Mobile Generator Service Flashcards

A technician is servicing a portable generator that starts and runs smoothly but produces no AC voltage at the receptacles. The circuit breakers are confirmed to be in the 'ON' position. What is the MOST likely initial diagnostic step?

Answer: Check for residual magnetism and flash the field windings if necessary.

Generators rely on a small amount of residual magnetism in the rotor to begin generating voltage when started. If this magnetism is lost due to long periods of storage or improper shutdown, the generator cannot 'excite' itself to produce power. Flashing the field with a DC voltage source (like a 12V battery) is a common, non-invasive first step to restore this magnetism before suspecting component failure.

What is the primary function of the Automatic Voltage Regulator (AVR) on a mobile generator?

Answer: To maintain a constant output voltage under varying load conditions.

The AVR's main purpose is to sense the generator's output voltage and adjust the DC current sent to the rotor's exciter windings. This action strengthens or weakens the magnetic field, which in turn regulates the output voltage, keeping it stable as electrical loads are added or removed.

A contractor wants to power a 13-amp circular saw (running watts: 1560W, starting watts: 2500W) and a 10-amp air compressor (running watts: 1200W, starting watts: 2000W) with a generator rated for 4000 running watts and 5000 surge (starting) watts. Which statement is correct?

Answer: The generator can run both tools but cannot start them simultaneously.

The total running watts (1560W + 1200W = 2760W) is well below the 4000W rating. However, the combined starting watts if both tools start at the exact same moment (2500W + 2000W = 4500W) is below the 5000W surge rating. The most common and correct method for calculation is to take the total running watts (2760W) and add the single highest starting watts (2500W), which equals 5260W. This exceeds the 5000W surge capacity. Therefore, the generator can run both tools, but attempting to start them at the same time will overload it.

A technician measures the output of a North American standard mobile generator and finds the frequency is stable at 62.5 Hz with no load connected. What is the most likely cause for this reading?

Answer: The engine speed is set slightly too high.

The output frequency of a generator is directly proportional to the engine's speed (RPM). For a standard 60 Hz output in North America, the engine must run at 3600 RPM. A frequency of 62.5 Hz indicates the engine is running slightly faster than required. This is often a deliberate setting (no-load droop) to ensure the frequency remains within an acceptable range once a load is applied.

Which of the following best describes the function of a Ground Fault Circuit Interrupter (GFCI) receptacle on a portable generator?

Answer: It protects the user from electric shock by detecting small imbalances in current flow.

A GFCI is a safety device designed to protect people. It constantly monitors the current flowing on the hot and neutral wires. If it detects a small imbalance (as little as 4-6 milliamps), it assumes the current is leaking to ground (potentially through a person) and trips the circuit in a fraction of a second to prevent a serious shock. It does not protect against overloads or short circuits.

A technician is troubleshooting a generator with a consistently low voltage output of 95V. The engine speed is confirmed to be correct (producing 60 Hz), and swapping the AVR with a known-good unit does not fix the problem. What is the most logical next diagnostic step?

Answer: Measure the resistance of the rotor windings through the slip rings and brushes.

With the engine speed and AVR ruled out, the problem likely lies in the excitation circuit or main windings. The brushes and slip rings are critical components that transfer excitation current from the AVR to the rotor. Worn brushes, dirty slip rings, or high resistance in the rotor windings can weaken the magnetic field, leading to low voltage output. Measuring the resistance across these components is the next logical step.