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Efficient motors Energy efficient motors
Types of motor
You will be familiar with the model motor on page 10. A simple d.c. motor works on the same principals except that it has more coils. This makes it more efficient.

However, it is still very wasteful because of:

Also, if the motor gets stuck (possibly through trying to lift too big a load), the rotor coils can overheat and melt.

Therefore, most industrial and heavy home appliances use an induction motor (below). They are used in washing machines, fridges, central heating pumps etc.

Induction motor Model motor
The table compares the parts of a simple d.c. motor with those of an induction motor. Roll over the component names to see where they are on the motors.
Component d.c. motor Induction motor
Rotor rotating coil of copper wire aluminium or copper bars in an iron core; copper bars are more efficient
Stator Permanent magnets coils of copper wire provide magnetic field
Connection Commutator and brushes carry the d.c. current to the rotor Terminal box connects the stator coils to the 3-phase a.c. supply
Cooling Fan to cool the rotor

Cutaway graphic of motor
Cutaway graphic of induction motor
Picture 3.8 A simple d.c. electric motor.
Picture 3.9 An induction motor.
Energy efficient induction motors
Induction motors can be made more energy efficient by:
  • using copper windings in the stator coils
  • using copper bars in the rotor
  • machining all moving parts to a high precision
  • using a special, high quality steel for the rotor and stator
  • keeping the rotor and stator as close as possible without touching - again through precision manufacture.

Graph of efficiency
Picture 3.10 Efficiency of motors with load.
Energy efficiency and load
Even the most efficient motors are going to lose energy in some places. Picture 3.10 shows how the efficiency varies with the load. This is measured by getting the motor to do work. This could be lifting up a weight or getting it to pull against a braking system.

The total efficiency of a motor is zero when there is no load (it is doing no useful work). As the load increases, so the efficiency increases because it starts to do some work. The efficiency then levels off. At this point, any extra load causes the motor to draw a bigger current. This increases both the useful work and the joule heating in the motor. They increase in proportion to each other, keeping the efficiency almost constant.

Where losses come from
A motor loses energy in a number of ways.
  • Friction – of moving parts
  • Core (iron) loss – due to changing magnetic fields in the rotor and stator cores
  • Stator loss – joule heating (I2R) when current flows through stator coils
  • Rotor loss – joule heating (I2R) when current flows through rotor.

These vary differently with the load (see picture 3.11). The friction and the core loss stay constant - they depend on the speed of the motor rather than the current being drawn. The joule heating losses increase with the load. This is because the motor has to draw a bigger current to do more work. And this bigger current causes more heating in the coils. However, although the losses are increasing with load, so is the useful work being done. So the efficiency of the motor is pretty constant after about 25% maximum load (see picture 3.10).

Graph of efficiency
Picture 3.11 Losses in different parts of a motor.
Question 10
This question relates to the graph shown in picture 3.11. Match the losses to the descriptions. Check all the appropriate boxes.
Friction Core
These losses increase as load increases
Zero when there is no load
Related to resistance to flow of current
Magnetic losses
Less than 2% loss at full load
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