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Soft starter and variable speed control for AC motor


Hi. I just found and built the attached schematic. It is a soft starter and a speed controller for AC motors. I checked the circuit and it worked with 70W/230V light bulb and then with a 720W angle grinder.
For making the soft starter working, I needed to make it start in the same time with the grinder, so I connected a new 3x1.5mmp cable from the grinder's switch to the circuit. Please find attached the connection schematic.
I would like to ask a few questions about this circuit:
1. Does the connection of the motor is correct ? I have 2 wires going from the switch to the stator windings, then from the stator windings 2 wires are going to the brushes. Is there any better solution for connecting the motor ?
2. Does the connection of the switch is correct (B1 on the schematic) ?
3. In order to turn off the circuit, does it needs to be disconnected from both wires (live and neutral) or it is Ok to disconnect only one of the wires ?
4. When I first opened the angle grinder's case, I found an 0.22uF/275V capacitor. I would like to know if this capacitor is still needed there ? If yes, then how can I connect it to the motor ?


Last edited:


Well-Known Member
Apparently it is a Universal motor, if 120v NA you only need to switch one conductor (Live), the field should remain in series with the armature, normally a simple triac or SCR following a bridge (for DC control) is all you normally need.


If I do not use the soft starter, then the motor starts suddenly when button pressed and it shakes my hand... this is why I am using this circuit...

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