An engine is used to rotate the impeller (s). A direct drive motor is connected directly to the impeller shaft, which rotates at the same speed as the motor. A gear-driven motor transfers its energy to the impeller shaft using one or more gear sets. This allows the impeller to rotate at higher speeds than the engine.
Direct drive requires fewer bearings and does not incur transmission losses. Also, since the compressor runs at lower speeds, it can be quieter.
