rating of contactor

They need to be rated above the full load current of the motor.

The voltage cannot be seen from here, so the best thing to do is to look at either the motor rating plate attached to it, or the manufacturer's type test and performance rating data that came with it.

If nether approach works, telephone the manufacturer with the serial number of the motor to hand, and ask directly. Once this information is to hand, telephone the contactor manufacturer for confirmation of the selection before installing and wiring it.

If the rating plate and the serial number cannot be found, then the installation is overdue for a replacement motor - one with known performance properties.

Assuming the 55kW refers to a 3-phase supply of 380-415V, here is the recommendation from Schneider Electric.

As you can see, the Main and Delta contactors are of 50A and the Star contactor is 40A. It is strongly recommended that you use (a) a mechanical interlock between Star and Delta contactors and (b) a special Star/Delta timer with a transition time delay.

Ideally, you should not use a ΥΔ starter at all, since it is something of a compromise. You will be better off using a SoftStarter.