DC Generator Ratings
The ratings of a DC generator are the parameters like voltage, current, speed, and power which are the specifications of the machine that defines the capacity of it or the norms it is to be operated. A machine can give its maximum efficiency and life only when it is operated under recommend ratings.
Current Rating
The current rating is the maximum level of current allowed to be operated. This value of current shall not exceed, any current higher than the current rating may cause damage to the machine. It is based on the size of the conductor and the amount of heat that can be dissipated in the generator.
Voltage Rating
The voltage rating of a DC Generator is the voltage that the machine should produce at the rated speed which is specified in volts.
Power Rating
The power rating of a DC generator is the rated maximum output when the terminal voltage is the same as the rated voltage. The conductors, mechanical and thermal limitations, etc. accounts the power rating of the machine.
Speed Rating
The speed rating of a DC generator is the rated speed at which the machine should be operated which is specified in revolutions per minute (R.P.M or r/min).
The machine is required to be operated at a rated speed in order to deliver the rated power at the rated voltage.
If the machine is operating at a speed below the rated value it causes overheating due to the increased field current. Also, it reduces the air flow through the windings which further increases the heating. And if operated at a speed higher than the rated value it results in mechanical damage of the machine. Excessive rotation produces strains in the armature, bearings, etc.
The operation at optimum RPM is very essential for the good condition of the machine.
Temperature
The various losses in the generator are converted into heat which results in the rise of the machine temperature above the ambient temperature.
For continuous operation, the temperature rises of the machine need to be considered that it is not exceeding the specified ratings. Because the insulation materials will subject to carbonization and oxidation when it is exposed to high temperature. In the long term, the quality of the insulation losses gradually due to this which reduces the life of the machine.
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What are the characteristics of a DC Generator?
The most important 3 characteristics of a DC generator is,
1. Open circuit characteristics – It is known as the magnetic characteristic or no-load saturation curves which shows the relation between no-load emf and field current at a constant speed.
2. Internal characteristics – The relation between generated emf and armature current on load.
3. External characteristics – The relation between the terminal voltage and load current. The voltage drop in the armature circuit causes the difference in terminal voltage from generated voltage.
Open Circuit Characteristics or O.C.C – E0/If
Internal Characteristic or Total Characteristic – E/Ia
External Characteristic – V/IL