Module 3.12 — DC Motor and Generator Theory
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What is the basic function of an electrical generator?
To convert mechanical energy into electrical energy.
What is the basic function of an electric motor?
To convert electrical energy into mechanical energy.
What physical principle allows a generator to produce an EMF?
Electromagnetic induction.
What is required between a conductor and a magnetic field to generate an EMF?
Relative movement that changes magnetic flux linkage.
What determines the magnitude of generated EMF?
The rate of change of magnetic flux linkage.
What happens to generated EMF when the rate of flux change increases?
It increases.
What happens when a coil rotates continuously in a magnetic field?
An alternating EMF is induced in the coil.
What is the main difference between an AC and DC generator's output collection system?
An AC generator uses slip rings; a conventional DC generator uses a commutator.
What collects current from the rotating winding in the simple BA generator?
Carbon brushes.
What is the basic construction of the generator in BA Figure 1?
A rotating wire loop, permanent magnet, two slip rings and brushes.
What does a slip ring allow?
Continuous electrical connection to a rotating conductor without reversing the connections.
What does a commutator do in a DC generator?
Reverses the external connections each half-cycle to provide unidirectional output.
Does a rotating generator loop initially produce AC or DC internally?
Alternating EMF.
How does a DC generator obtain a unidirectional output from an internally alternating EMF?
By using a commutator.
What three quantities determine the maximum induced EMF in BA's straight-conductor relationship?
Magnetic flux density, active conductor length and perpendicular velocity.
What is the BA equation for maximum induced EMF?
Emax = B × l × v.
What does B represent in Emax = Blv?
Magnetic flux density.
What does l represent in Emax = Blv?
Active conductor length.
What does v represent in Emax = Blv?
Velocity perpendicular to the magnetic field.
What happens to maximum generated EMF if flux density doubles at constant length and speed?
It doubles.
What happens to maximum generated EMF if active conductor length doubles?
It doubles, if other factors remain constant.
What happens to maximum generated EMF if perpendicular velocity doubles?
It doubles, if other factors remain constant.
What happens to maximum generated EMF if flux density and speed both double?
It becomes four times as large.
What happens to induced EMF when the conductor moves parallel to the magnetic field lines?
It becomes zero in the simple conductor model.
What happens to generated EMF when there is no relative cutting of magnetic flux?
No EMF is induced.
How often does a simple rotating loop produce maximum magnitude of EMF in one revolution?
Twice.
At which positions in BA Figure 2 is induced EMF zero?
Positions a, c and e.
At which positions in BA Figure 2 is induced EMF magnitude maximum?
Positions b and d.
How many complete alternating EMF cycles does one simple loop produce per revolution in BA's example?
One cycle.
Why does the induced EMF reverse polarity every half revolution?
The direction of conductor movement relative to the field reverses.
Which physical law governs the opposing direction of induced effects?
Lenz's Law.
Which hand rule determines generator induced-current direction?
Fleming's right-hand rule.
What does the thumb represent in Fleming's generator right-hand rule?
Conductor movement.
What does the first finger represent in Fleming's generator right-hand rule?
Magnetic field direction from north to south.
What does the second finger represent in Fleming's generator right-hand rule?
Conventional current direction.
What memory aid describes Fleming's right-hand rule?
Thumb = Motion; First finger = Field; seCond finger = Current.
Which direction of current is used for Fleming's right-hand rule?
Conventional current.
What happens to induced current direction if conductor movement reverses while the field remains unchanged?
It reverses.
What happens to induced current direction if field polarity reverses while conductor movement remains unchanged?
It reverses.
What happens to induced current direction if both movement and field direction reverse?
It remains the same.
Why do the two active sides of a rotating generator loop contribute to the total EMF?
Their induced EMFs act together through the connected loop.
What is a commutator?
A rotating segmented switching arrangement that reverses circuit connections at the appropriate points.
What is the simplest DC generator commutator made from?
Two insulated semicircular copper segments.
Where are the ends of a simple generator loop connected?
To opposite commutator segments.
Where are the stationary brushes positioned?
Against the rotating commutator.
What happens to the brush connections after each half-turn?
They transfer to the opposite commutator segments.
Why is the commutator connection reversed every half revolution?
To maintain one direction of current in the external load.
When should the commutator reverse the connections in the ideal single-loop example?
When the induced loop EMF passes through zero.
What is meant by unidirectional current?
Current that flows in only one direction.
Is the output of a simple single-loop DC generator perfectly steady?
No; it is pulsating DC.
How often does the simple single-loop DC generator's output fall to zero per revolution?
Twice.
What is generator output ripple?
Variation in the magnitude of the rectified output voltage or current.
How can ripple in a DC generator be reduced?
Use more suitably spaced armature loops and commutator segments.
What happens to generator output smoothness when more armature loops are added?
It improves.
What happens to commutator segment count when additional loops are incorporated?
More segments are required.
What does BA Figure 6 illustrate?
Additional generator loops and commutator segments for smoother DC output.
What does BA Figure 7 illustrate?
Reduced ripple from multiple generating loops.
What happens as one loop's induced EMF decreases in BA's multi-loop example?
Another loop's EMF increases.
How are loop voltages combined in the BA Figure 8 example?
Loops in each branch are connected in series.
How are the two loop groups between the brushes connected in BA Figure 8?
In parallel.
What happens to output ripple when the number of armature loops becomes large?
It becomes much smaller.
What two main structural parts form a DC machine?
The stator and rotor.
What is the stator?
The stationary part of an electrical machine.
What is the rotor?
The rotating part of an electrical machine.
What is the armature in the conventional BA DC generator?
The rotating winding and core assembly.
What are the principal constructional components of a DC generator?
Yoke, pole pieces, field windings, armature core, armature windings, commutator and brushes.
What is the yoke?
The outer frame of a DC machine.
What materials are used for the yoke?
Cast iron or steel.
What are the two main functions of the yoke?
Mechanical support and provision of a magnetic flux path.
Why must the yoke have suitable magnetic permeability?
It forms part of the magnetic circuit.
What are the poles of a DC machine?
Magnetic structures carrying the field windings.
How are poles attached to the yoke in BA's construction?
By bolts or welding.
What are pole shoes?
Extended pole faces attached to the poles.
What are the two main functions of pole shoes?
Support field coils and distribute magnetic flux across the air gap.
Why are pole shoes shaped to spread magnetic flux?
To produce a more uniform working magnetic field.
What is the field winding?
A winding that produces the main magnetic field when energised.
What material is generally used for field windings?
Copper.
How are field coils arranged around the poles?
To produce alternating north and south magnetic poles.
What is the armature core?
The laminated rotating magnetic core containing the armature winding.
What shape does BA describe for the armature core?
A cylinder with longitudinal slots.
What is the purpose of slots in an armature core?
To carry and support the armature windings.
What material is used for armature-core laminations?
Thin steel discs.
Why is the armature core laminated?
To reduce eddy currents and associated heating.
What cooling feature may be incorporated in an armature core?
Air ducts for axial cooling airflow.
How is the armature core attached to the shaft?
It is keyed to the shaft.
What is the armature winding made from?
Insulated copper conductors arranged in coils.
Where are armature windings installed?
In the armature slots.
Why must armature conductors be insulated from the core?
To prevent unwanted electrical contact and short circuits.
What two principal armature winding methods does BA describe?
Lap winding and wave winding.
What is the function of the commutator in a DC generator?
To collect internally generated current and provide unidirectional external output.
What is the function of the commutator in a DC motor?
To supply armature current with the reversals needed for continuous rotation.
What material forms the commutator segments?
Copper.
What electrically separates adjacent commutator segments?
Mica insulation.
What is the function of carbon brushes?
To transfer electrical current between stationary and rotating components.
What are generator brushes commonly made from?
Carbon or graphite.
What does BA Figure 9 illustrate?
A four-pole DC machine showing its principal components.
What does BA Figure 10 illustrate?
A segmented DC commutator.
Why are eddy currents undesirable in rotating magnetic cores?
They create wasteful heating and reduce efficiency.
What produces eddy currents inside a generator core?
Changing magnetic flux inducing circulating currents in the conducting core.
Why can a solid metal core support large eddy currents?
It provides large low-resistance paths for circulating currents.