ITMC Electrical & Electronic Systems 2 ā Questions and Answers
Question 1: A three-phase motor draws 18A per phase at 480V. What is the approximate total power consumption?
- 8,640 W
- 14,968 W (Correct answer)
- 25,920 W
- 43,200 W
Correct answer: 14,968 W
Three-phase power = ā3 Ć V Ć I = 1.732 Ć 480 Ć 18 ā 14,968 W.
Question 2: Which type of capacitor is commonly used for power factor correction in industrial motor circuits?
- Ceramic disc capacitor
- Electrolytic capacitor
- Run capacitor (oil-filled) (Correct answer)
- Tantalum capacitor
Correct answer: Run capacitor (oil-filled)
Oil-filled run capacitors are designed for continuous AC duty and are the standard choice for industrial power factor correction.
Question 3: When troubleshooting a PLC input module, the indicator LED is ON but the program logic shows the input as OFF. The most likely cause is:
- Blown fuse in the output module
- Wiring open between field device and input card
- Failed input module optocoupler (Correct answer)
- Incorrect PLC program logic
Correct answer: Failed input module optocoupler
If the LED lights (indicating voltage at the terminal) but the PLC reads OFF, the optocoupler inside the input module has failed to pass the signal.
Question 4: An insulation resistance test on a 480V motor winding reads 0.8 MĪ©. What action should be taken?
- No action needed; this is acceptable
- Motor can run at reduced load only
- Motor should be removed from service for rewinding or drying (Correct answer)
- Replace only the motor capacitor
Correct answer: Motor should be removed from service for rewinding or drying
IEEE 43 recommends a minimum of 1 MĪ© for motors operating below 1 kV; 0.8 MĪ© falls below this threshold and warrants removal from service.
Question 5: Which wiring method provides the best protection against electromagnetic interference (EMI) in a signal cable?
- Twisted pair with overall shield grounded at one end (Correct answer)
- Solid conductor in plastic conduit
- Flat ribbon cable with no shielding
- Coaxial cable grounded at both ends
Correct answer: Twisted pair with overall shield grounded at one end
Twisted pair cancels magnetically induced noise while a shield grounded at one end prevents ground loops and blocks electric field interference.
Question 6: A 24VDC control relay coil measures infinite resistance with an ohmmeter. This indicates:
- The coil is shorted to ground
- The coil winding is open (broken) (Correct answer)
- The relay contacts are welded closed
- Normal relay coil condition
Correct answer: The coil winding is open (broken)
An infinite (OL) resistance reading across a coil means the wire inside has broken, creating an open circuit.
Question 7: What is the purpose of a snubber circuit connected across a relay coil or motor contactor?
- Increase coil pull-in speed
- Suppress voltage transients caused by inductive kickback (Correct answer)
- Measure coil current during energization
- Provide backup power to the coil
Correct answer: Suppress voltage transients caused by inductive kickback
When an inductive coil is de-energized, the collapsing magnetic field produces a voltage spike; snubbers (RC networks or MOVs) clamp these transients to protect switching devices.
ITMC Electrical & Electronic Systems 2
A three-phase motor draws 18A per phase at 480V.
What is the approximate total power consumption?