Savings in electric motors

Bulletin reader Matt contacted me after seeing a presentation on replacement electric motors. The sales person claimed they could save 50% energy on a like-for-like replacement of an IE3 motor. Matt quite correctly challenged them to explain how they could save 50% on a motor that’s 90%+ efficient, and of course they could not give a scientific answer.

They might have meant that their motor technology halved the losses in the motor, taking it from say 90% to 95% efficient. But that would result in about 5.3% saving, not 50%. The only way to reduce the energy consumption of a motor by an order of magnitude is to reduce how much work it does. Actually that is entirely possible; it’s what variable-speed control does. On centrifugal fans, for example, a 20% speed reduction almost halves the mechanical power absorbed by the fan and that, thanks to the principle of an energy balance, translates into a corresponding reduction of the power delivered by its motor and hence the power that the motor draws from the mains. So maybe the vendor was not talking about a like-for-like replacement but the replacement of fixed-speed with variable-speed motors. In which case speed-control on the existing motors could be considered as an option.

Anyway, speed control would be one way that the vendor might have achieved their 50% savings. But that would be savings on motor power alone, while their web site trumpets 40-60% savings ‘overall’ on heating, ventilation and air conditioning systems. Also not completely impossible, but (again invoking the principle of an energy balance) it implies that the building’s demand for heating or cooling was reduced by that much. However, short of using the fan-speed control aggressively to cut back the ventilation rate drastically, it is hard to see how that could be achieved.