In thermal power plants, dc motors are employed for certain control and critical emergency operations which are supplied by dedicated batteries. DC motors operate as backup drives for normal ac drive systems when AC power supply to the plant is lost.
In thermal power plant, the dc motors find applications for performing control functions such as
- Turbine governor motor
- Governor limit setting
- Motor operated rheostats
- Emergency lubrication for the turbines (main, boiler feed pumps)
- Generator (H2 oil seal).
- DC motor operated valves
The emergency type services are of short duration. The emergency lube oil pump running time depends on the turbine-generator run-out time with vacuum or broken vacuum. The Hydrogen gas seal oil pump may be operated for 3 to 4 hours. The turbine-generator emergency bearing oil pumps provide lubrication, the emergency seal oil pumps maintain Hydrogen seal in case ac power is unavailable. The dc motor operated valves usually perform essential services, i.e., one AC and one DC operated valve in parallel or in series with each other will assure opening or closing, respectively.
DC motors employed in thermal plants are classified into two types based on the type of application.
- DC motors carrying out Control function
- Dc motors carrying out Emergency function
1. Control functions
This category consists of the turbine governor motor, governor limiting setting, motor operated rheostats, etc. These motors are small, about 1/8 hp or less. They are operated quite infrequently for short duration.
2. Emergency functions
This category consists of turbine-generator emergency (lubrication) bearing oil pumps and emergency seal oil pumps. Such pumps may also be provided for steam turbine drives of feedwater pumps, fans, and other large loads. The lack of lubrication during a shutdown without ac power will ruin the linings of the bearings and damage the shaft.
Hydrogen seal oil pump is provided to prevent the escaping of hydrogen (for large turbine-generators hydrogen cooling is provided for efficient cooling) from the casing by providing a tight seal with high-pressure oil.