Operation of Simple DC and AC Generators

Last Update: 18 January 2026

This is part of the HSC Physics course under the topic Applications of the Motor Effect.

HSC Physics Syllabus

analyse the operation of simple DC and AC generators and AC induction motors (ACSPH110)

Operation of Simple DC and AC Generators (Video)

This video analyses the operation of simple DC and AC generators and AC induction motors.

What are Generators?

A generator is a device which converts mechanical (namely kinetic) energy into electrical energy.

Simple AC and DC generators have similar components and set-up as simple DC motor.

The key difference is that in a DC motor, electrical energy is utilised (input) to rotate the armature, transforming into mechanical energy (output), whereas in a generator, the armature is manually rotated and the output is electrical energy.

As the armature is manually rotated in an external magnetic field, Faraday's law of induction applies - an emf is induced due to changes in magnetic flux. This is because the angle between the area of the armature and magnetic field is changing throughout rotation.

Since the armature is a closed electrical conductor, this emf will create a current, whose direction is determined by Lenz's law. Every 180° turn, the current direction will reverse. As such, and AC current flows in the armature for both AC and DC generators.

Generator Types: AC and DC

Despite similar set-up, the final current output is of course different between AC and DC generators. In an AC generator, the current is already alternating periodically, and so there is no need to reverse the current direction. This is why two slip rings are used to preserve the alternating nature of the induced current. Slip rings remain in contact with the brushes throughout the entire 360° rotation.

Conversely, in a DC generator, the output needs to be modified to become unidirectional i.e. direct current (DC). As such, a pair of split rings is used to reverse the current direction every 180°. Every half a revolution, each half of the split ring switches contact with the brush which acts as the stationary connection point to the external circuit. This 'switching' mechanic allows the alternating current to remain unidirectional when it flows into the external circuit.

Commutator Comparison - AC vs DC

Speed:

Slip Rings (AC)

                How it works: Two separate rings.
                
                The Blue Wire is always connected to the top ring.
                The Red Wire is always connected to the bottom ring.
                
                Result: As the coil flips, the current direction in the output wires flips too.

Split Ring Commutator (DC)

                How it works: One ring split in half.
                
                The brushes are stationary. As the coil rotates, the Blue and Red wires physically swap which brush they touch every half-turn.
                
                Result: This swap reverses the connections at the exact moment the current would reverse, keeping the output flowing in one direction.

Counter Torque in Generators

Furthermore, by Lenz's law, the direction of current is induced in such a way that the resultant magnetic field (of induced current) counteracts the change in magnetic field that first induced the emf. This leads to a counter torque against the armature rotation or 'resistance' that is felt when the armature is turned. This so-called resistance is present only when there is induced current. In the presence of induced emf but not induced current (e.g. in an incomplete circuit), the armature can be freely rotated without counter torque.

EMF Graphs for AC and DC Generators

Emf versus time graphs effectively illustrate the natural of the output current in AC and DC generators. The polarity of the emf (positive of negative) corresponds to the direction of current entails positive and negative values which indicate direction of emf or induced current.

DC generators produce unidirectional current so their emf are conventionally displayed as positive.

AC generators produce bidirectional current. The direction alternates twice every revolution.

Note that the DC generator emf vs time graph is the absolute value of the AC generator graph. This makes sense as the sole function of split ring commutators is to reverse the current direction (i.e. reflecting every negative portion about the x-axis).

Increasing the rotational velocity of the armature has two effects on emf and current:

Increases frequency of rotation - the period of one revolution becomes shorter
Amplitude of emf becomes greater. Increased rotation increases the rate of change in flux, thus increasing the magnitude of induced emf.

AC vs DC Generators Comparison Table

	AC Generator	DC Generator
Similarity	Both devices use Faraday’s law of electromagnetic induction. As a result, the rate of electrical energy production depends on rate of change in magnetic flux experienced by the coil i.e. rotational speed of coil/armature Fundamental components are the same e.g. external magnetic field, coil and external mechanical energy
Difference	Bidirectional Uses slip-ring commutator Can be used for transformers	Unidirectional Uses split-ring commutator Brushes are more important as they reverse current direction periodically (however, this means DC generators require more frequent maintenance
Applications	Common in household appliances and devices e.g. vacuum cleaner, toaster, transformers for laptops	Common for larger devices that require more efficient energy supply Charging batteries Electroplating and refining of metals e.g. coating coins with copper

Test Your Understanding

Practice Question 1

Explain the operation of AC generators by including important components and fundamental physics principles. (3 marks)

Practice Question 2

Generators, when not connected to an electric device, are easy to turn. However, when they are connected, the armature requires much more effort to turn.

Explain why this is the case. (2 marks)

Previous section: Back EMF in a Simple DC Motor

Next section: AC Induction Motor