There are two systems for powering burs: air and electrical. In the air system a distinction is made between a turbine and an air motor.
In the air system a distinction is made between a turbine and an air motor. With turbines the bur is directly driven by a rotor. The rotor has an impeller powered by compressed air. Turbines reach an idle speed of up to 400,000 min-1. The working speed depends on the pressure exerted and is approximately half the idle speed - i.e. approx. 150,000 to 250,000 min-1. The maximum power of 10-26 W is also reached in this speed range.
The air motor drives the bur indirectly via a contra-angle handpiece or straight handpiece. It reaches a maximum speed of 25,000 min-1. Contra-angle handpieces are available in various step-up and step-down ratios. An air motor with a 2:1 contra-angle handpiece step-down ratio thus reaches a speed of approx. 12,500 min-1.
Electric motors reach an idle speed of up to 40,000 min-1. This corresponds to a bur speed of 200,000 min-1 for 1:5 contra-angle handpieces. The maximum power is over 60 W and the torque is approx. 3 Ncm. This means that electrically powered contra-angle handpieces are not slowed or stopped when the bur cuts through different tooth structures or prosthetic materials. They continue cutting at virtually constant speed regardless of the load. Compared to turbines, burs are much more stable with contra-angle handpieces. Burs in a contra-angle handpiece vibrate much less than with turbines. Improved stabilization means greater precision, faster work and less heating of the tooth substance during preparation.
The trend towards electric motors was first noticed in Europe. One important reason for this was the potential expense of installing new air lines in existing buildings. It also became obvious that electric drives are not only easier to install but are also more efficient in use. Now, some decades later, electric motors have become very popular in both Europe and Asia.
Innovative developments in the fields of design, materials, torque and light are now also contributing to the popularity of electric motors in North America. They make work in the dental practice faster and easier.