Effects of ambient temperature
The motor rated efficiency is always given for a standard reference ambient temperature of 25 °C (see IEC 60034-2-1). Operation under cooler ambient temperatures will increase efficiency while operation under hotter ambient temperatures will reduce efficiency.
The mechanical output power of standard motors per frame size is not internationally standardized. However regional standards do exist (for example EN 50347 and NEMA MG1) which are widely recognized. It is therefore advantageous for the retrofitting of standard electric motors in existing applications to have energy-efficient motors with the same frame sizes and output powers that won’t require major refurbishments of driven equipments.
When the device being driven by an electric motor is producing a relatively constant and continuous level of useful work, the primary motor selection concern is its rated-load efficiency. However, many applications are cyclic in nature. In these cases specific application techniques can be used to obtain substantial energy savings.
Other applications require intermittent or continuous absorption of energy. Again there are application techniques that will recover a significant percentage of the otherwise wasted energy.
A few of these cases follow to illustrate the technology that is available to the user. The motor manufacturer should be consulted to determine the most effective solution.
Energy savings by speed control (variable speed drives, VSD)
In many applications, the largest energy savings can be created by varying the speed of the motor according to the application load demand. This is typically done by using a variable speed drive (VSD).
The additional losses in the frequency converter can easily be compensated for by the overall improvement of the application efficiency.
Many pump and fan applications currently involve the control of flow or pressure by means of throttling or bypass devices. Throttling and bypass valves are in effect series and parallel power regulators that perform their function by dissipating the difference between source energy supplied and the desired sink energy.
These losses can be dramatically reduced by controlling the flow rate or pressure by controlling the speed of the pump or fan with a variable speed drive.
Correct sizing of the motor
Energy efficient motors are specifically useful in applications with a high number of operating hours at greater than ¾ of full load.
In order to avoid significant operating hours with loads below 50% resulting in a lower efficiency the system should be sized according to the required peak-load and starting-torque.
Due to the low temperature utilization of more efficient motors their overload capacity is typically higher when compared to standard motors. Therefore, oversizing the motor for occasional peak-power demands is seldom required and certainly not cost effective.
When a replacement of a standard motor with a high efficiency motor is envisaged in existing applications, the correct power-demand and sizing of the motor should be evaluated.