Efficiency Gains in Electrical Drive Systems

Abstract

The environmental impact of the electricity generation is quite high because 70% of the primary energy used for electricity generation is based on fossile fuels. Another reason for this environmental impact is the poor overall efficiency of the electric supply if calculated from the primary energy to the useful work performed by an electric motor. The efficiency can be as low as 10% in case of inefficient process control like throttling of pumps.

Better process control can be achieved by variable speed control of pumps and fans. The efficiency can be doubled up to 20% and the primary power savings can be five times the rated power of the pump motor. The efficiency of the motors and drives is relatively high but not without importance when used with low efficiency systems. A more important feature of variable speed AC or DC drives is the possibility to save energy by using drives to improve process control efficiency.

Investments for energy savings have to be based on economic calculations. Many parameters have influence on the profitability of energy saving investments: savings in per cent, cost of energy, cost of investment etc. In many cases energy saving has a secondary meaning in VSD investments, for most cases the process control benefits are the most decisive. Especially the quality improvements of the produced end product are more important.

There are a number of cases where remarkable energy savings have been measured in pump and fan applications with variable speed drives. Two of such cases have been introduced, a district heating pumping case from Strausberg, Germany and an air conditioning fan application from Heathrow, England. Both of those cases had savings more than 50% and pay-off time for the investment less than one year.