AC drive speed tracking function (Runaway start)

The speed tracking function is an important technical feature of the frequency converter. It is mainly used when the motor is in a rotating state (such as inertial coasting, load dragging, etc.). The frequency converter can quickly detect the actual speed and phase of the motor and restart the motor at an appropriate frequency to avoid overcurrent, overvoltage or mechanical shock caused by frequency mismatch at the moment of startup. This feature is also known as "Runaway start", "sensorless speed tracking" or "automatic restart", and is commonly seen in scenarios where frequent starts and stops are required or where the load inertia is large.

I. Core Principles and Technical Implementation

1. Working principle

Detection stage: When the frequency converter receives the start signal, it first detects the residual voltage frequency and phase on the motor terminals through a current transformer (CT) or voltage transformer (PT), and calculates the current actual speed of the motor.

Synchronous stage: The frequency converter quickly adjusts the output frequency to a frequency point that matches the current motor speed based on the detected speed (for example, if the current motor speed corresponds to a frequency of 20Hz, the frequency converter outputs 20Hz first), avoiding current surges caused by frequency jumps during startup.

Smooth acceleration stage: After confirming the frequency synchronization, the frequency converter gradually increases the output frequency to the target value according to the preset acceleration curve (such as linear or S-shaped), completing the startup process.

2. Key technical points

Sensorless detection: No additional encoder installation is required. Only the built-in algorithm of the frequency converter is used to analyze the motor's counter electromotive force (EMF) or terminal voltage/current waveforms. It is suitable for renovation projects or low-cost scenarios.

Fast response: The detection time is usually within the range of 10 to 100 milliseconds, ensuring that the motor completes synchronization before a significant deceleration due to inertial coasting, thus avoiding start-up failure caused by excessive speed differences.

Adaptive algorithm: It can identify different motor parameters (such as inductance and resistance), and is compatible with asynchronous motors (IM) and permanent magnet synchronous motors (PMSM).

Ii. Typical Application Scenarios

High-inertia load equipment

Scene: Fans, water pumps, centrifuges, ball mills, conveyor belts and other equipment that continue to rotate due to inertia after being shut down.

Pain point: If the frequency converter is started directly before the motor has completely stopped, the traditional starting method will cause an overcurrent due to the superposition of the counter electromotive force and the power supply voltage caused by the mismatch between the motor speed and the output frequency of the frequency converter (which may trigger the overcurrent protection to trip), or damage the coupling and gearbox due to mechanical shock.

Value: The speed tracking function can directly start synchronously during the motor's coasting process, avoiding downtime waiting time and enhancing production efficiency (such as quick restart after an emergency shutdown of a fan in a cement plant).

2. Multi-motor linkage system

Scene: In equipment such as printing machines, textile machines, and papermaking production lines where multiple motors operate synchronously, when one motor stops due to a malfunction and is restarted.

Pain point: If the speed of a single motor is not synchronized with that of other running motors when it restarts, it will cause a sudden change in material tension (such as fabric breaking or paper wrinkling).

Value: By tracking the rotational speed, the restarted motor can quickly match the current operating speed of the system, maintaining multi-machine synchronization and reducing the scrap rate.

3. Scenarios for power outage recovery or fault reset

Scenarios: Equipment that needs to be quickly restarted when the power grid is restored or faults are eliminated after being shut down due to power grid fluctuations, inverter failure protection, etc. (such as sewage treatment pumps, agitators of chemical reaction vessels).

Pain point: The traditional start-up method requires waiting for the motor to completely stop rotating, which may lead to the interruption of the process flow or equipment damage (such as sewage backflow, material solidification).

Value: It can be started directly when the motor has not completely stopped, shortening the recovery time and reducing production interruption losses.

4. Energy feedback type load

In scenarios such as cranes lowering heavy objects and elevators moving up empty, motors in the power generation state continue to rotate due to the load when they stop.

Pain point: Direct startup may cause the DC bus voltage of the frequency converter to soar due to the motor being in the power generation state (overvoltage protection), or generate a large inrush current.

Value: The speed tracking function can first detect the motor's rotation direction and speed, start at a matching frequency, and at the same time consume the feedback energy through the braking unit to ensure a safe start.

Iii. Functional Advantages and Limitations

Core advantage

Avoid overcurrent impact: Limit the starting current to within twice the rated current (traditional starting may reach 5 to 7 times) to protect the frequency converter and motor.

Shorten the start-up time: There is no need to wait for the motor to completely stop. It can be started directly during coasting, improving system efficiency (for example, the fan restart time is reduced from 2 minutes to 30 seconds).

Reduce mechanical wear: Eliminate gear impact and belt slippage caused by the speed difference at the moment of startup, and extend the service life of mechanical components.

Enhance system reliability: Adapt to the demand for rapid recovery after emergency shutdowns, especially in continuous production scenarios (such as petrochemicals and steel smelting).

Limitations

The low-speed detection accuracy is limited: When the motor speed is lower than 10% to 20% of the rated speed (such as approaching the shutdown state), the back electromotive force signal is weak, which may lead to detection failure and requires switching to the traditional start mode.

Strong dependence on motor parameters: If the preset motor parameters of the frequency converter (such as rated power and pole number) do not match the actual situation, it may lead to a deviation in the speed calculation, and the parameters need to be re-optimized.

Optional braking unit is required: For high-inertia loads or energy feedback scenarios, an additional braking resistor or feedback unit needs to be configured to consume the regenerative energy that may be generated during the start-up process.