A V/F drive cannot regulate shaft torque directly because it controls the voltage-to-frequency relationship without resolving motor current into flux-producing and torque-producing components. It can limit total output current, but that is not the same as closing a torque loop. Open-loop vector control can estimate and limit torque when the drive exposes that function and the motor model remains observable; its accuracy normally degrades at very low speed, where closed-loop vector feedback provides the stronger solution.
Current, heat, and torque symptoms
The number that matters is the torque-producing current, not total motor current by itself. Total current includes magnetizing current needed to establish flux and load-producing current that creates torque. A V/F drive sees their combined electrical effect but does not independently command the two components.
This distinction explains the common symptoms: a current limit reduces acceleration or output frequency, yet shaft torque varies with speed, motor temperature, flux level, and slip. This is heat, not logic. High current raises copper loss approximately with the square of current, so holding a motor near its current limit can create substantial thermal load even when the shaft does not produce the expected torque.
| Quantity | What it indicates | Limit or decision | Where to read it |
|---|---|---|---|
| Output current | Total motor current, including magnetizing and torque-producing components | Compare with the motor and drive continuous-current ratings | Drive monitor and motor nameplate |
| Estimated torque | Model-derived shaft torque | Useful only when the selected control mode calculates it | Drive torque monitor, if provided |
| Speed error | Difference between commanded and actual speed | Persistent error indicates load, current limiting, or an inaccurate motor model | Drive speed monitor and independent speed measurement |
| Motor temperature | Accumulated thermal loading | Stop operation before the motor exceeds its permitted thermal condition | Motor sensor, thermal model, or measured winding/frame temperature |
| Operating time at limit | Duration of elevated current or torque demand | Compare with the overload curves for both motor and drive | Trend, event record, and manufacturer rating tables |
Control-mode interpretation
Read a V/F current-limit event as an electrical protection action, not proof that torque has reached a known value. Under scalar V/F control, the drive establishes a rotating stator field by commanding frequency and applying a corresponding voltage. Rotor slip develops the torque, but the drive has no directly controlled torque-current channel and no measured rotor-flux angle.
Open-loop vector control changes the decision. It uses measured terminal quantities and a motor model to estimate flux orientation and separate the commanded current into approximately orthogonal components. One component establishes flux; the other produces torque. Electromagnetic torque follows the relationship T_e ∝ ψ × i_q, where flux magnitude ψ and torque-producing current i_q must both be known well enough for the estimate to be useful.
A true vector-control implementation can therefore impose a torque-producing-current limit. Whether an operator can enter a torque limit depends on the drive interface and selected mode. Some products use the internal limit only for protection or current allocation and do not expose an adjustable torque command.
Torque-control mechanism
Closed-loop torque control compares a torque command with measured or estimated torque, converts the error into a torque-current demand, and regulates motor current through fast current controllers. The current controllers generate voltage commands for the inverter. A speed controller may sit outside this loop and generate the torque command, but torque control itself is not simply a frequency reference.
Closed-loop vector control adds rotor-position or speed feedback. Feedback fixes the relationship between the commanded stator field and the rotor, allowing the drive to maintain the intended flux angle during rapid load changes and at speeds where a sensorless estimate becomes weak. Open-loop vector operation instead derives this relationship from voltage, current, speed command, slip calculation, and motor parameters.
At low electrical frequency, stator-resistance voltage drop and inverter effects become large relative to the applied motor voltage. Small parameter errors then produce larger flux-angle and torque-estimation errors. A drive may still limit current, but the resulting shaft torque can differ materially from the displayed or commanded value.
Diagnostic decision path
- Confirm the selected control mode. Read the active mode from the drive status rather than relying only on the commissioned setting. Classify it as scalar V/F, open-loop vector, or vector control with feedback.
- Identify the controlled quantity. Determine whether the configured function limits total current, estimated torque, torque-producing current, speed-controller output, or acceleration demand. These limits can produce similar speed symptoms but act at different points in the control structure.
- Check motor data. Compare every entered motor value with the nameplate and commissioning record. Open-loop vector estimation depends on a valid motor model; incorrect data shifts the calculated flux, slip, and torque.
- Check feedback when fitted. Verify direction, scaling, signal quality, and the relationship between commanded and measured speed. Reversed or intermittent feedback prevents stable field orientation.
- Trend the event. Record speed command, actual speed, output frequency, output current, estimated torque, DC-bus condition, and motor temperature through acceleration and steady load. The trend separates a load-demand problem from a mode, model, or feedback problem.
- Compare the operating point with ratings. Use the drive and motor overload curves rather than treating a displayed percentage as an unlimited operating capability. Current magnitude and time jointly determine the thermal result.
Configuration procedure
- Select vector control when the application requires a defined torque limit. Choose feedback-based vector control when accurate low-speed or zero-speed torque is required and the drive supports the necessary feedback.
- Enter the exact motor nameplate data requested by the drive. Perform the manufacturer-defined motor identification or tuning routine when the product provides one, with the machine placed in the mechanical state required by that routine.
- Locate the drive function that limits torque or torque-producing current. If the interface offers only an output-current limit in V/F mode, treat it as motor and drive protection rather than calibrated shaft-torque control.
- Set acceleration, deceleration, speed-loop output limits, and process demand so they do not continuously force the torque controller against its limit. A saturated outer loop can make normal limiting look like unstable control.
- Apply load progressively while monitoring current, torque estimate, speed error, and temperature. Use an independent torque measurement when the application requires verified shaft torque rather than a model estimate.
- Validate operation at the lowest speed, highest load, and longest duty interval required by the machine. A successful test at rated speed does not validate sensorless torque accuracy near zero speed.
Verification and recurring pitfalls
A valid torque-limit test produces repeatable limiting at the commanded boundary without exceeding current or thermal ratings. During a controlled load increase, the torque indication or independent measurement should stop rising while speed or acceleration yields to the limit. Repeat the test after the motor reaches its normal operating temperature because resistance changes affect an open-loop motor model.
The most common mistake is equating current percentage with torque percentage. That shortcut ignores magnetizing current, flux weakening, parameter error, saturation, and motor temperature. Another recurring mistake is selecting open-loop vector control but skipping motor identification; the mode name alone does not create an accurate rotor-flux estimate.
Current clamping can also mask mechanical problems. Binding, excess inertia, an overloaded process, or incorrect gearing can drive the controller into its limit even when the electrical configuration is correct. Compare loaded and uncoupled operation where the machine permits a safe separation test.
Never raise the current limit to recover torque without checking the motor and drive overload curves. The additional current increases semiconductor and winding heating, while poor field orientation may still prevent the expected shaft torque.
FAQ
Can I control torque in V/F mode?
V/F mode can limit total output current but cannot directly regulate torque-producing current. Use vector control when the process requires a defined torque command or limit.
Does a V/F current limit equal a torque limit?
No. Total current contains both magnetizing and load-producing components, so the resulting torque changes with flux, slip, speed, motor temperature, and operating region.
Can I set a torque limit in open-loop vector mode?
Yes, when the drive calculates torque-producing current and exposes an adjustable limit in that mode. Confirm the active mode, motor-model setup, and available limit function in the drive interface.
Does open-loop vector control hold accurate torque at zero speed?
Its estimate weakens at very low electrical frequency because motor voltage drops and parameter errors dominate the model. Use feedback-based vector control when the application requires accurate low-speed or zero-speed torque.
Can I keep increasing the current limit when torque is low?
Stop when current, temperature, speed error, or operating time approaches the motor or drive rating, or when the torque estimate disagrees with an independent measurement. Escalate to the manufacturer's official support channel if motor identification fails, the selected mode lacks an accessible torque limit, feedback will not validate, or repeated tests show unstable torque with verified wiring and motor data.