Choosing the right variable frequency drive for an application involves several important considerations. For example, based on acceleration requirements, sensorless vector control may be more suitable than volts-per-hertz (V/f) control. While V/f control is effective in dragging logs up a slope, it’s not appropriate for dockside hoists that position 12-ton shipping containers to within inches.
What is your load type: constant or variable torque?
To get a constant-torque load, the torque is independent of speed (ignoring momentary shock loads). Examples consist of conveyors and hoists. To get a variable-torque load, torque varies as a function of speed. Examples consist of fans and pumps. This main distinction underlies each choice you will make concerning the kind of drive.
What are your acceleration requirements?
Does it matter how fast your load accelerates up to speed? For a fan, probably not. For a centrifuge, almost certainly. In the latter case, you may want to select sensorless vector control, rather than volts-per-hertz (V/f) control. While the V/f approach is effective for many applications, it doesn’t allow a motor to develop near-full torque at near-zero speeds (unlike sensorless vector control). V/f control can be appropriate for dragging logs up a slope, but not for a dockside hoist that needs to position a 12-ton shipping container to within inches.
Controlled deceleration presents its own challenges.
Throughout decelerations, the motor acts as a generator. The resulting power must go someplace, and is usually dissipated as heat inside a braking resistor. Controlled-deceleration capability is really a great answer for constant-torque loads, altering loads, or perhaps unbalanced loads.
What is your speed range?
Although a conveyor belt may operate consistently at 60 Hz, for an unspooling module on a printing line, the motor needs to deliver torque as effectively at 0.5 Hz as 60 Hz. This is another application where garden-variety V/f control won’t do the job. Sensorless vector control will (and most VFDs these days include it). Keep in mind, however, that not all offerings are created equal. Be sure to double check specifications against your requirements.
How do I select the proper size VFD?
For most applications, select a VFD that has an output current rating that equals or exceeds the nameplate current rating of the motor. Do not just select the VFD based on the motor Hp rating, as this can result in a VFD that is too small. Some severe applications, such as punch presses, vibratory conveyors, mixers, etc, may require oversizing the VFD in order to handle the peak current demands of the load. Please contact AC Tech's Application Engineering Department for more information.
VFDs have a wide variety of programmable functions. Not all manufacturers call these functions by the same name. Research is required to properly utilize a VFD to its best performance. We are fortunate to live in an age where a great wealth of information is available on the Internet. Utilizing manufacturers’ websites and educational information, plus a little time, can make most any controls person an expert in VFD selection.
What is your load type: constant or variable torque?
To get a constant-torque load, the torque is independent of speed (ignoring momentary shock loads). Examples consist of conveyors and hoists. To get a variable-torque load, torque varies as a function of speed. Examples consist of fans and pumps. This main distinction underlies each choice you will make concerning the kind of drive.
What are your acceleration requirements?
Does it matter how fast your load accelerates up to speed? For a fan, probably not. For a centrifuge, almost certainly. In the latter case, you may want to select sensorless vector control, rather than volts-per-hertz (V/f) control. While the V/f approach is effective for many applications, it doesn’t allow a motor to develop near-full torque at near-zero speeds (unlike sensorless vector control). V/f control can be appropriate for dragging logs up a slope, but not for a dockside hoist that needs to position a 12-ton shipping container to within inches.
Controlled deceleration presents its own challenges.
Throughout decelerations, the motor acts as a generator. The resulting power must go someplace, and is usually dissipated as heat inside a braking resistor. Controlled-deceleration capability is really a great answer for constant-torque loads, altering loads, or perhaps unbalanced loads.
What is your speed range?
Although a conveyor belt may operate consistently at 60 Hz, for an unspooling module on a printing line, the motor needs to deliver torque as effectively at 0.5 Hz as 60 Hz. This is another application where garden-variety V/f control won’t do the job. Sensorless vector control will (and most VFDs these days include it). Keep in mind, however, that not all offerings are created equal. Be sure to double check specifications against your requirements.
How do I select the proper size VFD?
For most applications, select a VFD that has an output current rating that equals or exceeds the nameplate current rating of the motor. Do not just select the VFD based on the motor Hp rating, as this can result in a VFD that is too small. Some severe applications, such as punch presses, vibratory conveyors, mixers, etc, may require oversizing the VFD in order to handle the peak current demands of the load. Please contact AC Tech's Application Engineering Department for more information.
VFDs have a wide variety of programmable functions. Not all manufacturers call these functions by the same name. Research is required to properly utilize a VFD to its best performance. We are fortunate to live in an age where a great wealth of information is available on the Internet. Utilizing manufacturers’ websites and educational information, plus a little time, can make most any controls person an expert in VFD selection.
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