#252 - Question to the ISA's “Ask the Automation Pros” program

Check out my answer to the ISA's “Ask the Automation Pros” program regarding variable frequency drives.

When selecting a VFD for an application, I consider a few key factors to ensure optimal performance. Understanding the relationship between torque and speed is fundamental. Applications generally fall into two categories: constant torque, like conveyors, material handling systems and hoists, or variable torque, where torque is proportional to the square of the speed, as in fans and pumps. Most VFD manufacturers provide models tailored to these categories, making it easier to choose the right one.

Matching the VFD to the motor is equally important. Selecting a VFD with a power rating slightly above the motor's ensures it can handle the application without strain or performance issues.

During commissioning, I configure parameters such as motor nameplate details and acceleration/deceleration ramp times and then perform an auto-tune operation. This allows the VFD to account for actual installation factors like cable distances and fine-tune its settings for optimal performance.

These are the basic steps to get the motor running, but fine-tuning is often necessary to tailor the system for the specific application. I monitor and adjust parameters such as duty type (heavy or normal, selecting heavy for higher overload tolerance), motor control mode (e.g., vector control or V/f), maximum motor frequency, control method (e.g., Ethernet or hardwired signals), brake release configuration (if the motor has a brake) and IGBT switching frequencies.

Many of these adjustments are guided by field observations, alarms and warnings displayed by the system. The ultimate goal is to achieve smooth operation during both starting and stopping. Key parameters I monitor include motor current and motor speed to ensure the system is functioning efficiently and reliably.