I3 Install DC Motors and Generators Flashcards
What is the term for a rotating machine that converts mechanical energy into electrical energy?
Dynamo
Define: armature
rotating part or rotor of the machine
List the two types of armature windings
Lap used for high current, low voltage applications
Wave used for low current, high voltage
What does the commutator do?
convert the induced AC within the armature within the armature coils to a uni-directional current in the external circuit.
What is the function of the brushs
they conduct the current to or from the armature coils via the commutator from or to the external circuit.
What are brushes commonly made of?
carbon
graphite
copper graphite
How much pressure is on the brushes?
1 to 2 psi
Define: field poles
they provide the magnetic field in which the armature will rotate.
What type of bearing are used in DC Motors
sleeve bearings
ball bearings
Define: magnetos
small dc generators that use permanent magnets to provide the magnetic field.
what two ways can field poles be excited
DC source external to the machine
from the armature of the generator itself
Define: Series field windings
connected in series with the armature and carry full armature current
Define: Shunt field windings
connected in parallel with armature therfore are made up of large number of turns of small-guage wire.
Self-exited generators must have what to produce voltage?
residual magnetism of the field core
What is Faraday’s law of electromagnetic induction?
e = Blv
Explain the generated voltage formula
Eg = k(flux)N Eg = generated voltage in volts k = constant Flux N = rotational speed in rpm
How can an absence of residual magnetism be rectified in a self exciting DC motor
by flashing the field
If a DC motor is rotated backwards it generates a field current that ________
strips away the residual magnetism and voltage will fail to build-up
Critical field resistance
for voltage build-up to occur, the field resistance curve must lie below the saturation curve therefore set field rheostats to their minimum value.
Generated voltage is proportional to
the product of field flux and armature speed and some constant
Define armature reaction
when an armature is delivering current to a load, a magnetic field surrounds the armature conductors
how does armature reaction affect the operation of a generator
reduced terminal voltage of the load
impairs commutation by shifting the mechanical neutral plane to the electric plane (sparking brushes)
what two ways can armature reaction be fixed
using interpoles
compensating windings in the field poles
Interpole coils consist of a
small number of turns of large guage wire and in series with the armature
List three types of losses in a DC generator
Mechanical
Copper losses
Core losses
List three types of DC generators
Series
Shunt
Compound
What do the letters A,F,S stand for
armature
shunt field coils
series field coils
Numerical subscripts are used to indicate current direction in the windings. Flow from even to odd
F2 to F1 and from S2 to S1
Define cumulative compound
when the shunt and series field coils work together
Define: differential compound
when the shunt and series field coil oppose each other.
how is a long shunt connected?
across the line
how is a short shunt connected?
across the armature
List the three types of cumulative compound DC generators
Flat-compound
Over-compound
Under-compound
what is the main application for a DC differential compound generator.
DC arc welding
what do the three finger represent in the Right-hand motor rule?
Thumb is thrust
First finger is direction of flux from North to South
Middle finger is direction of electron flow
Define: Torque
twisting or turning force which is capable of producing rotation about an axis
Torque is directly proportional to
the product of flux and armature current and some constant
Define counter emf
the motion of the armature conductors cutting the field in a DC motor results in an induced emf that opposes the applied emf that is producing the armature current
Define effective voltage
the product of armature current and armature resistance
Describe mechanical loading of a DC motor
Speed decreases counter emf decreases increase in effective voltage increase in armature current increase in torque
Describe how a DC motor is operated above base speed
by inserting rheostat in the shunt field cct.
field current will decrease
decrease in flux will cause counter emf to decrease
armature current to increase
armature current is greater than the decrease in excitation resulting in more torque.
Describe how a DC motor is operated below base speed
reduce armature current by adding resistance in series with the armature ( not practical)
Done with electronic devices
which way does the electrical plane rotate in a generator?
CW
which way does the electrical plane rotate in a motor
CCW
Shunt motor is commonly referred to as a
constant speed motor
To prevent run-away in a shunt motor an
over-speed senor (loss of field relay) is used
Always mechanically couple a series motor
directly to its load
common application of series DC motor
automotive starters
locomotives
mine haul trucks
Main characteristics of the series motor are
high starting torque
poor speed regulation
Describe compounding or degree of compounding
the degree of compounding can produce torque characteristics ranging from series to shunt.
compounding is the ratio of resistances of the shunt coils to series coils. ( I think)
Is it necessary to take precautions to prevent run-away of a compound motor?
No. shunt coils provide a no load base speed
