Where are VFDs Appropriate for Blowers?

Variable-frequency drives (VFDs) are adjustable-speed ac-motor controls that increase or decrease speed on non-explosion proof motors. Where their inclusion in designs is justified, they boost design performance and efficiency, minimize maintenance requirements and extend blower life. They allow for closed loop feedback, system monitoring — and real time control of the blower motor.

As background on the most common iteration, where VFDs pair with ac motors, induction motors are the most common of all motors used in large industrial blowers. With induction motors, energy consumption is a function of running rpm cubed, which means there is significant energy savings for even small speed adjustments. The catch is that blower motors must be sized to satisfy maximum-load conditions, even if such situations are relatively rare. For average operating conditions, blowers are oversized, and run inefficiently unless there’s some type of speed control to optimize flow to operating settings.

Here, VFDs can reduce and increase induction-motor speed as needed to satisfy application demands even while maximizing energy efficiency. That’s a capability that helps induction-motor-based blowers outperform comparable single-speed-motor or two-speed-motor arrangements. What’s more, VFD-based blower control can optimize overall design functions, especially in process industries. Plus, VFDs can provide dynamic braking as well as replacing motor-starter functions. This is important because motors otherwise come online at their full rated speed — which in turn spurs current inrush and physical wear on the motor.

In addition, many VFDs for pairing with blower motors accept 4-to-20 mA, 0-to-5 Vdc or 0-to-10 Vdc inputs for integrated automation installations. Some accept three-phase or single-phase inputs to drive three-phase motor-blower assemblies. VFDs with digital readouts (reporting Hz or rpm) and NEMA-rated keypad enclosures are available — and the latter can in some cases be mounted dozens of feet from the controls. (As covered in other AMETEK literature, installations that separate the controller and motor by 50 feet or more require a line reactor on the load side.

There are parameters such as allowable speed for sizing VFDs to pair with blower motors. Required rpm values for optimal performance (to get target pressure and air flow from the design’s blower) are listed in the manufacturer's literature. The required rpm must stay within the connected blower motor’s speed capacity. That said, VFDs can run motors at an above rated rpm where allowable and required.

One note that relates to inverter-duty motors; turn-down, the ability to reduce the air-flow rate exceeding 10:1 is uncommon on regenerative blowers. For applications with speeds exceeding 3,600 rpm, consult with the blower manufacturer regarding horsepower sizing.

Important! Speedup is not recommended on regenerative blowers due to the increasing horsepower demand per the fan laws.

At AMETEK Dynamic Fluid Solutions, we understand you’re looking for more than just an off-the-shelf part or one-time solution. You need a true technology partner who understands your engineering challenge that is focused on you, providing customized, collaborative solutions. We’ll also provide you with excellent customer service for a great total experience.