EETC Electrical Equipment & Maintenance 4 ā Questions and Answers
Question 1: What is the recommended maximum shaft voltage level on a VFD-driven motor before bearing damage becomes a concern?
- 500 mV peak (Correct answer)
- 1 V peak
- 5 V peak
- 10 V peak
Correct answer: 500 mV peak
Shaft voltages above approximately 500 mV peak can discharge through bearings, causing pitting and fluting damage known as electrical discharge machining (EDM).
Question 2: In a motor control center (MCC), what is the purpose of the stab assembly?
- Provide grounding for the enclosure doors
- Make electrical connection between the bucket and the vertical bus bar (Correct answer)
- House the overload relay elements
- Support the motor nameplate and wiring diagrams
Correct answer: Make electrical connection between the bucket and the vertical bus bar
Stab assemblies are the plug-in connectors on MCC buckets that make contact with the vertical bus bars when the unit is inserted into the structure.
Question 3: An overload relay is tripping on a motor that is lightly loaded. The MOST likely cause is:
- Incorrect overload relay heater elements installed for the motor FLA (Correct answer)
- Low supply voltage causing high current draw
- The motor is operating at high ambient temperature
- Loose terminal connections at the motor leads
Correct answer: Incorrect overload relay heater elements installed for the motor FLA
Heater elements sized for higher FLA will not trip under normal load, but elements sized too small will trip the relay even under light load conditions.
Question 4: What does the term 'dielectric withstand' (hi-pot) testing verify in electrical equipment?
- The minimum operating voltage of the equipment
- The ability of insulation to withstand overvoltage without breakdown (Correct answer)
- Continuity of protective earth conductors
- Current capacity of conductors under fault conditions
Correct answer: The ability of insulation to withstand overvoltage without breakdown
Hi-pot testing applies elevated voltage to insulation to verify it will not break down under transient overvoltages encountered in service.
Question 5: When performing contact resistance testing on a circuit breaker, results significantly higher than manufacturer specs indicate:
- The breaker is rated for a higher voltage class
- Pitted, burned, or corroded contacts that increase resistance and heating (Correct answer)
- The breaker has been properly de-rated for ambient temperature
- Normal aging that requires no corrective action
Correct answer: Pitted, burned, or corroded contacts that increase resistance and heating
High contact resistance causes excessive IĀ²R heating under load, which can cause contact failure, welding, or nuisance tripping.
Question 6: The synchronous speed of a 4-pole induction motor operating on a 60 Hz system is:
- 900 RPM
- 1200 RPM
- 1800 RPM (Correct answer)
- 3600 RPM
Correct answer: 1800 RPM
Synchronous speed = (120 Ć frequency) / poles = (120 Ć 60) / 4 = 1800 RPM; actual rotor speed is slightly less due to slip.
Question 7: What maintenance action is required when the insulating oil in a transformer tests at a dielectric breakdown voltage below 26 kV (per ASTM D877)?
- Continue monitoring with increased test frequency
- Filter, process, or replace the oil to restore dielectric strength (Correct answer)
- Add a corrosion inhibitor to the existing oil
- Drain 10% of the oil and replace with new oil
Correct answer: Filter, process, or replace the oil to restore dielectric strength
Oil below 26 kV dielectric strength per ASTM D877 must be reconditioned through filtration/drying or replaced to prevent insulation failure.
EETC Electrical Equipment & Maintenance 4
What is the recommended maximum shaft voltage level on a VFD-driven motor before bearing damage becomes a concern?