Ladder Diagram for Motor Control
This is a free printable ladder logic diagram for motor control: download the diagram as SVG or open it and print to paper or PDF.
A motor control ladder diagram is built from a handful of standard rungs: a three-wire start-stop seal-in circuit, an optional jog rung that bypasses the seal-in, and a forward-reverse pair with electrical interlocking. This page covers the rung-by-rung logic, contact types, and interlock wiring used to control a motor contactor from pushbuttons and an overload relay in a PLC or relay-based control panel.
The building block of motor control ladder logic is the three-wire start-stop circuit. A normally-closed Stop pushbutton sits first in the rung, in series with a normally-open Start pushbutton. The Start pushbutton is wired in parallel with a normally-open seal-in auxiliary contact taken from the motor contactor, M. A normally-closed overload relay contact, OL, sits in series with the whole rung, feeding the M coil. Pressing Start energizes M, the seal-in contact closes and holds the coil energized once Start is released, and either pressing Stop or an overload trip opens the rung and drops out M.
A jog rung uses a separate pushbutton wired directly to the M coil without passing through the seal-in contact. Because there is no latching path, the motor runs only while the jog button is physically held down and stops the instant it is released. Jog logic still runs through the OL contact for overload protection, but it deliberately skips the seal-in branch that makes Start-Stop hold in.
Forward-Reverse control uses two contactors, F and R, each wired to run the motor in the opposite direction. The direction change itself comes from swapping two of the three motor line connections on the reverse contactor's wiring, not from anything in the ladder logic. The ladder logic's job is interlocking: each contactor's coil rung carries a normally-closed auxiliary contact from the other contactor in series, so F and R physically cannot both energize at once, which would short two phases together through the motor. Many combination starters back this up with a mechanical interlock bar between the two contactors. Pushbutton interlocking is also common -- the Forward pushbutton's normally-closed contact sits in series with the Reverse coil rung and vice versa, letting an operator reverse direction directly on some panels, though many designs still require Stop first. A single shared OL contact protects both rungs.
How to wire ladder logic diagram for motor control
- List inputs and outputs Identify the field devices before laying out rungs: Start PB, Stop PB, Jog PB if used, Forward PB, Reverse PB, and the overload relay auxiliary contact as inputs; the Forward contactor coil, Reverse contactor coil, and any running indicator as outputs.
- Build the basic start-stop seal-in rung Place the normally-closed Stop PB contact first, in series. Follow it with the normally-open Start PB in parallel with a normally-open seal-in auxiliary contact from the M coil. Put the normally-closed OL contact in series with the rung, feeding the M coil.
- Add the jog rung if required Wire a separate rung with the normally-open Jog PB in series with the OL contact, feeding the same or a parallel path to the contactor coil, but without any seal-in contact, so the motor only runs while the button is held.
- Convert to two contactors for forward-reverse Duplicate the start-stop pattern into two rungs, one for the Forward contactor F and one for the Reverse contactor R, each with its own seal-in auxiliary contact and sharing the same Stop pushbutton and OL contact.
- Add the electrical interlock between F and R In the Forward rung, add a normally-closed auxiliary contact from the Reverse contactor in series. In the Reverse rung, add a normally-closed auxiliary contact from the Forward contactor in series, so neither can energize while the other is running.
- Wire the physical motor leads for direction reversal On the Reverse contactor's load-side wiring, swap two of the three motor phase conductors relative to the Forward contactor's wiring. This is what actually reverses rotation -- the ladder logic only decides which contactor runs.
- Add pushbutton interlocking if the design calls for it Wire the Forward pushbutton's normally-closed contact in series with the Reverse coil rung, and the Reverse pushbutton's normally-closed contact in series with the Forward coil rung, if the panel is meant to allow direct reversal without pressing Stop first.
- Fit or verify the mechanical interlock and test Confirm a mechanical interlock bar is installed between the Forward and Reverse contactors if the starter combination supports one, then bench test Start, Stop, Jog, Forward, Reverse, and the overload trip before connecting the motor load.
Specifications
| Control circuit voltage | 120VAC control transformer output is standard, 24VDC common on PLC-based panels |
|---|---|
| Control wire gauge | 14-18 AWG stranded, sized to the control voltage and terminal block rating |
| Contactor rating basis | Sized to motor full-load amps per NEMA or IEC rating, not just nameplate horsepower |
| Overload relay setting | Set to motor nameplate full-load amps, adjusted per the OL manufacturer's trip class |
| Discrete input points needed | 5-6 typical: Start PB, Stop PB, Jog PB, Forward PB, Reverse PB, OL contact |
| Discrete output points needed | 2-3 typical: Forward contactor coil, Reverse contactor coil, running indicator |
| Interlock contact type | Normally closed, wired in series in the opposite direction's coil rung |
| Reversal method | Swap two of three motor phase leads on the Reverse contactor's load-side wiring |
Safety warnings
- Lock out and tag out the motor disconnect and control power before working in the panel, and follow NFPA 70E arc-flash procedures for any work near live motor control conductors.
- Verify zero energy state with a meter on every phase conductor and control terminal, even after lockout.
- This work should be done by a licensed electrician or qualified controls technician familiar with motor starter wiring and interlocking.
- Never bypass the overload relay to get a motor running -- it is the primary protection against a locked rotor or sustained overcurrent condition.
- Confirm both the electrical interlock and any mechanical interlock are functioning before applying power to a forward-reverse starter -- a wiring error here can short two phases together.
- Keep hands and tools clear of the contactors during commissioning tests, since a miswired interlock can cause unexpected contactor operation.
Tools needed
- Digital multimeter
- Clamp-on ammeter
- Wire strippers and crimpers
- Lockout-tagout kit
- Torque screwdriver for terminal blocks
- PLC programming software and laptop
- Insulated hand tools rated for the panel's voltage class
Common mistakes
- Wiring the seal-in contact without the Stop pushbutton in series, so pressing Stop cannot drop out the contactor once it has latched in.
- Missing the electrical interlock between Forward and Reverse contactors, allowing both to close together and short two motor phases.
- Sizing the overload relay to the contactor rating instead of the motor's actual nameplate full-load amps.
- Wiring the jog pushbutton through the seal-in contact by mistake, which turns jog into a second start button instead of a momentary-only function.
- Confusing which two leads to swap for reversal and reversing all three, which changes nothing about rotation direction.
- Skipping the mechanical interlock bar on a combination starter that has one, relying on the electrical interlock alone.
Troubleshooting
- Motor stops as soon as the Start button is released
- Cause: Seal-in auxiliary contact missing, miswired, or pulling from the wrong coil's output status Fix: Verify the seal-in contact is in parallel with Start and reads the state of the same coil it feeds
- Jog button behaves like a normal Start button and latches on
- Cause: Jog rung was accidentally wired through or in parallel with the seal-in contact Fix: Re-route the jog pushbutton to feed the coil directly, bypassing the seal-in branch entirely
- Forward and Reverse contactors both attempt to close, tripping the breaker
- Cause: Electrical interlock contacts missing, wired to the wrong contactor, or a mechanical interlock is not installed or is damaged Fix: Trace each rung's interlock contact to the opposite contactor's auxiliary contact and confirm the mechanical interlock bar moves freely and blocks simultaneous closure
- Motor runs but overload trips soon after under normal load
- Cause: OL relay set below the motor's actual full-load amps, or sized to the wrong nameplate value Fix: Recheck the motor nameplate full-load amps and reset the OL relay to the correct value and trip class
- Pressing Reverse while running Forward does nothing
- Cause: Pushbutton interlocking was not implemented, and the panel design requires Stop before switching direction Fix: Confirm the intended design -- if direct reversal is required, add the pushbutton interlock contacts in series with the opposite coil rung; otherwise operators must press Stop first
Frequently asked questions
What is the difference between a seal-in contact and an interlock contact?
A seal-in contact is a normally-open auxiliary contact from a coil's own output, wired in parallel with the start button, so the rung holds itself on. An interlock contact is a normally-closed auxiliary contact from a different coil, wired in series, so that coil cannot energize while the other one is running.
Why does jog wiring skip the seal-in contact?
The seal-in contact is what makes a rung latch on and stay energized after the button is released. Jog needs the opposite behavior -- the motor should run only while the button is physically held -- so the jog rung feeds the coil directly without a seal-in path.
How does the ladder logic actually reverse the motor's direction?
It does not, on its own. The ladder logic only decides whether the Forward or Reverse contactor is energized. The reversal comes from the physical wiring, where two of the three motor phase leads are swapped on the Reverse contactor's load-side connections.
Can I skip the electrical interlock if I have a mechanical interlock bar?
No. The mechanical interlock is a backup, not a replacement. Standard practice is to wire both the electrical interlock, using normally-closed auxiliary contacts between the two contactor rungs, and the mechanical interlock where the starter hardware supports it.
What overload relay setting should I use?
Set the OL relay to the motor's actual nameplate full-load amps, adjusted for the relay's trip class, not to the contactor's current rating. Undersizing the OL trips nuisance faults; oversizing leaves the motor unprotected.
Does pressing the Forward button while the motor is in Reverse do anything?
It depends on the panel design. With pushbutton interlocking, the Forward pushbutton's normally-closed contact breaks the Reverse seal-in and allows direct reversal. Without it, the operator has to press Stop first before switching direction.
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