Construction Features Of DC Machines Flashcards
Define the term “dynamo”
Term originally given to a rotating machine that converted mechanical energy into electrical energy, primarily in form of DC current
What is the main difference between DC motors and generators?
Surrounding enclosure. Generators = open frame, motors = closed frame
Define “armature”
Rotating part, or rotor, of the machine
What does the armature core consist of?
Stack of soft iron laminations which are slotted to house a set of insulated coil windings
Why are laminations slotted?
To house a set of insulated coil windings
Why is the steel shaft of the armature supported by bearings?
Allows for uniform rotation of the complete armature assembly within the air gap
What are armature coils generally wound with?
Varnish-insulated magnet wire
How are armature coils protected?
Slot-cell insulation
What holds the coils of the armature tightly in place?
A wedge driven the full length of the slot
What are the ends of each coil connected to?
Commutator bars
What are the two types of armature windings?
Lap windings, wave windings
What applications are lap windings used?
High current, low voltage
What applications are wave windings used?
Low current, high voltage
What is the action of the commutator?
Convert induced AC within the armature coils to a uni-directional current in the external circuit
The combination of _________ and _________ acts as a _________ _________ ________ between the armature coils and the external load
Brushes, commutator, rotary selector switch
The commutator is an…….
Assembly of individually-insulated copper bars or segments
The copper bars in a commutator are…..
Wedge shaped segments with a V-shaped grove at each end for clamping together
What is the purpose of the brushes in a DC generator?
Conduct the current to or from the armature coils via the commutator from or to the circuit
What materials are brushes usually made of?
One or more of the following:
- carbon
- electro-graphite
- graphite
- copper graphite
What is the choice of brush material in a DC generator based on?
Application and cost
What is the brush holder designed to do?
Support the brush and provide suitable pressure against the commutator surface
What is the pressure applied by the brush holder?
1-2 psi
What is the purpose of field poles?
Provide the magnetic field in which the armature will rotate
How is the magnetic field provided in DC machines?
Permanent magnets or by direct current flowing through the field coils
What does the electromagnetic field poles consist of?
Soft iron, laminated core, field coils wound with varnish-insulated magnet wire
The frame or yoke…
Provides mechanical support for the machine and forms part of the magnetic circuit between the two field poles
End shields are….
Mounted at the end of the frame and contain recesses for the armature shaft bearings
What are the two common classes of bearings used in motors and generators?
Sleeve bearings and ball bearings
What do sleeve bearings consist of?
A shaft and oil ring
What do ball bearings consist of?
A shaft and an end plate
What are sleeve bearings lubricated with?
Oil, and applied with an oil ring or oil soaked ring
New sleeve bearings are….
Usually machined undersized and reamed for proper fit
What are ball bearings lubricated with?
Grease, and may be open or sealed
Sealed bearings are….
Factory lubricated for life of bearing
Open bearings require…
Grease nipples in the end shields
What happens if you over grease the bearings of rotating machines?
Produce heat and reduce bearing life
