DC Generators/Motors Flashcards
Define a generator
A device that converts mechanical energy into electrical energy
Voltage produced in the armature is DC or AC?
AC
Which part of the generator produces AC?
Armature
Which part of the generator produces DC?
Commutator
What is the purpose of the commutator?
To convert AC to DC by reversing the current flow in the winding
In a DC generator, increasing the number of loops has what purpose?
To create a smoother output voltage. More waves evens it out.
What is the smooth effect of multiple loops called?
Ripple
What makes up the armature?
Iron core, commutator, windings
What are the three types of armatures?
Lap-Wound, Wave-Wound, Frogleg-Wound
What is the most common type of armature?
Frog leg-Wound
What are the voltage and current properties of lap-wound armatures?
Low voltage, high current. Larger wire gauge.
What are the voltage and current properties of wave-wound armatures?
High voltage, low current. Smaller wire. Connected in series.
What are brushes?
Armature leads, carbon pieces that sit against the commutator to connect Eg to external circuit
What are the properties of the pole pieces?
Provide magnetic field for operation, made of soft iron or silicon steel.
Which windings have larger wire and fewer turns?
Series
Which windings have smaller wire and more turns
Shunt
What are the three types of DC generators?
Series, shunt, and compound
Which type of generator must be self-excited?
Series
What does a self-excited generator mean?
Residual magnetism produces an initial Power Out Voltage
The pole pieces retain magnetism
What are the three factors that determine Vout in a series generator?
- # of turns
- Strength of magnetic field
- Speed of rotation
What is Saturation of Iron in a series generator?
The max voltage that can be produced, max amount of flux produced. Past saturation the voltage output will decrease.
What is rated load in series generator?
Rated load is the point of saturation, where max voltage is produced.
What is the difference between a self excited and separately excited generator?
Self excited holds residual magnetism, separately excited needs an external source of energy to start
T or F: shunt generators can be both self excited and separately excited
True
T or F: series generators can be both self excited and separately excited
F. Only self excited.
What is the difference between series and shunt self-excited generators in regards to current?
A load is required to start current flow in a series SE generator, in a shunt SE the windings act as a loop so current is already generated before a load is connected. Full voltage can be obtained by a shunt SE generator before a load is connected.
What are the two advantages of separately excited shunt generators?
- Better voltage regulation
- Less voltage drop when a load is added
What is a difference between series SE and shunt SE generators in regards to voltage?
Series SE will only produce a small amount of voltage with no load, Shunt SE can produce full voltage with no load because the windings of the shunt act as a load creating a loop
Why are separately excited machines superior to self excited machines?
On a SE generator, once saturation is reached, when a load is added it decreases the voltage and current output because there is a maximum amount that can be produced through the magnetic field. On a Sep Ex generator, an external source keeps the flux magnetic field constant, so it doesn’t have the same amount of drop.
What is the difference between field poles and inter poles?
Field poles: permanent magnets that create magnetic field
Inter poles: windings connected in series with the armature. Made of smaller windings than the field poles
What is the purpose of interpoles?
Helps reduce arching between brushes and commutator
What are the connection codes for series and shunt?
S2 and S1 are series, F2 and F1 are shunt.
What is the formula to find Regulation?
Enl-Efl / Efl x100
When armature leads are reversed, what happens to the direction of rotation?
It also reverses.
What type of switch can be used to reverse the rotation of a shunt motor?
DPDT
When the shunt leads are reversed, what happens?
The field changes from cumulative to differential or vv
What is the most common compound configuration for a generator?
Short shunt cumulative compound. More magnetic flux and torque.
Your thumb indicates which rule?
Direction on force
Your forefinger indicates which rule?
Magnetic field
Your middle finger indicates which rule?
Direction of current
Your left hand rule is for _____
Generators
Your right hand rule is for _____
Motors
The standard rotation for a DC generator is CW or CCW?
CW
The standard rotation for a DC motor is CW or CCW?
CCW
What type of generator can be run in either direction?
A separately excited generator
How do you find E gen of a short shunt generator?
E across armature resistance + E across shunt.
How do you find efficiency of a generator?
Add both of the wattages produced in the generator (armature and resistor). Divide terminal power by the armature and resistor power.
Pa + Par / Pload
What are the four generator losses?
Windage/mechanical, copper/line, iron/core, brush
What are copper losses of a motor?
Resistance in the windings/loads. I^2 x R
In a DC generator, what is cumulative?
When shunt and series connections are made to aid in the development of the magnetic field.
Stronger magnetic field, more torque.
In a DC generator, what is differential?
When series and shunt fields are wound to oppose each others magnetic fields, reducing torque.
What is the formula to find frequency?
F = P x N / 120
Frequency = number of poles x RPM / 120
Generator EMF is proportional to what factors?
Field strength, number of turns in armature windings, rpm of armature
What is the formula for Output Voltage of a generator?
E gen - IR drop in armature and brushes
E gen - E of armature and brushes.
T or F: (for a generator) voltage regulation is determined by the resistance of the armature?
True
What are the four types of compounding?
Over, flat, under, differential
What is over compounding?
When voltage at full load is greater than no load.
Series windings are increased - higher voltage on long cable runs offsets I^2 x R losses at the load.
What is flat compounding?
When full load and no load output are equal
What is under compounding?
When output voltage is less at full load than no load.
How do you adjust the no load voltage of the generator?
With a field rheostat
How do you control the compounding and adjust the full load voltage of the generator?
With a diverter rheostat
How do you calculate power output of a motor?
Power input - all losses
What is the difference of A1 and A2 between a generator and motor?
Generator: A1 is usually positive, A2 is usually negative
Motor: determined by the source leads.
What is the purpose of a long shunt differential compound generator?
Lower the voltage to maintain a higher current.
Define counter torque
The turning resistance of the armature because of the attracting magnetic fields
What is dynamic or regenerative braking?
When the motor very quickly disconnects the armature from the source and reconnects it to a load resistor. Current now flows through the resistor instead of the source, slowing down the armature. This only works if the current is still flowing.
What is armature reaction?
Twisting or bending of magnetic lines of flux of the pole pieces, causing neutral plane to move, causing brushes to no longer connect to commutator.
Amount of armature reaction is proportional to what?
Armature current.
What are the three ways to correct armature reaction?
Move the brushes, interpoles, and compensating windings.
How does adjusting the brushes fix the armature reaction ?
Moving them to align with the neutral plane
How do interpoles correct the armature reaction?
The interpoles restore the magnetic field to its original condition
Interpoles in a motor must have the same polarity as ____
As the main field pole directly behind them (N or S)
Interpoles in a generator must have the same polarity as ____
As the main field pole directly ahead of them.
Interpoles are connected in 1. series or 2. parallel, and with what?
In series with the armature.
Full load speed of a shunt motor generally remains within ____% of no-load speed
10
What is speed regulation?
The change in speed from no load to full load
Speed regulation is proportional to _____
Supply voltage
What are the properties of series windings?
Less turns, larger wire, low resistance.
What are the properties of shunt windings?
More turns, smaller wire, higher resistance.
How do you determine total losses due to resistance?
Find all wattages across all loads and resistors, add them all together.
What is the formula for force of a motor?
F = mass x acceleration
What is the difference between load torque and acceleration torque?
Load torque is the amount of torque constantly required for application.
Acceleration torque is the torque required just for the maximum acceleration and deceleration rate for the load.
What is the formula for Torque?
T = Kt x I
Or
T = F x r
What is the formula for Kt?
2 x pi x RPM / 60
The strength of EMF can be measured at any moment with what formula
E = Blv
Maximum cutting of flux is reached at what degrees?
90 and 270
What is the formula to find instantaneous EMF?
E = BLVsintheta
If sin theta is not provided, what is it usually?
90
