3.12 DC Motors & Generators Flashcards
What are DC motors and generators?
Energy transfer devices that can function as either motors or generators.
What is the primary difference between DC motors and generators?
The direction of energy conversion.
What phenomenon do DC motors and generators utilize?
Electromagnetic induction.
What happens in the phenomenon of electromagnetic induction?
The motion of an electric charge in a magnetic field results in a force being exerted on the charge.
In a DC motor, what is the purpose of the electromagnetic force?
To turn a rotor.
In a DC generator, what is the purpose of the electromagnetic force?
To drive an electric current around a circuit.
What does the Motor Effect describe?
A current carrying conductor within a magnetic field experiences an electromagnetic force.
What is the direction of the electromagnetic force in the Motor Effect?
At right angles to both the magnetic flux lines and the current.
When is the Motor Effect strongest?
When the current and flux are at right angles to each other.
What happens to the Motor Effect when current and flux are parallel?
It falls to zero.
What does a DC motor transfer energy from and to?
From the electrical energy store of a power supply to the kinetic energy store of a turning armature.
What causes the movement of the armature in a DC motor?
The motor effect.
What is wrapped around the armature in a DC motor?
Many turns of wire, forming a rotating coil or inductor.
What creates the magnetic field in a DC motor?
Permanent magnets or electromagnets.
How is the armature connected to a DC power source?
Through a split-ring commutator and a pair of brushes.
What do the brushes in a DC motor do?
Provide a continuous connection between the battery leads and the coil.
What happens to the current when the commutator ring aligns with the brushes after a quarter turn?
The coil is momentarily disconnected from the supply and the current direction is reversed.
What is the effect of inertia on the armature in a DC motor?
It carries the armature past the momentary disconnection point.
How does the direction of the current affect the rotation of the coil in a DC motor?
The current flows in the same direction relative to the magnetic field, driving the armature in the same direction.
How does a DC generator differ from a DC motor?
The armature is rotated by an external force to induce an EMF across the armature.
What connects the ends of the armature coil to an external circuit in a DC generator?
A split-ring commutator.
Fill in the blank: In a working generator, there are several coils positioned at regular intervals around the _______.
armature.
What is the stationary part of a DC generator called?
Stator
The stator is the part of the machine which remains stationary.
What components make up the stator of a DC generator?
- A steel ring (yoke)
- Two main poles made of sheet metal with pole core and pole shoe
- Excitation windings around each pole
The excitation winding generates a stationary magnetic field in the stator.
What are the three main parts of the armature in a DC generator?
- The shaft
- The windings embedded in grooves
- The commutator
The armature is also known as the rotor.
What is the purpose of having several separate coils wound around the armature?
To increase efficiency by ensuring that at least one coil is ideally situated in the magnetic field of the stator magnets.
What is the function of the commutator in a DC generator?
Connects individual coils of the armature winding to commutator segments.
What materials are used to make commutator segments?
Hard copper separated by mica.
True or False: The brushes in a DC generator are generally made of carbon.
True
What generates the main field in modern DC machines?
Electromagnets.
What is the purpose of laminating the armature core?
To minimize eddy currents.
What is the ‘armature cross field’?
The field created by the armature that runs perpendicular to the magnetic field of the stator.
What happens to the neutral zone as the current in the armature increases?
The neutral zone is displaced.
Define ‘armature reaction’.
The impact of the armature cross field on the main field, which displaces the neutral zone and distorts the main field.
What is the neutral zone in a DC generator?
An imaginary zone where no induced voltage is generated in the armature.
What is ‘reactive sparking’?
Sparking at the commutator caused by the coils meeting the brushes outside the neutral zone.
What are interpoles?
Small poles located between the main poles that oppose the armature cross field.
How do interpoles and compensating windings work together?
Both are connected in series and supplied with current from the armature to cancel out the armature cross field.
Fill in the blank: The brushes must be adjusted according to the _____ to ensure they are always located in the neutral zone.
load
What are the three main factors that affect the output voltage of a DC generator?
The three main factors are:
* Number of turns on the armature windings
* Rotational speed
* Excitation current in the stator windings
Increasing any of these factors will also increase the output voltage.
How does the current drawn from a DC generator affect the output voltage?
Generally, the more current that is drawn from the generator, the lower the output voltage becomes.
This is due to the load on the generator.
How can the output voltage of a DC generator be regulated?
By varying the excitation current in the stator windings.
This helps prevent changes in output voltage due to changes in load current or rotational speed.
True or False: The direction of current flow from a DC generator is independent of the direction of rotation.
False
The direction of the output voltage and current flow depends on the direction of rotation.
What should be marked on the casing or main shaft of a DC generator?
The direction of rotation.
It is crucial to ensure that the generator is turning in the correct direction to avoid reversing the polarity of the circuit.
Fill in the blank: The output voltage of a DC generator is dependent on the number of turns on the armature windings, rotational speed, and _______.
[excitation current in the stator windings]
What are important performance parameters for DC motors?
Power, torque, speed, and direction of rotation
What is the rated Power Output of a DC motor?
The product of the motor’s rated torque and rotational speed
How is output power defined in terms of horsepower?
Divide the power in watts by 746
What does shaft rotation generate in a DC motor?
Torque
In what units is torque generally given?
lb-ft, oz-in, N-m, etc.
What are the two types of torque specifications in DC motors?
- Stall torque
- Continuous torque
What is stall torque?
The torque at which the shaft speed is zero
What is continuous torque?
The maximum torque at normal running conditions
What is the relationship between torque and armature current?
Torque is proportional to armature current
What does the torque constant indicate?
A high value limits the current to a low value
What is the effect of lower current consumption in a DC motor?
Lower power (heat) dissipation
What does voltage control do in a DC motor?
Calculates speed from standstill to rated speed
What happens when armature voltage is reduced while load increases?
The speed decreases
What is the purpose of a thyristor-controlled rectifier in motor speed control?
Provides a more efficient method for speed variation
What does field control involve in DC motors?
Reduces excitation current by series-connected resistance
What is a limitation of field weakening in DC motors?
It can only be applied to a certain degree to avoid damage
What determines the direction of rotation of a DC motor?
The directions of the magnetic field and current flow in the armature
What happens when both the magnetic field and current direction are reversed?
The motor continues to rotate in the same direction
What is required to change the direction of rotation in a DC motor?
Reverse the armature or magnetic field, but not both
What is a practical application of changing direction in DC motors?
Drive motors for gun turrets or missile launchers
What type of DC machine is used as a motor with a very high starting torque?
Series wound DC machines
Series wound motors must never be operated without load due to the risk of racing.
How is the armature winding connected in a series wound machine?
In series with the excitation winding
This means the same current flows through both windings.
What is the risk of operating a series wound motor off-load?
The armature races until destroyed
This characteristic is known as series wound characteristics.
List applications of series wound motors.
- Car starters
- Hoists
- Trains
- Trams
- Electric vehicles
- Windscreen wiper motors
- Shut-off valves
Series wound motors are used in various applications that require high starting torque.
What happens in a series wound motor when the load increases?
Current increases and speed decreases
This generates high torque during start-up and at high loads.
True or False: A series wound motor can operate as a generator.
True
The voltage induced in the armature winding opposes the applied terminal voltage.
What must be present for a series wound generator to be fully excited?
Total load current must flow
Very high loads can pose the risk of a short circuit.
What is the main characteristic of shunt wound motors?
Speed is nearly independent from the load
This means the speed only drops slightly at load.
What type of current flows through the excitation winding in a shunt wound motor?
The same at no-load operation as at full-load operation
The armature current, however, depends on the load.
What type of generator can a shunt wound DC machine operate as?
Self-excited or separately excited generator
The excitation method affects how the voltage is generated.
In a separately excited shunt wound generator, what determines the generated armature voltage?
The size of the exciting current
Increasing excitation current can raise armature voltage under load.
What is required for self-excitation in a self-excited shunt wound generator?
Residual magnetism and correct polarity
If these conditions are not met, the machine cannot excite itself.
What is a compound motor?
A motor with both shunt and series fields
This allows for flexibility in performance characteristics.
How does a shunt series DC machine behave at no-load operation?
Like a shunt wound motor
Racing occurs only if both excitation fields fail.
What are the components of a shunt series DC machine?
- Stator with two excitation windings
- Rotor with armature winding
- Carbon brushes
The windings can either strengthen or weaken the excitation field.
What is the effect of varying the size of the shunt and series windings in a shunt series DC machine?
Influences operational characteristics
It can lead to either series or shunt behavior prevailing.
There are two types of electric starting systems for gas turbine aircraft: direct cranking electrical systems and starter generator systems.
Direct cranking electric starting systems are used mostly on small turbine engines, such as Auxiliary Power Units (APUs) and some small turboshaft engines. Many gas turbine aircraft are equipped with starter generator systems which employ an electric motor to start the internal combustion engine.
Starter generator starting systems are similar to direct cranking electrical systems, except that they contain a second series of windings that allow them to switch from operating as a motor to operating as a generator after the engine has reached a self-sustaining speed. This saves both weight and space.
The starter generator is permanently engaged with the engine shaft through the necessary drive gears. By contrast, the direct cranking starter needs a way to disengage itself from the shaft after the engine has started. The starter generator unit is basically a shunt generator with an additional heavy series winding. This series winding is electrically connected to generate a strong field and a resulting high torque for starting.
Starter generator units are economically desirable, as they perform the functions of starter and generator within a single unit. This minimises the total weight of the starting system components and the number of spare parts required.
The starter generator internal circuit has four field windings
A series field (C field)
A shunt field
A compensating field
An interpole or commutating winding
When starting, the C field uses compensating and commutating windings. The unit is similar to a direct cranking starter, as all the windings used while starting are in series with the source. While acting as a starter, the unit makes no practical use of its shunt field.
Starting usually requires a source of 24 V and 1500 peak amperes.
When operating as a generator, the shunt field, compensating field, and commutating windings are used. The C field is only used for starting.
The shunt field is connected in the conventional voltage control circuit for the generator. Compensating and commutating, or interpole, windings provide almost sparkless commutation from no load to full load.
The undercurrent controller has a control block with two relays:
one is the motor relay that controls the input to the starter; the other is an undercurrent relay, which controls the operation of the motor relay.
