The facts behind saving energy with variable speed drives

When considering the energy saving benefits of using variable speed drives (VSDs) in motors applications, we quickly become involved in speed reduction percentages and payback periods. All relevant, but sometimes hiding the true costs of running electric motors in commercial and industrial applications.


Motors are good at moving fluids (mainly air and water) around, and under EU directives, motor efficiencies are continuing continue to improve. However, they need to as there are not many alternatives to using an electric motor. So, what are the numbers?


A 37 kW motor with a 94% efficiency consumes ~39 kW of energy. Assuming it runs 8 hrs/day, 5 days/week, 50 weeks a year. It has an annual running time of 2000 hours. Assuming a cost of £0.1 per kw, the annual running cost is:

– Annual savings are 39kW x 2000hrs x £0.1 = £7800 per year (or £156,000 over its operating life)


How does a small speed reduction cut energy consumption by nearly 50%?

A centrifugal pump or fan running at half speed consumes only one-eighth of the energy compared to one running at full speed. This is because the torque needed to run a pump or fan is the square of the volume. For instance, reducing the pump speed to 80% only requires 64% of the torque (0.8×0.8). Furthermore, to produce 64% of the torque only requires 51% of the power (0.64×0.8), as the power requirement is reduced in the same way.



By using a VSD to control flow rather than throttling/dampers, the above example gives a saving of about £3,900 per year (or £78,000 over its operating life). The explanation for this lies in the pressure difference across the impeller. When less pressure is produced, less acceleration of air or fluid across the impeller is required. It is the simultaneous reduction of acceleration and pressure that multiplies the savings.


Even further improved energy efficiency is offered through a technique called motor flux optimisation. This is particularly beneficial in pump and fan applications and under light loading conditions. The overall efficiency of an ABB low voltage AC drive and its motor is typically 1% to 10% higher than conventional AC drives. At 25% load, efficiency is up to 10% better.


High efficiency
Comparing energy consumption of various mechanical and electrical airflow control methods of a typical centrifugal fan reveals that: At 80% airflow, the energy consumption is: 97% of maximum with damper control; and 76% of maximum using guide vanes; but only 51% using ABB low voltage AC drives. The difference between these figures represents wasted energy – waste that any engineer or financial manager will find impossible to justify.



See the range of ABB energy saving drives at discounted prices here


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