Generating with a rotating coil

Definition

Alternator

An alternator is a generator in which a coil rotates in a magnetic field and connects to the external circuit through slip rings, so it produces an alternating potential difference.

Definition

Dynamo

A dynamo is a generator in which a coil rotates in a magnetic field and connects to the external circuit through a split-ring commutator, so it produces a direct potential difference.

A generator turns a coil of wire inside a magnetic field so that the field through the coil changes continuously and a potential difference is induced without pause.
As the coil rotates, the two long sides of the coil sweep through the field lines, and the rate at which they cut the field lines varies smoothly through each turn.
When the plane of the coil lies parallel to the field lines, the sides are moving straight across the field lines, so they cut them fastest and the induced potential difference is at its peak.
When the plane of the coil lies at right angles to the field lines, the sides are moving momentarily along the field lines, so they cut none of them and the induced potential difference is zero.
The coil is driven by an external source of energy, such as a steam turbine in a power station, a bicycle wheel or a hand crank, and energy is transferred from the kinetic store of the rotating coil to the electrical circuit.
Both types of generator can be built the other way round, with the magnet rotating inside a fixed coil, because only the relative movement matters.

The alternator and slip rings

Definition

Alternating current

Alternating current is a current that repeatedly reverses its direction of flow.

An alternator connects each end of the rotating coil to its own slip ring, and each slip ring is pressed against a fixed carbon brush.
Each end of the coil therefore stays connected to the same brush and the same side of the external circuit for the whole rotation.
Every half turn the two sides of the coil swap places in the field, so the induced potential difference reverses direction, and the external circuit sees that reversal.
The output over one rotation rises from zero to a positive peak, falls back through zero, reaches an equal negative peak and returns to zero, tracing a smooth curve.
One complete rotation of the coil gives one complete cycle of the output, so the frequency of the output equals the number of rotations each second.
The alternators in UK power stations turn at a rate that gives mains electricity a frequency of $Hz50\ \text{Hz}$ , so the output completes $50$ cycles every second.

The dynamo and the commutator

Definition

Direct current

Direct current is a current that flows in one direction only.

A dynamo connects the coil to a split-ring commutator, which is a single ring cut into two halves that each press against one of the fixed brushes.
At the moment when the induced potential difference in the coil would reverse, the two halves of the commutator swap brushes.
The connections to the external circuit reverse at the same instant as the potential difference in the coil reverses, so the two reversals cancel and the output keeps one direction.
The output over one rotation is two positive humps, each rising from zero to a peak and falling back to zero, because the negative half of each cycle has been flipped over.
The output is direct because it never crosses the zero line, even though its size changes constantly, so it is described as a pulsing direct potential difference.
The commutator is the only difference in principle between the two machines, so an alternator becomes a dynamo when its slip rings are replaced by a split ring.

Changing the output of a generator

Turning the coil faster raises the peak potential difference, because the coil sides cut the field lines more quickly.
Turning the coil faster also raises the frequency, because each rotation takes less time, so the peaks on a trace move closer together.
Adding more turns to the coil, using stronger magnets or using a coil of larger area raises the peak potential difference while leaving the frequency unchanged.
Placing the coil on a soft iron armature concentrates the field lines through the coil and raises the peak value further.
Drawing more current from a generator makes it harder to turn, because the induced current in the coil produces a field that opposes the rotation.

Practical

Investigating a model generator

Aim: to compare the output of an alternator and a dynamo and to find how the output depends on the rate of rotation.
Apparatus: demountable model generator fitted with interchangeable slip rings and a split-ring commutator, strong horseshoe magnet or a pair of magnadur magnets on a yoke, coil wound on a soft iron armature, hand crank or a small motor with a variable supply to drive it, oscilloscope or data logger with a voltage sensor, low-voltage lamp, tachometer or a stroboscope, and connecting leads.
Variables: the rate of rotation of the coil is the independent variable, the peak induced potential difference and the frequency of the output are the dependent variables, and the coil, magnets, armature and circuit are controlled.
Method, setting up:
- Mount the coil on the armature between the poles of the magnet so that it turns freely without touching them.
- Fit the slip rings first, check that both brushes press lightly on their rings, and connect the brushes to the oscilloscope.
- Set the oscilloscope timebase so that two or three complete cycles fill the screen, and note the volts per division and the time per division.
Method, comparing the two outputs:
- Turn the crank at a steady rate and sketch the trace, noting whether it crosses the zero line.
- Stop, replace the slip rings with the split-ring commutator without disturbing the coil or magnets, and repeat at the same rate of rotation.
- Sketch the second trace alongside the first and mark the zero line on both so the two can be compared directly.
Method, changing the rate of rotation:
- Refit the slip rings and drive the coil with the small motor so the rate of rotation can be set repeatably.
- Measure the rate of rotation with the tachometer, then read the peak height and the period of one cycle from the trace.
- Repeat for at least five rates of rotation, taking three readings at each rate and calculating means.
- Connect the lamp in place of the oscilloscope and observe that it lights more brightly as the rate of rotation increases.
Measurements and processing: read the peak potential difference as the greatest height of the trace above the zero line, read the period $T$ as the time for one complete cycle, calculate the frequency from $f=1Tf=\dfrac{1}{T}$ , and plot peak potential difference against rate of rotation.
Expected pattern: the slip rings give a trace that alternates about zero, the split ring gives humps that stay on one side of zero, and the peak potential difference and the frequency both rise as the rate of rotation rises.
Sources of uncertainty: hand cranking is uneven, the brushes may bounce or arc on the rings, the coil may not sit centrally between the poles, and reading a peak from a moving trace is imprecise.
Improvements: drive the coil with a motor rather than by hand, use a data logger to capture and average the peak values, and clean the brushes and rings so contact stays steady.
Safety: keep fingers and loose clothing away from the rotating coil and the drive belt, do not exceed the rated supply of the driving motor, and clamp the generator to the bench.

Reading a generator output trace

Identify the machine first: a trace that crosses the zero line and goes negative is from an alternator, and a trace of humps all on one side of zero is from a dynamo.
Read the peak potential difference by counting the vertical divisions from the zero line to the top of the trace and multiplying by the volts per division.
Read the period by counting the horizontal divisions for one complete cycle and multiplying by the time per division.
Convert the period into the rate of rotation using $f=1Tf=\dfrac{1}{T}$ , remembering that an alternator completes one cycle per rotation.
A dynamo trace shows two humps per rotation, so its period must be measured over one complete pair of humps rather than one hump.

Example

Reading an alternator trace

An oscilloscope trace from an alternator shows one complete cycle across four horizontal divisions, with the timebase set to $ms5\ \text{ms}$ per division.
The trace reaches three vertical divisions above the zero line, with the sensitivity set to $V4\ \text{V}$ per division.
The period is $sT=4\times5\ \text{ms}=20\ \text{ms}=0.020\ \text{s}$ .
The frequency is $Hzf=\dfrac{1}{T}=\dfrac{1}{0.020}=50\ \text{Hz}$ , so the coil completes $50$ rotations each second.
The peak potential difference is $V3\times4=12\ \text{V}$ .
Turning the coil twice as fast would give a period of $ms10\ \text{ms}$ , a frequency of $Hz100\ \text{Hz}$ and a peak of about $V24\ \text{V}$ .

Exam technique

Describing a generator

Name the connection first, slip rings for an alternator and a split-ring commutator for a dynamo, because that is the mark that separates the two machines.
Explain the output by saying that the coil sides cut field lines fastest when the coil is parallel to the field and cut none when it is at right angles to it.
When sketching an output, draw and label the zero line, then show two full cycles so the shape is unambiguous.
For a faster rotation, state both effects, a taller peak and a shorter period, since one alone is usually only half the marks.
Describe a dynamo output as direct but changing in size, rather than as steady.

Common Mistake

Do not describe a dynamo output as a smooth steady potential difference, because its size still rises and falls twice each rotation.
Do not swap the two connections, since slip rings keep the output alternating and a split ring makes it direct.
Do not say the induced potential difference is greatest when the coil is at right angles to the field lines, because that is where it is zero.
Do not confuse a generator with a motor, since a generator is turned to produce a potential difference while a motor is supplied with a current to produce rotation.
Do not claim that turning the coil faster changes only the frequency, because the peak potential difference rises as well.

Self review

State the part that makes an alternator produce an alternating output and the part that makes a dynamo produce a direct output.
Explain why the induced potential difference falls to zero twice during each rotation of the coil.
Sketch and label the output of an alternator and the output of a dynamo over two full rotations.
Calculate the frequency of an alternator whose output has a period of $ms25\ \text{ms}$ .
Describe two changes that raise the peak output of a generator without changing its frequency.

Generating with a rotating coil

Definition

Alternator

An alternator is a generator in which a coil rotates in a magnetic field and connects to the external circuit through slip rings, so it produces an alternating potential difference.

Definition

Dynamo

A dynamo is a generator in which a coil rotates in a magnetic field and connects to the external circuit through a split-ring commutator, so it produces a direct potential difference.

A generator turns a coil of wire inside a magnetic field so that the field through the coil changes continuously and a potential difference is induced without pause.
As the coil rotates, the two long sides of the coil sweep through the field lines, and the rate at which they cut the field lines varies smoothly through each turn.
When the plane of the coil lies parallel to the field lines, the sides are moving straight across the field lines, so they cut them fastest and the induced potential difference is at its peak.
When the plane of the coil lies at right angles to the field lines, the sides are moving momentarily along the field lines, so they cut none of them and the induced potential difference is zero.
The coil is driven by an external source of energy, such as a steam turbine in a power station, a bicycle wheel or a hand crank, and energy is transferred from the kinetic store of the rotating coil to the electrical circuit.
Both types of generator can be built the other way round, with the magnet rotating inside a fixed coil, because only the relative movement matters.

The alternator and slip rings

Definition

Alternating current

Alternating current is a current that repeatedly reverses its direction of flow.

An alternator connects each end of the rotating coil to its own slip ring, and each slip ring is pressed against a fixed carbon brush.
Each end of the coil therefore stays connected to the same brush and the same side of the external circuit for the whole rotation.
Every half turn the two sides of the coil swap places in the field, so the induced potential difference reverses direction, and the external circuit sees that reversal.
The output over one rotation rises from zero to a positive peak, falls back through zero, reaches an equal negative peak and returns to zero, tracing a smooth curve.
One complete rotation of the coil gives one complete cycle of the output, so the frequency of the output equals the number of rotations each second.
The alternators in UK power stations turn at a rate that gives mains electricity a frequency of $Hz50\ \text{Hz}$ , so the output completes $50$ cycles every second.

The dynamo and the commutator

Definition

Direct current

Direct current is a current that flows in one direction only.

A dynamo connects the coil to a split-ring commutator, which is a single ring cut into two halves that each press against one of the fixed brushes.
At the moment when the induced potential difference in the coil would reverse, the two halves of the commutator swap brushes.
The connections to the external circuit reverse at the same instant as the potential difference in the coil reverses, so the two reversals cancel and the output keeps one direction.
The output over one rotation is two positive humps, each rising from zero to a peak and falling back to zero, because the negative half of each cycle has been flipped over.
The output is direct because it never crosses the zero line, even though its size changes constantly, so it is described as a pulsing direct potential difference.
The commutator is the only difference in principle between the two machines, so an alternator becomes a dynamo when its slip rings are replaced by a split ring.

Changing the output of a generator

Turning the coil faster raises the peak potential difference, because the coil sides cut the field lines more quickly.
Turning the coil faster also raises the frequency, because each rotation takes less time, so the peaks on a trace move closer together.
Adding more turns to the coil, using stronger magnets or using a coil of larger area raises the peak potential difference while leaving the frequency unchanged.
Placing the coil on a soft iron armature concentrates the field lines through the coil and raises the peak value further.
Drawing more current from a generator makes it harder to turn, because the induced current in the coil produces a field that opposes the rotation.

Practical

Investigating a model generator

Aim: to compare the output of an alternator and a dynamo and to find how the output depends on the rate of rotation.
Apparatus: demountable model generator fitted with interchangeable slip rings and a split-ring commutator, strong horseshoe magnet or a pair of magnadur magnets on a yoke, coil wound on a soft iron armature, hand crank or a small motor with a variable supply to drive it, oscilloscope or data logger with a voltage sensor, low-voltage lamp, tachometer or a stroboscope, and connecting leads.
Variables: the rate of rotation of the coil is the independent variable, the peak induced potential difference and the frequency of the output are the dependent variables, and the coil, magnets, armature and circuit are controlled.
Method, setting up:
- Mount the coil on the armature between the poles of the magnet so that it turns freely without touching them.
- Fit the slip rings first, check that both brushes press lightly on their rings, and connect the brushes to the oscilloscope.
- Set the oscilloscope timebase so that two or three complete cycles fill the screen, and note the volts per division and the time per division.
Method, comparing the two outputs:
- Turn the crank at a steady rate and sketch the trace, noting whether it crosses the zero line.
- Stop, replace the slip rings with the split-ring commutator without disturbing the coil or magnets, and repeat at the same rate of rotation.
- Sketch the second trace alongside the first and mark the zero line on both so the two can be compared directly.
Method, changing the rate of rotation:
- Refit the slip rings and drive the coil with the small motor so the rate of rotation can be set repeatably.
- Measure the rate of rotation with the tachometer, then read the peak height and the period of one cycle from the trace.
- Repeat for at least five rates of rotation, taking three readings at each rate and calculating means.
- Connect the lamp in place of the oscilloscope and observe that it lights more brightly as the rate of rotation increases.
Measurements and processing: read the peak potential difference as the greatest height of the trace above the zero line, read the period $T$ as the time for one complete cycle, calculate the frequency from $f=1Tf=\dfrac{1}{T}$ , and plot peak potential difference against rate of rotation.
Expected pattern: the slip rings give a trace that alternates about zero, the split ring gives humps that stay on one side of zero, and the peak potential difference and the frequency both rise as the rate of rotation rises.
Sources of uncertainty: hand cranking is uneven, the brushes may bounce or arc on the rings, the coil may not sit centrally between the poles, and reading a peak from a moving trace is imprecise.
Improvements: drive the coil with a motor rather than by hand, use a data logger to capture and average the peak values, and clean the brushes and rings so contact stays steady.
Safety: keep fingers and loose clothing away from the rotating coil and the drive belt, do not exceed the rated supply of the driving motor, and clamp the generator to the bench.

Reading a generator output trace

Identify the machine first: a trace that crosses the zero line and goes negative is from an alternator, and a trace of humps all on one side of zero is from a dynamo.
Read the peak potential difference by counting the vertical divisions from the zero line to the top of the trace and multiplying by the volts per division.
Read the period by counting the horizontal divisions for one complete cycle and multiplying by the time per division.
Convert the period into the rate of rotation using $f=1Tf=\dfrac{1}{T}$ , remembering that an alternator completes one cycle per rotation.
A dynamo trace shows two humps per rotation, so its period must be measured over one complete pair of humps rather than one hump.

Example

Reading an alternator trace

An oscilloscope trace from an alternator shows one complete cycle across four horizontal divisions, with the timebase set to $ms5\ \text{ms}$ per division.
The trace reaches three vertical divisions above the zero line, with the sensitivity set to $V4\ \text{V}$ per division.
The period is $sT=4\times5\ \text{ms}=20\ \text{ms}=0.020\ \text{s}$ .
The frequency is $Hzf=\dfrac{1}{T}=\dfrac{1}{0.020}=50\ \text{Hz}$ , so the coil completes $50$ rotations each second.
The peak potential difference is $V3\times4=12\ \text{V}$ .
Turning the coil twice as fast would give a period of $ms10\ \text{ms}$ , a frequency of $Hz100\ \text{Hz}$ and a peak of about $V24\ \text{V}$ .

Exam technique

Describing a generator

Name the connection first, slip rings for an alternator and a split-ring commutator for a dynamo, because that is the mark that separates the two machines.
Explain the output by saying that the coil sides cut field lines fastest when the coil is parallel to the field and cut none when it is at right angles to it.
When sketching an output, draw and label the zero line, then show two full cycles so the shape is unambiguous.
For a faster rotation, state both effects, a taller peak and a shorter period, since one alone is usually only half the marks.
Describe a dynamo output as direct but changing in size, rather than as steady.

Common Mistake

Do not describe a dynamo output as a smooth steady potential difference, because its size still rises and falls twice each rotation.
Do not swap the two connections, since slip rings keep the output alternating and a split ring makes it direct.
Do not say the induced potential difference is greatest when the coil is at right angles to the field lines, because that is where it is zero.
Do not confuse a generator with a motor, since a generator is turned to produce a potential difference while a motor is supplied with a current to produce rotation.
Do not claim that turning the coil faster changes only the frequency, because the peak potential difference rises as well.

Self review

State the part that makes an alternator produce an alternating output and the part that makes a dynamo produce a direct output.
Explain why the induced potential difference falls to zero twice during each rotation of the coil.
Sketch and label the output of an alternator and the output of a dynamo over two full rotations.
Calculate the frequency of an alternator whose output has a period of $ms25\ \text{ms}$ .
Describe two changes that raise the peak output of a generator without changing its frequency.

13.1.3 Alternators and dynamos

Generating with a rotating coil

The alternator and slip rings

The dynamo and the commutator

Changing the output of a generator

Investigating a model generator

Reading a generator output trace

Reading an alternator trace

Describing a generator

Review 13.1.3 Alternators and dynamos by teaching Genie

13.1.3 Alternators and dynamos

Generating with a rotating coil

The alternator and slip rings

The dynamo and the commutator

Changing the output of a generator

Investigating a model generator

Reading a generator output trace

Reading an alternator trace

Describing a generator

Review 13.1.3 Alternators and dynamos by teaching Genie

13.1.3 Alternators and dynamos

Generating with a rotating coil

The alternator and slip rings

The dynamo and the commutator

Changing the output of a generator

Investigating a model generator

Reading a generator output trace

Reading an alternator trace

Describing a generator

Review 13.1.3 Alternators and dynamos by teaching Genie

13.1.3 Alternators and dynamos Revision Guide

13.1.3 Alternators and dynamos

Generating with a rotating coil

The alternator and slip rings

The dynamo and the commutator

Changing the output of a generator

Investigating a model generator

Reading a generator output trace

Reading an alternator trace

Describing a generator

Review 13.1.3 Alternators and dynamos by teaching Genie

13.1.3 Alternators and dynamos Revision Guide