Technical Aptitude & Knowledge Flashcards

A three-phase motor is running but drawing significantly higher current on one phase than the other two. Which condition MOST likely explains this?

Answer: One winding in the motor has a partial short to ground, creating a low-impedance path

A partial winding short to ground (turn-to-turn or winding-to-frame) reduces the impedance of that winding, causing it to draw disproportionately higher current than the other two phases. An open phase would cause the motor to single-phase and typically trip the overload or produce drastically unequal currents across all three. Overloading raises current on all phases similarly. A blown control fuse would prevent the motor from running at all.

A hydraulic cylinder is extending slower than normal even though the pump is at rated pressure and flow. A technician checks and finds the pressure at the cylinder port is 200 PSI lower than at the pump outlet. What is the MOST likely cause?

Answer: Excessive restriction in the supply line or directional control valve

A 200 PSI pressure drop between the pump and the cylinder port indicates a significant restriction in the circuit—most commonly a partially blocked or undersized supply line, a contaminated directional valve, or a kinked hose. A worn piston seal would cause the cylinder to extend slowly but would show little pressure differential between pump and port (the fluid would bypass internally at the piston, not in the line). Low viscosity or a relief valve issue would manifest differently and wouldn't create a localized pressure drop at that point.

When reading a micrometer, the thimble edge is positioned past the 0.350" mark on the sleeve but before 0.375". The thimble scale reads 11, and the vernier scale confirms a half-thousandth. What is the correct reading?

Answer: 0.3615"

The sleeve shows 0.350" (seven 0.050" marks visible). The thimble reads 11, adding 0.011". The vernier scale indicating a half-thousandth adds 0.0005". Total: 0.350 + 0.011 + 0.0005 = 0.3615". This is a vernier micrometer capable of resolving 0.0001", and the vernier line indicates the half-thousandth graduation, confirming the digit between the nearest thousandths.

A 480V/120V control transformer supplies a machine control circuit. A technician measures 120V at the secondary but finds that a pilot light rated 120V glows dimly and a relay rated 120V fails to pull in. What is the MOST likely fault?

Answer: The common (neutral) leg of the secondary has high resistance—indicating a poor connection

If the neutral/common return conductor has high resistance (corroded lug, loose terminal), the circuit forms a voltage divider: some voltage is dropped across the bad connection rather than across the load. Both the pilot light and relay see reduced voltage, even though an open-circuit voltmeter measurement (drawing minimal current) still reads close to 120V. A half-primary open would produce roughly half secondary voltage (60V), not nearly 120V. A transformer overload causes sag only under load—but the no-load voltage reading is still 120V. An intermittent open would cause an outage, not dim/weak operation.

A pneumatic system uses a 3/2 normally-closed solenoid valve to extend a cylinder. After a PLC output energizes the solenoid, there is a 4-second delay before the cylinder begins moving, but movement is then full speed. What is the MOST diagnostic conclusion?

Answer: A significant volume of air must compress in the supply line before sufficient pressure develops to overcome the load

When the valve opens and the line upstream of the cylinder has a large volume (long line, large accumulator absent, or undersized supply), the compressed-air column must build pressure before it can overcome the cylinder's load resistance. This creates a characteristic delay followed by normal-speed motion once working pressure is achieved. A worn rod seal causes continuous slow movement, not a clean delay then full speed. A saturated dryer would affect consistent operation, not just start-up. A slow solenoid actuation would delay valve opening by milliseconds, not 4 seconds.

A maintenance technician is performing a megohmmeter (megger) test on a 480V induction motor winding and obtains a reading of 1.2 MΩ. According to IEEE 43 guidelines, what is the CORRECT interpretation and recommended action?

Answer: The reading is marginal; the motor requires a polarization index (PI) test before energizing to assess winding condition trend

IEEE 43 sets a 1 MΩ minimum for motors above 1 kV and recommends at least 100 MΩ as a 'good' result for medium-voltage motors; for low-voltage (under 1 kV) motors, 1 MΩ is the absolute minimum. At 1.2 MΩ a 480V motor just barely passes the absolute minimum—it is not condemned, but it is not comfortably healthy. The recommended next step is a polarization index (PI) test: measure insulation resistance at 1 minute and 10 minutes; a PI ≥ 2.0 indicates the insulation can absorb moisture (good), while PI < 1.0 indicates a dangerous condition. Temperature correction (Answer D) is a real consideration but is not the primary required action here.