How the affinity laws deliver energy saving

With the recent increases in energy prices, it is time to look again at cost effective ways to save electricity. The affinity laws point the way, and it is not an exaggeration to say that most businesses identify huge savings in their electricity usage by focusing on electric motor applications.

Mid-sized motors cost thousands, if not tens of thousands of pounds a year to run. Yet for lots of applications savings of 50% are achievable by installing variable speed drives (VSDs) to reduce motor running speeds.

The first step is to consider what motors you have, what they do and how big (power rating) they are. After all, if it cannot be monitored, it cannot be managed. As an example*, a typical mid-sized fan or pump motor rated 37kW and assuming 90% efficiency has an input of 41 kW. Assume it runs for 8 hours a day, five days a week and 48 weeks a year, then it consumes:

8 x 5 x 48 x 41 = 78720 kWh of electricity a year

78720 x 17p = £13382 a year, or about £200,000 over its lifetime

According to the ‘Affinity Laws’, installing a VSD to reduce speed by 25% cuts the electricity consumption by about 50%. This would save over £6,600 every year over the life of this one motor.

So, if these savings are achievable, why is everyone not using variable speed drives? The answer is that for most new motor installations they are. All OEMs and machine builders optimise new machines for energy saving. Yet many organisations see motors as a cost of running their business, not as an opportunity for saving money.

Industrial motor applications

Mid-sized motors account for most of the power consumed by industrial and commercial organisations. It is estimated that motors consume 38% of the energy used in commercial buildings. However, in industry, motor systems consume 70% of total energy use. Overall, electric motors consume over 45% of the world’s electricity.

Many of these motors are larger than necessary for the applications at hand, and are often run at full speed, even when the extra power is not needed. This means that the potential reductions available from energy consumption and carbon footprints are dramatic.

Industrial motor applications include HVAC, pumps, conveyors, fans, and mechanical motion of all kinds. In certain older installations, pump motors run at full speed, with flow control for liquids by using valves, and dampers and louvres used to control the flow of air and gases.

Besides wasting electricity, running motors at full speed is noisy and wasteful of filters and drive belts and often unnecessary. For instance, in recent years engineers have started fitting drives to their numerous pumps and fans, with most achieving significant energy savings immediately.

Affinity Laws explained

VSDs save energy, particularly when used on variable torque loads such as water pumps or cooling fans. For such applications, where the maximum output is needed only part of the time, installing a VSD to respond to demand will improve control and provide rapid payback for most pumping applications.

Energy savings are achievable through Affinity Laws (also known as the cube law). These laws explain the relationship between the variables involved in pump or fan performance, including capacity, head, and power consumption. They show that flow is proportional to motor speed, pressure is proportional to motor speed squared and power required is proportional to motor speed cubed.

This means that if 100 percent flow requires full power, 75 percent requires the cube of 75% (0.75³) or 42 percent of full power. Furthermore, a 50 percent flow requires only 12.5 percent of full power. These are significant energy savings.

Modern VSDs also feature automatic energy optimisation functions to maximise energy savings. Often the biggest savings comes if a drive can reduce its speed to a slow idle over a large part of its duty cycle. Go here for further help to justify investment funding.

*Theoretical calculations are for guidance and an audit is the next step. Contact us if you need help to check your site.