A forward-reverse motor control diagram uses two contactors to swap two of the three power phases, altering the rotating magnetic field to reverse a 3-phase AC induction motor. The core mechanism relies on a mechanical and electrical interlock system to prevent both contactors from closing simultaneously, which would cause a catastrophic phase-to-phase short circuit. Below is a complete node-by-node trace, terminal mapping, and verification guide for a standard 480V AC power / 120V AC control reversing starter.
Decoding the Motor Control Diagram Forward Reverse Symbols
Before tracing the wires, you must understand the schematic shorthand. Most US industrial diagrams follow NEMA or IEC hybrid conventions. In a typical reversing schematic, you will encounter these specific symbols:
- Contactors (KM1 / KM2 or F / R): Represented by a box or coil symbol. The main power contacts are shown as heavy lines, while auxiliary contacts (used for interlocking and sealing) are shown as lighter lines with a distinct NEMA/IEC designation (e.g., 13/14 for Normally Open, 21/22 for Normally Closed).
- Pushbuttons (PB): Forward and Reverse are Normally Open (NO) momentary contacts. The Stop button is a Normally Closed (NC) momentary contact.
- Overload Relay (OL or F2): Shown as a heater element symbol in the power circuit and a set of NC contacts (usually designated 95/96) in the control circuit.
- Mechanical Interlock: Often denoted by a dashed line physically linking the two contactor armatures, indicating they cannot physically close at the same time.
Terminal and Pin Mapping Table
Physical devices use standardized terminal markings. When wiring a NEMA-style reversing starter (like a Square D Telemecanique or Allen-Bradley Bulletin 100), use this reference to map the schematic to the physical screw terminals.
| Component | Terminal ID | Function | Typical Wire Color (US) |
|---|---|---|---|
| Main Disconnect / Contactor Line | L1, L2, L3 | 3-Phase AC Power Input (480V) | Brown, Orange, Yellow (or Black/Red/Blue) |
| Contactor KM1 (Forward) Load | T1, T2, T3 | Output to Overload (Phase Sequence A-B-C) | Brown, Orange, Yellow |
| Contactor KM2 (Reverse) Load | T1, T2, T3 | Output to Overload (Phase Sequence C-B-A) | Yellow, Orange, Brown (Swapped L1/L3) |
| KM1 / KM2 Auxiliary NO | 13, 14 | Holding / Seal-in Circuit | Red (Control 120V) |
| KM1 / KM2 Auxiliary NC | 21, 22 | Electrical Interlock | Red (Control 120V) |
| Thermal Overload Power | 1, 3, 5 (In) / 2, 4, 6 (Out) | Pass-through for motor current | Match phase colors |
| Thermal Overload Control | 95, 96 | NC Trip Contact (Breaks control circuit) | Red / Black (Control 120V) |
| Control Transformer | X1, X2 | 120V AC Secondary (Hot / Neutral) | Red (Hot), Black/White (Neutral) |
Node-by-Node Trace: Source to Load
To truly understand the circuit, we must trace the current path from the source, through the control logic, to the final load. This trace assumes a 480V 3-phase power supply and a 120V AC control circuit derived from a Control Power Transformer (CPT).
The Power Circuit (480V AC)
- Source to Disconnect: 480V 3-phase enters the fused disconnect switch. The Equipment Grounding Conductor (EGC) is bonded directly to the panel ground bus and bypasses the disconnect.
- Disconnect to Contactors: L1, L2, and L3 feed the line side of both KM1 (Forward) and KM2 (Reverse) main contacts in parallel.
- The Phase Swap (Crucial Step):
- On KM1 (Forward), the load side connects straight through: L1 to T1, L2 to T2, L3 to T3.
- On KM2 (Reverse), two phases are swapped at the load side: L1 connects to T3, L2 connects to T2, and L3 connects to T1. L2 remains static. This swap reverses the rotating magnetic field in the stator.
- Contactors to Overload: The outputs of T1, T2, and T3 from both contactors are jumpered together and feed into the line side (1, 3, 5) of the thermal overload relay.
- Overload to Motor: The load side (2, 4, 6) of the overload relay feeds the motor terminal box (U1, V1, W1).
The Control Circuit (120V AC) and Ground Path
- Control Source: The CPT secondary X1 (120V Hot) feeds the Stop Pushbutton (NC). X2 (120V Neutral) connects directly to the common return bus for all contactor coils.
- Stop to Overload: Current passes through the Stop PB, then through the Overload NC trip contacts (95 to 96). If the motor overheats, 95-96 opens, killing all control power.
- Directional Split: The circuit splits at the Forward PB (NO) and Reverse PB (NO).
- Electrical Interlocks:
- Pressing Forward sends power through the KM2 NC auxiliary contact (21-22). If KM2 is engaged, this path is broken.
- Pressing Reverse sends power through the KM1 NC auxiliary contact (21-22).
- Coil Energization & Seal-in: Power reaches the KM1 or KM2 coil. Simultaneously, the corresponding NO auxiliary contact (13-14) closes, bypassing the pushbutton to maintain the circuit (seal-in) when the operator releases the button.
- Ground Path Distinction: The X2 neutral return completes the 120V control circuit. This is not a safety ground. The actual safety ground (EGC) is a dedicated green or bare copper wire routed from the main panel ground bar directly to the motor frame's designated pecking point and the contactor enclosure. The EGC carries zero current during normal operation and exists solely to clear faults.
Verifying Connections with a Multimeter
Before applying 480V power, you must verify the wiring with a digital multimeter (DMM) like a Fluke 87V. Ensure the main disconnect is locked out and tagged out (LOTO) and verify zero energy before proceeding.
- Verify the Phase Swap (DMM in Continuity/Diode Mode): Place one probe on L1 at the disconnect and the other on the T1 terminal at the overload relay. You should read continuity (near 0 ohms) through KM1, but an open circuit (OL) through KM2. Repeat for L3 to T1: it should be open through KM1, but continuity through KM2. This confirms the L1/L3 swap is correct and no dead shorts exist.
- Test the Electrical Interlocks (DMM in Continuity Mode): Place probes across the KM1 coil terminals. Manually push the KM2 armature in with a screwdriver or your finger. The meter should read open (OL) because the KM2 NC interlock in the KM1 coil path opens. If it reads continuity, your interlock wiring is bypassed or incorrect.
- Check Coil Resistance (DMM in Ohms): Measure across the KM1 and KM2 coil terminals (A1 to A2). A healthy 120V AC contactor coil typically reads between 10 and 50 ohms. A reading of 0 indicates a shorted coil; an OL reading indicates a blown internal coil wire.
- Verify Overload Trip (DMM in Continuity Mode): Place probes across the control circuit input and the coil common. Trip the overload relay manually using the test button. The meter should transition from continuity to OL, confirming the 95-96 NC contacts are wired in series with the control power.
For deeper troubleshooting on industrial control logic and component selection, refer to the Electrical Construction & Maintenance (EC&M) guide on motor control circuits, which details NEMA sizing and thermal coordination.
Frequently Asked Questions
What happens if both forward and reverse contactors close simultaneously?
If both contactors close at the exact same time, the swapped phases (L1 and L3) will bridge directly across the contactor load terminals, creating a massive phase-to-phase dead short. This will result in an explosive arc flash, vaporized copper busbars, and tripped upstream breakers. This is why both electrical interlocks (NC auxiliary contacts) and physical mechanical interlocks (a plastic/metal block preventing both armatures from pulling in) are strictly required by code.
Can I reverse a single-phase AC motor using this same diagram?
No. A 3-phase reversing diagram relies on swapping two power legs to alter the rotating magnetic field. Single-phase AC motors do not have a naturally rotating field; they rely on a start winding and a run winding phase-shifted by a capacitor. To reverse a standard single-phase motor (like a NEMA 56 frame compressor motor), you must physically swap the connection of the start winding leads (typically terminals 5 and 8 on the motor's internal terminal board) relative to the run winding, usually via a specialized drum switch or a DPDT relay configuration, not a 3-phase contactor swap.
Why does my motor hum but not start in the reverse direction?
A humming motor that fails to rotate usually indicates single-phasing (loss of one of the three phases). In a reversing circuit, this is most commonly caused by a blown fuse on the specific leg that was swapped (e.g., the L1 or L3 feed to KM2), a loose terminal connection on the KM2 load side, or a failed mechanical interlock that is physically preventing the KM2 armature from pulling in completely, leaving the main contacts barely touching and arcing. Check the fuses and measure the voltage at the T1, T2, and T3 terminals on the overload relay while the Reverse button is pressed.
How do I size the thermal overload relay for a reversing starter?
The thermal overload relay is sized exactly the same way as a standard non-reversing starter: typically 115% to 125% of the motor's Full Load Amps (FLA) as printed on the motor nameplate, in accordance with NEC Article 430.32. The physical placement of the overload relay must be on the common load side, after the outputs of both KM1 and KM2 have been jumpered together, ensuring that the motor current passes through the overload heaters regardless of which direction the motor is spinning.
