How to Choose a Variable Frequency Drive for an Electric Motor
A variable frequency drive (VFD, frequency converter, inverter drive) enables smooth speed control of an electric motor, reduces inrush currents, and saves energy. However, an incorrect choice of device leads to overheating, premature bearing wear, or emergency shutdowns of the production line. Below is a step-by-step algorithm that will help you select the optimal model without overpaying.
Step 1. Determine the Electric Motor Parameters
Before opening the variable frequency drive catalog, record the data from the motor nameplate:
- Rated power (Pn) — specified in kW. This is the mechanical shaft power, not the power consumed from the mains.
- Rated current (In) — the primary parameter for selection. The VFD is chosen based on current, not power.
- Rated voltage (Un) — 220 V (single-phase supply) or 380 V (three-phase). For single-phase input, use 220 V input to 380 V output frequency converters.
- Rated speed (nn) — typically 1,450 or 2,900 rpm for a 50 Hz supply.
- cosφ and efficiency — needed for accurate calculation of the consumed current.
If the nameplate is damaged or illegible, measure the actual motor current under load using a clamp meter.
Step 2. Determine the Load Type
The type of mechanism driven by the motor directly determines the operating mode of the drive. This affects the required overload capacity, control method, and power margin. For a detailed discussion on load characteristics, see the article Accounting for Load Characteristics When Selecting a VFD.
Variable Torque (Fan/Pump Load)
The resistive torque increases proportionally to the square of the speed: M ∼ n². Power varies according to the cube law: P ∼ n³. This applies to pumps, fans, induced-draft fans, and blowers. Reducing speed by 20% cuts energy consumption nearly in half. For such applications, the VFD should be sized with a 10–15% power margin. See detailed examples for ventilation systems and water pumps.
Constant Torque (Conveyor Load)
The resistive torque is independent of speed: M = const. Power is linearly proportional to speed: P ∼ n. This applies to conveyors, extruders, mixers, and screw feeders. The power margin should be at least 15–20%. The VFD overload capacity must be 150% of In for 60 seconds.
High Starting Torque
Crushers, ball mills, and hoisting mechanisms require drives with an overload capacity of 180–200% for 3–10 seconds. Compressors also exhibit elevated starting torque — see VFD setup for compressors.
Step 3. Calculate the Required Drive Current
Current-based selection formula:
I_vfd ≥ I_motor × K_margin
Where K_margin depends on the load type:
- Fan/pump load: K = 1.1 (10% margin)
- Conveyor load: K = 1.15–1.2 (15–20% margin)
- Heavy-duty start: K = 1.2–1.3 (20–30% margin)
Example: a 7.5 kW motor, 380 V, rated current 15.2 A, driving a pump (variable torque). Minimum VFD current: 15.2 × 1.1 = 16.7 A. Select a drive with a rated current of at least 17 A, which corresponds to a 7.5 kW rating in the catalog.
Step 4. Select the Control Method
The control method determines the accuracy of speed and torque regulation. For more detail, read the top questions about VFDs.
Scalar Control (V/f)
Maintains a constant voltage-to-frequency ratio. Simple, reliable, requires no tuning. Suitable for pumps, fans, and multi-motor drives (several motors on one VFD). Speed regulation accuracy: ±2–5%.
Sensorless Vector Control
A mathematical motor model inside the VFD provides precise torque and speed regulation without feedback. Suitable for conveyors, extruders, and single-motor drives. Accuracy: ±0.5–1%. Delivers full torque at low speeds.
Closed-Loop Vector Control (with Encoder)
Maximum positioning and torque accuracy. Used in hoisting mechanisms, winding machines, and CNC machine tools. Accuracy: ±0.01–0.1%. Requires an encoder mounted on the motor shaft.
Step 5. Consider Supply Voltage and Ingress Protection
Supply Voltage
- Single-phase 220 V — for motors up to 2.2–4 kW. The VFD converts single-phase 220 V to three-phase 220 V. For 380 V motors, models with 380 V output are used.
- Three-phase 380 V — standard industrial supply. Most VFDs from 0.75 to 500 kW operate on three-phase power.
- Three-phase 660 V — for high-power motors in heavy industry (90 kW and above).
Ingress Protection (IP Rating)
- IP20 — panel-mount installation inside an electrical enclosure. The most common option.
- IP54/IP55 — for mounting near the driven equipment, in dusty or humid environments.
- IP65/IP66 — for outdoor installation or harsh environments.
Step 6. Select Additional Features
Depending on the application, consider the following options:
- Built-in PID controller — for maintaining pressure or temperature (pump and ventilation systems).
- Built-in braking module — for mechanisms with frequent stops or reversals.
- Braking resistor — dissipates braking energy as heat. Required for elevators, cranes, and centrifuges.
- Built-in EMC filter — reduces electromagnetic interference. Essential when operating near sensitive equipment.
- Communication protocols — Modbus RTU, Profinet, EtherCAT for SCADA/DCS integration.
- Built-in line reactor — protects against voltage surges and reduces current harmonics.
Comparison Table: VFD Selection by Application Type
| Criterion | Pumps & Fans | Conveyors & Mixers | Compressors | Cranes & Elevators |
|---|---|---|---|---|
| Load type | Variable torque | Constant torque | Constant torque + heavy start | High starting torque |
| Power margin | 10–15% | 15–20% | 20–25% | 25–30% |
| Overload capacity | 110–120% / 60 s | 150% / 60 s | 150–160% / 60 s | 180–200% / 3–10 s |
| Control method | Scalar (V/f) | Sensorless vector | Sensorless vector | Closed-loop vector |
| Braking module | Not required | As needed | As needed | Mandatory |
| PID controller | Yes | No | Yes | No |
| Energy savings | 30–60% | 10–20% | 15–30% | 10–15% |
When to Choose a Soft Starter Instead of a VFD
A soft starter costs less than a VFD and addresses one specific task — reducing inrush currents. A soft starter is appropriate when:
- The motor operates at a constant speed with no speed adjustment required.
- You need to reduce mechanical shock during startup (conveyor belts, pumps with long pipelines).
- The budget is limited and speed control is unnecessary.
However, if you need to vary the rotational speed by even 10–20% of the rated value, choose a variable frequency drive. A comparison of these devices is covered in the article on VFDs and soft starters.
Common Mistakes When Selecting a Variable Frequency Drive
- Selecting by power instead of current. A motor with a non-standard power factor or reduced efficiency draws more current than a standard motor of the same power rating. Always verify the rated current.
- Ignoring overloads. If the mechanism has a heavy start and the VFD is chosen without margin, the overload protection will trip and the drive will shut down.
- Underestimating operating conditions. Elevated ambient temperature (+40 degrees C and above), altitude above 1,000 m, and dusty environments all require an increased power margin or a higher IP rating.
- Omitting a line reactor or filter. Without an input reactor, the drive generates harmonics into the supply network, which can damage other equipment.
- Running multiple motors on one VFD in vector mode. Vector control works with a single motor only. For multi-motor applications, use scalar (V/f) mode.
Summary: VFD Selection Algorithm
- Record the rated current and voltage from the motor nameplate.
- Identify the load type: variable torque, constant torque, or heavy-duty start.
- Calculate the minimum VFD current including the appropriate margin (10–30%).
- Choose the control method based on the required accuracy.
- Determine the IP rating according to the installation conditions.
- Add the necessary options: PID controller, braking module, EMC filter.
- Verify the availability of the required communication interfaces.
Browse the complete catalog of variable frequency drives with filters by power rating and series in the frequency converter section.
