CEM Electrical Power Systems & Motors 2

Question 1

What is the purpose of a variable frequency drive (VFD) in a motor application?

Accepted Answer

To control motor speed by varying supply frequency

Answer

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?

Accepted Answer

DOE 10 CFR Part 431

Answer

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?

Accepted Answer

2.78%

Answer

Slip = (Synchronous speed − Rotor speed) / Synchronous speed = (1800 − 1750) / 1800 = 2.78%.

Question 4

Which type of electrical load has a lagging power factor?

Accepted Answer

Induction motors

Answer

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'?

Accepted Answer

Total harmonic distortion of voltage or current waveforms

Answer

THD (Total Harmonic Distortion) quantifies the distortion of a waveform from its fundamental frequency due to harmonics, expressed as a percentage.

(CEM) Certified Emergency Manager Practice Test

(CEM) Certified Emergency Manager Practice Test

CEM Electrical Power Systems & Motors 2