The technological side of variable speed drives has improved rapidly over the past few years due to the enhancement of the AC drive and the increased capabilities of monitoring and diagnostic systems.  This resulted in AC drives becoming the choice for paper machine drive installations and rebuilds in place of the reliable DC drives.

The AC drive is an electronic power conversion device that controls the speed, torque, horsepower, and direction of various types of AC motors.   AC drives convert AC line power into adjustable-frequency AC power to control motor torques and speeds.

When deciding on an AC drive, there are a number of factors to be considered.

Reasons why the AC drive should rather be used than the DC drive are listed below:

• Elimination of commutator brush maintenance;
• Improved efficiency;
• Higher accuracy and resolution of control;
• Increased motor output; and
• Smaller cable site requirements.

Mechanical considerations to be aware of are the following:

• Motor lubrication method;
• Type of motor connection;
• Motor cooling;
• Natural frequency of the driven equipment;
• Tuning for hot response; and
• Critical speed.