3.12 DC Motors And Generators Flashcards
What type of devices are DC motors or generators?
Energy transfer devices
How does a motor work?
Transferring electrical energy to kinetic energy of a turning armature (the motor effect)
What is an armature in an electrical motor?
An armature that’s wrapped in turns of wire (a rotating coil or inductor)
How does a an armature connect to a power source in a dc motor?
By using a split ring and a pair of brushes
What happens when the brushes line up with the splits in a commutator of a dc motor?
It is momentarily disconnected from the supply but inertia carries it until it reconnects
How does a generator work?
By spinning a conductor in an electromagnetic field it creates electricity
What are the parts in a DC generator that stay still called?
Stator
What are the parts in a C generator that rotate called?
Armature or rotor
What are the stationary parts of a DC generator?
-the ring (yoke)
-two main magnetic poles
-excitation windings around each pole
In a DC generator what is a steel ring also known as?
A yoke
What is the purpose of the excitation windings in a DC generator?
To generate a stationary magnetic field in the stator
DC generators with a high output will usually come with?
Inter poles and compensation windings
What three parts does an armature consist of?
-the shaft
-the windings of the coil
-the commutator
Why does a DC generator and motor have multiple separate coils wound around the armature?
So there is always at least one coil ideally situated in the magnetic field
What happens when the brushes move across the commutator and go onto different sections?
It reverses the the polarity of the armature as the pass the neutral point
What are the brushes on a DC generator or motor made from?
Carbon
What happens when the brushes wear away over time?
They need to be replaced
What is used to create the main field in DC machines?
Electromagnets
Why is the armature core laminated in DC motors?
To minimise eddy currents
What is meant by ‘armature cross field’?
As the field of the armature crosses the main field
What happens to the cross field when the current in the armature increases?
It also increases
What is the armature relation?
The impact of the armature cross field on the main field
Where is electromagnetic induction at its highest in DC machines?
Under the poles
Where is the electromagnetic induction at its lowest in DC machines?
Between the poles
What is reactive sparking?
Where the brushes meet the coils in the neutral zone
What does brush sparking cause?
High wear on the brushes and the commutator
What are interpoles used for?
To cancel out the displacement of the neutral zone
Where are the interpoles located?
Between the main poles
What can be placed in the main poles to cover total armature cross field?
Compensating winding
How are the interpoles and compensating windings connected?
In series (both supplied by the current from the armature)
DC generators produce an output voltage which depends on what three main factors?
-number of turns on the armature
-rotational speed
-excitation current in the stator windings
DC generators have a stated rotational sped with produces a particular output voltage, why can the current still fluctuate?
It depends on the external load that its powering
What determines the current flow from a DC generator?
The direction of rotation
Where can the rotation direction be found for a DC generator?
Marked on the casing or the main shaft
What is the power output of a motor dependant on?
The motor rated torque and the rotational speed
Output power is defined in terms of?
Horsepower
To convert a calculated value of power from watts to horsepower, what do you have to do?
Divide by 746
What generates torque in a motor?
Shaft rotation
Torque is given in?
Force-distance units (lb-ft, n-m, etc.)
What is stall torque?
When the shaft speed is zero or the motor stalls
What is continuous torque?
The maximum torque at running conditions
What is meant by race in a motor?
When the machine infinitely increases its speed
What determines the the direction of rotation in a DC motor?
-direction of magnetic field
-the flow of current in the armature
What happens if you change the direction of the magnetic field and the direction of current flow in a DC motor?
Nothing it will spin the same way as you change both not just one
Series wound motors have a very high?
Starting torque
Why must series wound motors not be operated without load?
Because they can race causing the armature to break
How are series wound machines limited during start-up?
By using resistors
An operating series wound motor is also a?
Generator
In a series wound motor what does the terminal voltage and the speed depend on?
The load
Very high loads pose the risk of a?
Short circuit
What is the most common DC motor?
The DC hunt wound motor
When is a shunt wound motor used?
Whenever a constant speed at various loads is required
What does a DC shunt wound motor consist of?
-stator with excitation windings
-rotor with armature windings
-set of carbon brushes
What is meant by shunt characteristics?
-the speed is nearly independent from load
-the motor reaches its maximum speed at no load operation
What are the two types of electric starting systems for gas turbine aircraft?
-direct cranking electrical systems
-starter generator systems
Why are starter generator units economically desirable?
They’re a starter and a generator within a single unit
