Episode 415: Electric motors

The common electromagnetic machines are motors, generators and transformers. All three are described in this episode but you will need to check your specification to find out which you need to cover and in what detail.

Summary

  • Demonstration or student experiment: A model motor (20 minutes)
  • Discussion: Motor torque (30 minutes)
  • Discussion: Back EMF and power (10 minutes)
  • Demonstrations: The practical importance of motors (20 minutes)
  • Student questions: DC motor (20 minutes)

Demonstration or student experiment: A model motor
Although motors could be discussed in episode 412, there is much sense in waiting until after electromagnetic induction has been covered so that a full account of back emf can be included. Again your students are likely to have met motors at pre-16 level. Working with a simple motor will cover the essential physics involved.

Episode 415-1: A simple electric motor (Word, 91 KB)

Westminster motor kit

Discussion: Motor torque
From this practical work (and previous knowledge), it should be clear to your students that a simple motor is a (rectangular) coil of wire that rotates in a magnetic field when a current is passed through the coil.

The diagram shows a section through a coil that is pivoted at ´ so that it can turn about a horizontal axis. The coil has sides of length L and a width w, so that its area is A. There are N turns of wire in the coil carrying a current I. B is the flux density between the magnetic poles.

The force F on each side of the coil is F = NBIL

Geometry for a motor coil

The direction of the forces is found by Fleming's left hand rule and the two forces together produce a couple.

The torque produced = 2 ´ (F ´ w/2) = NBILw = NBIA

This picture is only valid if B is uniform and B and I are perpendicular. The design of commercial motors tries to make this true for a significant part of the rotation by including a lot of shaped soft iron, both in the armature and in the pole pieces. At the same time this increases the value of B.

The current has to be reversed each time the coil is perpendicular to the field so that the forces reverse and the circular motion is maintained. A commutator and brushes are used for this.

Discussion: Back emf and power
Once the motor starts to turn, the movement of the coil through the magnetic field leads to an induced emf E in the coil. This emf will generate a current that opposes the motor current. (Why? - either Lenz's law or conservation of energy).

Now if the supply voltage is V and the armature has a resistance R, we have:

V = E + IR

VI = EI + I2R

or electrical power supplied = mechanical power output + rate of heating of armature

Demonstrations: The practical importance of motors
These two experiments will show some of the ideas discussed.

Episode 415-2: Torque from a motor (Word, 27 KB)

Episode 415-3: Using an electric drill (Word, 28 KB)

Student questions: DC motor
These questions emphasize the design features of a dc motor.

Episode 415-4: Thinking about the design of a simple DC motor (Word, 43 KB)

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Episode 415: Electric motors (Word, 179 KB)