A variable frequency drive (VFD) controls the speed of an AC motor by changing the frequency and voltage it receives. Choosing one is mostly a matter of matching the drive to the motor, the load and the site. Getting any of the three wrong leads to nuisance trips, overheating or a drive that is larger and more expensive than the job needs.
This guide walks through the information you need before you ask a supplier for a quotation.
1. Start with the motor nameplate
Collect these values from the motor nameplate or datasheet:
- Rated power (kW) and rated current (A). Size the drive on motor current, not only on kW.
- Rated voltage and frequency, for example a three-phase supply at 50 Hz.
- Number of poles / rated speed, which tells you the base speed.
- Motor type: standard induction motor, or a motor designed for inverter duty.
- Service factor and insulation class, if the motor will run at low speed for long periods.
2. Know your load type
The load decides how much current the drive must deliver, especially at start-up and during overloads.
| Load type | Typical examples | What it means for sizing |
|---|---|---|
| Variable torque | Centrifugal pumps and fans | Torque falls as speed falls, so the overload demand is low and a normal-duty rating is often enough |
| Constant torque | Conveyors, mixers, compressors, hoists | Full torque is needed across the speed range, so a heavy-duty rating or a larger frame is usually required |
| High starting torque | Crushers, loaded conveyors | Check the drive's overload capability and the starting current you actually need |
Drive manufacturers publish normal-duty and heavy-duty ratings for the same frame. Read the datasheet carefully and compare like for like.
3. Check the current rating, not just the kW
A common mistake is to pick a drive with the same kW as the motor. Use the motor's rated current and confirm that the drive's continuous output current at the site conditions is equal to or higher. Allow margin if the motor is often loaded close to its limit, or if the application needs repeated overloads.
4. Allow for the environment
Drive ratings are given under stated conditions. Check the datasheet for:
- Ambient temperature. Many drives must be derated above a stated temperature. This matters in hot plant rooms and in enclosures without good ventilation.
- Altitude and humidity.
- Dust and moisture, which decide the enclosure protection (IP) rating.
- Cooling and enclosure size. A drive inside a panel releases heat that the panel must remove.
5. Decide where the drive will live
You can mount drives in a motor control panel, in a motor control centre, or in a dedicated drive enclosure. Think about cable length between drive and motor, because long motor cables may need output filters. Your drive datasheet states the maximum cable length for the standard setup.
6. Plan the protection and the control
- Short-circuit and overload protection upstream of the drive: see our guide to MCB vs MCCB for choosing the right breaker.
- Line reactors or filters where the supply quality or the standards require them.
- Control interface: start/stop, speed reference (analogue or fieldbus), feedback signals and safety functions.
- Bypass or redundancy, if the process cannot stop when the drive faults.
Confirm the requirement for your project with your consultant or the authority.
7. VFD or soft starter?
If you only need a smooth start and stop and the motor then runs at full speed, a soft starter may be simpler. If you need speed control or energy savings at partial load, a VFD is the usual choice. We compare the two in soft starter vs VFD: how to choose.
Information to send with your enquiry
To get an accurate quotation, send the supplier:
- Motor nameplate data (kW, A, V, Hz, poles).
- Load type and duty cycle.
- Speed range required.
- Supply voltage and site ambient conditions.
- Preferred mounting (panel, MCC or standalone) and any cable length.
- Control requirements and communication protocol.
Where Arab Arc Trading fits
Arab Arc Trading supplies motor control equipment, including variable frequency drives and soft starters, and builds panel assemblies in Jeddah. See Motor Control and Electrical Panel Assemblies, or contact us with your motor data and we will help you check the selection.
FAQ
Can one VFD run several motors? Yes in some set-ups, but each motor then needs its own overload protection and the drive must be sized for the combined current. Check the drive manual first.
Do I need an inverter-duty motor? Standard motors are often used with drives, but long cables, very low speeds or high switching frequencies can stress the insulation and cooling. Check the motor manufacturer's guidance.
What is the difference between VFD and inverter? In everyday use the terms are close to interchangeable; "VFD" and "variable speed drive" are the common names for the complete unit.
