Page 140 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

MG set and rotary converter

Objectives: At the end of this lesson you shall be able to:
• list the advantages of direct current over alternating current
• list the methods of converting AC to DC
• state the advantages and disadvantages of MG-set
• describe the rotary converter construction and its working.

The AC system has been adopted universally for the generation, transmission and distribution of electric power. It
is more economical than a DC system of generation, transmission and distribution. There are applications where
DC is either essential or more advantageous over AC.
DC is essential in the following applications.
• Electrochemical process such as electroplating, electro-refining etc.
• Storage battery charging.
• Arc lamp for search light and cinema projectors.

Direct current is more advantageous in the following applications.
• Traction purposes - DC series motor.
• Operating telephones, relays, time switches.
• Rolling mills, paper mills, elevators where fine speed control, frequent starting against heavy torque and
rotation in both directions are required, DC motors are more suitable.
The conversion of AC to DC has become a necessity due to the above reasons.
Methods : The methods of conversion of AC to DC

• Motor-generator set
• Rotary converter
• Mercury arc rectifier
• Metal rectifiers
• Semi-conductor diodes and SCR
Out of the above five the motor generator sets and semi-conductor rectifiers are now mostly in use. The other
types have become obsolete for obvious reasons.
Motor generator set : It consists of a 3-phase AC motor directly coupled to a DC generator. In the case of larger
units, the AC motor is invariably a synchronous motor and the DC generator is usually compound.
Advantages
1 The DC output voltage is practically constant. The output (DC) voltage is not affected by changes in AC supply
voltage.
2 DC output voltage can be easily controlled by the shunt field regulator.
3 The M.G set can also be used for power factor correction, where synchronous motor is used for driving the
generator.
Disadvantages
1 It has a comparatively low efficiency.
2 It requires more floor space.
Rotary or synchronous converter

A rotary converter is used when a large DC power is required. It is a single machine with one armature and one
field. It combines the function of a synchronous motor and a DC generator. It receives alternating current through
a set of slip rings mounted on one side of the armature rotating synchronously (Ns = 120 f/P) and delivers direct
current from the opposite end through the commutator and brushes.

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