#### 6.2.4. Excitation

Every rotating electrical machine requires an exciter field. The exciter field of the direct current machine, generated by the main poles, is a permanent magnetic field of constant value. We differentiate between various exciter categories.

Permanent excitation

An exciter field is realised by means of permanent magnets. This exciter category is mainly used for lower-power machines.

Separate excitation

The necessary voltage to generate an exciter field is attained from a voltage source (e.g. accumulators) outside the machine. Natural excitation is a particular excitation category.

In this case the necessary excitation voltage is provided by a generator (exciter machine) which is coupled directly to the main machine.

Self-excitation

Because of remanance (residual magnetism), the main poles evidence a weak exciter field. In accordance with U0 = c • Φ • n the rotation of the rotor winding induces only a small rotor voltage in the exciter field. Rotor winding rotation however enables a weak current to pass through the exciter winding. This current increases the exciter field whereby a greater rotor voltage is induced. This is, moreover, a continuous process leading to a fully fledged exciter field. This alternating effect is termed “dynamoelectric principle”. One differentiates between the following self-excitation categories:

Shunt excitation

The exciter winding has been parallel connected in the rotor winding.

Series excitation

Both exciter and rotor winding have been positioned in series.

Compound excitation

Each main pole features two main types of exciter windings: a shunt winding parallel to the rotor and a series winding in series with the motor current circuit.