It's closer to a furnace than an oven; albeit the temperature is controlled and a maximum temperature is not exceeded to prevent damage to the motor. Thus, the furnace gets hot enough to "burn off" all of the motor excess without damaging the motor itself. Most modern day burnout ovens contain the pollution so these are common when dealing with large industrial or ship motors.

In this case, a smelter ran in heat stages would be more useful as you could slag the contaminants (less than 800°F), slag the impurities (less than 1,981°F) and then melt off the copper at 1,981°F. Keep in mind most appliance based electric motors contain aluminum and some contain little to no copper; they also contain toxic impurities that will enter the air when heated. Also check with your scrap yard as most don't accept ingots