VFD control methods: V/f, vector, DTC, torque and current
How a frequency converter controls the motor: V/f characteristics, sensorless and closed-loop vector control, direct torque control, torque and current limits — and which method to choose.
V/f (scalar) control
The drive keeps the ratio voltage/frequency constant so that the motor flux stays roughly constant: 400 V at 50 Hz → 200 V at 25 Hz. Options:
- Linear characteristic for constant-torque loads (conveyors).
- Quadratic characteristic for pumps and fans (less voltage at low speed → energy saving).
- Voltage boost at low frequency to compensate stator resistance.
- Slip compensation to hold speed under load.
Advantages: simple, robust, works with several motors on one drive. Weak torque and speed accuracy at low speed.
Vector control
The drive uses a motor model (from nameplate data and the identification run) to control the flux-producing and torque-producing components of the current separately — like a DC motor.
| Variant | Feedback | Typical performance |
|---|---|---|
| Sensorless vector | None (model) | Good torque down to a few Hz, speed accuracy ≈ slip-compensated |
| Closed-loop vector | Encoder | Full torque at zero speed, high speed accuracy, positioning |
ABB's DTC (Direct Torque Control) reaches similar results by controlling torque and flux directly without a modulator.
Torque and current
- Current limit protects drive and motor; acceleration that hits the limit becomes longer.
- Torque limits (motoring/generating) protect mechanics and are used for tension or pressing applications.
- In torque control mode the drive follows a torque reference and speed results from the load — needs speed limits for safety (winders, master/follower).
Choosing
| Application | Method |
|---|---|
| Fans, pumps | V/f quadratic or sensorless vector with energy optimization |
| Conveyors, mixers | Sensorless vector |
| Hoists, cranes, winders, positioning | Closed-loop vector with encoder |
| Several motors on one drive | V/f |
Identification (motor data measurement) is essential for vector control — repeat it after replacing the motor or changing the cable significantly.
Connections
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Sources
Updated 2026-10-02