What is torque boosting?

Torque boosting is a general term for methods that increase the usable turning moment in an application or improve torque capability at low speed. However, a drive's torque boost parameter, a motor's short-term overload capability and gearbox torque multiplication are different mechanisms. First establish whether the aim is to increase motor-shaft torque or load-shaft torque.

Three different approaches

How does torque boost work in V/f control?

In scalar control of an induction motor, output voltage generally changes with frequency according to the V/f curve. At low frequency, the voltage is also low, so the voltage drop across stator resistance can become more significant. This may weaken magnetic flux and the motor's low-speed torque capability.

Torque boost or IR compensation helps counter this effect by providing additional voltage support in the appropriate operating region. The aim is to maintain the required motor flux conditions, rather than simply raise voltage without a defined purpose. The function need not be a power burst lasting only a few seconds; its operation depends on the drive and control mode.

Manual versus automatic boost

Manual boost adjusts low-frequency voltage support through a manufacturer-defined parameter. On suitable drives, automatic boost uses motor data and information such as load current to adapt that support to operating conditions. Correct motor data are important. Loaded starts requiring starting torque and low-speed applications requiring constant torque illustrate where this distinction matters.

A 5% boost setting does not mean 5% more torque. Check the manual for the voltage or reference used to define the percentage, its frequency range and its interaction with automatic control. A percentage setting on one drive brand cannot simply be transferred to another.

FOC and DTC differ from simple voltage support

Field-oriented control (FOC) manages motor current through components associated with flux and torque. It can also be used with suitable BLDC and PMSM/permanent magnet motor systems. Torque demand is met by controlling current appropriately for the motor type and operating region. Torque control therefore involves more than increasing a voltage percentage.

Direct torque control (DTC) estimates and controls motor flux and torque using a model and electrical data. “Direct” does not necessarily mean there is a torque sensor on the shaft. FOC and DTC can make better use of available motor capability, but they do not remove the physical limits of the motor and drive.

Current, duration and cooling limits

A higher motor torque demand usually requires more torque-producing current. Check the drive's current limit, the motor's continuous and peak torque ratings, permitted duration and repetition rate. Peak torque is not a continuous-duty rating. Higher current also increases winding losses: with resistance held constant, I²R loss varies with the square of current.

A motor cooled by a shaft-mounted fan receives less cooling at low speed. Sustained high torque at low speed therefore requires a separate thermal check. The torque curve can also change at high speed. In a voltage-limited field-weakening region, adjusting low-frequency boost may not solve the problem. Assess the torque-speed balance across the entire operating range.

Gearbox torque multiplication: a worked example

For a gearbox, approximate relationships are T_output = T_motor × i × η and n_output = n_motor / i, where i is the speed reduction ratio and η is transmission efficiency. This method delivers more torque to the load rather than increasing the motor's own torque. Selection requires load and transmission calculations.

If a motor produces 12 N·m at 1,500 rpm, the gearbox ratio is 8:1 and efficiency is 90%, output torque is approximately 86.4 N·m and output speed is 187.5 rpm. Torque increases as speed falls; losses make output power lower than input power. The difference between torque and power and the torque-speed relationship explain this result.

What should be checked during adjustment?

If starting performance is inadequate, first check motor nameplate data, winding connection, control mode, current limit, acceleration ramp and mechanical binding. Assess the boost setting using the drive manual. Overcurrent, excessive heating or unstable operation may indicate too much boost or another underlying problem.

Sudden torque increases can impose shock loads on couplings, shafts and gears. Consider the effects on the mechanical system as well. A drive's torque display may not be a direct measurement, so distinguish measured torque from estimated torque when assessing the result.

Is automotive “boost” the same thing?

In an internal combustion engine, turbocharging and supercharging aim to increase the mass of air entering the cylinders; an intercooler cools the compressed air. With suitable fuelling and engine control, this can increase torque. It is a different mechanism from low-frequency voltage support in an electric drive. More air and fuel do not necessarily mean a richer air-fuel ratio.