© Terrella Consulting Limited July 2019

Variable Speed Drive (VSD)

Variable Speed Drive (VSD), also known as inverters or Variable Frequency Drives, is the term that describes the equipment used to regulate the rotational speed and hence torque of an electric motor. Basically VSDs are electronic devices that can be attached to a motor to fluctuate its speed through a control mechanism, such as temperature or pressure. Initially VSDs were developed for achieving better process control in the industrial sector and recently they have further developed as a successful solution for smaller industries and building systems due to their potential to save significant amounts of energy. It is possible to couple them with any motor exhibiting a variable load, but the most usual applications are pumps and fans that operate in industrial processes or as part of heating, ventilation and air-conditioning systems (HVAC). Millions of motors are used by commercial and industrial users around the world, which, according to ABB Group, account for more than 65% of industrial electricity demand. During the design phase of a mechanical project, the exact motor loads is often unknown, this in turn leads to motor selection being oversized to safely meet the maximum system requirements. This leads to energy wastage as the motor operates outside its optimum zone. In order to prevent this and to achieve more control over operations and processes, many commercial and industrial users turn to airflow control vanes, two speed drives and other solutions. These solutions however are inefficient compared to VSDs from an energy saving perspective   A variable speed drive can reduce energy consumption by as much as 60%. For a 90 kW motor in continuous duty, this can mean over £9,000 per year. This is because the variable speed drive reduces the amount of energy drawn by the motor. Even a small reduction in speed can give significant savings. For instance, a centrifugal pump or fan running at half speed consumes only one-eighth of the energy compared to one running at full speed. This is because the torque needed to run a pump or fan is the square of the volume. For instance, reducing the pump speed to 80% only requires 64% of the torque (0.8x0.8). Furthermore, to produce 64% of the torque only requires 51% of the power (0.64x0.8), as the power requirement is reduced in the same way. The explanation for this lies in the pressure difference across the impeller. When less pressure is produced, less acceleration of air or fluid across the impeller is required. It is the simultaneous reduction of acceleration and pressure that multiplies the savings. Even further improved energy efficiency is offered through a technique called motor flux optimisation. This is particularly beneficial in pump and fan applications and under light loading conditions.