Glossary · Control theory and process control
Speed control
Also known as: Speed regulation, Velocity control
German: Drehzahlregelung
In drive engineering, speed control is the closed-loop control of the rotational speed of a motor, in which a speed controller compares the setpoint with the measured or estimated speed and sets the torque or current setpoint of the inner loop.
- Control theory
In one sentence
Speed control keeps a motor's rotational speed at its setpoint by adjusting the torque or current setpoint from measured or estimated speed.
Example
The frequency converter of a conveyor runs speed control with encoder feedback so the belt keeps 1.2 m/s regardless of load.
How it applies
- Engineering: In servo and frequency converter drives, speed control is the middle loop of a cascade: position (outer), speed, current (inner). It uses encoder feedback or sensorless estimation. Mechanical resonances show up in the speed loop first.
- Safety: Speed control is a control function. Monitoring that speed stays below a limit for the protection of people is a separate safety function, such as Safely-Limited Speed (SLS), with its own requirements.
- Documentation: Document speed ranges, feedback type, ramp times and controller parameters. Service manuals should explain the effect of changing ramps and gains on the machine, and which changes require a new verification.
Speed control vs. open-loop frequency control
In open-loop V/f operation, a converter sets the frequency and the speed follows with slip. Speed control measures or estimates speed and corrects deviations, so it holds speed more accurately under changing load.