When sizing a Variable Frequency Drive (VFD), the most important consideration is to match the size of the VFD to the AC induction motor. AC induction motors have nameplates that list critical specifications. Examples of these specifications are Manufacturer, Model Number, Part Number, Serial Number, Design Type, Voltage rating, Horsepower, Hertz, Full Load Amps, RPM, Service Factor, and Maximum Ambient Temperature Rating. When considering a VFD voltage rating, horsepower and full load amp are most critical. TIP: Take a picture of the motor nameplate when possible – it will come in handy during commissioning also. Ultimately current is THE MOST IMPORTANT parameter to understand. Too many times people look at HP only and when a VFD is specified if the current rating to too close to the rating the VFD my not be able to drive the load. Avoid this drama by focusing on the amperage from the nameplate as the critical starting point.
Now that the nameplate data is covered the application must be considered. Typically, VFDs are rated as Normal Duty or Heavy Duty. So how do you know which one is needed? Normal Duty VFDs are for Variable Torque Applications such as centrifugal pumps, fans, or screw compressors. Heavy Duty VFDs are for Constant Torque applications. A constant torque application is any motor application where the motor can start with a load on its process. Examples of Constant Torque applications are conveyors, escalators, elevators, cranes, mixers, extruders, rock crushers, or saws. Be sure to get this point correct as the wrong decision can cause technical issues and even money. Utilizing a HD VFD in a normal duty application is overkill and adds extra cost to a project. Understand the application and size accordingly.
Some special applications have regeneration loads such as an elevator, crane or downhill conveyor. These applications could damage the VFD electronics with excessive voltage. Braking resisters are required for regenerative load applications or specify an active front end Drive to handle the excessive voltage generated by the load would be required.
An often overlooked item when sizing a VFD is the environment where the VFD will be located. The ideal environment is a clean electrical room at a maintained temperature. This is considered a NEMA 1 – IP20 location (IP = Ingress Protection). Dusty environments require NEMA 12 – IP 52 rating for protection from dust and dripping liquids. If the drive is used on the rooftop by an air handling unit or outdoors by a pump, a NEMA 3R – IP52 enclosure is needed for protection from dust and rain. For food processing operations where equipment is subjected to wash downs and caustic cleaning solutions, a NEMA 4/4X – IP56 enclosure is required.
If the VFD is placed in a UL508A rated industrial panel, a compact drive may be considered to save space, reduce heat and electrical noise interference (EMI) to other electrical components. If the drive is used in a building with sensitive electronics or with another AC to DC switching device, a low harmonics drive may need to be considered. A 3% or 5% line reactor inductive filter can reduce harmonics produced at the VFD six pulse diode rectifier input. Most larger VFD sizes of 30 HP and above have a built-in DC choke to decrease line harmonics. VFD’s used in high altitudes or high temperature environments may need to be upsized to allow for additional current carrying capacity and cooling of the electronics with thinner air. If the drive is placed inside a UL508A panel, the heat sink should protrude from the rear of the panel for optimum drive cooling.
By following the guidelines above a VFD can be sourced, installed and commissioned with upmost confidence.
