AC motors topdrive in drilling rig

I would think you need to ask Varco to be certain. But from what I can tell, it is probably associated with applying braking power via a DC source, such as a battery, in the event of a power failure. The term "chopper" is typically used to describe a DC to DC drive system, usually where you have a fixed DC power source such as a battery and need a variable power output, such as a brake. From what I've heard, drilling rigs have a "twist" on the drillstring pipes as they get longer and longer. If the power fails for some reason, the drive motor torque stops and the drillstring begins to "untwist", spinning the motor backwards. If the power returns and the motor is restarted as the pipes are untwisting, you can snap the drillstring and/or damage the motor. So people apply DC powered holding brakes that lock on as soon as the power fails, then stay on until the drillstring has completely untwisted.

AC drives by themselves normally won't regenerate power. So if braking is required the DC bus link volts increases. The Chopper braking circuit turns on and starts to discharge the energy into a resistor grid. You can see that as the duty cycle increases more energy will be dissipated in the resistors.

So to be precise, the chopper "braking unit" does just that, provides controlled braking of the AC motor.

A brake chopper on an AC drive is used to bleed off power from the DC bus when the drive is ramping down. The act of slowing the motor with the drive will actually put power back into the VFD. As has been previously stated this power is "chopped" and bled off into a resistor bank to convert into heat. Generally the larger the motor/load and the faster you ramp down, the larger the resistor bank needs to be.