Reversing Starters and Interlocking

Reversing Starters and Interlocking

Why Reverse a Motor?

Many construction and industrial applications require a three-phase induction motor to run in both directions — hoists that raise and lower loads, conveyor systems that need to clear jams, and HVAC damper actuators are common examples. A standard motor starter runs the motor in only one direction. A reversing starter adds a second contactor that swaps any two of the three line connections to the motor, which reverses the rotating magnetic field and therefore reverses shaft rotation.

How Reversal Works Electrically

A three-phase motor's direction is determined by the phase sequence delivered to its terminals. If the line is connected T1-T2-T3 in the forward direction, swapping any two phases — for example, connecting T1-T3-T2 — produces the opposite sequence and the motor runs in reverse. The reversing starter accomplishes this by wiring two contactors (Forward and Reverse) so that when the Reverse contactor closes, it crosses two of the motor leads relative to the Forward contactor.

The Critical Hazard: Phase-to-Phase Short Circuit

Here is the central safety problem: both contactors must never close at the same time. If Forward and Reverse contactors close simultaneously, two phases of the supply are connected directly together through the contactor power contacts. This creates a bolted phase-to-phase short circuit — an extremely high fault current that can destroy equipment, cause an arc flash, and injure or kill anyone nearby. Preventing simultaneous closure is not optional; it is the primary design requirement of every reversing starter.

Mechanical Interlocking

A mechanical interlock is a physical linkage — typically a bar or bracket supplied in a mechanically interlocked mounting kit — that connects the two contactors so that when one closes, it physically prevents the other from closing. It works regardless of the control circuit wiring. Think of it as a mechanical deadbolt between the two contactors. Always install the mechanical interlock before wiring the control circuit; it is your last line of defense if control wiring fails.

Electrical Interlocking

An electrical interlock uses each contactor's own auxiliary contacts wired into the opposing contactor's coil circuit. The wiring works as follows:

• The Forward contactor has a normally closed (NC) auxiliary contact wired in series with the Reverse coil. When Forward is energized, that NC contact opens and kills power to the Reverse coil — even if the Reverse button is pressed.
• The Reverse contactor has a NC auxiliary contact wired in series with the Forward coil. When Reverse is energized, the Forward coil circuit is broken.

This is sometimes called cross-interlocking or anti-plugging interlock wiring. Used alone, electrical interlocking is reliable but can fail if contacts weld shut. Used together with mechanical interlocking, the two methods provide the redundancy required by good practice and expected by NCCER and industry standards.

Control Circuit: Push-Button Station

The forward-reverse-stop station contains three buttons:

• Stop (NC): Breaks both coil circuits when pressed; de-energizes whichever contactor is running.
• Forward (NO): When pressed, completes the Forward coil circuit (provided the Reverse NC auxiliary is closed and Stop is closed).
• Reverse (NO): When pressed, completes the Reverse coil circuit (provided the Forward NC auxiliary is closed and Stop is closed).

Each run button is also sealed in (held) by a normally open (NO) auxiliary contact from its own contactor wired in parallel with the run button — this is the holding circuit (also called seal-in or memory circuit) that keeps the motor running after the operator releases the button.

Safe Wiring Procedure

1. De-energize and lock out / tag out (LOTO) the training panel before any wiring. Verify absence of voltage with a calibrated meter.

2. Install and verify the mechanical interlock on the two contactors before any other wiring.

3. Wire the power circuit (line side → Forward contactor → motor terminals; Reverse contactor wired to swap two phases) according to the schematic.

4. Wire the control circuit: Stop NC → Forward NC auxiliary → Forward coil; Stop NC → Reverse NC auxiliary → Reverse coil; add seal-in contacts.

5. Perform a pre-energization continuity check using a multimeter: verify each coil circuit path, verify NC auxiliary contacts open when the opposite contactor is manually depressed by hand, and verify no continuity between phase conductors through the power contacts.

6. Have the instructor verify all wiring and both interlocks before energizing.

7. Energize under instructor supervision. Test Stop, Forward run, Stop, Reverse run. Attempt direct Forward-to-Reverse (without Stop) and confirm the motor does not reverse without stopping.

PPE and Safety Rules for This Lab

• Safety glasses required at all times on the lab floor.
• Insulated gloves and face shield when instructor energizes the panel.
• Never defeat or bypass an interlock for any reason.
• If a contactor hums excessively, smells burned, or trips the breaker, stop, open the disconnect, and notify the instructor immediately.
• Follow LOTO every time you modify wiring, even between tests.

Overload Relay

The overload relay protects the motor from sustained overcurrent. In a reversing starter it is typically placed in the motor branch after both contactors' load-side contacts so it protects in both directions. If the overload trips, identify the cause (mechanical jam, motor fault, incorrect setting) before resetting.