- Energy Saving
Variable Speed Drives - what are they and how do they work?
Variable speed electric drives (VSDs) enable what are otherwise fixed speed motors to operate a range of speeds and torques from zero to 100%. They can give great process flexibility, reduce wear and tear on machines, and dramatically cut running costs.
VSDs work by electronically converting mains voltage to a variable frequency and voltage supply hence reducing motor speed without reducing torque. They are generally only available for three-phase machines, although some single phase options can be found.
Having speed control of electrically driven devices like pumps, conveyors and fans can bring great benefits. In process control, conveyers can now be slowed gearlessly to the correct speed. Pumps and fans can be set to deliver the correct output volume or pressure and the parameters can be automatically modulated in sympathy with some measurement from a sensor. So temperature, pressure or flow can determine the speed of the motor to give the right output.
In horticulture, example applications are for heating circulation pumps, irrigation and ventilation. With heating pumps for instance, during times of low heating demand pump output can be reduced to keep the differential between flow and return temperature the same. Not only does this produce savings in electricity, but it also helps the boiler to operate more efficiently. Irrigation pumps can be made to respond to water pressure, so as solenoid valves cut in and out, water pressure and volume is adjusted to exactly what the system requires.
Energy use will also be minimised. One characteristic of many motive loads is that energy use is proportional to a cube of the motor speed. So for instance, reducing a pump or fan speed by 20% will reduce energy consumption by more than 50%. There are other useful spin-offs from using variable speed drives. Motor starting currents are reduced, and wear and tear on motors and attached mechanical components are minimised. Generally, VSDs are much gentler on systems compared with on / off controls.
The good news is that, following 20 years of constant development, drives have increased in flexibility, sophistication and reliability — and their cost has fallen dramatically. So, next time you are using a motor for pumping, conveying or ventilation applications, consider how a variable speed output could be of advantage.