Operating Characteristics

Author: E.E. Kimberly

The voltage at the d-c terminals bears a fixed ratio to the a-c voltage at the slip-rings, and can be varied only by varying the a-c voltage. The a-c voltage may be varied by changing the transformer taps or, indirectly, by changing the d-c excitation as in a synchronous motor. Over-excitation causes the converter to take a leading current. The leading current flowing through the reactance of the transformers causes an increase in their secondary voltages and, hence, an automatic compounding of voltage. Because of excess heating of armature conductors at and near the tap points¹ when the power factor departs more than 10 per cent or so from unity, this method of voltage control by field rheostat can be used only within small confines near unity power factor. The same heating characteristic which limits the range of compounding also limits to almost negligible proportions the capacity of the converter for correcting the power factor of a power system, as may be done so readily with a synchronous motor. The relative merits of the synchronous converter and the synchronous motor and d-c generator set are as follows:

Advantages of converter:

1. Cost is less per kilowatt of capacity.

2. Efficiency is higher.

3. Weight is less, and foundation may be smaller.

4. Requires minimum of space.

Advantages of synchronous motor and generator set:

1. Wider range of d-c voltage is available.

2. Motor may be used to correct power factor.

1	Principles of Alternating Current Machinery, Chapter 27, by R. R. Lawrence, McGraw-Hill Book Co.