What is constant torque in a VFD system?

In a variable frequency drive (VFD) system, constant torque describes a load that needs roughly the same shaft torque as speed changes. For the motor and drive, it describes the ability to deliver that torque over a defined speed range. The torque demanded by the machine and the torque available from the motor are separate considerations. A constant torque load does not mean the motor can provide the same torque at every speed for an unlimited time.

A conveyor carrying the same amount of material illustrates the difference: halving belt speed does not necessarily halve the required pulling force. The torque demand can remain similar while mechanical power falls with speed. Constant torque does not mean constant power or constant current.

How torque, speed and power are related

For a rotating shaft, P = T × ω, where P is mechanical power, T is shaft torque and ω is angular speed. With practical engineering units, P (kW) = T (Nm) × n (rpm) / 9550. Rearranging gives T (Nm) = 9550 × P (kW) / n (rpm). Use the motor's mechanical output power in this calculation. Electrical input power also includes losses.

Worked example: A motor rated at 7.5 kW and 1450 rpm has a rated shaft torque of approximately 49.4 Nm. If it delivers the same torque at 725 rpm, its mechanical output is approximately 3.75 kW. Its rated power has not changed; its actual shaft output at that operating point has. This calculation alone does not establish that the motor can sustain that load thermally at the lower speed.

How a VFD provides a constant torque region

For an induction motor, V/F control raises voltage with frequency to maintain usable motor flux. With correct motor data and a suitable V/F curve, the system can provide approximately constant torque capability within its voltage and current limits. A simple voltage-to-frequency ratio cannot perfectly compensate for every loss or load change, particularly at very low speed.

At low frequency, the voltage drop across stator resistance becomes significant. Torque boost can compensate for part of that drop, but an excessive setting can increase current and heating. For more demanding applications, vector control regulates the flux-producing and torque-producing current components using a motor model. Actual performance depends on the drive's control system and the operating conditions.

Base frequency and field weakening

Base frequency must be considered together with the motor voltage and winding connection. Once the available drive output voltage reaches its limit, increasing frequency without a proportional voltage increase leads to field weakening. This boundary is not automatically 50 Hz in every installation. Motor nameplate data, star/delta connection and the drive's voltage capability determine the usable range.

Over part of the permitted field weakening range, approximately constant power operation may be possible. Available torque then falls approximately in inverse proportion to speed: T ≈ P / ω. If the example system maintains 7.5 kW at 2900 rpm, available shaft torque is approximately 24.7 Nm, compared with 49.4 Nm at 1450 rpm. This is a calculation example, not an approval for operation at 2900 rpm.

Check the motor's maximum mechanical speed, bearings, rotor balance, driven-machine speed limit and breakdown torque margin separately. The field weakening region is not an unlimited constant power guarantee. The illustration below is an idealised operating envelope; it does not plot transient maximum torque or motor current.

Constant torque, torque control and CT/HD ratings

A constant torque load describes the machine's mechanical demand. Torque control describes how the drive regulates developed torque against a reference. Constant torque capability describes the operating envelope available from the motor and drive under stated conditions. The terms are related, but they are not interchangeable.

On a conveyor running in speed control, the drive attempts to maintain speed and develops the torque required by the load. In torque control, the torque reference takes priority, so suitable speed limits are needed to prevent excessive speed if the load decreases. The difference between speed control and torque control matters particularly in winding, unwinding and traction applications.

CT, Heavy Duty (HD) and High Overload (HO) usually identify a load category and an associated current/overload rating. Terminology and duty cycles differ between manufacturers. Read the actual continuous output current, short-term overload current, permitted duration and recovery interval for the selected heavy duty drive rating. There is no single overload percentage that applies to every CT drive.

Which machines have approximately constant torque loads?

Conveyors, screw feeders, some extruders and mixers can have an approximately constant torque characteristic. Actual demand also depends on viscosity, fill level, material compaction, temperature and mechanical friction. The machine category provides an initial guide; the measured load profile and loaded starting behaviour determine the final selection.

For a positive displacement pump, torque demand can remain broadly constant when differential pressure remains constant. Screw compressors require a separate assessment of pressure conditions and loading/unloading control. Centrifugal fans and pumps can follow approximately T ∝ n² and P ∝ n³ under suitable similarity conditions; static head and the system curve affect the actual result. The word “pump” or “compressor” alone is insufficient to select CT or VT duty.

A crane drive application also requires consideration of brake opening/closing sequence, low-speed stability and an overhauling load that drives the motor. Load holding must not rely solely on an electrical torque command; an appropriate mechanical brake and application protection arrangement are required.

Why continuous torque can be limited at low speed

Two separate limits apply: electrical torque production and thermal loadability. A standard self-cooled motor receives less airflow from its shaft-mounted fan as it slows down. Even when shaft power is lower, the current needed for high torque and the resulting copper losses can remain substantial. Low speed therefore does not automatically mean a cooler motor.

For extended high-torque operation at frequencies such as 10–15 Hz, assess the manufacturer's torque-speed loadability curve, ambient temperature and operating time. Possible measures include an independently powered cooling fan, a motor designed for low-speed duty or a suitable gearbox ratio. There is no universal safe minimum frequency for all motors. Danfoss's low-speed derating guidance likewise considers cooling, load and operating duration together.

The near-zero-speed capability of sensorless vector control (SVC) varies between drives. Continuous torque at zero speed requires explicit support from the chosen motor/drive combination and adequate thermal capacity. Encoder feedback in closed-loop control improves speed information; it does not independently solve motor cooling. At zero shaft speed, mechanical output power is zero, but electrical losses and heating can continue.

Does motor current remain constant?

It can remain broadly similar, but it is not guaranteed to be constant. In the constant flux region, similar load torque generally needs a similar torque-producing current component. Total motor current also depends on magnetisation, motor modelling, slip, saturation and the operating point. ABB's motor current explanation likewise distinguishes total current from its torque-producing component.

Motor output current and supply input current are not the same quantity. At low speed, motor current may remain high to provide the required torque while shaft power and active power drawn from the supply fall. Motor-side amperes alone cannot determine supply power without accounting for efficiency, losses and power factor.

Starting and acceleration torque

Keeping a machine moving differs from starting it at rest. Breakaway torque may exceed running torque because of static friction, compaction or material loading. During acceleration, the motor must also accelerate the system inertia. With all quantities referred to the same shaft, T_motor = T_load + J_eq × dω/dt, where load torque includes the relevant friction and mechanical losses.

Excessively short acceleration and deceleration times can push an otherwise adequate system into its current limit. A longer acceleration ramp reduces acceleration torque, but it does not remove the machine's continuous torque demand or breakaway requirement. For repeated starts, assess overload duration together with how often the overload occurs.

Drive selection and commissioning

First, verify the nameplate data and winding connection. Enter rated voltage, frequency, current, power and speed correctly, and confirm that the star/delta connection matches the drive output. If motor identification is required, follow the manufacturer's auto-tuning procedure. For rotating identification, establish whether movement of the connected machine is permissible.

Next, verify usable current capacity. The drive's continuous output current under the selected HD/CT rating must meet the motor and load requirements. Apply manufacturer-specified derating for ambient temperature, altitude, enclosure cooling and carrier frequency. Moving up one kW size is not a substitute for checking the selected rating's actual current and duty cycle.

Then, test with the actual load. Observe loaded starts, minimum continuous speed, normal production and repeated acceleration. Monitor motor current, temperature feedback, speed stability and drive warnings together. Persistent overcurrent or inadequate torque calls for an assessment of motor data, mechanical restriction, control method and load profile. Interpret VFD fault codes in the context of those conditions rather than simply raising limits.

Finally, assess stopping and energy flow. An overhauling load or rapid deceleration of high inertia can return regenerative energy and raise DC-link voltage. A brake resistor with a suitable brake chopper, or an appropriate regenerative solution, must be assessed against the actual energy and duty cycle. The ability to produce torque does not establish the ability to dispose of braking energy.

Frequently asked questions

Can any VFD operate a constant torque machine? It may be suitable if its control method, continuous current, starting capability and duty cycle meet the application. A matching catalogue kW rating alone does not establish suitability.

Will more torque boost solve a low-speed problem? It can address some effects of low-frequency voltage drop. It cannot correct wrong motor data, overload, mechanical jamming or inadequate cooling, and an excessive setting can increase heating.

Does halving speed halve energy consumption? At the same torque, shaft power approximately halves. Electrical consumption also depends on motor/drive losses, efficiency and operating time, so the same reduction is not guaranteed. Include the time required to complete the same amount of production.

Can a 50 Hz motor provide the same torque at 100 Hz? This cannot be assumed. If the voltage limit has been reached, flux and torque capability decrease. The permitted motor and driven-machine speeds must also be verified separately.

Technical references

ABB — Technical guide No. 7: Dimensioning of a drive system (motor current, constant flux/field weakening and load types); Danfoss — VLT AQUA Drive FC 202 Design Guide (section 5.1.2, low-speed derating and motor cooling). Numerical examples are calculated illustrations. Use the specific motor and drive documentation for actual selection.