Page 98 - Offshore Electrical Engineering Manual

Parallel Operation 85

OVER COMPOUNDED

RATED VOLTAGE

FLAT COMPOUNDED
VOLTAGE UNDER COMPOUNDED

FULL LOAD

AMPERES
FIGURE 2.6.3
Compounding characteristics.

ADJUSTMENT OF COMPOUNDING
Voltage characteristics may be modified by adjusting the airgap during manufactur-
ing by inserting or removing shims between the poles and the yoke. Other methods
are

• the use of a diverter resistance connected in parallel with the series winding,
• adjustment of the commutator brush position.

The shunt field may be connected in ‘short shunt’ (Fig. 2.6.1A) or ‘long shunt’
(Fig. 2.6.1B). The effect on the characteristics is usually minimal, but machines
should always be operated as originally built. In small generators, the effects of
armature reaction are reduced by actually mechanically shifting the position of the
brushes. The practice of shifting the brush position for each current variation is not
practiced except in small generators. In larger generators, other means are taken to
eliminate armature reaction. Compensating windings or interpoles are used for this
purpose (see Fig. 2.6.4).

PARALLEL OPERATION
The general rule is for the use of machines with flat-compounded characteristics
when running in parallel. When two DC machines are operating in parallel, a change
in load will cause change in speed and therefore in terminal voltage. Therefore,
‘equaliser’ connections will be required to avoid the machine with higher revolutions