The M200-ST3 speed protection relay, available from Cambridge Transducers , is used to detect the speed of engines used in generating sets. The pick up, situated close to the flywheel, produces a high frequency pulse train which is directly proportional to the number of teeth passing it. The frequency is converted, by the ST3 into a mA signal that is directly proportional to the rotational speed of the flywheel.
The speed protection relay provides the user with the following adjustments, which allows control of start up, normal running and protects against under and over speeds of the generator.
- Adjustment of crank speed is 10 to 50%.
- Adjustment of under speed is 50 to 100%.
- Adjustment of over speed is 100 to 133%.
- A mA output signal proportional to the input frequency.
Typical start up is as follows: When the speed of the morot (or as frequency rises) reaches the crank set point, the crank relay energises (disengaging the crank starter) and when the under speed set point is reached, the under speed relay is energised. The motor will be in normal running condition with all relays energised. If an under or over speed condition occurs, the appropriate relay is de-energised.
Two other safety features are also incorporated. If the pick up sensor input lead breaks, the over speed relay will de-energise. The crank relay will only de-energise when the input frequency goes below 20% of the setpoint. The mA signal can be fed to digital or analogue meters scaled in speed, or to provide a control signal to a PLC.