Soft starters for electric motors protect and extend the life of drive belts, chains, gearboxes, bearings and machine mountings at relatively low cost, writes James Bowler.
Soft starters provide an electronic solution to mechanical problems at relatively low cost. They protect and extend the life of belts, chains, gearboxes, shafts, bearings and machine mountings. In addition, soft starters can achieve real savings in energy for users by providing the facility to switch motors off, without fears for the torque stresses they will cause upon restarting.

Added to these factors, the traditional fit-and-forget reliability of soft starters ensures security of operation even in the most critical of tasks.

Statistics show that over 90% of the motors in UK industrial have no form of control, other than simple electro-mechanical switching.

As a result, the sudden impact at start- up of these motors, followed by the rapid acceleration to full speed, causes problems across a wide range of equipment types.

Sudden torque stresses cause excessive wear on belts, pulleys, gears, chains, couplings and bearings, and also cavitation in pumps, which reduces their efficiency and life.

In conveyor systems, too, loads may be displaced or damaged upon start-up, and products may become contaminated.

Clearly then, mechanical engineers have plenty of reasons to press for the fitting of soft starts in fixed speed motor applications.

Especially so, as the cost savings resulting from reduced downtime and from not having to replace bearings, gears, pulleys and bearings so frequently, will ensure quick payback on any soft starter unit.

A additional argument for the fitting of soft starters is that equipment previously left running, due to concerns about it restarting - and concerns, also, regarding belts braking, shafts shearing and bearings failing - can now be switched off, enabling real savings in energy to be achieved.

In addition to their primary protective function, soft starters can also safeguard power transmission components in applications where loads are likely to jam suddenly - mixing, wood sawing and rock crushing, for example - often with disastrous results for production.

The traditional method of achieving this safeguarding was via a mechanical 'shear pin' that consisted of a pin of a deliberately weak material inserted into two concentric shafts at a convenient point in the drive train to the load.

If the load became jammed, the sudden rise in torque would cause the pin to shear so that the two shafts could then rotate independently, thereby disconnecting the motor from its load.

Before the load could be re-started, the old pin would need to be removed and a new one inserted - an obviously inconvenient and time-consuming process.

The 'Electronic Shear Pin' facility integrated into Fairford's QFE series of soft starters eliminates the need for a mechanical shear pin entirely.

The speed and extent of a sudden and rapid rise in motor torque is immediately detected by the QFE which will then decide on a course of actions ranging from instantaneous shutdown to monitoring for recurrences if the blockage is released rapidly.