3.12 DC Motor and Generator Theory Flashcards
What is the function of the commutator in a DC generator?
Mechanically reverses the armature loop connections to the external circuit. This occurs at the same instant that the polarity of the voltage in the armature loop reverses., this action is known as COMMUATION!
What is the function of the brushes in a dc generator?
Brishes ride on the surface of the commutator and act as an electrical contact between the armature coils and the external circuit.
When is the ideal time for commutation to take place in a DC generator.
The same time the armature current changes direction. (short circuit of a brush)
Which law when applied to power generation in a DC Generator gives motor reaction?
Lenz Law
What are the efficiency losses of a DC Generator?
Armature losses, copper loss in the winding, eddy current loss in the core, hysteresis loss.
Why are the armatures of a DC Generator laminated?
To prevent eddy currents.
What factors affect the output voltage generated by a DC generator?
Number of conductor loops in series in the armature, armature speed, magnetic field strength
What is the difference between the output of a series connected DC generator and a shunt DC generator?
The shunt wound generator has a much more stable output then a series wound generator.
What provides the initial magnetism to start a shunt generator producing power?
Residual magnetism or residual flux in the generators field assembly.
In a DC Generator which way is armature reaction acting
In the direction of rotation.
In a DC motor what is the armature reaction direction?
Against the direction of rotation.
What does the series DC motor get used for?
Develops a large amount of turning force, called torque. Used on electric tools, cranes winches, hoists.
What are shunt DC motors used for?
Used where uniform speed, regardless of load is needed. not a very good starting torque.
What is done to reverse the direction of rotation of a DC motor?
Reversing the direction of current flow in the armature or the field windings. Another method is to use a SPLIT FIELD! winding
How is a series DC motor adjusted above normal speed?
Rheostat connected in parallel with series field.
