Explanation:

In a compound DC generator, the field winding consists of both series and shunt windings. The series winding is connected in series with the armature while the shunt winding is connected in parallel with the armature.

Advantages of over compound machines:

1. Adjustable compounding: Over compound machines have the advantage of allowing the degree of compounding to be adjusted. This adjustment is achieved by diverters across the series field. By changing the position of the diverters, the amount of series field current can be varied, thereby adjusting the degree of compounding. This flexibility allows for better control of the generator's performance.

2. Better voltage regulation: Over compound machines provide better voltage regulation compared to other types of compound generators. Voltage regulation is the ability of the generator to maintain a relatively constant output voltage, regardless of changes in load. The over compound design helps to compensate for voltage drops that occur due to armature reaction and resistive losses in the generator. This ensures a more stable output voltage, making it suitable for applications with strict voltage requirements.

3. Cost-effective: Over compound machines are generally more cost-effective than shunt machines. This is because a shunt machine would require additional external equipment, such as voltage regulators or other control devices, to achieve the same level of voltage regulation. The adjustable compounding feature of over compound machines eliminates the need for such external equipment, reducing overall costs.

4. High voltage direct current (HVDC) applications: Over compound machines are often preferred for HVDC applications due to their better voltage regulation characteristics. HVDC systems require precise control of voltage levels, and over compound generators can provide the necessary stability and regulation.

Conclusion:

Commercial compound DC generators are usually supplied as over compound machines because they offer adjustable compounding, better voltage regulation, cost-effectiveness, and suitability for HVDC applications. The ability to adjust the degree of compounding using diverters across the series field provides greater control over the generator's performance. This makes over compound machines a preferred choice for various industrial and commercial applications.