ITMC Electrical & Electronic Systems 3 — Questions and Answers
Question 1: A VFD display shows an 'OC' (overcurrent) fault immediately at startup. The most likely cause is:
- Undersized braking resistor
- Short circuit in motor windings or output cable (Correct answer)
- Incorrect keypad parameter entry
- Low input line voltage
Correct 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.
Question 2: 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)?
- 100%
- 115%
- 125% (Correct answer)
- 150%
Correct 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.
Question 3: Which semiconductor device is most commonly used as the output switching element in a modern PWM variable frequency drive?
- Silicon Controlled Rectifier (SCR)
- Bipolar Junction Transistor (BJT)
- Insulated Gate Bipolar Transistor (IGBT) (Correct answer)
- MOSFET only
Correct 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.
Question 4: 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?
- The meter is set to the wrong range
- Both the line and neutral conductors are inside the clamp jaw (Correct answer)
- The circuit is on a GFCI breaker
- AC current cannot be measured with a clamp meter
Correct 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.
Question 5: A photoelectric sensor is used in a retroreflective mode. What component is required in addition to the sensor head?
- A separate receiver unit
- A retroreflective target or reflector (Correct answer)
- A fiber optic cable
- A separate amplifier module
Correct 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.
Question 6: What does a 4-20 mA analog signal represent when used for process control?
- 4 mA = 50% of range, 20 mA = 100% of range
- 4 mA = 0% of range, 20 mA = 100% of range (Correct answer)
- 4 mA = error condition, 20 mA = fault
- 4 mA = minimum safe value, 20 mA = setpoint
Correct 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.
Question 7: 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?
- Measure motor winding resistance
- Measure line voltage on all three phases at the motor disconnect (Correct answer)
- Perform a megger test on the stator
- Check the motor nameplate FLA rating
Correct 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.
ITMC Electrical & Electronic Systems 3
A VFD display shows an 'OC' (overcurrent) fault immediately at startup.
The most likely cause is: