Whether your onshore or offshore, Drawworks and Top Drive/Rotary Table drilling rigs drill holes into the earth in order to extract petroleum. Most of these rigs utilize variable speed drives to control the raising and lowering of the travelling block (done via drawworks), as well as the spinning of the drill bit (done via top drive or rotary table).
Time is money in the oil industry. The faster the drawworks lowers the drill string back into the borehole after adding an extension, the better. The drill string is dropped and then stopped rapidly before hitting the bottom. This rapid deceleration requires drive overvoltage protection, commonly in the form of a Braking Transistor and Resistor.
A Top Drive or Rotary Table spins the drill string. Drill strings can be thousands of feet in length, so it is common for one end of the string to be spinning faster or slower than the other end (just as if you were to spin on end of a shoe string). Overvoltage protection on the top drive or rotary table is necessary in these cases to control torque on the drill string.