Variety best describes the number of design aspects that can be chosen with a variable frequency drive. While many are under the impression that drive selection is simple (match drive HP to motor HP), there are eight key questions that must be answered before a drive is selected.

1. What is the system voltage?

Drives can operate at voltages from 120 VAC to 600 VAC. The operating voltage of the control system will need to be specified as part of VFD selection.

2. What is the Motor’s Full Load Amp Rating?

Drives produce amps which then power a motor.  The drive’s output amps must be equal to or large than the motor’s full load amps or the drive and motor system may fail to operate as expected. 

3. What is the Application?

Drives have a wide range of applications: fans, pumps, extruders, etc.  Centrifugal pumps and fans are considered normal duty applications, while all others are considered heavy duty applications.  Drives have different power ouput profiles depending on their duty rating, so make sure you  are aware of the application type when specifying a drive.

4. In What Environment is the Drive Located?

Handling environmental concerns such as dust, rain, and corrosives are a fundamental part of drive selection and implementation.  Careful consideration should be given to environmental ratings when choosing drives and drive enclosures.

5. What is the Control Voltage?

Both 24 VDC and 110 VAC are common control voltages; however, the drive in question will have to be outfitted appropriately for said voltage.  Be sure to specify control voltage when selecting a drive.

6. What is the Preferred Communication Method?

From Ethernet to Profibus, Allen Bradley drives can be outfitted to suit many different communication methods. However, the communication method of choice will have to be specified during drive selection.

7. Is Safe Torque Off Required?

The Safe Torque Off (STO) function is used to prevent unexpected motor rotation in the event of an emergency while the drive remains connected to the power supply. When STO is activated, the torque power cannot reach the drive, thus stopping and preventing any motor shaft rotation.  Many Allen Bradley drives feature a STO option, and this should be specified before purchase if required.

8. How Will the System Be Grounded?

Allen Bradley drives are outfitted with both metal-oxide varistors (MOV) and common mode (CM) capacitors. For 7-series drives, you may specify as to whether these features come with their connection jumpers either installed or removed. If the grounding system in question is center tapped wye, then the MOV and CM jumpers should be installed. For any other grounding system, the MOV’s and common mode capacitors should NOT be connected.