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Open-loop vs Closed-loop Control of Frequency Inverters
Source: | Author:Admin | Published time: 2026-09-05 | 10 Views | 🔊 Click to read aloud ❚❚ ▶ | Share:

An intuitive way to understand open-loop and closed-loop control for frequency inverters: Open-loop control means executing commands without follow-up checks, while closed-loop control means monitoring real-time results and making corrections on-the-fly.

These two modes differ fundamentally in structure, performance and application scenarios. The table below summarizes their core differences for a quick overview:

Comparison Item

Open-loop Control

Closed-loop Control

Core Feature

No feedback path; output does not affect input

Equipped with feedback path; output is compared against setpoint for control correction

Typical Implementation

V/f control, Sensorless Vector Control (SVC)

PG-encoder-based Vector Control (VC)

Control Accuracy

Low (speed accuracy approx. ±0.5%~3%)

Extremely high (speed accuracy up to ±0.02%)

Speed Adjustment Range

Narrow (e.g. 1:40 to 1:100)

Very wide (up to 1:1000)

Low-speed Performance

Low starting torque; performance degrades easily

Delivers 200% rated torque even at 0 Hz; excellent performance

System Structure

Simple, low cost

Complex; requires feedback devices such as encoders; higher cost

Typical Applications

Fans, water pumps and general-purpose machinery with low accuracy requirements

Lifting equipment, elevators, rolling mills, high-precision machine tools, etc.

Detailed Explanation of Key Differences

Understanding the indicators in the table above helps clarify their distinctions:

Dramatic gap in control accuracy 

Under open-loop control, for instance, you set the inverter output to 40 Hz, yet actual motor speed fluctuates with load variations and cannot be precisely known. Closed-loop control monitors real-time speed and compares it with target values. Deviations trigger automatic adjustments to guarantee accurate performance. According to manufacturer data, closed-loop vector control achieves speed accuracy of ±0.02%, versus roughly ±(2-3)% for open-loop V/f control.

Low-speed performance as a core advantage 

This represents the major strength of closed-loop control. At low motor speeds, open-loop V/f control suffers insufficient output torque caused by stator resistance voltage drop. With an encoder, the inverter acquires precise rotor position and speed information. Consequently, it delivers high torque at extremely low speeds, even at 0 Hz. This capability is critical for anti-sway protection in cranes and smooth elevator start-up.

How to Make the Right Selection？

With the above differences in mind, the selection logic becomes straightforward. Ask yourself two questions:

Does the process require high speed or pressure accuracy?

1)Low requirement: For example, fans and water pumps that only need coarse speed regulation based on temperature or pressure. Standard open-loop V/f control suffices with minimum cost.

2)Moderate-to-high requirement: Where speed synchronization and steady-speed accuracy are needed but load does not fluctuate violently. Sensorless Vector Control (open-loop) improves performance without extra hardware costs.

Is high torque output at low speed or fast dynamic response required?

 Yes: For crane hoisting, elevators, rolling mills, winding tension control and other cases demanding full torque at very low or even zero speed plus rapid response to load changes. Encoder-aided closed-loop vector control is the only viable option.

Summary

To put it simply, open-loop control is an economical, general-purpose solution. Closed-loop control acts as a high-performance “ace” for demanding precision-critical applications. Closed-loop control is not merely an optional premium feature. It is a mandatory requirement for safety- and process-critical applications such as cranes and elevators.