Operation of Simple DC and AC Generators

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

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

What are AC and DC Generators?

A generator is a device which converts mechanical (kinetic) energy into electrical energy. Simple AC and DC generators have the exact same anatomy as a DC motor (with the exception of a slip ring or a split ring). The key difference is that in a generator, the armature is manually rotated and the output is electrical energy. Conversely, in a DC motor, the input was electrical energy, and this caused the armature to rotate.

As the armature is manually rotated in an external magnetic field, Faraday's law of induction states that it will experience an induced emf. 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.

However, the final current output between the two generators are different. In an AC generator, the current is already AC and so there is no need to reverse the current direction with a split ring. This is why a slip ring is used to preserve the AC current. Slip rings remain in contact with the brushes throughout the entire 360° rotation.

Conversely, in a DC generator, the output needs to be DC. As such, a split ring is used to reverse the current direction every 180°. This changes the AC current into DC as it flows through the split rings out the coil.

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 all positive. AC generators produce bidirectional current. The direction alternates twice every revolution. Notice that the DC generator graph is the absolute value of the AC generator graph. This makes sense as all we are doing with the split ring is reversing the direction (i.e. reflecting every negative portion about the x-axis).

EMF vs Rotational Speed

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

• 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

 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

Practice Question 1

Explain the function of 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 becomes quite difficult to turn. Explain why this is the case. (2 marks)

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