PlantFlow 3D › Guides › How to read electrical control circuit diagrams (schematics)
How to read electrical control circuit diagrams (schematics)
How to read motor control schematics: power and control circuits, device letters KM, KA, KT, F, S, H, contacts 13-14 and 21-22, and tracing the current path.
Two circuits on one drawing
A motor control drawing almost always shows two separate circuits. The power circuit carries the motor current from the supply through the breaker, the contactor main contacts and the overload relay to the motor. The control circuit is a low-power circuit, usually 230 V AC or 24 V DC, that decides when the contactor coil is energised.
Read the power circuit first to understand what the machine does, then read the control circuit to understand when it does it. In PlantFlow 3D the power circuit is on the left of the 2D diagram and the control circuit on the right.
Device letters and numbers
IEC 81346 letter codes tell you what each device is: Q or QF for a breaker, KM for a power contactor, KA for a control relay, KT for a timer, F for protection such as an overload relay or MCB, S for push buttons and switches, H for pilot lamps, M for motors and T for transformers.
Terminal numbers follow a pattern. Main contacts are 1-2, 3-4 and 5-6. Auxiliary contacts ending in 3-4 are normally open (13-14, 43-44); those ending in 1-2 are normally closed (21-22, 31-32). Coils are A1-A2. On an overload relay 95-96 is the normally closed trip contact and 97-98 the normally open alarm contact.
Contacts are drawn at rest
Every contact is drawn in its de-energised, unpressed state. A normally open START button is drawn open; a normally closed STOP button is drawn closed. When you read a schematic, ask “what changes when this coil is energised?” and mentally flip every contact belonging to that device.
Trace the current path
Start at the control supply L, follow the line through each contact and stop at the coil, then continue from A2 back to N. If any contact in the path is open, the coil is off. This is exactly how a technician fault-finds with a voltmeter: measure from N to each point along the path until the voltage disappears.
PlantFlow 3D draws live current in colour on the diagram, so you can see the path change when you press a button.
Try it yourself
Open the DOL starter in the 2D diagram, turn on Current flow and press START. Follow the coloured path from X1.L through S0, S1 and the 13-14 contact to KM1 A1.
Open the PlantFlow 3D simulator →Safety: this guide explains principles for learning. Real installations must follow the applicable codes, the equipment manufacturer's instructions and the approval of a competent, qualified person. Isolate, lock off and prove dead before touching any circuit.