3.6 Motor Troubleshooting, Sequential Starting, and Programmable Logic Controllers (PLC)

3.6 Motor Troubleshooting, Sequential Starting, and Programmable Logic Controllers (PLC)

Quick Summary: The written syllabus and the practical test both demand structured motor fault-finding: isolate and lock out first, verify the supply, then diagnose from symptoms. The classic signatures are no-start, abnormal humming (single-phasing), stalling under load, overheating, and reverse rotation. Modern panels coordinate motors with sequential starting, soft starters, and PLCs — but emergency stops remain hard-wired.

1. Safe Diagnostic Sequence

1. Isolate and Lock-Out/Tag-Out (LOTO): open the isolator, apply a personal padlock and warning tag, and prove dead with a tested voltage indicator before touching terminals (Chapter 6.1 SLD duties and Chapter 8 LOTO practice).
2. Verify the supply: measure all three line-to-line voltages (≈ 400 V) and line-to-neutral (≈ 230 V) at the starter input; check the main MCB/MCCB and HRC fuses.
3. Check the control circuit: control fuse, stop and start pushbuttons, thermal overload relay (95–96) status, and contactor coil voltage (A1–A2).
4. Measure the motor: insulation resistance to earth (≥ 1 MΩ at 500 V DC) and winding continuity/balance across U–V, V–W, W–U.

2. Symptom Diagnosis Matrix

Single-Phasing Deep Dive

If one line fuse blows or one contactor pole welds open, a running 3-phase motor continues on two phases: the healthy windings carry current well above FLC, torque collapses, and the motor hums and overheats. Modern thermal overload relays include differential (phase-loss) tripping (Chapter 3.2); without it, the winding burns within minutes. A humming motor must be stopped immediately — the locked-rotor current is 6 × FLC or more.

3. Sequential Starting of Multiple Motors

Starting several DOL motors simultaneously multiplies inrush and collapses busbar voltage. Sequential starting staggers motors using timer relays or PLC logic:

• Start the largest or process-critical motor first, then subsequent motors at 3–10 second intervals as each reaches speed.
• In conveyor trains, the sequence is often downstream-first: the discharge conveyor starts before the feed conveyor so material never piles onto a stopped belt.
• Interlocks enforce the order: the start circuit of motor 2 passes through a running-contact of motor 1 or a timer contact.

4. Soft Starters and Reduced-Voltage Alternatives

• Soft starter: thyristor (SCR) voltage ramping limits starting current to roughly 2–4 × FLC with adjustable ramp time (2–30 s) and torque — smoother than star-delta, with no open transition; a bypass contactor carries the run current after ramp-up.
• Comparison: DOL (6–8 × FLC, cheapest, typically ≤ 3.7 kW), star-delta (⅓ current and torque, light loads), soft starter (2–4 × FLC, adjustable), VFD (full speed control plus soft start, highest cost).
• DC machines and welding sets: DC motors and DC welders deliver smooth torque and stable arcs; the syllabus expects awareness that DC welding sets are preferred over AC sets for general site welding because the rectified DC arc is more stable and easier to strike.

5. Programmable Logic Controllers (PLC), Relays, and Contactors

A PLC replaces hard-wired relay logic with a programmable unit: input devices (pushbuttons, limit switches, TOR contacts) are wired to input terminals; the stored program (traditionally ladder logic, mirroring the hard-wired diagrams of Chapter 3.2) evaluates rungs each scan cycle and drives outputs wired to relays and contactors that switch the motor power circuit.

• Advantages: logic changes by reprogramming rather than rewiring; built-in timers and counters for sequential starting; diagnostics and status indication; easy addition of interlocks.
• Limits: a crashed program or welded output relay must never defeat safety.
• The hard rule: emergency stops, safety gates, and overload trips are hard-wired in series with the contactor coil (NC, direct-opening action). A PLC may add stop logic but must never be the sole means of removing power in an emergency.