Why is your ac compressor smoking when ac is on?

The Air conditioning compressor is likely seized. There is a clutch in the pulley that engages the compressor when the A/C is turned on. You can easily check the compressor (with engine off make sure the pulley has had time to cool off as it will be very hot) Try to turn the most center part of the pulley. If you cannot turn it by hand the compressor is seized. The smoke was MOST likely from the serpentine belt rubbing on the seized pulley when the A/C compressor was engaged. Please note that the same belt drives the power steering pump. This belt will be destroyed from the heat and must be replaced to prevent a possible steering failure.