for EMF generated in a DC generator?

Answer 3

E = Ke ω

where Ke is the EMF constant and ω is the angular speed.

Note:
Ke can be written as: Ke = Z/a BL D/2

where Z is the number of conductors in the active armature coil, a is the number of conductors connected with ,a, parallel paths; D is the diameter of the armature coil; finally L is length of the armature coil cutting the magnetic flux

Question 4

What is the equation for EMF constant, Ke, for DC generators or motors?

Answer 4

Ke = (Z/a) BL (D/2)

where Z is the number of conductors in the active armature coil, a is the number of conductors connected with ,a, parallel paths; D is the diameter of the armature coil; finally L is length of the armature coil cutting the magnetic flux

Question 5

Draw the general equivalent circuit and the equivialent circuit when current is constant, of a DC generator?

State to the differences between them.

Answer 5

Differences:
* The general equivalent circuit has an inductor in it, unlike the other circuit. As current isn’t steady so voltage is still across the inductor.
* The inductor in this case is based on the coils in the armature.

Circuits shown below: