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Nearly all Soft Starts are incorporated into a control panel and this section considers how best to achieve this and gives some useful information to assist panel builders.

Heat loss, size of panel and By-pass operation

The Soft Starts provided by Softstart Direct are designed to stay in circuit continuously, however in doing this the thyristors switching will create a heat loss. This heat loss is 4-5W per amp and must be considered, along with other heat producing components when sizing the cubicle. A typical heat calculation will consider total heat loss, ambient temperature and maximum internal temperature to determine the size of the cubicle and any additional cooling required.

In many installations the soft start will be by-passed and this will mean only nominal heat will be generated from the soft start. The soft start can therefore be ignored from the heat calculations and it could be that additional cooling may not be necessary as a result of this. Soft Starts can therefore be installed into IP55 boxes or higher with no problem. As a general rule 50-100mm should be left around the soft start in any panel installation.

EMC Issues

As part of the International Standard on Electronic Soft Starts IEC60947-4.2, it is necessary to perform a number of routine and type tests on electronic soft starts. These tests include EMC Tests in accordance with the generic EMC Regulations. All Softstart Direct Soft Starts have been type tested and comply fully with the EMC Regulations and no additional filtering is required external to the Soft Start.

When mounting Soft Starts into control panels it is good practice to ensure power and control cables are kept separate and to ensure control cable lengths are not longer than 2M. When fitting Soft Start to large power motors above 200kW it is also recommended to fit contactor coil suppression to eliminate switching spikes. When using the Softstart Direct Softdrive 300 door mounting keypad, please ensure only the supplied 2M lead is used as this is the tested and screened lead required for compliance.

Fuse protection/type 2 co-ordination

The recognised standard to which Soft Starters are manufactured and tested is EN60947-4-2. In particular clause 9.3.4 deals with the performance of the Soft Starter under short-circuit conditions to ensure the starter operates safely, giving protection to the soft starter and to protect individuals against injury.

Thyristors used in Soft Starters must be rated not only for the motor running duty but for the motor starting duty which is typically 3 times or even 5 times the motor running current. Under short circuit output conditions, distribution or motor rated fuses will not protect the thyristors and High Speed Fuses must be used.

High Speed Fuses have been designed specifically for use with thyristors to interrupt safely, very high prospective fault currents in extremely short times to limit the value of current allowed to pass through to the device. To ensure rapid melting of the fuse elements the necks have a different design to industrial fuses and typically operate at higher element temperatures. Under high prospective fault currents the correctly selected High Speed Fuse will clear the fault much faster than a conventional fuse and will therefore protect the thyristors as the current through them is restricted.

In stand alone starters and MCC sections, HRC fuses are generally required for type 2 co-ordination of the contactor/overload combination. Similarly high speed fuses are required for type 2 co-ordination of the soft start. The system builder must fit both fuse types in the panel unless the panel is fused with HRC fuses externally. If external fusing is confirmed then only high speed fuses are necessary. With Softstart Direct soft starts the type 2 co-ordination is achieved through Bussmann fuses and they must be fitted as close to the soft start as possible. The high speed fuses cannot be fitted in the fuse switch disconnector as this deviates from the type 2 tests in terms of both fuse type and fuse position in the circuit. Unfortunately one single type of fuse will not offer type 2 co-ordination for both soft start and the contactor/overload combination.

During tests carried out by Softstart Direct at Falcon Testing Laboratories on our Soft Starters to the requirements of clause 9.3.4, when a short circuit was applied to the soft starter the high speed fuses protected the fault safely and after replacement of the fuses, the Soft Starters functioned correctly with no damage to any of the components other than the fuses.

What do I have to fit with a Soft Start

The recommended circuit for the Soft Starts with and without By-Pass contactors are shown in the respective manuals in the documentation section. However as a quick overview it is necessary to fit the Soft Start into a panel with the following equipment as a minimum.

  • Door Interlocked Isolator or Breaker
  • Automatic Mains Switching Contactor
  • Motor Overload

A By-Pass Contactor and High Speed Fuses are optional

Please note that the Softdrive 300 has its own Integral Protection Package this includes: 

  • I2t Electronic Overload
  • Instantaneous Overcurrent
  • Under Current
  • Motor Thermistor
  • Stack Overtemperature
  • Phase Imbalance
  • Earth Fault

This means external relays may not be necessary. Whilst the Soft Start has Integral Earth Fault separate Core Balanced CT's are still required. Please click here for more details. No other CT's are required in circuit for the protection circuits. The overload/protect features are designed to stay in circuit during starting and running. This is facilitated by maintaining connections through the CT's. Therefore on Softdrive 300 by-pass contactor connections are provided as well as output connections. On the basis that by-pass contactors are used the Softdrive 300 has an additional feature of disabling the cooling fans after 10 minutes in this mode.