Commercial Generators In Vermont And New Hampshire

When the Lights Flicker but the Engine Runs

Understanding how to diagnose your excitation system can be the difference between a five-minute fix and a $10,000 alternator replacement. Commercial Generators In Vermont And New Hampshire

1. The “Loss of Residual” Problem (Shunt Systems)

Since Shunt (Self-Excited) systems rely on the residual magnetism in the rotor to “boot up,” they can lose that magnetism if the generator sits idle for months.

• The Symptom: The engine runs, but the voltmeter reads zero or very low (under 10V).
• The Fix: “Flashing the Field.” This involves applying an external DC source (like a 12V battery) to the exciter field to restore that magnetic memory.
• Pro Tip: If your company is in a high-humidity region like Florida, moisture can accelerate the loss of residual magnetism. Ensure your space heaters are operational.

2. Troubleshooting the AVR: SCR vs. FET

As mentioned in our last post, the Automatic Voltage Regulator (AVR) is the “brain.” If your voltage is hunting (fluctuating), the type of AVR you have dictates your troubleshooting steps.

• SCR-Type AVRs: If you are running non-linear loads (like a massive data center UPS), an SCR-type AVR might get “confused” by the harmonic noise, causing voltage instability.
• FET-Type AVRs: These use Pulse Width Modulation (PWM) and are far more resilient. If an FET AVR fails, it’s often due to a blown fuse or a disconnected sensing wire rather than “noise” interference.

3. The “PMG Advantage” in Fault Clearing

Why do we push Permanent Magnet Generators (PMG) for hospitals? It’s all about the “Fault Current.”

• The Scenario: A short circuit occurs in the building.
• Shunt Failure: A Shunt system’s voltage will collapse because the stator (its power source) is shorted. The breaker may never trip because the generator “died” first.
• PMG Success: Because the PMG is a separate power source on the shaft, it keeps “shoveling” current into the fault for up to 10 seconds, forcing the circuit breaker to trip and isolating the problem.

4. Visual Inspection: The “Burned Smell” Test

Regardless of the excitation method, technicians should look for three physical “red flags”:

1. The Rotating Rectifier (Diodes): If the generator produces voltage but cannot hold it under load, a “blown diode” is the likely culprit. Look for discoloration or a burnt smell near the exciter rotor.
2. The Varistor (Surge Suppressor): This protects your diodes from voltage spikes. If it looks cracked or charred, your excitation system is at risk.
3. Loose Sensing Leads: 90% of AVR failures aren’t failures at all—they are loose wires. Vibration is the enemy; ensure all connections to the AVR are torqued and secure.

Which System is on Your Site?

• Standard Commercial (Linear Loads): You likely have a Shunt system. Reliable, but keep a field-flashing kit handy.
• Industrial/Data Center (Non-Linear): You likely have PMG. Check your PMG air gap annually to ensure the magnets haven’t shifted.
• Emergency Backup (Motor Starting): You might have EBS. Ensure the EBC module hasn’t been bypassed during previous repairs.