Ac Drives

AC Drives

Load Sharing Applications for AC Drives

Load Sharing Applications for AC Drives

Overview Load sharing is a term used by many to describe a system where multiple drive and motor sets are coupled and used to run one mechanical load. In the strictest sense, load-sharing means that the amount of torque applied to the load from each motor is prescribed and carried out by each driveand motor set. Identifying Load Sharing Applications The first step in addressing the needs of an application with multiple motors is to determine if the motors are required to work together to share the load. Load sharing involves multiple motors and drives powering the same process. Each drive and motor set must contribute its proportional share of power to the driven load. Multiple motors thatare run from a single drive is not load sharing because torque control of individual motors is not possible. Motors that are controlled by separate drives without any interconnection are also not sharing the load. The lack of interconnection defeats any possible comparison and error signal generation that is required to compensate for differences in the load that is applied to any single driveand motor set. Control Topologies Three categories are Droop, Torque Follower, and Speed Trim Follower. Control Topologies Three categories of load sharing techniques will be presented, each having unique characteristics. The subtle differences will be addressed to better identify how to implement each to ensure a successful application. The categories are Droop, Torque Follower, and Speed TrimFollower. Droop This is the simplest form of load sharing to set up and, therefore, the least precise and less flexible. The precision of this control is dependent on three factors, the drives control algorithm, the motor characteristics, and the type of load being controlled. Since a volts/hertz drive does not have the ability to run in “torque mode”, a more loose interpretation of the term “loadsharing” is sometimes used. Load sharing on a volts/hertz drive is much less controllable and to a large extent dependent on the motor and type of load coupling. Since a loaded induction motor has “slip” as an inherent characteristic, load sharing is a natural response to loading.

Load Sharing Applications for AC Drives

When two motors are coupled to the same load each will slip according toits portion of load, forcing load to be picked up by the other motor. In this regard, a high slip motor is better for load sharing applications. However in a general sense, the slip of a motor is representative of its efficiency rating. High slip motors tend to have high starting torque but have low efficiency. Though high efficiency motors will slip when loaded, they may not have enough slip atfull load to achieve a reasonable degree of load sharing. For this reason many drives offer a feature called “Droop”. The Droop feature enhances the natural slip of the motor by reducing the output frequency in proportion to load current. Ideally it is only the torque-producing component of the load current that determines the droop. The amount of droop can usually be programmed in the drive inhertz or % speed. Torque Follower This type of load sharing requires a drive having the capability of operating in “torque mode”, such as the 1336 IMPACT™ drive. If speed regulation is required, one of the drives may be in “speed mode” (called the “Master”). Keep in mind that a drive in speed mode is really still in torque mode. While in speed mode the speed regulator provides a torque command output.This torque command output can be distributed to the other drives on the system (called the “Slaves” or “Torque Followers”). This torque signal may be scaled at the master output or the slave input to divide the load sharing in any ratio desired. In this case, it is possible to have drive and motor sets of varying horsepower sizes each pulling load to the extent of their capability. For...