CEM Electrical Power Systems & Motors 3 — Questions and Answers
Question 1: What is the typical full-load efficiency range of a NEMA Premium 100 HP, 4-pole induction motor?
- 85–88%
- 89–92%
- 95–96% (Correct answer)
- 97–99%
Correct answer: 95–96%
NEMA Premium 100 HP motors typically achieve full-load efficiencies in the 95–96% range, significantly above standard or energy-efficient tiers.
Question 2: A facility has a 500 kVA transformer operating at 60% load. Approximately what percentage of transformer losses are core (no-load) losses versus copper (load) losses?
- Core losses dominate because they are constant regardless of load (Correct answer)
- Copper losses dominate because the load is above 50%
- Both losses are always equal at any load
- Core losses disappear below 50% load
Correct answer: Core losses dominate because they are constant regardless of load
Core (no-load) losses are constant and exist whenever the transformer is energized, so at low loads they form a larger fraction of total losses.
Question 3: What is the primary energy benefit of using soft starters on large centrifugal pumps?
- They eliminate reactive power entirely
- They reduce mechanical stress and inrush current during starting, extending equipment life (Correct answer)
- They allow motors to run at double synchronous speed
- They convert the motor to regenerative braking automatically
Correct answer: They reduce mechanical stress and inrush current during starting, extending equipment life
Soft starters ramp voltage gradually, reducing inrush current (often 6–8× FLA) and mechanical shock, lowering maintenance costs and improving reliability.
Question 4: Which IEEE standard is the accepted method for measuring 3-phase induction motor efficiency in the US?
- IEEE 519
- IEEE 112 Method B (Correct answer)
- IEEE 1547
- IEEE 141
Correct answer: IEEE 112 Method B
IEEE 112 Method B is the US standard for measuring motor efficiency using input/output power testing with specified loss segregation.
Question 5: When rewinding a failed motor instead of replacing it, what is the main energy efficiency concern?
- Rewound motors always have better efficiency than new motors
- Poor rewind practices can reduce motor efficiency by 1–2% or more (Correct answer)
- Rewinding changes the motor's synchronous speed
- Only single-phase motors lose efficiency during rewinding
Correct answer: Poor rewind practices can reduce motor efficiency by 1–2% or more
If rewind temperature and process controls are inadequate, core losses increase and efficiency drops by 1–2%, costing more in energy over the motor's life.
Question 6: A 25 kW motor runs 6,000 hours/year at 75% load with 92% efficiency. What is the approximate annual energy consumption?
- 122,283 kWh (Correct answer)
- 112,500 kWh
- 150,000 kWh
- 98,000 kWh
Correct answer: 122,283 kWh
Input power = (25 kW × 0.75) / 0.92 = 20.38 kW; annual energy = 20.38 × 6,000 = 122,283 kWh.
Question 7: Which type of motor is best suited for constant-speed, high-starting-torque applications such as compressors and conveyors?
- NEMA Design A
- NEMA Design D
- NEMA Design B
- NEMA Design C (Correct answer)
Correct answer: NEMA Design C
NEMA Design C motors have high starting torque (200–250% of full-load torque) with normal starting current, suitable for hard-to-start constant-speed loads.
CEM Electrical Power Systems & Motors 3
What is the typical full-load efficiency range of a NEMA Premium 100 HP, 4-pole induction motor?