Methods of excitation and types of DC generator

Excitation means supplying current to the field winding to produce the main flux. On this basis DC generators are divided into two groups.

1. Separately excited generator

The field winding is fed from an independent DC source (battery or another generator).

  +----[Rheo]---[ Field ]        Armature
  | Ext. DC                        (E, Ra)---->  Load

• Field current is independent of the armature load, so flux is nearly constant.
• Terminal voltage falls only slightly with load (armature drop and armature reaction).
• Used where wide voltage control is needed (e.g. Ward-Leonard system, testing).

2. Self-excited generator

The field current is taken from the generator's own armature. Residual magnetism is required to start.

(a) Shunt generator: the field winding (many turns of thin wire, high resistance) is connected across the armature.

$$I_a = I_L + I_{sh}, \qquad V = E - I_aR_a$$

It gives nearly constant voltage with moderate drooping; used for battery charging and lighting.

(b) Series generator: the field winding (few turns of thick wire) is in series with the armature and load.

$$I_a = I_{se} = I_L, \qquad V = E - I_a(R_a + R_{se})$$

The voltage rises with load current. Used as a booster and in arc lamps.

(c) Compound generator: has both shunt and series fields.

• Short shunt: the shunt field is across the armature only; the series field is in series with the load.
• Long shunt: the shunt field is across the armature and series field combination.
• Cumulative compound: the series field aids the shunt field (flat, over- or under-compounded).
• Differential compound: the series field opposes the shunt field (drooping voltage; used in welding).

 Short shunt                 Long shunt
 +--[Ra]--+--[Rse]---o       +--[Ra]--[Rse]--+---o
 |        |                  |               |
 |      [Rsh]   Load         |             [Rsh]  Load
 +--------+------------o     +---------------+---o