When working with motors that need to change direction, such as in industrial automation equipment, it is essential to ensure safe and reliable control. One common method to achieve this is by using relays for motor reverse protection. Relays act as solenoid-driven contacts that can isolate high-power loads involved in motor operation while allowing PLC signals to manage the direction changes.
To begin, you need two relays: one for forward rotation and one for reverse. These should be configured for the drive’s power rating. The motor has two power leads, and reversing the current flow direction across these terminals changes the turning direction. The relays are wired in such a way that when the forward relay activates, it establishes forward polarity, and when the REV relay engages, it swaps the connections to reverse the direction.
It is critical to prevent both relays from being activated at the same time. If both relays close simultaneously, it will create a voltage bypass condition, which can warp relay contacts or the relays. To avoid this, implement a dual-guard relay logic. A physical linkage uses a rigid coupling that prevents both relays from being engaged together. An NC relay interlock uses the normally closed contacts of each relay in the control circuit of the other relay. This means that if the forward relay is on, its normally closed contact opens and disables the reverse relay’s control circuit, and the reverse relay blocks forward.
Additionally, include a neutral position that deactivates both relays before switching direction. This ensures the motor fully decelerates before reversing, reducing bearing wear and voltage spikes. Use a latching stop relay in the relay power path to interrupt power to both relays before activating the reverse relay.
Use UL-listed overcurrent protection to protect against excessive current. These should be calculated for LRA and FLA, as motors pull 5–7x rated current on energization. Place the fuses on the power supply side of the relay circuit to protect the wiring and relay coils.
Always ensure that the wiring is secure and insulated properly, especially since motor circuits can emit RF interference and arcing. Use shielded cables for PLC outputs if the environment has high-power equipment. Test the system thoroughly with the motor unloaded to verify that the only one relay activates at a time.
Relays are a proven industrial technique for bidirectional motor switching. With compliant installation practices like mechanical and electrical safeguards, they provide long-term operational safety and extend the life of both the motor and the control system. Always follow NEC, IEC, or OSHA standards when assembling relay-based motor circuits.
