In electric machines, slip rings used on AC generators and motors are usually made of which material to provide good conductivity and durability?

Introduction / Context:
Slip rings are important components in AC machines such as alternators and some types of induction motors. They provide a continuous electrical connection between stationary and rotating parts. Their material must have good electrical conductivity and mechanical strength, and must withstand wear due to contact with brushes. Choosing the right material for slip rings ensures reliable operation and long service life of the machine. This question asks you to identify the material that is usually used for slip rings.

Given Data / Assumptions:

• Slip rings are circular conducting rings mounted on a rotating shaft.
• They are in sliding contact with carbon or graphite brushes.
• The material should have good conductivity, hardness and wear resistance.
• Common conductor materials include copper, aluminium and copper based alloys such as phosphor bronze.

Concept / Approach:
Pure copper offers excellent electrical conductivity but is relatively soft and can wear quickly under sliding contact. Aluminium is lighter but has lower conductivity and can form an oxide layer that increases contact resistance. Carbon is commonly used for brushes, not for the slip ring surface itself, because it provides a controlled wear surface against the harder metal ring. Phosphor bronze is a copper alloy containing tin and phosphorus, which improves hardness, strength and wear resistance while maintaining good electrical conductivity. These properties make phosphor bronze a suitable and commonly used material for slip rings.

Step-by-Step Solution:
Step 1: Recall the function of slip rings: to carry alternating current between the rotating coil and external circuit through sliding contacts.Step 2: Recognise that the material must be a good conductor to minimise power loss.Step 3: Understand that the surface must also resist mechanical wear caused by brushes rubbing on it during rotation.Step 4: Evaluate copper: excellent conductivity but relatively soft and prone to wear and surface damage.Step 5: Evaluate aluminium: lighter but lower conductivity and more problematic surface oxidation compared to copper alloys.Step 6: Evaluate phosphor bronze: a copper alloy with improved hardness, strength and wear resistance, making it a good compromise for slip rings.Step 7: Conclude that phosphor bronze is usually chosen for slip rings.

Verification / Alternative check:
Machine design handbooks and electrical engineering references often mention phosphor bronze or similar copper alloys as preferred materials for slip rings, due to their ability to maintain a smooth, conductive surface under repeated brush contact. Maintenance manuals for large generators commonly refer to bronze slip rings with carbon brushes. In contrast, carbon is almost always associated with brushes, and aluminium is rarely used as a slip ring surface in heavy duty machines. These real world practices support the choice of phosphor bronze.

Why Other Options Are Wrong:
Aluminium, though conductive and light, tends to oxidise easily, and the oxide layer is less conductive, which is undesirable for sliding contacts. Pure copper has very high conductivity but is soft and can wear rapidly under brush contact, leading to frequent maintenance. Carbon is used as brush material to provide a sacrificial contact surface and controlled wear, but it is not typically used to form the ring itself in standard designs. Therefore, these materials are less suitable than phosphor bronze for slip rings.

Common Pitfalls:
Students may confuse the material used for brushes with the material used for slip rings because both parts work together in the same assembly. It is common to remember that brushes are made of carbon and mistakenly think that slip rings are also carbon. Another error is to assume that the best electrical conductor is automatically the best choice, without considering mechanical wear and oxidation. To avoid these mistakes, remember that slip rings are usually metal (often phosphor bronze or similar alloys), while brushes are often carbon.

Final Answer:
Slip rings in AC machines are usually made of phosphor bronze, a durable copper alloy that combines good conductivity with wear resistance.