Open-loop and closed-loop control describe two fundamental ways in which a Variable Frequency Drive (VFD) can control a motor. In a simple open-loop system, actual shaft speed or position is not measured directly. In a closed-loop system, an encoder or another feedback device reports the motor's real mechanical motion to the drive.

That does not mean open loop is “uncontrolled” and closed loop is automatically perfect. Modern sensorless drives can estimate motor behaviour extremely well from current, voltage and a mathematical motor model. Open Loop Vector Control is a good example: it uses no shaft encoder, yet it can deliver much better low-speed and dynamic performance than basic V/Hz control.

What is open-loop motor control?

In open-loop motor control, the drive does not directly measure actual shaft speed or shaft position. It applies the required voltage, frequency and current according to its control algorithm and estimates how the motor should respond.

The simplest example is V/Hz control. The drive produces an output voltage appropriate for the commanded frequency. As induction-motor load increases, slip can increase, so actual rotor speed may move further away from synchronous speed.

But not every open-loop system is simple scalar V/Hz. Sensorless Vector Control uses measured electrical quantities and a motor model to estimate flux and torque-producing current components. This can provide substantially better low-speed torque and speed regulation without a mechanical encoder.

So the statement “without an encoder the drive knows nothing about the motor” is also too simplistic. The drive may not directly measure shaft motion, but it continuously measures its electrical output and can infer a great deal about motor behaviour.

What is closed-loop motor control?

In closed-loop motor control, actual motor speed or rotor position is returned to the drive through a feedback device. In industrial AC-drive systems this is commonly an encoder.

The drive compares the speed reference with measured shaft speed. If an error develops, the control algorithm adjusts motor torque-producing current and other internal variables to reduce that error.

Some drives include an encoder interface as standard, while others require a separate PG or encoder feedback option card.

One important technical distinction is that the encoder generally measures speed and position, not motor torque directly. Vector drives normally calculate or regulate torque from motor current components and the motor model.

Closed-loop vector control becomes particularly valuable where the application requires very low-speed operation, tight speed regulation, rapid disturbance rejection, controlled operation close to zero speed, synchronization or position-related functions.

The real difference

An open-loop drive operates from command + motor model + electrical measurements.

A closed-loop drive adds direct measurement of the motor's actual mechanical motion.

A practical way of saying it is:

Open loop: “I know what I am applying to the motor and I estimate how the motor is responding.”

Closed loop: “I also measure what the motor is actually doing.”

Is Open Loop Vector the same as V/Hz?

No. Both may operate without an encoder, but the control algorithms are very different. V/Hz control primarily manages the relationship between output voltage and frequency. Open Loop Vector Control uses a mathematical motor model and measured current to control flux and torque-producing components more accurately.

A modern open-loop vector drive can therefore provide much stronger low-speed torque and tighter regulation than a basic scalar V/Hz drive.

The exact performance difference between open- and closed-loop operation depends on the drive family, motor, auto-tuning quality and application. Manufacturer-specific performance figures should never be treated as universal values for all VFDs.

What does the encoder actually add?

An encoder reports actual motor speed and, depending on encoder technology and control method, rotor angular position. The drive no longer has to rely entirely on an estimated mechanical response.

When load changes suddenly, the drive can directly detect the resulting speed error and adjust torque-producing current to correct it. This is valuable in hoists, elevators, winders, extruders, test stands, spindles and precision machinery.

Encoder voltage, signal format, resolution and maximum pulse frequency must be compatible with the drive. Some drives accept encoder feedback directly, while others require an additional PG or feedback card.

Is closed-loop control always necessary?

No. Adding an encoder to a normal fan or centrifugal pump often adds cost and maintenance without adding useful process performance. Many conveyors and general machinery applications are also handled very effectively by modern sensorless vector drives.

Closed-loop control becomes more attractive when very low-speed torque, extremely small speed error, controlled operation close to zero speed, precise synchronization or actual shaft position is important.

In lifting applications, encoder feedback does not replace a mechanical holding brake. Producing motor torque at zero speed and safely holding a suspended load are separate engineering functions.

Motor feedback loop and process feedback loop are not the same thing

The words open loop and closed loop are also used outside the motor-speed loop. This causes a lot of confusion.

A pump VFD may operate the motor using open-loop V/Hz or sensorless vector control with no shaft encoder. At the same time, a pressure transmitter may send a 4–20 mA signal to the drive's PID controller so the drive automatically maintains constant system pressure.

In that example:

Motor-speed loop: open loop.

Pressure process loop: closed loop.

The correct question is therefore not simply “Does the system have feedback?” but “Which variable is being fed back?”

Why does auto-tuning matter?

Vector-control methods depend heavily on the motor model. Incorrect rated current, voltage, frequency, speed, resistance or inductance values reduce the quality of that model.

Many drives therefore provide an auto-tuning routine. Static and rotational auto-tuning methods identify different motor parameters depending on drive design.

Adding an encoder does not magically correct poor motor data. Closed-loop systems still require correct motor and encoder configuration.

Which one should be used?

Fans and pumps: V/Hz or sensorless vector is normally sufficient. Shaft feedback is rarely needed.

General conveyors: Sensorless vector is often a strong practical solution. Encoder feedback can be added where very accurate synchronization is required.

Extruders and winders: Open-loop vector or closed-loop vector may be appropriate depending on torque, line-speed and tension requirements.

Hoists and cranes: Low-speed torque, brake sequencing, zero-speed behaviour and the safety architecture must be evaluated together. Encoder feedback can be highly valuable in demanding applications.

CNC spindles: Sensorless operation may be sufficient for general speed control, while spindle orientation, precise synchronization or high low-speed accuracy can make encoder feedback necessary.

Positioning systems: Where actual shaft or load position must be known, feedback becomes a core part of the system. Advanced systems may use a second load-side encoder or linear scale for full closed-loop control.

Editor's Note – how I separate them in the field

The first thing I ask is: what is the machine doing at low speed? If a fan loses a few rpm at 5 Hz, nobody may care. If a crane has to creep into position or a winder starts losing tension, the same speed error suddenly matters.

The second question is whether the drive really needs to know the actual shaft speed. On a lot of machinery the answer is no. Modern sensorless vector drives are far more capable than old basic drives, and fitting an encoder simply because it is “better technology” adds cable, coupling, feedback hardware, parameters and another possible failure point.

But avoiding encoder feedback on a machine that genuinely needs it is also false economy. When very low-speed operation, tight synchronization or near-zero-speed torque matters, knowing what the motor is actually doing makes the control problem much easier.

One misconception I still see is that installing an encoder somehow turns every VFD system into a servo. It does not. Drive algorithm, motor, encoder resolution, mechanical backlash, loop tuning and the load itself all determine final performance.

What should you read next?

For the simplest open-loop approach, continue with What Is V/Hz Control?. For the higher-performance sensorless approach, continue with Vector Control. The existing detailed page on feedback operation is Closed-Loop Control.

For the physical feedback device itself, continue to Encoders. For higher-performance positioning systems, the Servo Systems section is the natural next step.

Related OtomasyonAVM sections

For real drive products and control modes, see AC Drives / VFDs. For feedback hardware, use the Sensors section. For higher-precision motion applications, continue into Servo Systems, and for machine-level command logic see PLC and Modules.

Technical note: Current Yaskawa GA800 documentation explicitly separates Open Loop Vector Control, which does not require a motor encoder, from Closed Loop Vector Control, which uses encoder feedback for higher-performance speed and torque control. Exact control range, accuracy and starting-torque specifications are product-specific and should always be checked in the manual for the drive being used.