D.C Machines – DC machine is a highly versatile energy conversion device it can meet the demand of loads requiring high starting torque, high accelerating and declerating torques. At the same time DC machine is easily adaptable for drives requiring wide-range speed control and quick reversals. These in herent characteristics can further be modified,if desired, by feedback circuits. In view of these outstanding features, D.C. machine possesses a high degree of flexibility. These are therefore widely used in industry, particularly for tough jobs as are encountered in steel-mill drives inspite of their initial cost.
Methods of Excitation of D.C. machines – A D.C. machine can work as an electromechanical energy converter only when its field
winding is excted with direct current, except for small D.C. machines employing permanent magnets.
There are, two methods of exciting the field windings of D.C. machines are :
(a) Separate excitation and
(B) Self-excitation
(a) Separate excitation – The separately excited field winding consists of several hundred turns of fine wire and is connected to a separate or external D.C. source.
The voltage of the external D.C. source has no relation with the armature voltage, i.e. the field
winding energised from a separate supply can be designed for any convenient voltage.
(B) self excitation –
When the field winding is excited by its own armature, the machine is said to be a self-excited D.C. machine. In these machines, the field poles must have a residual magnetism, so that when the armature rotates, a residual voltage appears across the brushes. This residual voltage should establish a current in the field winding so as to
reinforce the residual flux.
A self-excitation D.C. machine can be sub-divided as follows :
(1) Series excitation – The field winding consists of a few turns of thick wire and is connected in series with the armature, In the series field current depends on the armature current and in view of this, a series field may be called a current
operated field.
(2) Shunt excitation – The field winding consists of a large number of turns of fine wire and is connected in parallel (or in shunt) with the armature, Therefore, the voltage across the armature terminals and the shunt field is the same and it is for this reason that a shunt field may be called a voltage-operated field.
Remember that series and shunt field windings are characterised by low and high
resistances respectively.
(3) Compound excitation – A compound excitation involves both the series excited winding and the shunt-excited winding. In some applications, a shunt-excited winding may be replaced by a separately-exicted winding.
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