Episode 416: Generators and transformers

In a generator, motion of a conductor in a magnetic field induces an EMF. In a transformer, it is the changing field that induces an EMF in a fixed conductor.

Schematic generator

Summary

  • Discussion: Generators (30 minutes)
  • Demonstrations: A motor in reverse (15 minutes)
  • Student questions: On ac generators (20 minutes)
  • Demonstration: Transformers (20 minutes)
  • Student experiment: Testing the relationships (30 minutes)
  • Student questions: Transformer equations (20 minutes)
  • Further discussion: Practical transformers (10 minutes)

Discussion: Generators
The structure of a simple generator is essentially the same as a motor. The difference is that now mechanical energy is converted into electrical energy. The electrical current to a load is via a commutator for an ac generator or slip rings if ac is required.

Basic ideas can be understood by thinking about a coil rotating in a uniform magnetic field.

Plane of coil at an angle to the magnetic field

Consider a coil of area A with N turns of wire rotating at a constant angular velocity w in a uniform magnetic flux density B. As the coil rotates, it cuts through the lines of flux. Another way to express this is to say that the flux linking the coil is changing.

At what point is the rate of flux-cutting greatest? (When it is horizontal in the diagram above; when it is vertical, the rate of flux cutting is instantaneously zero.)

Rate of flux cutting = induced EMF = BANwcos wt

with a maximum value, Eo = BANw when the coil is parallel to the field.

Demonstrations: A motor in reverse
Show that a motor can operate in reverse, as a generator. One starting point is simply to attach a weight to a small motor and to drop the weight. The motor works in reverse as a generator; the induced EMF can be monitored with a meter.

Further experimental work will reinforce the discussion.

Episode 416-1: The effect of loading a generator (Word, 57 KB)

An alternative approach is to think about a magnet rotating to give changing flux in one or more pairs of coils. A useful demonstration follows but you may decide to show only the first stages of this.

Episode 416-2: Building up an alternator (Word, 304 KB)

Student questions: On ac generators

Episode 416-3: Alternating current generators (Word, 80 KB)

Demonstration: Transformers
Experiments with transformers can be used as a way of investigating and confirming the laws of electromagnetic induction and could be done earlier. This work can also be a means of rounding off the whole of this section of post-16 work.

Transformer construction

The aim is to show that a transformer is an electrical machine that converts one ac voltage into another ac voltage. Working through parts or all of the following presentation will illustrate both the structure and the operation of a transformer.

Episode 416-4: Building up a transformer (Word, 172 KB)

Discussion:Transformer equations
Having defined the quantities involved, you can deduce the transformer equations. Emphasise that the deduction of these equations depends on an assumption of 100% efficiency; in most transformers the efficiencies are so high that the inequalities can be treated as being approximate equalities.

Student experiment: Testing the relationships
The theory presented above can be tested by experiment.

Energy losses become very apparent with the apparatus described in this experiment. It is worth repeating the measurements with 120/240 turn coils (if available) or with a small commercial transformer.

Episode 416-5: Transformers (Word, 31 KB)

Student questions: Transformer equations

Episode 416-6: Transformers (Word, 108 KB)

Further discussion: Practical transformers
Discuss reasons for energy losses in real transformers. These are readily identified as:

  • ohmic heating of the coils
  • eddy current heating of the core
  • hysteresis effects which heat the core
  • magnetic flux escaping

But even with these it is not unusual to find efficiencies of 95% and higher. Large transformers used in power transmission may be as much as 99.5% efficient.

Where electronics are being used, low voltage ac supplies are usually required so step-down transformers will be an essential part of the power supply. The output from a transformer is ac, so there will have to be some form of rectification (with diodes) and smoothing (with capacitors).

Transformer on the grid

A second widespread use is within the 'Grid' that supplies electricity to the consumer. The connection from a power station to the consumer involves a long length of wire and often, high currents. For a given section of the grid, the resistance, R is fixed and the rate of heating generated in the wire will be I2 R; this energy is wasted. To minimise this energy loss, the current should be as small as possible. To deliver a particular power (VI), a smaller current can be achieved by using as high a voltage as possible. The grid is designed so that transformers are used to step up the voltage at the power station before transmission. Step down transformers reduce the voltage in stages to the level required by industrial and domestic consumers.

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Episode 416: Generators and transformers (Word, 753 KB)