I would like to know the principle behind the working of Motor protection circuit breaker

Short circuit protection which a circuit breaker provides is generally different that motor protection. I'll desl quickly with motor protection.

The circuit would be short circuit protected at the panel by a circuit breaker.

At the motor starter:
a) Fuse size will be selected since it fine tunes protection.
b) An electronic overload or mechanical overload would be selected.

The mechanical overload is composed of a low melting point alloy held under tension. If the overload is severe enough, the alloy melts allowing the latch mechanism to work

In the electronic version wires pass through either a coil or hall effect sensor to measure the current. This electronically looked at to see if the overload and time is severe enough to warrant a trip.

The motor starters also contain a definite purpose contactor for HP rated loads. That contactor opens when the load is exceeded. It does not reset automatically.

A motor protection circuit breaker consists of a magnetic short circuit protection mechanism and a thermal overload mechanism specifically rated for the motor it will protect instead of the conductors. The sudden excessive short circuit current will increase the magnetic flux and operate the mechanism to trip the breaker. The thermal overload operates like the thermal overload of a motor starter to prevent the motor coil from burning. Typical overload trip current is within 1.5 x full load curent of the motor, usaully 1.25 x full load. The breaker must be chosen with the rating of the motor in mind. Most of these breakers that I have seen are actually a combination of a starter and a circuit breaker in that they have push buttons or other operating handles on them. Some have other auxiliary contacts for other motor control functions.

I think that Cuttler-Hammer (formerly Westinghouse) do make some which look exactly like an ordinary circuit breaker i.e. without control operators.

These breakers are primarily found in motor control centers.

Mpcb gives both short circuit and overoad protection while mccb gives only short circuit protection.