Reading drawings · Lesson 4 of 7

Reading a control circuit

The control circuit is the most important drawing for troubleshooting. You will read it one current path at a time, then press the buttons yourself and see where the current flows and at which contact the voltage stops. You will also see it in the European IEC style and the American ladder style.

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1How to read any control circuit

Start at the supply: L (or +24V) at the top in IEC, or the left rail in a ladder.
Follow each current path to the load: every path ends with something that operates: a coil, a lamp or a PLC input.
Things in series = AND: all of them must close.
Things in parallel = OR: one of them is enough.
Remember: the drawing is drawn with everything de-energised. Ask: "When this coil is energised, what will its contacts in the other paths do?"

The path sentence: coil -K1 is energised if: the fuse is healthy and the overload has not tripped and Stop is not pressed and (Start is pressed or -K1 13-14 is closed).

2DOL circuit: read it and run it

This is a direct on line (DOL) starter circuit for a motor. Read it step by step, then press the buttons (press and release). The red line = live, and the moving yellow dots = current flowing. You can switch to the American ladder and see the same behaviour.

Read it path by path

Run the circuit

Focus on the seal-in: the Start button springs back by itself, so what keeps the coil energised? Contact -K1 13-14, wired in parallel with the button. As soon as the coil picks up, this contact closes and holds the coil in by itself.

3Reversing circuit and interlocks

To make a 3-phase motor run in reverse, we swap two phases. This is done with two contactors in the power circuit: -K1 forward and -K2 reverse. If both pick up together = a phase-to-phase short circuit! That is why there is an interlock:

Electrical interlock: contact -K2 21-22 (NC) in the coil path of -K1, and -K1 21-22 in the path of -K2. Whichever picks up first opens the other one's path.
Mechanical interlock: a part fitted between the two contactors that stops both from closing together even if both get power. It is drawn as a dashed line between the two coils, or written next to them.
And sometimes a push-button interlock: an NC contact of the REV button in the FWD path, and vice versa.

Run the reversing circuit

4From the field

Things you will come across:

The motor runs only while you hold the button: the 13-14 seal-in contact or its wire. (You will try this in the lesson 7 game.)
The contactor chatters: low coil voltage, or a loose contact in the path.
Someone removed the interlock "so the motor reverses faster": the first time both pick up, the contacts will weld or the breaker will trip. Put it back.

Safety: a 230V control circuit can kill just like the power circuit. And sometimes it carries voltage fed from another panel (foreign voltage) that does not go off when you open the main switch. The drawing marks it with an orange colour or a note: look for it.

?Lesson quiz

Get at least 5 of 6 right to complete the lesson.

Symbols follow IEC 60617 and ANSI/NEMA (IEEE 315 / NFPA 79); device tags follow IEC 81346. Every company draws a little differently: the legend page at the front of the drawing set is the reference. Your progress is saved on this device only.
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