Thyristor implies an scr or triac.
Since your supply is 180 VDC, it may be best not to connect the 555 to a thyristor, if the thyristor is connected to the supply. I'm not sure what amount of DC is able to come back through the gate of a thyristor, and harm the 555.
I was going to suggest an N-mosfet for the switching device.
10 HP converts to 7600 W. At 180V the current will be 42 A.
There are mosfets that can handle this, as long as you heat sink it sufficiently.