Specifying VFD's

The basic application for a VFD is one where an AC motor must be run with some aspect of control over the speed, accel/decel or torque. Cranes, fans, pumps, mixers, conveyors, and similar applications are all candidates for using a VFD.

VFD’s can control the speed and torque of an AC motor which optimizes the energy (electricity) used to run the motor. In other words, instead of just turning the motor on at full voltage, like flipping a switch, the VFD can vary the speed to better meet application requirements, start more smoothly, and conserve energy. All of us have been in a business or at home when the lights dim for a moment while the air conditioner, an air compressor or other such motor turns on. Now imagine a factory with dozens of motors turning on and off many times each throughout the day. Controlling those same motors with VFDs can eliminate that sudden power draw on the circuit.

Constant Torque

Conveyors, elevators, punch presses, cranes, printing machines require the same amount of torque at low speed and high speed. Maintaining this torque can sometimes use more than the rated torque of the motor, so the drive needs to have an overload rating.  The VFD needs to respond to changes in the load on the motor to keep the speed constant. “Overload torque” refers to the characteristic of the VFD to run above 100% of capacity for a brief time, typically one minute or less.  Different drives have different overload capacity, some up to 250%.

Variable Torque

Fans, pumps, and other spinning inertial loads are another type. It is uncommon for most of these types of applications to exceed the rated capacity of the VFD. These types of loads need lower torque to run at low speeds, so the VFD can vary the torque to use just what is needed, conserving energy.

First figure out what type of application you have. Some VFD manufacturers will categorize their VFDs by application type: Fan, conveyor, pump, crane, etc. Essentially is it an overload application or not? Another way some manufacturers classify the type of VFD required is by the “duty” required: Light duty, normal duty, and heavy duty for example. Most fans, pumps and other spinning loads are light or normal duty. Punch presses, cranes and other heavy loads are heavy duty applications. Conveyors can be light duty to heavy duty depending on the size and the material conveyed. Moving small medical parts is less demanding than moving cement bags.

The motor nameplate information or specification is the first place to start collecting the info needed to specify a basic VFD. Motor voltage, electrical phases (single or three phase) and the full load amps (FLA) comprise the bulk of the specification. Horsepower (HP) can be used if FLA is not available.

Control architecture, speed setting method and start/stop method will finalize most basic VFD selection specifications.  Will the VFD be connected to a fieldbus like Ethercat or Ethernet/IP, which will allow an upstream controller to set the speed and other parameters and start, stop, and select direction? Or will the drive be controlled with digital IO that typically allows a PLC to set the direction and select 1 of 3 speeds before turning on a “Start” input. Another option is to have a digital operator panel connected to the drive so that operators can change parameters as needed and start/stop the drive.

Additional considerations/options

Communications/interface options

Some manufacturers include special functions in their drives. Some of these features may make a more expensive drive more desirable because the special function or functions make using the drive easier in the specific application for which it was designed. Some Mitsubishi drives have a “Derag” or anti-clog, or self-clean (different names for the same thing) function for example. This function can be configured to automatically reverse a pump motor in a sewage treatment application to keep pump blades clear of solid debris like, well, rags that get in the treatment system. There are multiple other examples of special functions one should look for in the spec sheet of the VFD being selected.

Finally, configuration method and assistance should play a key role in the selection.  Getting a VFD at rock bottom pricing does not save anything if the manufacturer offers no support or is difficult to get support from. Considerations in this category are personal in a way, but one should consider if there is easy software to use for setting parameters and monitoring for troubleshooting, or is only the keys on the front of the drive? If many VFD’s are going to be commissioned for similar uses, such as dozens of the same fans in a building ventilation system, how easy is it to get one set up correctly and then just put a copy of the parameter set into all of them without having to set each parameter individually?  Software helps this, but sometimes computers are inconvenient to use, so some manufacturers allow you to copy parameters from one VFD to a USB drive and then use that USB to program the rest of the VFDs. As always, as a value-adding distributor of multiple brands of VFDs, we at Valin cannot stress enough that you should consider who is selling you the drive. Will they have easily accessible support for you after UPS drops off the shiny new VFD, or are they going to suggest finding the phone number of the manufacturer on the website? Time is money and the faster your supplier can help you, the less expensive the VFD costs.