Servers have some of the most strict requirements for their fan motor drivers. The low noise requirement is more for the vibration than for the sound. These fans are in close proximity to hard disk drives which are sensitive to vibration. Because there are usually many servers located together it is important that they use as little power as possible. Closed loop speed control with precise revolution accuracy is also a feature that is needed. In many cases, soft-start is required to reduce the load on the power supply on power up because multiple fans are driven off of the same supply. A three phase motor is commonly used to reduce reactive current and lower vibration.