Concept Notes
1. Star (Y) & Delta (Δ) Connections
In 3-phase systems, loads can be connected either in Star or Delta.
Star (Y) Connection
- Three windings’ similar ends joined at a neutral point.
- Phase voltage:
= Line voltage / √3 - Phase current:
Formulas:
Delta (Δ) Connection
- Windings connected end-to-end to form a closed loop.
- Line voltage = Phase voltage
- Line current = √3 × Phase current
Formulas:
👉 Power in both connections:
Example:
If a balanced star system has 
:
.
2. Energy Concepts in Electrical Systems
Electric Energy
Work done by an electric circuit over time:
(Joules)
Electrical Power
- Instantaneous Power:
- AC Power (with power factor):
Energy Consumption in kWh
(in Wh or kWh)
1 kWh = Energy consumed by 1 kW load in 1 hour = 1000 × 3600 = 
Example:
A 100W bulb runs for 10 hours → Energy = 
.
⚙️ Formulas
- Star:
- Delta:
- 3-phase Power:
- Energy:
- 1 kWh =
🔟 10 MCQs
Q1. In a star connection, line voltage is 400V. Phase voltage is:
a) 400V
b) 230V
c) 115V
d) 690V
Q2. In a delta system, line current = 10A. Phase current is:
a) 10A
b) 5.77A
c) 17.32A
d) 20A
Q3. For a balanced 3-phase star load with 
, find power.
a) 2000W
b) 3000W
c) 3464W
d) 4000W
Q4. In star connection, if line voltage = 415V, then phase voltage is approximately:
a) 415V
b) 240V
c) 720V
d) 120V
Q5. In delta connection, if 
, then = ?
a) 400V
b) 230V
c) 690V
d) 115V
Q6. Energy consumed by a 2kW heater running for 3 hours is:
a) 2kWh
b) 3kWh
c) 5kWh
d) 6kWh
Q7. A 60W fan runs for 5 hours daily for 30 days. Energy used = ?
a) 9kWh
b) 12kWh
c) 15kWh
d) 18kWh
Q8. A load takes 10A at 230V, unity power factor for 2 hours. Energy consumed = ?
a) 2.3kWh
b) 4.6kWh
c) 1.15kWh
d) 5kWh
Q9. In a 3-phase delta, line current is related to phase current by:
a)
b)
c) 
d) 
Q10. 1 unit of electricity (1 kWh) is equal to:
a) 
b) 
c) 
d)
✅ Answer Key
| Q.No | Answer |
|---|---|
| 1 | b |
| 2 | b |
| 3 | c |
| 4 | b |
| 5 | a |
| 6 | d |
| 7 | a |
| 8 | a |
| 9 | b |
| 10 | d |
🧠 Explanations
- Q1:
→ (b). - Q2:
→ (b). - Q3:
→ (c). - Q4:
→ (b). - Q5: In delta,
→ (a). - Q6:
→ (d). - Q7:
→ (a). - Q8:
. For 2h → 
→ (a). - Q9: By definition, → (b).
- Q10:
→ (d).
🎯 Motivation / Why Practice Matters
Star/Delta and energy concepts are repeatedly tested in ECET exams because they link directly to power systems, machines, and utilisation topics. Mistakes often happen in line-to-phase conversions and unit conversions (kWh to Joules). If you master these basics now, later problems in transformers, AC machines, and power systems will feel easier.
📲 CTA
👉 Join our dedicated ECET 2026 WhatsApp Group for daily notes, quizzes & mock tests:
🔗 Join Now
