Energy Saving Using Variable Speed Drives – Explanation of How to Save Energy Using Drives
Energy Saving – An introduction
Many systems use constant speed motors and control process flow rates or pressures by mechanically regulation using throttling speed cube valves, dampers, fluid couplings or variable inlet vanes etc. These devices generally do not control flow or pressure efficiently because energy is dissipated across the throttling device.
Running a motor at full speed while throttling the input or output is like driving a car with one foot on the accelerator and the other on the brake; a part of the produced output immediately goes to waste.
A variable speed drive can save over 60% of the energy. This is possible as it controls the energy at source, only using as much as is necessary to run the motor with the required speed and torque – much in the same way as the accelerator in the car controls the engine revs and without the foot on the brake. Types of loads – which are suitable for energy saving?
Drive applications are categorized with respect to power and torque changes in response to the motors speed. It is important to understand the type of load for a particular application because not all are equally good energy saving opportunities for the application of a variable speed drive. In fact, if a variable speed drive is used on some loads there will be little or no energy savings.
Variable speed drives and the loads they are applied to can generally be divided into 3 groups:
– Constant power
– Constant torque
– Variable torque
Constant Power Loads
In constant power applications, the power requirement remains constant at all speeds, and the torque requirement varies inversely with speed. One example of this type of load would be a lathe. At low speeds, the machinist takes heavy cuts, using high levels of torque. At high speeds, the operator makes finishing passes that require much less torque. Other examples are drilling and milling machines.