From the experience of operating an asynchronous motor in a 1-phase network.

 First, turn on the bag and press the book. start for connecting starting capacitors. This is how it starts.

But in the process of operation, it sometimes happens that you have to reconnect the starting capacitors, or even connect them permanently.

 But it is better if it is in semi-automatic mode, and use the information from the current sensor while the engine is running. 
Example. The engine is on the circular in idle mode, the current Imin, we start dissolving the material at the engine power is not enough Imax., here our circuit monitors the increase in current and connects an additional capacitor, thereby automatically increasing the engine power at the right time for this.

Of course, you can not bother with any circuit solutions, just connect the starting capacitors to the engine permanently for the entire period of operation, and in principle, no problems … no problems if the engine runs in this mode for 5 – 15 minutes … ..

Then, for any reason, the engine will overheat, as there is constantly increased energy consumption,

and if you need long-term work, then in this mode the engine will not work for a long time,

in this case, the semi-automatic connection of capacitors is just right.

That’s why such a scheme was made, “semi-automatic” on NE555 .

The whole principle of operation of the circuit occurs by controlling the current consumption of the switched on load, the threshold of which is determined by the current transformer ZMCT103C  T1  and  R1 .

Next timer 555 , this is the exception of the small jerking of the relay, and also its main role, keeping the starting capacitors on at a time convenient for you (set using R2 ) . When you first turn on the first start will be like this, since C1 will be discharged ( if it is charged, then during the start  V1 will  discharge it )   
 this first switching on of the circuit will always begin simultaneously with the switching on of the relay P1 , and there will be the first switching on of the starting capacitors, well, then everything happens automatically from the load.

After assembling the circuit, the first setting is performed in the following sequence, the resistors are set to R1 maximum –  R2 minimum ( Kn1 for testing the timer circuit, in principle, this button has no value in the circuit at all ).

After applying power to the circuit and starting the engine in idle mode (the “ turbo ” LED is on), smoothly adjust R1 to reduce the response threshold until the “ turbo ” LED turns off.

Further operation of the engine at a small load, the “ turbo ” LED turns on, which means that the sensitivity threshold for increasing power is working correctly.

After that, adjust R2 for the required holding time of the connected starting capacitors.

All. Successful operation.

Photo of the finished device.

If anyone is interested in what size the transformer is for this printed circuit board, then this transformer was taken from the power supply of the “Polish antenna” and you may have one lying idle.

Printed Board files , and model in proteus.