VSD energy saving payback times halved

With the doubling of electricity prices, the payback time for installing variable speed drives (VSDs) has halved. Most VSD energy saving fan and pump applications deliver payback periods of under 18 months. Recent hikes in the cost of electricity further reduces these payback times by up to 50%. Now is a suitable time to look again at some shelved projects.

Fan and pump motors offer the greatest opportunity for saving energy. They are big, expensive to run and there are plenty of them. Makers of newer motor driven systems optimise them for energy saving, so start VSD energy saving with the older installations. VSDs automatically reduce motor speeds to match the application and as a result a 10% speed reduction reduces the energy needed by about 27%.

Why is it possible to reduce the motor speed by these amounts?

Why does changing the motor speed not affect how the application runs? In fan and pump applications, engineers match the motor size to the maximum demand of the system. Yet most systems do not run at this level all the time, so  the motors run at full speed and mechanical valves, or dampers control the rate of flow. This is wasteful.

A popular analogy is driving a car with the engine running flat-out and using the brakes to control the speed. Installing a VSD gives you back control. It adjusts the motor speed to meet the needs of the system. It reduces the running cost yet also improves control, lowers noise levels, and extends the life of consumables like drive belts and filters.

Why is a VSD energy saving much bigger than the speed reduction?

It is wrong to assume that reducing a motor’s speed by 10% saves 10% of the power. Similarly switching motors on and of will not deliver the same level of savings and can be more stressful on the system. Changing the speed of an industrial motor is complex and needs special electronics like those found in VSDs. What makes these savings achievable is that reducing the speed of a motor has a non-linear effect on the energy needed to drive it. This is known as the cube law.

The cube law

According to the cube law, flow is proportional to motor speed, pressure is proportional to motor speed squared and power required is proportional to motor speed cubed. Another name for the cube law is the affinity law.

In practice, if 100 percent flow requires full speed, 90 percent flow needs the cube of 90% (0.9³) or 73 percent of full power. Moreover, a 50 percent flow requires only 24 percent of full power. These are large  energy savings at today’s energy prices and apply to both larger and small motors. The value of the savings increases with larger motors.

As an example, a typical mid-sized fan or pump motor rated 18.5 kW and assuming 90% efficiency has an input of 20.5 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 20.5 = 39360 kWh of electricity a year.

39360 x 34p = £13382 a year, or about £268k over its lifetime.

Installing a VSD to reduce speed by 20% cuts the electricity consumption by 49% to 19286 kWh a year. This would save over £6557 a year every year, or £131k over the lifetime of that single motor. Consider then that the cost of an 18.5 kW VSD is about £1500 plus installation, which can often be managed by inhouse engineers. Importantly, whatever method a company uses to calculate return on investment (ROI), our variable speed drives will meet it.


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