Soft starters provide increased control

Modern soft starters offer virtually all the control features of inverter drives with the exception of speed control of the motor.

Smaller-rating inverter drives offer everything that a soft starter does for only a modest price premium, but with higher power and medium and high-voltage applications, soft starters become highly competitive when starting control and not variable speed is the application requirement.

With the application of digital control, modern soft starters offer virtually all the control features of inverter drives with the exception of speed control of the motor and in some cases can be the more efficient alternative.

Once a motor is started, a bypass circuit can, if required, take the soft starter out of the circuit, eliminating the losses and the heat associated with large motor control controllers.

Softstart UK now offers soft starters in ratings up to 25MW and 13.8kV.

The HRVS-DN medium-voltage soft starter is used for starting current control and for starting torque control.

In addition to applications in very large fans and pumps, these units are designed to suit marine and offshore applications where the electrical supply is limited and control of starting current is critical.

At 25MW, they are big enough for main propulsion drives where variable pitch propellers provide the speed control for the vessel.

Softstart UK offers a wide range of soft starters, covering digital and analogue models from as small as 8A at 220V.

Softstart UK has released the only soft starter on the market suitable for starting EX certified explosion proof motors.

The digital control circuitry offers a range of programmable motor protection features.

While providing smooth, step-less controlled start up, these units also protect the motor from voltage variations, phase loss, stalls and can even offer a motor insulation check.

Standard field-bus options such as Modbus, Profibus and TCP allow the soft-start units to form a fully integrated part of the plant control system.

There are many application areas where soft starters offer distinct benefits, particularly with considering larger motor loads.

Quarries, often on the end of an overhead line, frequently need to have well-controlled startup of their equipment.

Star-delta provides some current limitation but the torque characteristics are far from smooth.

Starting a loaded conveyor requires smooth control, or belt breakage or motor shaft damage could result in lengthy production downtime.

In applications where the electrical supply is generated in-situ, controlled starting of large loads is critical.

Large starting requirements could lead to the necessity to increase the rating of the generator sets, merely to cope with starting currents.

Soft start units offer a generator control operation whereby if the generator voltage output starts to drop, the output ramp of the soft-start unit is suspended until the generator recovers.

This is a critical for marine applications, where everything runs off the vessel's on-board generator sets.

Shock loading within pump systems can also be a major source of damage to the pipelines and valves.

The "water hammer" caused by sudden stopping of the pump, or inappropriate deceleration where the motor torque reduces too quickly and the pump stalls, causes pressure surges that can do enormous damage.

Modern digital soft-starters offer sophisticated pump control operation that avoids dangerous pressure curves during starting and stopping.

The world's tallest building, Taipei 101, pumps water to 101 floors, over 508m and uses Softstart UK RVS-DN units rated at 105A to eliminate voltage drops and water hammer that might otherwise damage their pipe system.

Spinning off from the development of soft starters is a new generation of digital motor protection relays and thyristor power controllers.

The MPS-3000 controller offers a sophisticated menu of control features for large medium and high-voltage motors where full protection, advance warning of potential problems and external overload bias is essential, as in the process and chemical industries.

Three-pase ground currents are monitored and with 10 temperature inputs, every aspect of motor voltage, current and operating condition is protected.

Thyristor Power Controllers have also benefited from the application of digital microprocessor control.

By controlling the voltage supplied to large electrical heating loads and electrolysis processes, continuous accurate temperature control is possible without continuous contactor switching (which can lead to over or under heating).

At the same time, a high degree of fast digital load protection is offered for applications in the plastics, petrochemical, metallurgical and glass industries.

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