CEM Electrical Power Systems & Motors 2 — Questions and Answers
Question 1: What is the purpose of a variable frequency drive (VFD) in a motor application?
- To increase motor voltage
- To control motor speed by varying supply frequency (Correct answer)
- To improve motor insulation resistance
- To measure motor shaft torque
Correct answer: To control motor speed by varying supply frequency
A VFD controls motor speed by varying the frequency and voltage of the power supplied to the motor, enabling significant energy savings at partial loads.
Question 2: Which motor efficiency standard defines the NEMA Premium efficiency tier in the US?
- NEMA MG1 Table 12-12
- IEEE 112 Method B only
- DOE 10 CFR Part 431 (Correct answer)
- ANSI/ASHRAE Standard 90.1
Correct answer: DOE 10 CFR Part 431
DOE 10 CFR Part 431 establishes minimum efficiency standards, and NEMA Premium refers to levels defined therein for electric motors sold in the US.
Question 3: A 3-phase induction motor operates at 1,750 RPM under load while the synchronous speed is 1,800 RPM. What is the slip?
- 2.78% (Correct answer)
- 5.56%
- 1.39%
- 8.33%
Correct answer: 2.78%
Slip = (Synchronous speed − Rotor speed) / Synchronous speed = (1800 − 1750) / 1800 = 2.78%.
Question 4: Which type of electrical load has a lagging power factor?
- Resistive heating elements
- Capacitor banks
- Induction motors (Correct answer)
- Incandescent lighting
Correct answer: Induction motors
Induction motors draw lagging reactive power because their magnetic fields require energy stored in inductance, causing current to lag voltage.
Question 5: What does a power quality analyser measure when it reports 'THD'?
- Total harmonic distortion of voltage or current waveforms (Correct answer)
- Thermal heat dissipation rating
- Total hourly demand in kWh
- Transformer heat differential
Correct answer: Total harmonic distortion of voltage or current waveforms
THD (Total Harmonic Distortion) quantifies the distortion of a waveform from its fundamental frequency due to harmonics, expressed as a percentage.
Question 6: In a delta-connected 3-phase motor, if the line voltage is 480 V, what voltage appears across each winding?
- 277 V
- 480 V (Correct answer)
- 240 V
- 830 V
Correct answer: 480 V
In a delta connection, the voltage across each winding equals the line-to-line voltage, which is 480 V.
Question 7: Which strategy best reduces reactive power demand charges for a facility with many induction motors?
- Installing synchronous condensers or capacitor banks (Correct answer)
- Replacing all motors with DC motors
- Adding resistance in series with each motor
- Increasing supply voltage by 10%
Correct answer: Installing synchronous condensers or capacitor banks
Capacitor banks or synchronous condensers supply reactive power locally, reducing reactive current drawn from the utility and lowering demand charges.
CEM Electrical Power Systems & Motors 2
What is the purpose of a variable frequency drive (VFD) in a motor application?