Copper and electricity. Using electric motors.

page 9

Using electric motors

	Making more of motors

Picture 3.1 Systems driven by electric motors consume half of all the electricity generated in the UK.

Electric motors are vital in industry. They account for two-thirds of all the electricity used in industry and half of the total electricity used in the UK. In UK industry:

over 10 million electric motors are at work
3000 new motors are sold each day
industrial motors have an average life expectancy of 13 years.
A lifetime’s supply of electricity can cost over 200 times the initial capital cost of the motor.

Motors are relatively cheap, but are expensive to run. A modest-sized 11 kW induction motor could cost as little as £ 300 to buy, but its running costs over ten years could be £ 44, 000. Up to 15% of this cost could be due to waste within the motor.

	Making motors more efficient

		Electric motors can be made more efficient in a number of ways. Some of these are subtle improvements to design and to the precision construction of components within the motor. However, using more copper in motors has a big impact. The high conductivity of copper means that components can be made smaller and kept closer together. Induction motors (see page 11) are widely used. Their efficiency can be improved using cast copper rotors – again because currents will flow so easily in copper. Electric motors are typically 85 to 95 % efficient in transferring energy to their load. However, the difference between 85 % and 95 % is enormous. Saving 5 % of the energy supplied would cut running costs for the 11 kW motor by £ 2, 200 over ten years, and reduce CO₂ emissions by about 25 tonnes.

Picture 3.2 In fact, the lifetime cost of running a motor may be up to 100 times the original purchase price, so it's worth making a careful selection.

Making savings

It is often better to buy a more expensive, more efficient motor if it will consume less electricity than a cheaper one. This is particularly true if the motor is used for 24 hours a day. If it has a shorter duty cycle (i.e. it is used for only an hour or so a day), then replacing an existing motor with an energy efficient one may not make such a good saving.

Component	Existing motor	New motor
Component	Existing motor	New motor
Cost	Already paid for	£400
Efficiency	90 %	95 %
Annual electricity cost	£2 800	£2 600

The new motor will pay for itself in 2 years and make savings from then on.

Picture 3.3 See question 8.

Question 8

Picture 3.3 shows how the efficiencies of two motors depend on their load. (100% load means that they are turning the maximum load for which they were designed.)

a) What would the efficiency be at zero load? Explain your answer.

b) At what load are these motors operating most efficiently?

c) Suppose the standard motor cost half the price of the more efficient motor. Use the graph to explain why this doesn't necessarily make it a good choice.

d) In 2002, UK power stations generated about 3.6 x 10¹¹ kWh per year. About 40 % of this was used in standard electric motors (and 10 % in energy efficient ones). Each kWh releases about 0.51 kg of CO₂ into the atmosphere*. What could be the annual saving in CO₂ emissions by changing all motors to energy efficient ones?

Assume the standard motors are 90% efficient and the efficient motors are 95% efficient.

[* This is the actual figure for the UK and takes into account that only 85% of electricity generation uses fossil fuels]