Handbook of Electrical Installation Practice - BeKnowledge

Fig. 15.11 Series-wound d.c. motor.
Rotating Machines 405
is weak and the motor tends to run fast. As the load increases so does the flux with
large speed reduction and high torque capability. For this reason series-wound d.c.
motors are used only where they cannot run unloaded and are found particularly
in applications such as traction.
Motor generator sets
Motors are invariably used to convert electrical to mechanical energy and vice versa
in the case of generators. Motor generator sets, on the other hand, are used to
convert one form of electrical energy to another.
This may be d.c. to a.c. such as in traction auxiliaries applications (Fig. 15.12)
or when battery power is required in an a.c. system (e.g. uninterruptible power
supplies). Battery charging, welding and Ward Leonard drive systems are typical
applications for a.c. to d.c. converters.
Both d.c. to d.c. and a.c. to a.c. sets may be required, the former for voltage raising,
lowering or regulation of a d.c. power source, and the latter for frequency changers.
Fig. 15.12 Unit construction motor alternator set. Input 600V d.c., output 6kVA, 240V,
single-phase 50Hz.