In order to adapt the dynamo to the varied conditions of service, its design is modified in numerous ways, giving rise to the different “types.” These may be classified with respect to:
1. Field magnets; 2. Field excitation; 3. Field winding.
The first division relates to the number of magnetic poles, as unipolar, bipolar, and multi-polar dynamos; also inter-polar dynamos. Under the second division are included the following:
1. Self-exciting machines of which the magneto is the simplest. Its magnetic field is obtained from permanent magnets, hence the electromotive force generated is comparatively small. The more important type of self-exciting machine is provided with electromagnets in which the field of force is “built up” from the residual magnetism of the soft iron or steel cores of the field magnets of the dynamo itself. Nearly all commercial types of dynamo are of this class.
2. Separately excited machines in which the field magnets are magnetized when the machine is in operation by current supplied from a separate source such as a battery or magneto generator.
With respect to the third division, based on the field winding, dynamos are classed as:
1. Series wound; 2. Shunt wound; 3. Compound wound.
In addition to the foregoing there are further distinctions with respect to the mechanical features. Most dynamos have a revolving armature and stationary field magnets; however, in some cases, both the armature and field magnets are stationary, a revolving iron inductor being provided to intercept the magnetic lines intermittently which produces the same effect as is obtained in cutting the magnetic lines by a revolving armature.
Ques. What may be said of bipolar and multi-polar dynamos?
Ans. Dynamos with bipolar field magnets were universally used prior to 1890, but since that time machines of this type are only made in very small sizes; the multi-polar dynamo is the type now in general use.