Motor disconnect

What does A784 mean?

The drive has detected that all three output phases are disconnected from the motor. This can occur due to an open motor circuit, open switches, or faulty cabling, preventing the drive from controlling the motor and potentially causing damage to the drive if it attempts to operate into an open circuit.

• Open motor circuit due to a disconnected motor cable, broken wire, or loose terminal connection at either the drive output terminals (U, V, W) or the motor terminal box.
• Motor thermal overload relay (if external to the drive and wired in series with motor phases) has tripped, opening the circuit to the motor.
• Motor contactor (M1, M2, M3) is not pulling in or its main contacts are faulty/burned, preventing power from reaching the motor.
• Internal drive output phase detection circuit fault, although less common, could falsely report a motor disconnect.
• Very high impedance fault on all three motor phases (e.g., motor insulation extremely degraded but not fully shorted), causing the drive to detect an open circuit.

Click steps to track your progress.

1. 1

   1. Power down the drive and lock out. Use an ohmmeter to measure the resistance between the drive's output terminals (U, V, W) and the motor terminal box to check for continuity on each phase.

2. 2

   2. Visually inspect all motor power cabling from the drive to the motor for signs of physical damage, loose connections, or corrosion at terminals (e.g., U, V, W at drive, T1, T2, T3 at motor).

3. 3

   3. If an external motor contactor is used, verify its coil voltage is present when the drive is commanded to run, and check continuity across its main power contacts (L1-T1, L2-T2, L3-T3) when energized.

4. 4

   4. Check the state of any external motor thermal overload relays or protective devices to ensure they are not tripped or faulty. Reset if tripped.

5. 5

   5. Measure the motor winding resistance directly at the motor terminals (U-V, V-W, W-U) with a multimeter to ensure all three windings are intact and balanced.