Why soft starters make sense

A REO product story
Edited by the Engineeringtalk editorial team Jun 19, 2000

Soft starters provide a properly engineered means of powering AC motors in fixed speed applications as John Symonds of power conversion specialists Reo (UK) Limited explains.

Soft starters provide a properly engineered means of powering AC motors in fixed speed applications as John Symonds of power conversion specialists Reo (UK) Limited reports.

Of the millions of AC motors in use within industry it is estimated that about 95% have no control.

While AC inverter manufacturers seek to remedy this, there remain countless applications where variable speed control is not necessary and a simple soft starter is all that is required.

AC induction motors are frequently used at fixed speeds and the overwhelming majority of these are powered at full rated voltage direct on line (DOL).

The problem this gives is that high inrush current on starting can be 4-8 times the full load current of the motor.

This causes far higher torque than is delivered at full load and creates the customary "snatch" from the motor that can result in wear and mechanical damage to couplings and transmission equipment and even to pipe work in pump applications.

Star-Delta starters work by first connecting the motor in star configuration so that only 58% of the full line voltage is applied to the motor.

After a short period, this connection is changed to delta thereby applying the full line voltage.

This method relies on all six stator terminals being available for use, which is not always the case.

The transition from star to delta also introduces some current peaks, which can be as bad as using DOL starting.

The contactors used for this method also require regular maintenance.

Primary resistance starters work by introducing additional resistance into the stator windings, thus reducing the current drawn from the line.

The value of these resistors is chosen to provide a compromise between reduced current on starting and the required initial motor torque.

As motor speed increases, the in rush effect is reduced and so the resistors can be switched out of circuit.

The resistors needed for this type of starting can be very large and can require forced air cooling.

The peripheral circuitry needs regular maintenance.

Starting transformers are essentially automatic transformers with a variety of secondary tappings which each representing a percentage of full line voltage - typically, 50%, 65% and 80%.

Initially, the motor is connected to the 50% tapping, then the 65% and 80% tappings in turn.

As with the resistance starters, this method of starting can occupy significant space and requires even more external circuitry.

The transitions between tapping can also cause current spikes.

Soft starters provide the best engineered solution and are the most cost effective method.

Reo's designs use 6 high power thyristors, to gradually increase the voltage applied to the motor, starting slowly and limiting start up currents.

Once the adjustable ramp up time has been reached, the thyristors are by passed via a contactor so that the mains voltage is applied directly to the motor.

This means the thyristors are not in continuous operation as with most electronic soft starts and no additional cooling is required.

This feature, coupled with the small size of the starter, means panel space is kept to a minimum.

The soft starters can also be used to ramp down where heavy dynamic loads are encountered.

Soft starter operation uses the principle of phase angle control.

This technique uses triacs and thyristors with the level of voltage delivered by the control system being determined by the point on the mains sine wave at which the device fires.

The longer the line delay, or greater the firing angle, the lower the output voltage.

The device switches itself off as the current drops to zero and is fired again on the next half sine wave.

Reo's soft starters control all three phases in this manner.

Reo offers soft starters in its Vareosoft range in 11 sizes ranging from 4kW up to 160kW.

Acceleration and deceleration ramps are variable between 0 and 10 seconds and starting torque is variable between 0 and 50%.

Technical details of all the products are available on Reo's excellent web site at www.reo.co.uk.

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