What is Torque Boost in a VFD?

Torque boost helps an induction motor produce enough turning force when it starts or runs slowly. In a drive using V/f control, this commonly means adding a little voltage at low frequency to compensate for voltage lost in the motor windings. Think of a loaded conveyor that will not quite begin to move: the setting can help the motor overcome its initial resistance, provided the motor and drive are sized for the load.

The name can sound as though the drive simply orders the motor to produce more torque for a few seconds. Actual settings vary by model. Some drives apply compensation throughout a selected low-speed region; others offer a separate starting or acceleration boost. The Omron inverter guide describes manual and automatic methods. Torque boost does not increase the motor’s rated power or override the drive’s current limit.

On this page: why voltage helps · original animation · boost settings · motor torque questions · practical setup · more OtomasyonAVM guides.

Why does a motor need voltage compensation at low frequency?

In a basic V/f curve, the drive reduces motor voltage as it lowers frequency. At very low frequency, the stator winding resistance consumes a noticeable share of the available voltage. Less voltage remains to establish magnetic flux, so the motor may struggle to start a load. The drive’s boost or IR compensation restores part of that missing voltage where it is needed.

Here is a simplified numerical picture. A straight 400 V at 50 Hz V/f line would suggest about 40 V at 5 Hz. If the chosen motor and drive require 5 V of compensation at that point, the illustrative output becomes 45 V at the same 5 Hz. These numbers describe the approximate line-to-line RMS fundamental voltage for teaching; they are not suggested parameter settings. The point is that a voltage correction does not require a frequency increase.

The extra voltage needs restraint. Too little may leave the motor struggling to start; too much can increase magnetizing current, heating or rough operation. The proper amount depends on the motor, load and selected control mode. Danfoss drive documentation describes V/f behavior, boost settings and the limits of motor cooling.

Can a VFD increase motor torque?

A VFD can help an appropriately rated motor deliver the torque required by the load, especially where an uncompensated V/f setting gives weak starting performance. It does so within the motor current, heating and drive overload limits. This is different from torque control, where the controller works toward a torque reference, and from a constant-torque load, which describes what the machine needs over its operating speed range.

It is misleading to say that raising voltage and frequency together always raises torque. Frequency primarily changes rotating-field speed. In the usual low-frequency boost example, voltage is adjusted relative to that frequency. Above the motor’s base frequency, the available voltage has a ceiling and the motor generally enters a field-weakening region where available torque can fall; base frequency and rated motor frequency explain these boundaries.

Manual boost, automatic boost and starting boost

Manual boost: a technician sets a voltage correction or low-frequency characteristic according to the drive manual, then checks starting and running current. Automatic boost: a capable drive adapts compensation to operating conditions; it still needs correct motor information and, where required, motor identification. Starting or acceleration boost: some models offer a separate function active during startup or the acceleration ramp.

These are different possible implementations, not three mandatory settings on every VFD. Siemens SINAMICS documentation distinguishes continuous, acceleration and starting boost. ABB drive documentation describes an IR compensation frequency range. A so-called boost threshold is therefore model specific; check its name, units and supported control mode in the actual drive manual.

See the original torque boost animation

Press the button to follow the red dot. Its rising shape is a schematic way to show added starting support, not a measured torque curve. The return to the baseline represents one possible starting-only boost; a drive set for continuous low-frequency compensation would behave differently. The baseline does not mean the motor has zero torque.

Reading the animation: You can change the duration of a drawing without changing the underlying motor physics. Check the drive manual to learn whether its own torque boost follows a frequency range, motor current or a starting interval.

Does using a VFD reduce torque?

A properly selected VFD can supply a motor’s required torque across a useful speed range below rated frequency. Reducing frequency does not, by itself, guarantee a loss of torque. Very low speed brings different practical limits: winding voltage drop may need compensation, and a motor cooled by a fan mounted on its own shaft receives less airflow. Long periods at low speed and high load may call for a larger motor or independent cooling.

Torque can also be limited because the VFD has reached its current limit, the motor or drive is undersized, the motor data are wrong, or the V/f setting does not match the application. A heavy-duty drive may be needed for demanding starts. If a machine meets a large load step at normal speed, first inspect the mechanism and drive rating; turning up the low-speed boost may not address that problem. See the separate motor stall fault guide for a related symptom.

How should torque boost be set?

Start with the motor: verify its nameplate voltage, rated current, frequency and winding connection. Select the drive’s supported V/f or vector control mode for the machine. Then try a loaded start and watch the current, startup behavior and motor temperature. Increase the boost only as far as needed for reliable starting, following the model’s manual; Schneider Electric’s setup note highlights the effect of incorrect low-speed settings on a hot motor.

Check the entire duty cycle: a motor that starts successfully once may still overheat after hours at low speed. Confirm the overload rating, ventilation and current limit before treating a boost adjustment as a cure. The basic VFD parameters guide explains other setup values that affect the result.

Quick answers

Is torque boost the same as torque control? No. Boost often corrects low-frequency voltage in a V/f drive; speed control versus torque control covers the wider distinction.

Is it active only for a few seconds? Sometimes a drive has a start-only function. On other drives, low-frequency boost remains active across a chosen frequency range; Omron’s inverter terminology gives further context.

Can I increase boost until the motor starts? Use the manufacturer’s limits and monitor motor current and temperature. Excess boost can worsen heating, while a stalled or overloaded machine may need mechanical attention or a differently sized drive.

Explore the OtomasyonAVM drive library

Voltage and low-speed behavior: V/f control · V/f curve · IR compensation · motor rated frequency · base frequency.

Motor torque and drive control: torque control · constant-torque applications · speed versus torque control · vector control · field-oriented control · flux vector control · slip compensation.

Setup and applications: motor identification · current limiting · acceleration and deceleration · heavy-duty drives · winding and unwinding · motor stall fault.

Browse further: AC drives · professional drives · crane drives · essential drive settings · complete VFD technical library.

Manufacturer references: Omron · Danfoss · ABB · Siemens · Schneider Electric. Parameter names and limits vary by model and firmware.