Current limiting is a control function that prevents a Variable Frequency Drive (VFD) from commanding motor current beyond a configured or internally permitted level. Depending on the drive and motor-control mode, the controller may restrict torque, slow acceleration or modify output frequency to remain inside the allowed current envelope.

A useful way to think about it is: “The machine is demanding more torque, but producing that torque would require more current than the drive is allowed to supply. The controller therefore has to limit something.”

This makes current limiting an active control function, not merely a protection event after a fault has already occurred.

It is equally important to understand that current limiting, an overcurrent trip and short-circuit protection are not the same function.

Why does motor current increase?

Motor current generally increases as the machine demands more torque. High current is therefore common during loaded starting, aggressive acceleration, conveyor jams, sudden crusher loads, high extrusion pressure or other heavy mechanical conditions.

For an induction motor, total stator current should not be treated as if every ampere directly produces torque. Part of the current establishes magnetic flux, while another component contributes to torque production. Advanced vector-control algorithms can separate these components mathematically.

For that reason, 150% motor current does not universally equal exactly 150% motor torque. Motor design, control mode, speed, flux level and saturation all influence the relationship.

What does the VFD do when current limit becomes active?

The exact response depends on drive manufacturer, control mode and operating condition.

During acceleration: the drive may reduce the acceleration rate or temporarily hold the frequency ramp when motor current approaches the limit. When load demand falls, normal acceleration can resume.

At constant speed: some V/Hz drives may reduce output frequency to decrease motor current. In vector-control systems, a torque reference or torque-producing current component may instead be limited.

During a sudden load increase: the controller attempts to remain within its permitted current envelope. If the mechanical load continues to demand more torque than the system can produce, motor speed can fall and an overload or overcurrent fault may eventually occur.

Current limiting therefore does not promise to maintain perfect speed under every condition. Sometimes mechanical performance has to be temporarily sacrificed to remain inside the electrical limit.

Current Limit vs Overcurrent Trip

Current limiting is a normal control function. As current approaches the limit, the drive changes its control action in an attempt to remain operational.

An overcurrent trip is a protective shutdown that occurs when the drive detects an unsafe electrical condition beyond its permitted operating region.

For example, an overly aggressive acceleration ramp can cause current to rise and current limiting may intervene before a trip. A serious phase-to-phase short circuit on the motor cable is a completely different event; the drive should protect its power stage rather than simply reduce the acceleration rate and continue operating.

This is why current limiting should not be described as an electronic fuse.

Motor thermal protection is another layer

A motor can tolerate some temporary overload without being able to carry that current continuously. Winding temperature accumulates over time.

Electronic motor thermal models, I²t calculations and motor temperature sensors such as PTCs therefore perform a different job from current limiting.

Current limiting manages the immediate electrical operating boundary. Thermal protection monitors the cumulative heating effect of motor or drive loading.

Heavy Duty rating and current limit

Before selecting a current limit, establish the actual duty rating of the drive. A Heavy-Duty VFD may permit more temporary overload current than the same hardware under its Normal Duty rating.

A manufacturer might specify 150% for 60 seconds in Heavy Duty and 110% for 60 seconds in Normal Duty. Another product family can use different values.

The current-limit parameter should therefore never be set to 150%, 180% or 200% simply because those numbers are common in drive literature. The exact continuous-current and overload-current/time ratings of the installed drive are what matter.

What happens if current limit is set too low?

A limit that is too low can prevent the motor from developing the torque required by the machine.

The result may be:

• Failure to reach commanded speed.
• Excessively slow acceleration.
• A loaded conveyor that cannot break away.
• An extruder or mixer losing speed under load.
• Insufficient starting torque in a lifting application.

The objective is not to select the lowest possible limit. It is to find a value that protects the drive and motor while still allowing the machine to produce the torque it genuinely requires.

What if the limit is set too high?

Increasing a parameter beyond the practical capability of the drive does not create a more powerful power stage. Drives normally have additional internal software and hardware limits.

An unnecessarily high user current limit can expose the motor to additional heating and the machine to higher torque before another protective boundary is reached.

The setting should be coordinated with motor rated current, drive current rating and actual machine duty.

Acceleration time and current limiting

Accelerating an inertia to target speed more quickly requires more acceleration torque. More torque generally requires more motor current.

If current limiting is frequently active during startup, one of the first questions should therefore be: is the acceleration ramp unrealistically short?

Increasing acceleration time can reduce the torque and current required to reach operating speed.

If the machine genuinely requires high breakaway torque, however, extending the ramp may not solve the underlying problem. Drive sizing and motor-control strategy must also be checked.

Interactive current-limit demonstration

Does current limiting solve braking problems?

Not necessarily. High motor current during acceleration and regenerative energy during deceleration are different electrical problems.

When a high-inertia load is decelerated rapidly, mechanical energy can flow back into the DC link and raise DC-bus voltage. The relevant solutions may include a longer deceleration ramp, DC-bus overvoltage control, a braking resistor, brake chopper or regenerative drive.

In simple terms:

Acceleration + heavy load → motor current / current-limit problem.

Deceleration + high inertia → regenerative energy / DC-bus overvoltage problem.

Is current limiting an energy-saving function?

That is not its primary purpose. Current limiting is used to keep motor and drive operation within an allowable current envelope while maintaining operation where possible.

Energy savings in centrifugal fan and pump systems mainly come from reducing motor speed according to process demand. A correctly sized machine should not normally spend its life continuously against the current limit.

Frequent current-limit operation is usually a reason to investigate load, ramp time, motor data, drive sizing or mechanical condition.

Do not confuse it with a “current limiting circuit” fault

Some VFD manuals also use the phrase current limiting circuit for the precharge or inrush-current limiting circuit used when the DC-link capacitors are initially energized.

That circuit is different from motor-output current limiting.

If a fault code says “current limiting circuit fault,” do not automatically start changing the motor current-limit parameter. Read the fault definition for the exact drive model.

Editor's Note – what current limit tells me in the field

I do not treat current limit as a magic protection parameter. To me it is often a clue about what the machine is asking from the drive.

If the drive hits current limit on every start, I want to know why. Is the machine starting fully loaded? Is the acceleration time too short? Is something mechanically tight? Is the drive undersized? Are the motor parameters correct?

One common shortcut is to increase the current-limit value until the problem disappears. That can hide the symptom without explaining the load. Raising the limit simply tells the drive that it may use more current; it does not make the motor, IGBTs or gearbox stronger.

The opposite problem also exists. Set the limit too low and a perfectly healthy machine starts behaving weakly. A loaded conveyor cannot accelerate, the drive stays against its current ceiling and everyone starts blaming the motor.

During commissioning I prefer to watch motor current, ramp behaviour and the machine load at the same time. One parameter on its own rarely tells the whole story.

And a genuine short circuit is not a current-limit tuning problem. Once the motor or cable has an electrical fault, you are troubleshooting protection and insulation—not optimizing the process.

What should you read next?

If current limit is active during every start, continue with Acceleration / Deceleration Time. For demanding load profiles, read Heavy-Duty VFD. For more advanced torque production at low speed, continue to Sensorless Vector Control.

For regenerative energy during stopping, the natural next subject is Braking Resistors.

Related OtomasyonAVM sections

For actual drive models and current ratings, see AC Drives / VFDs. For braking, filtering and surrounding hardware, continue to VFD Accessories. For machine-level logic and load-management control, the PLC section provides the next layer of the automation system.

External technical references

For current manufacturer implementations, consult the Yaskawa GA800 Programming Manual, particularly its current-limit acceleration and stall-prevention functions, together with the ABB ACS880 firmware documentation showing the current limiter inside the motor-control limitation chain. Current-limit behaviour is product-specific, so the programming manual for the exact VFD always remains the primary source.