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Built-in bypass helps to shrink soft-starts

01 July, 2003

Built-in bypass helps to shrink soft-starts

Rockwell Automation has introduced a series of high-current (up to 480A) soft-starts with the claim that they are half the size of traditional products, and generate 80% less heat. The SMC-Flex soft-starts are an upward extension of a low-current range launched late last year.

One reason for the compact size and cool operation of the new devices is that they have a built-in automatic bypass. Once the motor reaches its nominal speed, the bypass closes, reducing the heat given off by the thyristors. Heat generation is also restricted by using an electronic overload with an adjustable trip class from 10-30.

There is a choice of direct-on-line or star-delta starting, with rated currents ranging from 108-480A for DOL and 94-831A for star-delta. Operating options include soft-stop, kick-start, current limit, and pump start and stop for controlling pipe "hammer" effects.

As well as protecting against faults such as overloads, under- and over-voltages, phase reversals, line faults, current imbalances, and excessive starts per hour, the soft-starts can also measure variables such as three-phase currents, kW and kWh, power factors and thermal capacity. There is an input for thermistors incorporated in motor windings and, using an optional current transformer, the starters can also detect earth faults.

By adding a communications interface, the soft-starts can be linked into networks such as DeviceNet, Profibus, ControlNet and Ethernet IP. They can be programmed either via a network using the same tools as used for the PowerFlex range of drives, or locally using their backlit displays.

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