PU M PS 201 Determining the Ideal Control Mode for a VFD Factors to consider when selecting a control type for an application. BENJAMIN STRONG | Mitsubishi Electric Variable frequency drives (VFDs) are everywhere in modern life—operating the fan in home air conditioners, controlling the pump in the backyard pool or operating submersible pumps in the lift station in the neighborhood, to name a few. As the presence of VFDs grows in our society, so does the sophistication of the control algorithms governing their output. This article will describe the common control types for VFDs and the criteria to consider when selecting a control type for an application. IMAGE 1: Typical volts per hertz curve (Images courtesy of Mitsubishi Electric) Control Mode Pros & Cons Each method of controlling the voltage and current output of the drive has its advantages and disadvantages. Drives confi gured in volts per hertz (V/Hz) mode require fewer steps to commission than other control options but provide the lowest level of performance. On the other hand, fl ux vector mode provides better torque characteristics at low speed but requires more steps to commission. Vector (or sensorless vector) control offers the highest level of performance at a higher cost. To achieve the best results, it is essential to consider the application’s criteria before selecting a VFD-control method, rather than choosing the default or easier method to commission the drive. Most pump and drive fans default to V/Hz mode out of the box. V/Hz mode is best used for “shaft turner” applications— applications that require simple setup and operation—and additional voltage control methods are rarely necessary. Just enter the motor rated current and base frequency, then confi gure the speed reference, and the drive is ready to run. IMAGE 2: Volts per hertz curve with torque boost range In V/Hz mode, the drive output voltage follows a pattern where the voltage is relative to the output frequency. The drive follows this pattern until it reaches the motor base frequency. In North America, this is usually 60 Hz; however, imported or special-purpose motors may have a different base frequency. Fan and pump drives are preconfi gured with a V/Hz pattern that is optimized for variable torque loads (torque varies with motor speed), such as centrifugal pumps or fans. A general-purpose drive will default to a V/Hz pattern that better fi ts a constant torque load (torque does not vary due to motor speed), such as positive displacement pumps, conveyors or rolls. However, V/Hz mode cannot be used with permanent magnet or synchronous reluctance motors. Challenges of Default Se ings While default settings may be convenient and require fewer steps to commission, there are challenges to consider. The downside of V/Hz control is poor torque output at low speed. For some pump designs, applying larger torque at low, or no, speed to overcome static friction of seals and bearings is critical to prevent motor stalling when starting. Most VFDs 56 PUMPS & SYSTEMS JUNE 2025
Pumps & Systems June 2025: Page 56
