hii,i am new to plcs and need to know about NC and NO.
when power is supplied through both NO contact,it closes and powre flows through the rest ofthe circuit.
when a NC contact is used,it must open and power should not flow,but in case of a stop switch,this doesnot happen
i mean when using a start and stop in a ladder program,the normally open strt switch closes and motor starts and if this happpens,then the normally closed stop switch must open and power should not flow...but this doesnt happen
i hope a few of them understaand what i meant
Normally Open and Normally closed can be confusing in a PLC. Allen Bradley uses the "Examine if closed" (XIC) and "Examine if open" (XIO) instructions. Many other PLC's use "Load" (LD) and "Load inverse" (LDI), represented by the normally open and normally closed contacts respectivly in the ladder.
Remember, that a PLC, loads descrete inputs into registers, which is either a logic "1" or "0". An "XIC" or "LD" contact (normally open symbol) is true, when there is a "1" in the addressed register. "XIO" and "LDI" contacts (normally closed symbol) is true, when there is a "0" in the addressed register.
This can become confusing in the case of an Stop pushbutton, when an actual Stop condition is logic "0" in the register. In a motor control circuit, a stop button is wired through the N.C. contacts (shown on the print). However, when the logic is transfered to a PLC, the stop button is still wired normally closed (loss of power = stop condition), but the logic in the PLC is a Normally open symbol. (Logic 1 lets the output be true)
I'll often try to label the registers in my PLC programs to reflect the N.C. nature of an input for clarity. Therefore the true condition (1) reflects the comment. For example a low pressure switch, which is pressure to close (Logic 0 = low pressure stop running). I would label this "Not Low Pressure" so that when I am reading the logic, a -| |- I would read if "Not low pressure" and -|/|- would be if Not "Not low Pressue" In other cases, I may simply put a comment, that it is a N.C. contact. Another way would be to label it "Above low Pressure"
The key with this is to be consistant... I have seen some programs, in which registers are labeled both ways, irreguardless of the inputs wiring, and it becomes very difficult to keep track of what a logic 1 or a logic 0 acutally means.
-MUR