Home > FAQ >

How can you further improve the energy efficiency of your drive installation ?

If practical, motors and drives should be switched off when not in use.
For all new installations, energy-efficient motors should be used. The combination of a VSD and an energy-efficient motor ensures an economical and future-proof installation.
For retrofit applications, you should consider replacing standard-efficiency motors with equivalent energy-efficient ones. Energy-efficient motors are also included in the ACA, and those that have qualified included on the specified list.
The mechanical efficiency of the driven equipment and transmission system (i.e. pumps, fans, belts, etc) directly influences the efficiency of the overall drive system. It’s important that this equipment be regularly maintained and lubricated and that wear-and-tear of mechanical parts be monitored.
Example of a VSD payback calculation
Consider an 11kW 2-pole EFF1 motor driving a product transfer fan for a milk powder processing plant. The fan motor operates 6,000 hours per annum. Air flow is controlled via a manual damper set to 80% open. The motor efficiency is 90.5%.
From the curve representing the system in Figure 2 below, we see how the damper setting reduces the input power requirement by a factor of about 0.9. A cost of electricity of 0.14 per kWh is assumed.

Figure 2: Damper vs. VSD control efficiencies (courtesy of The Carbon Trust)

Without VSD
The annual cost of running the motor without VSD is as follows:

ANNUAL RUNNING COST WITHOUT VSD =
Input power * Input power reduction factor * Run hours * Electrical cost
= (11kW/0.905) * (0.9) * (6000) * (0.14)
= 9,188.95


With VSD
If the damper is replaced with a VSD, the curve in Figure 2 shows that the input power is now reduced to 58% of maximum when running at 80% of full load. If we assume that the combined efficiency of the motor and the VSD is now 86%, then the annual running cost of the motor combined with VSD can be calculated as follows:


ANNUAL RUNNING COST WITH VSD =
Input power * Input power reduction factor * Run hours * Electrical cost
= (11/0.905) * (0.58) * (6000) * (0.14)
= 6,231.63

Thus the annual cost savings achieved by replacing the damper with the VSD are as follows:
Cost savings with VSD = 9,188.95 - 6,231.63 = 2,957.32 p.a.
If we assume a cost of 6,000 to supply and install the VSD, taking support from the ACA scheme into account, this gives us the following payback period:
Payback period = 6,000 / 2,957.32 = 2.03 years
In this simplified example, a payback of two years has been calculated. The load profile has been simplified to a constant 80% of full load. In practice, a more detailed examination of a varying load profile would be needed to calculate the true annual running costs.