Motor and Generator Testing Flashcards

During a surge comparison test on a three-phase motor winding, two of the three surge waveforms are identical but the third shows a slight loop deviation that disappears after a few pulses. What is the most likely cause?

Answer: Turn-to-turn insulation weakness that self-heals due to corona activity

A waveform deviation that appears and then self-heals (loop closes after a few pulses) is a classic indicator of a turn-to-turn insulation weakness where corona discharge temporarily ionizes the degraded insulation path, causing a measurable impedance shift. As the corona dissipates, the impedance normalizes, closing the loop. This is a precursor to insulation failure and warrants immediate attention. Loose connections cause consistent deviations, not self-healing ones, and rotor bars do not influence surge waveforms on a de-energized winding.

A 4160 V synchronous generator is tested with a DC high-potential step-voltage test. The leakage current readings at each voltage step are: 10 μA, 19 μA, 29 μA, 41 μA, 56 μA. What does this pattern most specifically indicate?

Answer: A nonlinear leakage current rise suggesting insulation approaching breakdown

A healthy insulation system exhibits a nearly linear leakage current increase with each voltage step. In this dataset, the increments are +9, +10, +11, +15, +15 μA — the rate of increase is itself increasing (nonlinear), which is a diagnostic flag indicating that the insulation is exhibiting increasing conductivity as voltage rises, a hallmark of insulation nearing dielectric breakdown or containing significant voids. Linear increases would suggest acceptable insulation. Moisture would typically show high but steady leakage, not an accelerating trend.

When performing a no-load test on a wound-rotor induction motor with the rotor circuit open, the measured power factor is 0.08 lagging. If the same test is repeated with the rotor circuit short-circuited through external resistors set to maximum resistance, how will the no-load power factor most likely change?

Answer: It will increase slightly, because rotor copper loss now adds a small real-power component

At no-load with the rotor shorted through high external resistance, slip remains very small but the rotor circuit is now closed. Even at maximum external resistance, a small rotor current flows, introducing additional rotor copper losses. These real-power losses are reflected into the stator, slightly increasing the real-power component and therefore slightly raising the power factor above the open-rotor value. However, the increase is small — not dramatic — because slip at no-load is minimal. The dramatic reflection effect of rotor resistance occurs at load, not at no-load, ruling out option D.

A 13.8 kV generator insulation resistance test yields a Polarization Index (PI) of 4.2, but the one-minute IR value is only 800 MΩ. According to NETA/IEEE 43 evaluation criteria, what is the correct interpretation?

Answer: The PI is excellent but the low absolute IR value warrants caution — temperature correction and trending are required before a definitive accept/reject decision

IEEE 43 establishes that the PI is most meaningful when the absolute IR value is also acceptable. While a PI of 4.2 is classified as 'excellent' (PI ≥ 4.0), an IR of 800 MΩ for a 13.8 kV machine is below the often-cited guideline threshold and may reflect temperature effects or moisture. IEEE 43 explicitly warns that PI interpretation requires consideration of the absolute IR and that temperature correction (to 40°C reference) is essential. A single low IR with a high PI is not a blanket pass — trending over time and temperature correction are required. Option A is an oversimplification that ignores absolute IR context; option B is incorrect as no universal rewind threshold exists at exactly 1000 MΩ; option D is incorrect because the test is still valid.

During shaft voltage measurement on a large inverter-driven induction motor, a technician measures 3.2 V RMS at the drive's switching frequency (typically >1 kHz) across the bearing journal. A second technician argues this reading is harmless because it is below 10 V. Who is correct and why?

Answer: The first technician is correct; high-frequency common-mode voltages as low as 1–2 V can cause electric discharge machining (EDM) damage to bearings when the lubricant film breaks down

Bearing fluting (EDM damage) on inverter-driven motors is caused by high-frequency capacitively-coupled common-mode voltages that charge the shaft until the lubricant film dielectric is overcome, at which point discharge current flows through the bearing. The breakdown voltage of a thin lubricant film is typically only 0.5–5 V at high frequencies, making 3.2 V at switching frequency a legitimate concern. The 10 V threshold is sometimes cited for low-frequency or DC conditions and does not apply universally to high-frequency PWM-induced shaft voltages. Shaft current (not just voltage) causes the damage, but voltage is the driving potential — both matter, and voltage measurements at high frequency are a valid diagnostic indicator.

A motor's rotor is tested using rotor influence check (RIC) during a motor current signature analysis (MCSA) test. The sideband amplitudes at (1 ± 2s)f₁ are −28 dBc and −31 dBc relative to the fundamental. The motor nameplate lists 4 poles, 60 Hz supply, and full-load slip of 3%. What condition do these sideband levels most likely indicate, and what additional test would definitively confirm it?

Answer: Broken rotor bars; confirm with a locked-rotor test measuring rotor bar resistance asymmetry via a four-wire milliohm measurement per bar

Sideband amplitudes of −28 and −31 dBc at the rotor bar pass frequency sidebands (1 ± 2s)f₁ are elevated — industry guidelines (e.g., EPRI) typically flag sidebands above −35 dBc as suspect and above −25 dBc as indicating probable broken bars. The asymmetry between the two sideband amplitudes (3 dBc difference) further supports rotor asymmetry rather than pure eccentricity (which produces more symmetric sidebands). The definitive follow-up for broken bar diagnosis is a low-resistance four-wire milliohm test measuring each rotor bar's resistance for asymmetry, or a rotor growler test. Option C is incorrect because higher load (higher slip) actually increases sideband amplitude and separation, making detection easier, not the opposite.