Question 1
A 15 hp, 600 V, 3-phase motor is supplied from a fused starter. The motor will not start and blows fuses immediately on each attempt. Insulation tests are satisfactory, but phase-to-phase resistance readings show one phase noticeably lower than the others. What is the most likely cause?
Question 2
A small three-phase motor hums but does not start when energized. Which is the most likely cause?
Question 3
A three-phase motor fails to start and only hums when the start button is pressed. Which is the most likely electrical cause?
Question 4
A single-phase motor hums but fails to start when energized. Which is the most likely cause?
Question 5
During troubleshooting of a control circuit, an electrician finds that a contactor coil is rated 120 V but is being supplied from a 277 V lighting circuit through a small control transformer. The coil has failed open. What is the most likely cause of the coil failure?
Question 6
A 3-phase motor is overheating. You measure the currents: L1=22A, L2=22A, L3=22A. The nameplate FLA is 24A. What is the most likely cause?
Question 7
Which test is used to identify a shorted turn in a motor winding?
Question 8
If a 3-phase motor is vibrating excessively, what is the most likely mechanical cause?
Question 9
A 10A motor trips its overload relay during startup. The overload is correctly sized at 125%. What is the likely cause?
Question 10
An electrician is troubleshooting a 3-phase motor that hums but refuses to start. Current measurements show 15A on Phase A, 15A on Phase B, and 0A on Phase C. What is the most likely diagnostic conclusion?
Question 11
A three-phase motor is running hot and drawing higher current than its nameplate rating on all three phases. The supply voltage is within limits. Which is the most appropriate corrective action?
Question 12
A three-phase, 208 V, 10 hp motor has a full-load current of approximately 30 A. What size three-pole breaker would generally be selected for running overload protection (assume typical values and ignore specific code tables)?
Question 13
In a PLC-controlled motor starter, the output from the PLC to the starter coil is not energizing, even though the PLC output indicator is on. What should you check next?
Question 14
A motor fails to start when the start button is pressed, but the control circuit fuse is intact and line voltage is present at the starter. Which is the next logical check?
Question 15
A programmable timer is used to control exterior lighting and is fed from a contactor. The lights occasionally remain on during the day. Which issue is most likely?
Question 16
A Level 4 apprentice is troubleshooting a motor that trips on overload after running for several minutes. Which issue is the most likely cause?
Question 17
A small motor starts normally but occasionally trips its circuit breaker immediately on startup. Which is the most likely cause?
Question 18
A three-phase motor controlled by a magnetic starter fails to start when the start pushbutton is pressed. The control circuit fuse is intact, and line voltage is present at the starter line terminals. What should the electrician check next?
Question 19
A motor fails after running for several hours under normal load, and inspection shows the windings are darkened and insulation is brittle. Which condition is most likely responsible?
Question 20
A journeyperson is troubleshooting a three-phase motor that hums but will not start. Which condition is MOST likely causing this symptom?
Question 21
During troubleshooting, a journeyperson finds that a three-phase motor overload relay is tripping intermittently. The measured line currents are balanced and below full-load amps, but the overload still trips. What is the MOST likely cause?
Question 22
When troubleshooting a three-phase motor that hums but fails to start, which of the following is the most likely initial cause to check if the motor starter is engaging properly?
Question 23
When troubleshooting a three-phase motor with a clamp-on ammeter, a Journeyperson measures $15 \text{ A}$ on L1, $15 \text{ A}$ on L2, and $0 \text{ A}$ on L3. The motor is running quietly at full speed. What is the most likely cause?
Question 24
When troubleshooting a tripped motor overload, what should be checked before resetting?
