This is a very common thing among drilling machines. If you get a burning smell while using the drilling machines, it basically suggests that the motor is being overly stressed or overheating. This is applicable not only in drills but also in any power tools. This can also be an indication that the drill is being used on something that it isn’t meant for or used incorrectly. This occurs when the motor’s fans are not able to cool the motor windings fast enough leading to overheating and melting of varnish coating. It is not an alarming situation though. If you get the smell again, just withdraw it from whatever you are working on and leave the machine running. This will give time to the motors to cool down the windings and you can start working again as usual.