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3-Phase Servo AVR (AC Voltage Stabilizer) — Parts, Tests, Repair & Maintenance

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3-Phase Servo AVR (AC Voltage Stabilizer) — Troubleshooting, Repair & Maintenance By Edwin Ogie • December 18, 2025 • -- AC Voltage Stabilizer — 3-phase servo control type (example from user photo) A practical, step-by-step guide to diagnose, repair and maintain 3-phase servo Automatic Voltage Regulators (AVR) / servo voltage stabilizers. Written in simple terms for technicians and maintenance teams working with generators, UPS rooms and factories. Includes videos, spare-parts list, safety checklist, troubleshooting flow and links to internal/external resources. Contents Why this matters In environments with unstable mains (frequent sags, surges or phase imbalance) a servo AVR protects sensitive equipment by continuously adjusting an autotransformer tap via a small servo motor. A well-maintained stabilizer saves equipment, reduces downtime and prevents costly damage. ...
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FRICTION

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Lesson Note: Friction

Friction

Objectives:

  1. Differentiate between static and dynamic (kinetic) friction.
  2. Determine the coefficient of limiting friction.
  3. Compare the advantages and disadvantages of friction.
  4. Suggest ways to reduce friction.
  5. Analyze factors affecting viscosity and terminal velocity.
  6. Apply Stoke’s law in practical situations.

Key Concepts

Friction: The resistive force occurring when two surfaces interact, opposing relative motion.

Static Friction: The frictional force that must be overcome to initiate motion; it increases with applied force up to a maximum (limiting friction).

Dynamic (Kinetic) Friction: The frictional force acting on an object already in motion; it is generally lower than static friction.

Friction Illustration

Figure: Illustration showing friction between two surfaces.

Coefficient of Limiting Friction

The coefficient of limiting friction (μl) is the ratio of the maximum static frictional force (Fmax) to the normal force (N) acting on an object:

μl = Fmax / N

This value indicates the “stickiness” between surfaces. A higher value means more friction; a lower value means less friction.

Coefficient of Friction Diagram

Figure: Diagram illustrating forces used to determine the coefficient of limiting friction.

Advantages, Disadvantages & Reduction of Friction

Friction has both beneficial and detrimental effects. While it provides necessary traction and stability, it also causes energy loss and wear.

Advantages of Friction: It enables vehicles to grip the road, prevents slipping, and is essential for many everyday actions such as walking.

Disadvantages of Friction: It results in energy loss (as heat), causes wear on surfaces, and reduces machine efficiency.

Reduction of Friction: Friction can be reduced using lubricants (oils, greases), ball bearings, and by polishing surfaces to make them smoother.

Advantages & Disadvantages of Friction

Figure: Illustration showing the pros and cons of friction.

Viscosity, Terminal Velocity & Stoke’s Law

Fluids experience friction through viscosity – a measure of resistance to flow. Terminal velocity is reached when the gravitational force on a falling object equals the drag force, and Stoke’s Law describes the drag force on spherical objects in a viscous medium.

Viscosity: A measure of a fluid's resistance to flow; higher viscosity means the fluid is thicker and flows more slowly.

Terminal Velocity: The constant speed an object reaches when the force of gravity is balanced by the drag force (including friction and air resistance).

Stoke’s Law: For a sphere moving through a viscous fluid, the drag force is given by:

Fd = 6πηrv

Where:

  • η is the fluid's viscosity
  • r is the sphere's radius
  • v is the sphere's velocity

Viscosity and Terminal Velocity Diagram

Figure: Diagram illustrating viscosity and terminal velocity.

Calculation Worked Examples

This section presents six calculation worked examples to illustrate friction concepts and the application of Stoke’s Law.

Example 1: Maximum Static Friction Force

Given: Normal force (N) = 100 N, coefficient of limiting friction (μl) = 0.5
Calculation: Fmax = μl × N = 0.5 × 100 = 50 N

Example 2: Kinetic Friction Force

Given: Normal force (N) = 150 N, coefficient of kinetic friction (μk) = 0.3
Calculation: Fk = μk × N = 0.3 × 150 = 45 N

Example 3: Drag Force Using Stoke’s Law

Given: A sphere with radius r = 0.05 m falls in a fluid with viscosity η = 0.2 Pa·s at velocity v = 2 m/s
Calculation: Fd = 6πηrv = 6 × π × 0.2 × 0.05 × 2 ≈ 0.38 N

Example 4: Determining Viscosity from Measured Drag

Given: A sphere (r = 0.1 m) falls at v = 1 m/s, and measured drag force Fd = 1.2 N
Calculation: η = Fd / (6πrv) = 1.2 / (6 × π × 0.1 × 1) ≈ 0.64 Pa·s

Example 5: Friction Force on a Vehicle

Given: Mass of car = 1200 kg, Normal force N = 1200 × 9.8 = 11760 N, coefficient of friction = 0.8
Calculation: Friction force = 0.8 × 11760 = 9408 N

Example 6: Kinetic Friction on a Sliding Block

Given: Mass = 30 kg, Normal force = 30 × 9.8 = 294 N, coefficient of kinetic friction = 0.4
Calculation: Friction force = 0.4 × 294 = 117.6 N

JAMB CBT Quiz on Friction

Total time: 900 seconds

This lesson covers: Static and dynamic friction and their differences Coefficient of limiting friction and its determination Advantages and disadvantages of friction Methods to reduce friction Qualitative treatment of viscosity and terminal velocity Application of Stoke’s Law Calculation examples for friction and drag forces 30 multiple-choice questions on friction covering all key aspects of the topic.

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