An over speed trip is a safety feature provided on the diesel engine of the ship to restrict uncontrolled acceleration of the engine, leading to mechanical failure or untoward accidents. In order to prevent the speed of a diesel engine to go beyond the pre-set speed range, an over speed trip is used in the diesel engines.
How Over Speeding can be Harmful?
A Diesel engine is designed for the mechanical stress associated with the centripetal and centrifugal forces of the moving parts inside it in a specified operational range. Centripetal force is directly proportional to the square of the rotational speed, stress increases rapidly with increase in speed. Mechanical connection strength can be overcome by the exceeding stresses due to the increase in operational speed. These can result in breaking of rotating parts or damage to the machinery itself. Over speed is thus a serious safety hazard and can lead to a fatal situation.
What does Over Speed Trip Does?
Due to sudden changes in the load on the diesel engine, the speed of the engine may vary. Though a governor is provided to control the speed of the diesel engine, the speed might go out of control, damaging the engine. Thus, for this reason over speed trips are used.
No matter what type of the over speed trip the engine uses, the main aim of the over speed trip is to cut the fuel supply to the engine cylinders in case the engine speed rises above a specific level.
Preventing Over Speeding of Engine
Reducing the likelihood of an uncontrolled and catastrophic over speed is essential and can be done by two methods:
a) Mechanical over speed trip
b) Electronic over speed trip
In this article we will have a look at the electronic over speed trip.
Electronic Over Speed Trip
To understand the electronic over speed trip, a normal lay out of the system is described below. The electronic over speed trip consists of
a) Fly wheel mounted speed sensor
Magnetic speed sensor is preferred in generator engines. Due to the discontinuity of actuator surface (gear tooth of flywheel) voltage is excited in the pick off coil of sensor, producing an electric analog wave. This cyclic wave created by the flywheel is read by the sensor.
b) Signal condition unit
This unit act as a receiver to the speed sensor. Basic function of the signal conditioner is to convert one type of electronic signal which may be difficult to read into another type into a more easily read format. This can be achieved by amplification, excitation and linearization of an electrical signal.
c) Detection and Comparison unit
There is a set value which is normally 10 % above the rated speed and acts as base value for this unit. Signal condition unit output is continuously detected and compared with the set value.
d) Trip signal unit
If the difference between the set value and detected value is above the limit, then this unit gives a trip signal which in turn shuts down the generator.