Softstart UK claims to have designed and built the UK’s first home-grown, digitally-controlled, medium-voltage soft-starter. The first installation, rated at 3.3kV, 110A, is due to be commissioned soon.

The currents drawn by high-power motors usually require them to be installed on medium voltage supplies – typically 3.3 or 6.6kV. However, the starting torque surges of these motors can do a lot of damage and soft-starts can offer protection as well as smooth current and torque control.

Softstart’s MV soft-starters cover supplies up to 6.6kV and 720A, with a 400% overload for 30s, making them suitable for motors up to 6.7MW. The company is planning to extend the range up to 11kV, 700A. A 3.3kV soft-starter costs about a fifth as much as a variable speed drive.

The range also offers soft-stopping, gradually reducing the output voltage until the motor comes to a halt. This avoids abrupt and potentially damaging stops and is a particular advantage in pumping and conveyor applications.

The IP54-protected soft-starts incorporate an industrial PC with an 8.1-inch front-panel touchscreen display, and Ethernet and Modbus communications. Bidirectional motor control is optional, as is power factor correction and slip-ring motor control with rotor resistance shorting. The design also accommodates generator set operation with 45–65Hz auto frequency tracking.

Other options include withdrawable line and bypass contactors, and switchfuse disconnects. The lead time for the soft-starts is six weeks, instead of several months often quoted for high-power soft-starts.