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 10^{11} 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 CO_{2} into the atmosphere*. What could be the annual saving in CO_{2} 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]
