Even when you have electronic speed control fitted, use the pulleys to provide appropriate gearing for whatever you are turning. If you use the 'high speed' configuration of pulleys then use the speed control to turn very slowly, the motor is turning very slowly. This can overheat the motor as it use an internal fan on the motor shaft to cool it. By using the low speed configuration, you will be turning slowly but the motor will be running at a reasonable speed, thereby keeping itself cool. This will improve the torque slightly but a good motor/inverter combination should not lose much torque throughout it's speed range.
By the way, if you are 'doing it yourself' bear in mind that most inverters offer the facility to reverse the motor direction very easily. But if you do that don't forget to look at your chuck and make sure it's suitable.
