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Wiring a soft starter


Sep 25, 2011
Garbsen, Germany
I am (remotely) helping someone in the UK to fix the electrics on a Schaublin 135 lathe built in the early 1970s. The lathe has a two-speed 3-phase 4kW spindle motor with two independent sets of windings for low and high speed.

The only remaining unresolved issue is that (most of the time) when the motor high-speed windings are turned on, the lathe switches off. This is because the voltage sags low enough that the "emergency stop/power fail" contactor drops out.

The fix that we are going to try is to add a soft starter to the high speed motor windings. He has gotten a simple Siemens Sirius slow starter, and I've given him instructions about how to add it to the lathe. I was hoping that someone here could double check my instructions. I have lots of VFD experience but have never installed or configured a slow starter. In this case, the slow starter can not replace the contactor for the motor high speed windings, because the contactor has an NC auxiliary contact, whereas the slow starter has only NO auxiliary contacts. So my instructions detail the addition of the slow starter to the system, with a control signal coming from the same set of contacts which controls the mechanical contactor for the motor.

My instructions are here (along with a more-or-less correct schematic of the lathe wiring).
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