Question

Sketch a diesel electric propulsion arrangement for a ship

Describe the operation

Outlining how reversal of the propulsion motor is achieved.

In a synchrodrive system as shown in Fig., the computer receives a command (set speed) input and many feedback signals (voltage, current, power, frequency etc.) but the obvious regulating item is the actual shaft speed feedback forming a closed control loop.

The principal parameters to be controlled are the size of motor stator current (to set motor torque) and the motor frequency to set the shaft speed.

In addition, the d.c. motor field current has to be continually controlled from the propulsion regulator via the excitation converter.

In normal running and full-away with both propulsion motor speeds within 5% of each other, the bridge can select a shaft synchro-phasing mode which applies momentary acceleration/deceleration to bring the propeller blades into an alignment which minimises shaft vibration into the hull.

Speed and position are derived from detectors on the non-drive end of the motor shaft.

At speeds of less than 10%, the motor does not generate sufficient back e.m.f. to cause automatic thyristor switch-off (line commutation)

A thyristor can only switch off when its current becomes zero.

This problem is overcome by pulse-mode operation where the current is momentarily forced to zero by the thyristors in the controlled rectifier stage.

This allows the inverter thyristors to turn-off so that the controller can regain control.

The decision is now which thyristor and which sequence of switching is required to maintain the required shaft direction of rotation.

It is necessary to know exactly the position of the rotor poles and this is provided by the shaft position encoder for low-speed, pulse-mode operation.

When kicked above 10% speed, the motor e.m.f. will be large enough to allow the converter to revert to its normal line-commutation mode for synchronous operation

For normal running, above about 10% speed, the operation is switched to synchronous mode where the thyristors in both bridges are switched off naturally (line commutated) by their live a.c. voltages from supply and motor

To reverse the shaft rotation the forward/ ahead phase sequence of motor supply currents is reversed by the inverter thyristors.

This reverses the direction of stator flux rotation and hence shaft direction to astern.

The rate of deceleration to zero speed must be carefully controlled before a shaft reversal to avoid large power surges in the system.

For a motor braking operation, the inverter bridge can be considered as a rectifier bridge when viewed from the live a.c. supply produced by the motor emf.

If the network (rectifier) bridge thyristors are switched with a delay angle greater than 90° the d.c. link voltage reverses causing power flow from the motor back to the supply (motor braking).

In this mode the roles of the network and machine bridges are swapped over.