Reversing an AC motor’s direction depends on the motor type. For three-phase motors, you swap any two of the three power leads. For single-phase motors, you reverse the connections on the start winding. Both methods change the direction of the rotating magnetic field inside the stator, which is what determines which way the shaft spins.
Why Swapping Wires Changes Rotation
Every AC motor works by creating a rotating magnetic field inside the stator (the stationary outer part). The rotor (the spinning inner part) chases that field. The direction the field rotates is set entirely by the sequence in which current flows through the windings. Change that sequence, and the field spins the other way, taking the rotor with it.
In a three-phase motor, three sets of currents flow 120 degrees apart in phase. That timing creates a smooth, constant-strength magnetic field that sweeps around the stator in one direction. Swapping any two of those phases flips the sweep direction. In a single-phase motor, a separate start winding creates an artificial phase difference to get the field rotating. Reversing that start winding’s polarity flips which way the field kicks off.
Reversing a Three-Phase Motor
This is the simplest reversal in all of motor wiring. Disconnect power, then swap any two of the three supply lines at the motor terminals. It doesn’t matter which two you choose. If your lines are labeled R, Y, and B (or L1, L2, L3), you could swap R and Y, R and B, or Y and B. The result is the same: reversed rotation.
In practice, most industrial installations handle this with a pair of contactors wired in a reversing starter configuration. One contactor connects the phases in the original order, the other connects them with two phases swapped. An interlock prevents both from energizing at the same time. If you’re wiring a reversing starter, always confirm the motor has come to a complete stop before switching direction. Reversing a motor while it’s still spinning (called plugging) combines the supply voltage with the motor’s own back-EMF, creating enormous mechanical stress on the shaft, coupling, and driven equipment.
Reversing a Single-Phase Motor
Single-phase motors are more involved because they have two windings: a main (run) winding and an auxiliary (start) winding. The start winding is offset from the main winding and creates just enough of a phase difference to get the rotor moving in one direction. To reverse the motor, you swap the two leads of the start winding only. You leave the main winding connections exactly as they are.
The key requirement is that you can isolate both ends of the start winding. On motors designed for reversible operation, the manufacturer brings all four winding leads out to the terminal box, making the swap straightforward. NEMA standard terminal markings label the main winding leads as T1 and T4 (sometimes T8), and the start winding leads as T5 and T8 (with color codes of black for T5 and red for T8). If your motor has these markings, swap the connections on T5 and T8 relative to the line and neutral.
On motors not designed for easy reversal, the start winding leads may be connected internally, often soldered or crimped inside the motor housing. In that case, you’d need to open the motor and physically reroute the start winding connections. One important point: if you swap both the start and run winding leads at the same time, the relative polarity stays the same and the motor spins in the original direction. You must reverse only one winding.
Capacitor-Start and Capacitor-Run Motors
Capacitor-start motors use a capacitor in series with the start winding to create a larger phase shift, giving them stronger starting torque. The reversal method is identical: swap the start winding leads. The capacitor stays in the same circuit, it just sees current flowing the other direction through the start winding. The centrifugal switch that disconnects the start winding at speed doesn’t need any changes either.
Permanent split-capacitor (PSC) motors, common in fans and blowers, work the same way. If the motor has accessible start winding leads, reversing them reverses rotation. Many PSC motors used in ceiling fans and small appliances are specifically wired with accessible leads for this purpose.
Motors That Can’t Be Easily Reversed
Shaded-pole motors are the main exception. These small, inexpensive motors (found in bathroom exhaust fans, small desk fans, and microwave turntable drives) use copper shading rings physically embedded in the stator poles to create the phase difference that starts rotation. The rings are part of the iron core, not wired connections. You cannot reverse a shaded-pole motor by changing any wiring. The only way to reverse one is to disassemble the motor and physically flip or rotate the stator relative to the housing, which is impractical in most cases. If you need the opposite rotation from a shaded-pole motor, replacing it with one wound for the desired direction is usually the better option.
Safety Considerations
Always disconnect and lock out power before changing any motor wiring. After making your changes, bump-test the motor briefly to confirm the new direction before running it under load. Some equipment, like pumps, compressors, and screw conveyors, can be damaged by running in the wrong direction even for a few seconds.
Avoid reversing direction while the motor is still spinning. When you apply power in the opposite direction to a spinning motor, the supply voltage and the motor’s back-EMF add together rather than opposing each other. This produces a massive current spike and a strong mechanical shock load through the shaft, coupling, and connected equipment. Let the motor come to a complete stop first. In automated systems where mid-motion reversal is necessary, purpose-built reversing starters with time delays or braking resistors handle the transition safely.
On three-phase systems, if you’ve just had utility work done or connected to a new power source, the phase sequence may have changed without any wiring changes on your end. A phase rotation meter (an inexpensive handheld tool) lets you verify the sequence at the disconnect before energizing the motor.