Design of a voltage control unit for small three phase alternators

Abstract

The scope of the project is to design of a voltage control unit intended to be used on relatively small three phase alternators. It is intended to be used mainly to replace old voltage control units on existing installations fitted with electromechanical or transductor regulators. The cost is to be kept as low as possible without sacrificing performance, and the unit is to be easily adaptable to different alternators. Such differences being the operating voltage, the frequency and the KVA rating among other things. It is very common to find backup generators fitted with very old voltage control units in places such as remote pumping stations and other installations. These units are usually either the mechanical or transductor types and are often in a bad state of service. Another problem is that it is often very difficult to obtain the replacement parts for such units. The Voltage control unit described here is intended to avoid these problems both by using a fully electronic circuit with no moving parts and to use components which are readily available locally and which are unlikely to be out of stock at any time. The unit presented here only provides voltage regulation and does not provide any overload or short circuit protection. The reason for this is simply that with most fixed frequency alternators, the residual magnetism in the rotor is often enough to generate an output voltage of over one third of the rated voltage. It is thus not possible to limit the short circuit current by reducing the field current. This could actually be done by reversing the field current by the correct amount to neutralize the residual field but this would unnecessarily complicate the circuit, the protection unit being more complex than the rest of the circuit. It was thus decided that short circuit protection would be provided separately by conventional methods such as automatic circuit breakers and/or fuses. [ .... ]