Electric motor-driven systems account for more than 50% of all electricity generated in the world and often represent the highest energy users in a facility. Therefore, for organizations looking to strengthen energy management or meet corporate sustainability goals, optimizing motor-related energy usage can be highly beneficial. This optimization often relies on process improvement/modification, but this approach tends to be very involved and may not be suitable for every industry. In contrast, a relatively simple — though often overlooked and misunderstood — way to reduce motor-related energy usage is by utilizing variable frequency drives (VFDs).
What is a VFD?
A VFD is a device used in combination with an AC induction motor. Grid power is connected to the input terminals of the VFD and the output terminals of the VFD are connected to a motor. The VFD is a medium to control motor speed and torque by varying the frequency and voltage at the motor terminals. To get technical, most VFDs do this using an AC-DC rectifier on the front end and a DC-AC inverter on the back end with a DC Bus in between. Insulated-gate bipolar transistors (IGBTs) are switched at frequencies in the range of 2-20 kilohertz to deliver the pulse-width modulated (PWM) signal seen at the motor terminals.
VFDs are particularly useful with centrifugal loads, such as pumps and fans. Popular commercial and industrial VFD applications include variable-loaded air compressors (typically rotary screw type), boiler and chiller feedwater pump motors, cooling tower fans, air handler supply and return fans, exhaust fans, and more. VFDs are especially valuable in applications with high operating hours, as there is more time to capture energy savings to offset the cost of the equipment.
What are the benefits?
VFDs take advantage of a principle known as the pump/fan affinity laws to produce energy savings. These laws state that the input power of a pump or fan is directly related to the speed of the motor shaft according to the following equation:
Traditionally, if you wanted to restrict water/air flow in a system, you would need to use a mechanical damper or valve. This method might limit flow, but it would require almost the same amount of incoming power as had the damper or valve not been present, making it inefficient. In lieu of a damper or valve, a VFD can speed up or slow down flow by physically reducing the speed of the motor shaft, drastically reducing the amount of energy needed to produce the same amount of flow. When you consider how many fans/pumps you might have in even one facility, the savings can start to add up quickly. In addition, the cost of VFDs has become more reasonable in recent years, making paybacks shorter.
VFDs provide additional benefits beyond energy savings. One is precise speed control for critical processes, like rolling applications. Adding an encoder, which is a sensor for mechanical motion that provides feedback to a VFD, allows for precise control of the roller speed so material isn’t damaged during the process.
Another benefit is reduced starting current of the motor load it is controlling. Induction motors have starting or inrush currents approximately six times higher than rated current. Transient currents at initial startup may be even higher. VFDs provide a bridge between the grid and the motor, allowing for lower voltage starting and thus lower current during inrush. However, if inrush current is the only problem being experienced by a motor-driven application, adding a soft starter may be a more cost-effective solution than a VFD.
What are the downsides?
VFDs do not make sense in all scenarios. They are generally unnecessary and can even be detrimental in non-variable-load situations unless precise speed control or reduced starting current is needed. For example, if you implemented a VFD on a motor that is 100% loaded all the time, it would use more electricity than if you did not have a VFD. This is because the VFD requires a small amount of energy to operate and would not likely be slowing the speed of the motor. Therefore, make sure to carefully consider which motors could benefit from a VFD.
In addition, because of how VFDs operate, motors paired with them need to be inverter-duty rated. Inverter-duty-rated motors are designed to better handle higher voltages (peaks can be 3-4 times rated voltage) that are fed from a VFD through its PWM signal output that can damage motor windings and bearings. VFDs should also be placed as close to the motor as possible to avoid this issue, which is amplified at long cable lengths.
Many VFDs add current harmonics to the circuit as well, which can cause problems with other devices on the circuit. Harmonic issues can typically be mitigated with ancillary equipment from the VFD manufacturer, but it’s something to keep in mind during the purchasing and installation phases of your project.
VFDs are a great way to moderate energy usage in many commercial and industrial environments, and they can improve speed control and starting current as well. With increasing focus on decarbonization and cost savings, implementing VFDs can be a start toward reducing energy consumption at your facility.