(a) Carbon
(b) Nickel
(c) Phosphor bronze
(d) Manganese
✅ Answer (Detailed Solution Below)
✔️ Option: (c) Phosphor bronze
📖 Detailed Solution
🔹 Slip rings are used to transfer current between the rotating rotor and stationary external circuit.
🔹 They must possess good electrical conductivity.
🔹 They should have high resistance to wear and corrosion.
🔹 They must provide smooth contact with brushes.
🔹 Phosphor bronze offers an ideal balance of these properties.
🔹 It ensures long service life and stable performance.
🔹 Hence, it is commonly used for slip rings.
👉 Therefore, phosphor bronze is the preferred material for slip rings.
⚙️ Main Parts Related to Slip Ring System are:
✔️ Slip Rings (Phosphor Bronze)
✔️ Carbon / Graphite Brushes
✔️ Rotor Windings
✔️ External Resistance (Rheostat)
✔️ Shaft
✔️ Bearings
🔄 Slip Ring Assembly:
🔹 Three slip rings are mounted on the rotor shaft.
🔹 Each ring is connected to one phase of rotor winding.
🔹 Brushes rest on the rings.
🔹 Brushes are connected to external resistance.
🔹 Resistance is transferred to rotor circuit.
🔹 Enables controlled starting and speed regulation.
🚀 Function of Slip Rings
🔹 Allow insertion of external resistance.
🔹 Improve starting torque.
🔹 Reduce starting current.
🔹 Enable smooth acceleration.
🔹 Support heavy-load starting.
❌ Why Other Options are Incorrect
(a) Carbon ❌
🔹 Carbon is used for brushes.
🔹 Not used for slip rings.
(b) Nickel ❌
🔹 Does not provide ideal conductivity and wear properties.
🔹 Not commonly used for slip rings.
(d) Manganese ❌
🔹 Mainly used as alloying element.
🔹 Not used as base material for slip rings.
📌 Important Points
⭐ Slip Ring Material → Phosphor Bronze
⭐ Brush Material → Graphite / Carbon
⭐ Number of Slip Rings → 3
⭐ Purpose → External Resistance Connection
⭐ Frame Material → Closed Grained Cast Iron
⭐ Shaft Material → Mild Steel
📝 Final Summary
📍 In a slip-ring induction motor:
✔️ Slip rings are made of phosphor bronze
✔️ Brushes are made of carbon/graphite
✔️ External resistance is connected through slip rings
✔️ Ensures reliable starting and operation
👉 Hence, Option c is Correct.