Applicable Systems
MNS-MCC
Symptoms
The bus voltage has dropped below 85% of the rated 480V, reaching 408V. The undervoltage relay trips and disconnects the bus to protect connected motors.
Possible Causes
- Supply transformer overloaded, causing voltage sag under heavy load
- Loose or corroded bus connection causing voltage drop
- Large motor starting inrush causing temporary voltage dip below the relay threshold
- Power factor correction capacitor bank failure causing increased reactive current
Troubleshooting Steps
- Step 1: Measure the three-phase bus voltage at the main incoming terminals
Voltage should be 456-504V (480V +/-5%); if below 456V, investigate the supply - Step 2: Check all bus connections for tightness using a calibrated torque wrench
Tighten to the manufacturer's torque specification; look for discoloration from heating - Step 3: Review the load profile to identify if a large motor start caused the trip
If motor inrush is the cause, consider a soft starter or VFD to reduce starting current - Step 4: Test the power factor correction capacitors for capacitance value and integrity
Replace any failed capacitor cans; verify the bank is properly fused
How to Reset
Reset: After the bus voltage is restored above 460V, reset the undervoltage relay and close the main breaker.
Common Replacement Parts
- Undervoltage relay: Adjustable, 70-90% of rated voltage (Set to 85% for motor protection)
- Power factor capacitor: 3-phase, 25 kVAr, 480V (Match to existing bank rating)
- Soft starter: 3-phase, 480V, motor-rated (Size to motor FLA x 1.15)
Frequently Asked Questions
- Q: What causes voltage sag?
Large motor starts (6x FLA inrush), transformer overload, loose connections, and utility-side faults are the most common causes.
- Q: How do I calculate voltage drop across a connection?
Measure voltage at both ends of the connection under load. The difference is the drop. A drop exceeding 3% indicates a problem.
- Q: What is the power factor correction target?
Aim for 0.95-0.98 lagging. Over-correction to leading power factor can cause resonance and overvoltage.
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Source: OEM Technical Manual
Category: Electrical
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