Page 405 - Handbook of Electrical Engineering

394 HANDBOOK OF ELECTRICAL ENGINEERING

torque-slip characteristic for operation at various supply frequencies. This method, and its variants,
is applied to the largest induction motors used in the oil industry. The method is also applied to
submersible motors for ‘down-hole’ pumping in oil and water wells.

14.4 VARIABLE SPEED DC MOTORS

In the oil industries, DC motors are used for driving drilling rig systems, e.g. rotary table, draw-
works, mud pumps and winches. Modern drilling rigs use thyristor rectifiers to supply DC power to
the motors which are in the range of 700 to 800 kW. Drilling rigs do not normally require very fast
reversal of speed and so reversal by means of field control is usually adequate, or if a series motor
is used than a reversing switch will need to be used at zero speed and zero current.

Often the driven machine requires to be braked e.g. draw-works, anchor winches. It is possible,
although not yet common practice, to use the motor as a brake for the mechanical load. This is
achieved by using the load to drive the motor as a generator and to pass its current through the
thyristors in their inversion mode. This requires the thyristor bridge circuit to have six thyristors and
some additional electronic control circuits. Thus the motor ‘supplies the supply’ with energy during
the braking period. The transition from motoring to braking, and from braking to motoring can be
arranged to be fully automatic, without any kind of surge or disturbance at the point of changeover.
The electronic circuit makes this possible.
The very flexible nature of the thyristor controller allows the motor to have accurate control
plus excellent overload protection. Most thyristor controllers are furnished with maximum current
limits for motor armature current and for short-circuit current protection. During conditions of rapid
acceleration or heavy load the armature current will rapidly become high and so the maximum current
limiter will automatically hold the armature current until the duty is reduced. Thyristor controllers
also make it possible to gain accurate control of the torque or load at zero speed. This is very desirable
when handling anchors and the drill string.

14.5 ELECTRICAL SUBMERSIBLE PUMPS

14.5.1 Introduction

There are many methods by which well-bore fluids can be raised to the platform or land level. In
situations where high flow rates are required, the main options are:-
i) Gas lift, assuming adequate gas supply.
ii) Water flood.
iii) Electric submersible pumping (ESP).

Gas lift and water flood systems give higher reliability over ESP systems (by virtue of their
operating environment at ground level or on a platform deck) but are disadvantaged by equipment
weight and space requirements plus the large power demands involved. Moreover, gas lift cannot
easily be installed gradually across a field on a well-to-well basis.
Electrical submersible pumps, albeit less reliable than gas and water flood systems, are utilised
worldwide due to the major advantages of minimum topside weight and space requirements. The units,