Page 141 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Construction : In general construction and design, a rotary converter is more or less like a DC machine. It has
interpoles for better commutation. Its commutator is larger than that of a DC generator of the same size because
it has to handle a larger amount of power.

The only added feature are -
• a set of slip-rings mounted at the end opposite to the commutator end
• dampers in the pole faces as in a synchronous motor.
A simple sketch illustrating the main parts of a rotary (synchronous) converter is shown in Fig 1.

The fact that the emf induced in the armature conductors of a DC generator is alternating and that it becomes
direct (unidirectional) only due to the rectifying action of the commutator, the slip-rings are to be connected to
some suitable points on the armature winding to use this machine as an alternator.
The rotary converter armature is mostly lap wound. The number of parallel paths in the armature is equal to the
number of poles. Therefore the number of equi-potential points on the armature is equal to the number of pairs
of poles. The number of tappings taken to each slip-ring is, therefore, equal to the number of pairs of poles.
For a 3-phase lap wound rotary converter, it is essential that the number of armature conductors per pole should
be divisible by 3.

Operation : In its normal role, the machine is connected to a suitable AC supply through the slip-rings and it
delivers direct current at the commutator. In this application the machine runs as a synchronous motor receiving
AC power from the slip-ring side and as viewed from the commutator end, it runs as a DC generator delivering
DC power.

Converter aspects M.G.Set Rotary converter
for comparison

Machinery Two machines i.e. one AC Single machine
another one DC generator
Cost Very costly Costly
Noise Noisy Noisy
Efficiency Very low because of two Low
rotating machines
Maintenance cost High High
Overloading capacity Cannot be over loaded Cannot be overloaded
Power factor of AC factor Low power factor Good power

Attention during its operation Less attention required No attention required
Space required Very high Low

128

CITS : Power - Electrician & Wireman - Lesson 76-85 CITS : Power - Electrician & Wireman - Lesson 76-85