Oh Yes Rashu !!

Now I get you. What you are talking about is interlocks when you are mentioning "auxilliary relays".

Putting it in a logic parlance would be to say that K is interlocked with K3 which is turn is interlocked with K1 and K2. All such relationships can be transformed into Boolean logic.

In this case it will be simply K = K1&K2&K3

ANd when you transform this into a MCU setup an interesting things happen.

Case 1:
If K1,K2 and K3 are used for some other functions also then simple would be to use thier NO contacts as inputs to MCU. SO in this case K1,K2,and K3 will feature both in DI as well as DO listings. And when you want to switch on K, you simply check if K1,K2,K3 inputs are active.

However since you want to elimnate "auxilliary" relays another way would be,

Case 2:
If K1 represents the OverCurrent relay and if K2 represents the Over Temp relay, then the NC contacts of both these sensors can directly be wired as inputs to the MCU. And whenever you want to switch on K, you simply check if these 2 input bits are active. Thats it - one shot you have knocked off K1,K2,K3.

Caution : The real story does not end there however.

A. It is not only enough to check K1 and K2 bits before switching on K, but also at frequent intervals of 10 ms or so AS_LONG_AS motor K is running. This can be easily handled in a Timer ISR.

B. If K1 and K2 are also serving some other functions like annunciation, starting a cooling system etc then that has to be accounted for also.

That should do for a start I think.

Raghu