While speed control regulates how fast a motor rotates (RPM), torque control dictates how much rotational force the motor exerts regardless of speed. In demanding industrial applications—such as web tensioning, extruders, cranes, and heavy crushers—controlling torque with high precision is vital to prevent mechanical failure, product damage, and motor stalls.
Variable Frequency Drives (VFDs) manage motor torque by regulating the magnitude and phase angle of the stator current supplied to an AC induction motor.
This technical guide covers the key torque control algorithms inside modern VFDs, how vector control separates flux from torque, and how to select the right control mode for your application.
1. Speed Control vs. Torque Control in VFDs
In standard Speed Control Mode, the drive adjusts motor frequency to maintain a set RPM. If a heavy load suddenly hits the motor shaft, the VFD increases torque automatically to keep speed constant.
In dedicated Torque Control Mode, the VFD ignores speed feedback (within safe limit thresholds) and continuously adjusts output frequency to maintain a precise torque setpoint:
Winding & Unwinding (Paper/Textiles/Foil): As a roll diameter expands or shrinks, the drive maintains uniform material tension without tearing or slacking.
Hoists & Winches: Holds full load torque at zero speed before releasing mechanical brakes to prevent load dropping.
Extruders & Mixers: Prevents drive shaft shearing by capping maximum allowable torque output.
2. VFD Torque Control Modes Compared
Modern frequency converters utilize three primary control architectures to govern torque output:
| Control Method | Operating Principle | Torque Response Time | Typical Applications |
| Scalar Control (V/f) | Maintains a linear voltage-to-frequency ratio. Torque is controlled indirectly. | Slow (100 ms+) | Centrifugal pumps, simple fans, conveyors with light loads. |
| Sensorless Vector Control (Open-Loop SLV) | Uses internal mathematical motor models to separate magnetizing current from torque current. | Fast (10 – 20 ms) | Machine tools, mixers, positive displacement pumps, material handling. |
| Closed-Loop Vector Control (Field Oriented Control) | Uses quadrature encoder feedback to monitor exact shaft position and speed in real time. | Ultra-Fast (< 5 ms) | Overhead cranes, hoists, paper machines, winders requiring torque at 0 RPM. |
| Direct Torque Control (DTC) | Calculates motor flux and torque directly without decoupling PWM modulators. | Instantaneous (< 2 ms) | Heavy mining crushers, locomotives, test benches. |
3. How Vector Control Separates Flux and Torque
In a standard DC motor, controlling torque is simple because the field current (flux) and armature current (torque) are physically decoupled at 90 degrees.
In a 3-phase AC induction motor, both currents flow together in the stator winding. Vector control (Field Oriented Control) uses complex mathematical transformations (Clarke and Park transforms) to split stator current into two decoupled vectors:
Magnetizing Current Vector ($I_d$): Establishes the magnetic field in the motor core.
Torque-Producing Current Vector ($I_q$): Controls the actual mechanical torque generated at the shaft.
By dynamically adjusting $I_q$ independently of $I_d$, a vector-equipped VFD maintains 100% rated motor torque down to zero speed.
4. Key Engineering Considerations for VFD Torque Setup
To achieve stable torque control in industrial environments:
Perform an Auto-Tuning Run: For Sensorless Vector Control to function, the VFD must measure the motor’s exact stator resistance, leakage inductance, and magnetizing current. Always execute a “Static” or “Rotational” Auto-Tune during commissioning.
Thermal Management at Low RPM: When a motor produces high torque at low speeds, the shaft-mounted fan spins too slowly to cool the windings. Use an inverter-duty motor with an external forced-cooling fan (blower kit) to avoid thermal overload.
Braking Resistors & Dynamic Braking: Overhauling loads (e.g., lowering a hoist) force mechanical energy back into the VFD. Ensure a properly sized braking chopper and external resistor are installed to dissipate regenerated power.
We invite you to explore our online VFD shop, where you can discover a wide selection of high-quality frequency converters designed to meet your specific industrial needs. From precise torque control to energy-efficient operations, our VFDs empower you to optimize your motor systems and unlock new possibilities for success. Visit our shop today and take the first step towards transforming your industrial processes.
