Open a NEMA reversing starter — an Allen-Bradley Bulletin 505, an Eaton Freedom, a Square D 8736. Inside the enclosure you will find two identically-rated contactors mounted side by side, a standalone overload relay downstream of both, and a mechanical interlock bar linking the two armatures. The line-side bus distributes three-phase power to both contactors. The load-side wiring from one contactor crosses two of the three phases before reaching the overload relay. That crossing is the entire mechanism of motor reversal.
Why Swapping Two Leads Reverses the Motor
A 3-phase motor runs on a rotating magnetic field. Three phase voltages, each displaced 120 electrical degrees from the next, energize three sets of stator windings arranged around the bore. The resulting magnetic field rotates at synchronous speed, and the rotor follows it.
The direction of that rotating field depends on the phase sequence at the motor terminals. For a given winding arrangement, if L1 leads L2 leads L3, the field rotates in one direction. Reverse the sequence so L3 leads L2 leads L1, and the field rotates the opposite way. The motor reverses.
Swapping any two of the three phase connections at the motor terminals reverses the sequence. Swap L1 and L3: the field that was L1-L2-L3 becomes L3-L2-L1. Swap L1 and L2: L2-L1-L3, which is also a reversed sequence. Any single pair swap produces reversal. Swapping all three leads does nothing — the sequence rotates but the direction stays the same.
Convention crosses T1 and T3, keeping T2 straight. This is the standard taught in Petruzella, Rockis and Mazur, and most motor control curricula. The choice of which two to swap is arbitrary from a physics standpoint, but standardizing on T1/T3 means every technician reading the prints expects the same crossing point.
The Reversing Contactor Arrangement
A reversing starter uses two contactors sharing one 3-phase source. One contactor handles the forward direction. The other handles reverse. Only one operates at a time.
Forward contactor — straight-through wiring. Line terminals connect directly to the overload relay inputs: FWD L1 to OL L1, FWD L2 to OL L2, FWD L3 to OL L3. The phase sequence at the motor terminals matches the source. Motor runs forward.
Reverse contactor — load-side phase swap. The line-side connections are identical to the forward contactor: source L1 to REV L1, source L2 to REV L2, source L3 to REV L3. The swap happens on the load side. REV T1 routes to OL L3. REV T2 routes to OL L2 (straight). REV T3 routes to OL L1. T1 and T3 cross. The phase sequence at the motor reverses. Motor runs in the opposite direction.
Both contactors feed the same overload relay. The OL heaters see motor current regardless of which contactor is closed. From the overload relay outputs, the wiring runs straight to the motor: OL T1 to motor T1, OL T2 to motor T2, OL T3 to motor T3.
The load-side swap is the taught standard. There is an equally valid field convention that swaps L1 and L3 on the line side of the reverse contactor instead, keeping the T-terminal wiring straight. Both produce the same phase reversal at the motor. The load-side method is more common in instructional material because it keeps the line-side bus wiring identical for both contactors, which simplifies panel layout and bus bar routing.
Why You Need Interlocking
If both contactors close simultaneously, the motor terminals see two conflicting phase sources. Through the forward contactor, T1 connects to L1 and T3 connects to L3. Through the reverse contactor, T1 connects to L3 and T3 connects to L1. The result: L1 and L3 are shorted together through the motor windings, a bolted phase-to-phase fault. Fault current reaches thousands of amps. Upstream protective devices clear it rapidly, but not before the arc can weld contactor tips, damage bus bars, and create a maintenance shutdown.
Reversing starters use two independent layers of protection to prevent this.
Electrical interlocking uses normally closed auxiliary contacts. Each contactor carries an NC aux contact wired in series with the opposing contactor's coil circuit. When the forward contactor energizes, its NC aux opens and breaks the path to the reverse coil. The reverse contactor cannot pull in while the forward contactor is active. The protection is symmetrical — the reverse contactor's NC aux blocks the forward coil in the same way.
Mechanical interlocking is a physical linkage between the two contactor armatures — a slide bar or rocking beam that prevents one armature from closing while the other is engaged. This is a last line of defense. If an auxiliary contact welds from arc damage, the electrical interlock fails silently. The mechanical interlock still physically blocks the opposing contactor.
NFPA 79 requires both electrical and mechanical interlocking on reversing starters. The rationale is defense in depth: either protection layer can fail independently, but both must fail simultaneously for a phase-to-phase short to occur. Every NEMA-rated reversing starter ships with mechanical interlocks factory-installed. NEMA ICS 2 separately addresses the overload protection side, specifying that a single overload relay must sense motor current regardless of direction of rotation.
The Standalone Overload Relay
In a standard motor starter, the overload relay is built into the starter assembly. The OL heaters sit between the contactor's main contacts and the T-terminals. Current flows through the contactor, through the heaters, and out to the motor. The heaters sense every amp the motor draws.
This integrated arrangement does not work for reversing. The problem is geometry. In an integrated starter, the OL heaters are physically between the contactor's power contacts and its T-terminals. If you use one integrated starter for forward and a bare contactor for reverse, the reverse current enters the starter's T-terminals, passes through the motor, and returns through the reverse contactor — bypassing the OL heaters entirely. The overload relay only protects the forward direction.
The solution is a standalone overload relay — a separate device with its own L1/L2/L3 heater inputs and T1/T2/T3 outputs to the motor. Both contactors feed into this single relay. Forward current flows through the forward contactor and into the OL heaters. Reverse current flows through the reverse contactor and into the same OL heaters. The motor is protected in both directions through one set of heaters, one OL trip mechanism, and one NC contact (95-96) wired in series with both coil circuits.
This is why every NEMA reversing starter you open in the field contains two bare contactors and a standalone overload relay, whether it is an Allen-Bradley Bulletin 505, an Eaton Freedom, or a Square D 8736. The contactor-plus-integrated-OL assembly that works for non-reversing starters is not used here.
Where You See Reversing Starters
Overhead hoists and cranes. The most common application. The motor drives the hoist drum in both directions, up and down. Hoist circuits add limit switches in series with each coil to prevent over-travel at the top and bottom of the hook's range. NEC 610.53 limits pendant push-button station voltage to 150V AC maximum. Control voltage is commonly stepped down to 24V through a control transformer, well within the limit and a practical safety margin when the operator is gripping a metal enclosure in a wet industrial environment. A cable break on the pendant opens the control circuit and stops the hoist — fail-safe by design.
Conveyors. Reversing allows operators to clear jams, back product off a line, or run a conveyor in both directions during different production phases. The control circuit is simpler than a hoist — no limit switches, no pendant, often line-voltage control at 208V.
Machine tools. Lathes, mills, and drill presses use reversing starters for spindle direction control. Some operations require clockwise cutting; others require counter-clockwise. The operator selects direction from the machine's control panel.
Mixers and agitators. Reversing the motor periodically prevents material from settling or channeling in the tank. Some mixer controls alternate direction on a timer without operator intervention.
Reading is one thing — wiring it yourself is another. Open the interactive trainer and build this circuit from scratch.
Wire a reversing hoist circuit with electrical interlocking in the trainer →Frequently asked questions
How do you reverse a 3-phase motor?
Swap any two of the three phase leads at the motor terminals. Convention crosses T1 and T3, keeping T2 straight. This reverses the phase sequence, which reverses the rotating magnetic field in the stator, which reverses the direction of the rotor. A reversing starter automates this by using two contactors — one wired straight-through for forward, one with T1 and T3 crossed on the load side for reverse.
Why does swapping two leads reverse the motor but swapping all three does not?
Three-phase reversal depends on phase sequence, not on which wire is on which terminal. Swapping two leads reverses the sequence (e.g., A-B-C becomes C-B-A). Swapping all three rotates the sequence (A-B-C becomes B-C-A) without reversing it. The field still rotates in the same direction. Any single pair swap reverses; a full three-lead swap just cycles the sequence back to the original rotation.
What happens if both contactors in a reversing starter close at the same time?
A phase-to-phase dead short. Through the forward contactor, T1 connects to L1 and T3 connects to L3. Through the reverse contactor, T1 connects to L3 and T3 connects to L1. L1 and L3 are shorted together through the motor windings. Fault current reaches thousands of amps — enough to weld contactor tips and damage bus bars. This is why NFPA 79 requires both electrical and mechanical interlocking on every reversing starter.
What is the difference between electrical and mechanical interlocking?
Electrical interlocking uses NC auxiliary contacts. Each contactor has an NC aux wired in series with the opposing coil circuit — when one contactor is energized, its NC contact opens and blocks the other. Mechanical interlocking is a physical bar or beam between the two contactor armatures that prevents one from closing while the other is engaged. Both can fail independently — a welded aux contact defeats the electrical interlock, a broken lever defeats the mechanical one. NFPA 79 requires both.
Why do reversing starters use a standalone overload relay instead of a starter with a built-in OL?
In an integrated motor starter, the OL heaters sit between the contactor's main contacts and the T-terminals. If the reverse contactor feeds current into the T-terminals from the opposite direction, that current bypasses the heaters — the OL only protects one direction. A standalone overload relay sits downstream of both contactors, so all motor current passes through the heaters regardless of which contactor is closed. Both directions are protected through one device.
Does it matter whether the phase swap is on the line side or the load side of the reverse contactor?
No — both produce the same phase reversal at the motor. The load-side swap (crossing T1 and T3 between the reverse contactor and the overload relay) is the standard taught in most motor control curricula. The line-side swap (crossing L1 and L3 at the reverse contactor's input terminals) is equally valid and common in the field. Load-side is more common in instructional material because it keeps the line-side bus wiring identical for both contactors.