How to Select a VFD: A Practical Guide to Mitsubishi FR-Series Drives
By AB Automation Team3 min read
A variable frequency drive (VFD) is one of the simplest ways to save energy and gain control over an AC motor. Yet many drives are bought by matching the motor kW and nothing else, which leads to overcurrent trips, overheating and short life. This guide explains how to select a VFD properly and how the Mitsubishi Electric FR-series range maps to common applications.
Step 1: Size by motor current, not just kW
Read the motor nameplate full-load current (FLC) and choose a drive whose rated output current is equal to or greater than it. Two motors with the same kW can have quite different currents, especially older, rewound or low-speed (6- or 8-pole) motors. If the drive will run more than one motor in parallel, add up their currents and allow a margin.
Step 2: Identify the load type
| Load type | Examples | What it means for the drive |
|---|---|---|
| Variable torque | Fans, blowers, centrifugal pumps | Light duty rating is usually fine; biggest energy savings. |
| Constant torque | Conveyors, mixers, extruders, positive displacement pumps | Use the normal/heavy duty rating; needs good low-speed torque. |
| High inertia / impact | Centrifuges, crushers, punch presses | Oversize the drive and plan braking for deceleration. |
| Hoisting / vertical | Lifts, hoists, cranes | Needs braking resistor, brake sequence logic and high starting torque. |
Step 3: Check braking and deceleration
When a high-inertia load is stopped quickly, the motor acts as a generator and pushes energy back into the drive's DC bus. Without a braking resistor this causes overvoltage trips (E.OV1/E.OV2/E.OV3 on Mitsubishi drives). Calculate the required stopping time; if it is short or the load is vertical, specify a brake resistor or brake unit at the start.
Step 4: Consider the environment and installation
- Ambient temperature: panels in Indian summers often exceed 45°C. Provide ventilation or derate the drive.
- Dust and fibre: textile and cement plants need filtered panels and regular cleaning of heatsinks.
- Cable length: long motor cables cause voltage spikes and leakage currents. Use output reactors or filters beyond the length recommended in the manual.
- Harmonics: add a DC or AC reactor where many drives share a transformer, or where power quality is an issue.
Step 5: Decide how it will be controlled
Will speed be set by a potentiometer, a 4-20 mA signal, multi-speed inputs, or over a network from a PLC? Mitsubishi FR-series drives support RS-485 (Mitsubishi inverter protocol and Modbus RTU) as standard on most models, and Ethernet or fieldbus options on others. Network control reduces wiring and lets the HMI display current, frequency and fault history.
Which Mitsubishi FR series?
| Series | Positioning | Good for |
|---|---|---|
| FR-D700 / FR-D800 | Compact, simple | Small conveyors, fans, pumps and machine auxiliaries. |
| FR-E800 | Compact, high function | OEM machines needing better torque control, Ethernet and safety functions. |
| FR-F800 | Fans and pumps | HVAC, water and blower applications with energy-saving and PID functions. |
| FR-A800 | High performance | Cranes, winders, extruders and demanding constant-torque loads; vector control. |
Common selection mistakes
- Using a single-phase input drive for a large motor without checking the supply.
- Ignoring the motor's insulation class on old motors, which can fail under PWM voltage stress.
- Running a standard self-cooled motor at low speed for long periods without forced cooling.
- Bypassing the drive with a contactor on the output while it is running.
Get a VFD sized for your application
AB Automation supplies genuine Mitsubishi Electric FR-series drives, builds complete VFD panels, and repairs drives of all major brands. Share your motor nameplate and application on our contact page and we will recommend the right model.
Need help with your automation project?
Talk to AB Automation’s engineers — authorised Mitsubishi Electric partner & Oriental Motor distributor.