Question

Sketch and describe a Ward Leonard speed control method

Give 2 examples where Ward Leonard speed control method may be employed for ship board operation.

Ward-Leonard system

This system, used for fine control of d.c. electric motor speed from zero to full in either direction, is also able to give the motor a robust torque characteristic.

The working motor (Figure) which powers the steering gear, windlass or other equipment is a d.c. machine, because speed control of these is easy.

The method is to alter the voltage applied through the brushes to the armature windings of the d.c. motor; no change is made to the current in the field windings.

The voltage is increased or decreased, not with the use of resistances but by arranging an individual d.c. generator with controllable output voltage as the power supply for the armature of the working motor.

Speed and direction of the working motor vary with the magnitude and direction of applied voltage.

Current for the windings of the field poles is derived from the source of main power.

Where there is a.c. main power, current for the windings is transformed to lower voltage and rectified.

A constant speed induction motor drives a d.c. generator which in turn supplies one or more d.c. motors.

The generator output voltage is controlled by adjusting its small excitation current via the speed regulator.

The d.c. motor speed is directly controlled by the generator voltage.

Switch of direction of current flow through the field poles, also with the potentiometer, will cause the direction of the pole magnetic fields to change.

This changes the direction of generated current supplied to the motor and thereby also the running direction of the motor.

The control lever can, by moving the potentiometer contacts in opposite directions away from the mid-position shown in the sketch, set the polarity and strength of the generator field poles.

By governing generator output to the armature of the working motor, this in turn gives stepless speed control of the working motor, in either direction.

The system as used for steering gear operation on d.c. ships is complicated by the feedback which automatically brings the potentiometer to its mid/neutral position, when the rudder gear reaches the desired position.