Electrical Power Systems & Motors Flashcards

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

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.

Which motor efficiency standard defines the NEMA Premium efficiency tier in the US?

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.

A 3-phase induction motor operates at 1,750 RPM under load while the synchronous speed is 1,800 RPM. What is the slip?

Answer: 2.78%

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

Which type of electrical load has a lagging power factor?

Answer: Induction motors

Induction motors draw lagging reactive power because their magnetic fields require energy stored in inductance, causing current to lag voltage.

What does a power quality analyser measure when it reports 'THD'?

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.

In a delta-connected 3-phase motor, if the line voltage is 480 V, what voltage appears across each winding?

Answer: 480 V

In a delta connection, the voltage across each winding equals the line-to-line voltage, which is 480 V.

Which strategy best reduces reactive power demand charges for a facility with many induction motors?

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.