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Motion System Design

AC vector technology provides efficient and flexible alternative to servos

AC vector technology continues to increase in servo applications, such as in an automotive timing-chain manufacturing plant recently serviced by distributor Eastern Bearings Inc., Waltham, Mass.

The manufacturer's chain-life testing equipment required quick, dynamic response times to maintain a constant tension by the chain. It also had to be extremely flexible for changeovers into new engine designs.

Several attempts to design a drive system for life testing of the product failed because the system could not modify speed, torque, and acceleration profiles quickly. It was also unable to regulate tight speed and torque throughout the ramp. Various technologies, including eddy current, were unsuccessful in overcoming these challenges and did not address a problem inherent in Dynamometer systems: how to handle regenerative energy from the load drive.

Eastern Bearings reviewed several profiles with about 900 points in a single test sequence and near vertical deceleration ramps and identified ac closed-loop vector technology as the most cost-effective solution. Based on the flexibility of its function block programming, Eastern Bearings selected Eurotherm's 690+ vector drive.

The main drive — acting as the power plant in the system — was set up as a speed-regulated drive with closed-loop encoder feedback. The load drive was set up as torque-regulated with a speed demand signal from the main drive; a slight reference bias drove the control into torque limit.

The new system is more efficient than local generating devices, such as eddy current and water brakes, which dissipate generated energy as heat. The dc bus of multiple drives connects to absorb regenerative energy from the load motor into the drive motor; therefore, an ac vector drive results in less wasted energy.

During product testing , monitoring and collecting information on several feedback data points (including torque and speed) were required. Previously, externally mounted sensors provided feedback.

The new system provides easy access to this information from the control through the drive's built-in intelligence and DeviceNet communication bus. When monitoring digital communications to the drive, the output can be tracked and electrical noise eliminated.

Authors Drew Tucci and Bill Spiers, of Eastern Bearings Inc., are members of the Power Transmission Distributors Association. For more information, visit

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