Question

(a) (i) Describe the characteristics of a d.c. motor. (8)

(ii) Explain the advantages of such a motor for deck machinery.

(b) Describe with the aid of a sketch a control system for the motor in (a). (8)

Answer 1

Part (a)

A d.c. motor has a high starting torque, which is important for deck machinery such as winches, windlasses and capstans that must start under load.
The speed can be controlled smoothly over a wide range by varying the armature voltage or the field flux.
The torque-speed characteristic can be shaped by the type of winding: a series motor gives very high starting torque and a falling speed characteristic (constant power), while a shunt motor gives a nearly constant speed.
For deck machinery, a compound (cumulative) motor is often used, combining high starting torque with a reasonably stable speed.
The motor can be reversed easily by reversing the armature or field connections.
It provides smooth, stepless speed control and can hold a load (e.g. a suspended anchor) without running away.
(ii) Advantages of such a motor for deck machinery:
High starting torque to lift heavy loads from rest.
Smooth, precise speed control for delicate handling of loads.
Easy and smooth reversal.
Good speed regulation and ability to hold a load.
Can be controlled remotely and provides regenerative braking.
Robust and reliable for marine service.

Part (b)

A typical control system is a Ward Leonard system or a thyristor (SCR) d.c. drive.
Ward Leonard: a three-phase induction motor drives a d.c. generator; the generator supplies the d.c. motor armature. The motor field is separately excited and kept constant. By varying the generator field current (via a field rheostat or reversing switch), the voltage applied to the motor armature is varied, giving smooth speed control from zero to full speed in either direction. Reversal is by reversing the generator field.
Thyristor drive: the a.c. supply is converted to variable d.c. by a controlled rectifier (thyristor bridge). The armature voltage is varied by controlling the firing angle, giving smooth speed control. Reversal is by reversing the armature current or using a reversing contactor.
The control circuit includes: start/stop push buttons, speed control (rheostat or firing-angle control), overload protection, field failure protection, and limit switches for the deck machinery.
The sketch shows the supply, the controller, the motor with its armature and field, and the control/protection devices.

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