Motor control · lesson 7 of 12
Forward / reverse
A reversing starter is two contactors: F feeds the motor straight, R feeds it with two lines swapped. The one rule that matters: they must never both be closed, because together they short one line to another. The interlocks enforce that. The first lab lets you take them out and find out why they are there.
Electrical interlocks
Look at each branch: in front of the F coil is a normally closed R contact, and in front of the R coil is a normally closed F contact. Whichever contactor is in has opened the other's path. To change direction you must press Stop first.
Forward / reverse with electrical interlocks
Turn on JavaScript to press the buttons and run this circuit.
Do this
Two contactors: F feeds the motor L1-L2-L3, R feeds it with two lines swapped. They must never close together.
Pushbutton interlocks: reverse without Stop
Give each direction button a second, normally closed contact block and put it in the other direction's rung. Now pressing Rev opens the forward rung first; F drops; its NC interlock recloses; R pulls in. The electrical interlocks are still there as the backstop.
Forward / reverse, pushbutton and electrical interlocks
Turn on JavaScript to press the buttons and run this circuit.
Do this
Each direction button also has an NC contact in the OTHER branch.
The power side
On the power diagram, F's three main contacts connect L1, L2 and L3 to the motor's T1, T2 and T3. R's three contacts connect the same motor terminals with two lines crossed — typically L1 and L3 swap places. Close both and L1 meets L3 directly through the contacts. That is why a factory reversing starter also has a mechanical interlock: a bar that physically stops the second contactor closing, which works even if someone has wired the coils wrong.
Questions people ask
How do you reverse a three-phase motor?
Swap any two of the three supply leads. A reversing starter has two contactors: forward feeds the motor L1-L2-L3 straight through, reverse feeds it with two lines crossed.
What is an electrical interlock?
A normally closed contact of each contactor wired in series with the OTHER contactor’s coil. While forward is in, its NC contact in the reverse rung is open, so reverse cannot energize — and the other way round.
What is a mechanical interlock?
A lever or bar between the two contactors that physically stops both closing at once, even if both coils were energized. Reversing starters normally have both kinds, because both closing together is a line-to-line short.
What is a pushbutton interlock?
Each direction button also has a normally closed contact in the other direction’s rung. Pressing Reverse then drops Forward first, so you can change direction without pressing Stop.
Learn one thing a week
Now and then a new diagram, drill, or reference sheet goes up. Leave your email for a short heads-up when it does — no spam, no selling it on, one click to leave.