Three-phase power worksheet with answers

Free printable Three-phase power worksheet: 6 problems from easy to exam level, with an answer key. Print it or save it as a PDF.

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1.Easy
ABCVL​ = 600 VIL​ = ?NZ = 8 + j6 Ωper phase

A balanced Y-connected load has an impedance of 8 + j6 Ω per phase. It is supplied from a balanced three-phase source with a line-to-line voltage of 600 V. Find the magnitude of the line current IL.

VL = 600 VZ = 8 + j6 Ω per phase
IL = A
2.Easy
ABCVL​ = 400 VZ = 18 + j24 Ωper phase

A balanced Δ-connected load has an impedance of 18 + j24 Ω per phase. It is supplied from a balanced three-phase source with a line-to-line voltage of 400 V. Find the magnitude of the phase current Iph flowing through each impedance.

VL = 400 VZ = 18 + j24 Ω per phase
Iph = A
3.Medium
ABCVL​ = 208 VIL​ = 100 Acos φ = 0.75lagging

A balanced three-phase load draws a line current of 100 A from a supply with a line-to-line voltage of 208 V. The power factor is 0.75 lagging. Find the total real power P drawn by the load in kW.

VL = 208 VIL = 100 Acos φ = 0.75
P = kW
4.Medium
ABCVL​ = 400 VIL​ = ?M90 kW, η = 95%cos φ = 0.85

A three-phase induction motor delivers 90 kW of mechanical output power at full load. At this load its efficiency is 95% and its power factor is 0.85 lagging. The supply has a line-to-line voltage of 400 V. Find the line current IL.

Pout = 90 kWη = 95%cos φ = 0.85VL = 400 V
IL = A
5.Hard
P = 50 kWφ1​S1​Q2​QC​ = ?

A balanced three-phase load takes 50 kW at a power factor of 0.8 lagging from a 400 V (line-to-line), 50 Hz supply. A three-phase capacitor bank in parallel with the load must raise the overall power factor to 0.9 lagging. The real power stays the same. Find the total reactive power QC the capacitor bank must supply, in kvar.

P = 50 kWpf1 = 0.8pf2 = 0.9VL = 400 Vf = 50 Hz
QC = kvar
6.Hard
P = 300 kWφ1​S1​Q2​QC​

A balanced three-phase load takes 300 kW at a power factor of 0.7 lagging from a 480 V (line-to-line), 60 Hz supply. A three-phase capacitor bank in parallel with the load must raise the overall power factor to 0.9 lagging. The real power stays the same. The capacitors are connected in Δ. Find the capacitance Cph of each capacitor in µF.

P = 300 kWpf1 = 0.7pf2 = 0.9VL = 480 Vf = 60 Hz
Cph = µF

Answer key

  1. 1. IL = 34.64 A
  2. 2. Iph = 13.33 A
  3. 3. P = 27.02 kW
  4. 4. IL = 160.9 A
  5. 5. QC = 13.28 kvar
  6. 6. Cph = 617 µF

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