Low-voltage operation augments soft-start versatility

Milwaukee, WI -Automated facilities typically use contactors or solid-state soft-start controllers to control motor operation. Contactors, magnetically operated devices that work like a light switch, simply turn motors on or off. Soft starts use a microprocessor to control the firing of a set of power semiconductors to work more like a dimmer switch that gradually ramps motors up to speed in order to avoid facility-wide brown outs or voltage dips, and minimize mechanical stress on power transmission components. Engineers typically use soft starts to reduce the initial current draw of larger(>50 hp) motors a problem when you consider that soft starts have a reputation for getting big and costly as motor hp increases.

Cutler-Hammer's Intelligent Technologies (IT) Soft Starters make soft-start technology pragmatic for more applications by reducing the voltage. The company claims these units may prove cost-effective for control of motors as small as 10 hp, and that they retrofit into existing equipment because they are 30 to 80% smaller than all current industry offerings. A 24V dc control power provides safer operating voltages, simplifies safety-standard compliance, and makes interfacing with PLCs and distributed-control logic systems more reliable. We use the micro- processors' intelligence to reduce the power draw wherever we can," explains Cutler-Hammer's Manager, Standard Power Control Marketing Charlie Kane, "and we separate power and logic circuits to reduce the control power requirements." This allows users to overcome obstacles that have prohibited greater use of 24V dc control power, such as power supply costs, transistor and hard contact ampere ratings, and voltage drop on long cable runs.

The design integrates both voltage ramp for traditional loads and current-limiting start for difficult loads to save panel space and wiring costs. IT Soft Starters eliminate the need for add-on bypass contactors and overload relays, and the full six bridge SCR (Silicon Controlled Rectifier) design does not require the use of isolation contactors. "This resulted in a significant size reduction," says Kane. "Now a 135A device fits in a 12-inch motor control center (MCC) bucket and weighs only nine pounds.'' Designed to meet the industrial requirements of compressors, blowers, fans, pumps, mixers, crushers, and grinders, claimed benefits include:

Reduced stress on the mechanical drive system due to high starting torque increasing the life and reliability of belts, gear boxes, chain drives, motor bearings, and motor shafts.

A kick start feature allows motors to develop additional torque (up to 550% current or 90% torque for a time of 0 to 2 sec) at start-up to break inertia. Additionally, soft stop averts sudden stops of high friction loads by allowing an extended coast to rest by ramping down the voltage to the motor. IT Soft Starters also include advanced motor protection in the form of overload, stall, jam, phase loss, phase reversal, and SCR over-temperature protection. DIP switches and LEDs, in an easy-to-use device-mounted display, make configuration quick and easy.

Additional details...Contact Charlie Kane, Cutler-Hammer, 4201 N. 27th St., Milwaukee, WI 53216; Tel: (414) 449-6000; Fax: (414) 449-6221.

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