CEM Electrical Systems and Motors 4 — Questions and Answers
Question 1: Which motor starting method reduces voltage applied to the motor during startup by approximately 58% of line voltage, reducing starting torque to about 33% of full voltage starting torque?
- Part winding start
- Autotransformer start at 65% tap
- Wye-delta start (Correct answer)
- Soft starter at 50% voltage
Correct answer: Wye-delta start
Wye-delta starting connects windings in wye (reducing voltage by 1/√3 ≈ 58%) during starting, then switches to delta; torque is proportional to voltage squared, so torque = (0.577)² ≈ 33%.
Question 2: In an industrial facility, 'demand ratchet' clauses in utility tariffs typically penalize customers how?
- By charging for energy at a higher rate during peak hours
- By billing demand based on a percentage of the highest demand in the past 11-12 months (Correct answer)
- By requiring minimum monthly energy consumption
- By adding a flat surcharge for low power factor
Correct answer: By billing demand based on a percentage of the highest demand in the past 11-12 months
Demand ratchet clauses bill the customer based on a percentage (often 75-100%) of the highest peak demand recorded in the previous 11-12 months, even if current demand is lower.
Question 3: What is the primary energy efficiency advantage of an EC (Electronically Commutated) motor compared to a PSC (Permanent Split Capacitor) motor in HVAC fan applications?
- EC motors operate at higher RPM
- EC motors use permanent magnets and variable speed control for significantly higher part-load efficiency (Correct answer)
- EC motors require no lubrication
- EC motors generate less electromagnetic interference
Correct answer: EC motors use permanent magnets and variable speed control for significantly higher part-load efficiency
EC motors combine permanent magnet rotors with built-in variable speed electronics, achieving efficiencies of 70-85% across a wide load range versus PSC motors at 25-50% efficiency.
Question 4: A facility's power factor is 0.78 lagging. The utility applies a power factor penalty below 0.90. To avoid the penalty, the facility installs capacitors. This is an example of correcting what type of reactive power?
- Leading reactive power from capacitive loads
- Lagging reactive power from inductive loads (Correct answer)
- Harmonic reactive power from VFDs
- Resistive power from heating elements
Correct answer: Lagging reactive power from inductive loads
Inductive loads (motors, transformers) create lagging reactive power (kVAR); capacitors supply leading reactive power to offset it, raising the displacement power factor.
Question 5: What is the function of a 'soft starter' and how does it differ from a VFD in steady-state operation?
- A soft starter varies frequency continuously; a VFD only controls starting
- A soft starter reduces voltage only during start/stop; a VFD continuously controls speed (Correct answer)
- Both control speed continuously but soft starters use thyristors
- A soft starter is used only for synchronous motors
Correct answer: A soft starter reduces voltage only during start/stop; a VFD continuously controls speed
Soft starters use SCRs to ramp voltage during starting and stopping only; once the motor reaches full speed, it bypasses the SCRs and runs at line voltage with no speed control.
Question 6: Which IEEE standard defines voltage distortion limits and current distortion limits for harmonic control in electrical power systems?
- IEEE 519 (Correct answer)
- IEEE 1547
- IEEE 141 (Red Book)
- NFPA 70E
Correct answer: IEEE 519
IEEE 519 establishes recommended harmonic voltage and current distortion limits at the point of common coupling (PCC) between the utility and customer.
Question 7: A 100 kW motor load operates at 70% load factor for 4,000 hours/year and 30% load factor for 2,000 hours/year. What is the annual energy consumption?
- 440,000 kWh
- 360,000 kWh
- 340,000 kWh (Correct answer)
- 280,000 kWh
Correct answer: 340,000 kWh
Energy = (100×0.70×4,000) + (100×0.30×2,000) = 280,000 + 60,000 = 340,000 kWh.
CEM Electrical Systems and Motors 4
Which motor starting method reduces voltage applied to the motor during startup by approximately 58% of line voltage, reducing starting torque to about 33% of full voltage starting torque?