Soft Starter Fault Connection

I have involved with a soft starter study recently which was about a 750 kW motor had failed to start. In the investigation, we found that there were many flaweds in the system configuration as follows:

* The power supply was slightly weak to handle the full starting current this is why a soft starter was deployed - of course - but again with the soft starter there was some voltage dip during the operation.

* The cables were laid flat rather than in trefoil. This caused the cable impedance to increase and hence less voltage available to the soft starter-motor combination.

* When the soft starter was configured in delta mode six cables were required (two cables per phase windings. this means that the cables are connected as two in series. This has aggravated the problem even further.
Soft starter
In an application that has problem with ABB soft starter which is connected to motor 160kW, when try to start is shown "Fault Connection". It's already doing remedy with checking the cable and motor and found OK. But still not start, and also try with another cable and small motor 7.5 kW but same problem has come. And then try to start with option DOL START from local control, and the contactor is working and motor 7.5 kw is running.

Another application faced similar problems with Gozuk soft starter fitted with around 300 HP induction motor. Soft starter running for several months suddenly says - 'Fault Connection', without any disturbance in motor cables and other connections.

In such cases it looks like there is problem with motor internal configuration (Delta / Star) and the soft starter configuration (Inside Delta / inside Line). It is important to realize that they should not match or mismatch. In other words:

  1. If the soft starter is configured as (inside Delta), the motor phase windings should be available as two connections per phase (W1- W2, V1-V2 and U1-U2).
  2. If the soft starter is configured as (inside Line) then the motor can be configured as either star or delta. Having said that, it's better keep the motor in delta configuration for maximum power.

Make sure that you do your homework regarding the voltage, current and power calculations in all cases. If this doesn't help then I would suggest that either the motor, the soft starter or both are faulty.

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