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Product category: Drives, motors and power transmission, couplings, clutches
News Release from: Siemens Automation and Drives | Subject: Sirius 3RW3 soft starters
Edited by the Manufacturingtalk Editorial Team on 09 August 2000

Choosing and Using Soft Starters

Due to the improved operating characteristics they give to the equipment they control, electronic motor soft starters are increasingly widely applied. Peter Costello of Siemens explains

Due to the improved operating characteristics they give to the equipment they control, electronic motor soft starters are increasingly widely applied Escalators, pumps, elevators and conveyor belts all operate more effectively if they are soft started

However, it is not simply the ergonomics of an airport, process plant or shopping mall that are improved by soft starters.

A major factor in the growth in popularity of soft starters is the reduced wear and tear that they place on motors and their associated drive systems.

In turn, this reduces maintenance, conserves energy and plays a significant part in improving plant performance and operating costs.

The current drawn by a three-phase motor at start up is several times more than its rated operating current.

This can vary from 3 to 15 times depending on the characteristics of the motor and is typically at least a factor of 7 more than the operating current.

In addition, problems associated with torque surge are encountered when a motor is started direct.

Extra stress on the gearbox, couplings, belt drives and other parts can soon lead to wear and even failure.

To overcome the problems associated with current and torque surges, designers have developed different systems over the years.

These can be categorised as follows: Direct-on-line starting; Star delta starting; Frequency conversion; and Solid state, stepless control, or soft starting.

Direct-on-line starting is common up to 7.5kW.

However for higher currents, some form of start-up reduction is required.

Traditionally, the star delta method of reducing the start-up voltage electro-mechanically has proved popular.

The technique, so called because the motor windings are switched from a star connection to a delta connection, reduces both start-up current and torque by about two thirds.

However, at the point where switching occurs there is still a current surge that can be as high as those experienced in DOL starting.

The alternative method of frequency conversion takes the AC voltage, converts it to DC and then to a start-up voltage of any desired frequency.

However this can be both complicated and expensive.

Soft start systems however, offer an excellent alternative at low cost and complexity.

Soft start devices provide stepless motor control, allowing both the start-up torque and current to be adjusted in small increments.

Not only can these parameters be controlled; soft starters can also vary the time taken to run the motor up to its normal operating speed.

The devices operate by gradually increasing the voltage to the motor during the run-up period, this being done by phase control of the input AC voltage.

The starting current is reduced in proportion to the reduction in voltage, the starting torque by the square of the reduction.

Soft starting is now finding application in control situations as diverse as water treatment plants and crisp factories.

The benefits of longer motor life, reduced maintenance and improved torque control mean that this type of starter is now being used in many areas where inverters would have previously been considered the only option.

Conveyor drive is a good example, as KP Foods in Billingham discovered.

At the crisp making plant, between 6 and 7 tonnes of washed potatoes per hour are transferred on conveyors from a hopper on to the peeling process.

With 50kg of spuds on the conveyor at start up, the initial load on the motor was 7.5A.

When direct-on-line starting was used, the conveyor jolted into motion and overload tripped.

Slipping conveyor belts and lost production were a constant problem.

KP Foods engineers asked Siemens Automation and Drives if a solution could be found.

The possibility of installing variable speed drives was considered but was dismissed for technical and commercial reasons.

Instead, Siemens recommended the installation of their new SIRIUS 3RW3 electronic soft starters.

The new system is designed to take a signal from the soft starter, apply this to the motor brake and thereby completely eliminate conveyor belt slippage.

The motor terminal voltage then increases steadily until it reaches full mains voltage without causing mechanical wear to either motor or conveyor.

The smooth torque change also ensures that the potatoes stay on the belt and don't jump onto the floor.

The Sirius 3RW3 soft starters used at KP Foods are a good example of the way this type of device is developing.

One of the main advantages of soft starting has always been the relatively small amount of space these modules occupy.

Generally, no wiring is necessary to satisfy the RFI suppression regulations, commissioning is easy and there are only three motor connection leads required.

The Sirius family, which covers an output range up to 55kW at 400V in only four frame sizes, measures just 45mm wide for the 4kW version and 70mm wide for the 55kW device.

Hybrid technology is used to maximise the packing density in modern soft starters.

In the case of the Sirius, the power semiconductors (two anti-parallel thyristors for each phase) are bypassed using integrated contacts after the motor has run up.

This reduces power loss to a minimum and no external bypass contactors are needed.

Another development that has helped to establish soft start technology in production and process plants is the opportunity to control the motor via a PLC (programmable controller).

No additional interfaces are required on the latest generation units, which means that an entire plant can be controlled via a PLC and SCADA system that integrates the motor and drive controls as well.

Soft starting of larger motors up to 1000kW can also be achieved.

There are now several options for the connection of isolation and protection devices in this way, the user has the option of the protection and isolation devices being installed in series with the soft starter.

This uses the three normal connections to the motor.

An optional mode (known as the Inside Delta Connection) can also be used, which connects the motor as if the starting device was a star delta starter.

This has the advantage that the soft starter only has to conduct the motor phase current, which is 58 per cent of the rated motor current.

The use of serial communications enables additional information to be made available such as motor current and overload capacity.

This data is particle useful during commissioning and enables the user to 'fine-tune' the starter to suit the particular application.

Adding a serial bus connection can enable the motor to be started stopped and monitored via a remote system, a PLC for example.

With the control and monitoring technology now available as part of a standard soft starter package, not to mention communications capabilities, soft starting now offers significant cost and reliability advantages over conventional electro-mechanical solutions.

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