Electrical & Electronic Systems Flashcards
7 cards from real ITMC practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Electrical & Electronic Systems flashcards as text
A VFD display shows an 'OC' (overcurrent) fault immediately at startup. The most likely cause is:
Answer: Short circuit in motor windings or output cable
An immediate overcurrent trip at startup typically indicates a short circuit in the motor or the wiring between the VFD output and the motor.
According to NEC Article 430, a branch circuit supplying a single motor must be sized at a minimum of what percentage of the motor's full-load current (FLC)?
Answer: 125%
NEC 430.22 requires motor branch-circuit conductors to be rated at no less than 125% of the motor's full-load current.
Which semiconductor device is most commonly used as the output switching element in a modern PWM variable frequency drive?
Answer: Insulated Gate Bipolar Transistor (IGBT)
IGBTs combine high switching speed with high voltage/current handling, making them the standard output switch in modern VFDs.
When using a clamp-on ammeter to measure current in a single-conductor cable, the reading is zero despite the circuit being energized. What is the most likely cause?
Answer: Both the line and neutral conductors are inside the clamp jaw
If both conductors are clamped together, their equal and opposite magnetic fields cancel, producing a zero reading.
A photoelectric sensor is used in a retroreflective mode. What component is required in addition to the sensor head?
Answer: A retroreflective target or reflector
Retroreflective sensors emit light toward a reflector and detect the returned beam; only a reflector is needed, not a separate receiver.
What does a 4-20 mA analog signal represent when used for process control?
Answer: 4 mA = 0% of range, 20 mA = 100% of range
In the 4-20 mA standard, 4 mA represents the low end (0%) of the process range and 20 mA represents the high end (100%), with 0 mA indicating a broken wire fault.
An induction motor is running but making a loud humming noise and is very hot. Single-phasing is suspected. What is the first test to confirm?
Answer: Measure line voltage on all three phases at the motor disconnect
Measuring voltage at all three phases of the supply identifies a missing phase (single-phasing) quickly and safely before any winding tests.