Star Delta Timer Working:
A simple ON-delay timer which is associated with the star delta starter is called star delta timer. It consists of two Normally open contacts. In a starter, closed contact (after receiving input) is used in the star side circuit and Normally Open contact is used in the delta side starter circuit.
For understanding let we take Siemens to make 3RP1576-1NP30 model electronics timer.
Construction & Timer circuit:
Construction front, it consists of three input terminal of A1, A2 and A3. In this A1-A2 terminal will be used for AC input and A3-A2 Terminal will be used for DC 24Volts Input. Here A2 terminal works as a common terminal for both AC and DC input.
Note that while connecting the input terminal, we should be very careful since interchanging the supply (ie. If you give AC 230 Volts in the DC terminal means it will burn the circuit) lead to damage the timer.
17NC => common terminal
18NO => As default without input supply it will be NO Contact. While providing input supply to the timer, it will become NC after the set time it will become NO (delayed NO)
28NO => As default without input supply this terminal will be as NO and even after giving the input supply, this will be NO only. After reaching preset timing, it will become NC.
Timer Dial: It consists of 0-60 sec rotating dial. You can adjust the timing by pointing the arrow.
Working principle of star delta timer:
Initially, the terminal 17NC will be common, and the terminal 18 will be connected to the star coil and 28 terminals will be connected to the delta coil. While giving input supply to the timer A1 and A2 terminal, the 18NO become 18NC hence the star coil receives the power. But 28NO become NO only.
After reaching set timing, the 18NC become 18 NO and 28 NO become 28 NC then delta coil receives the power until you turn off the A1 and A3 of the input power.
Also, you must see: Why 17/28 contact will not be used for delta coil.
Look at the operation:
How to convert Star delta timer into on delay timer?
Look, on delay timer means “delayed ON” it means NO become NC after certain timer period. The same function will happen in this timer also. By using 28NO terminal we can use star delta timer as on delay timer.