Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Apply motor control principles (DOL, Star-Delta).
• Solve single-phase and three-phase transformer calculations.
• Understand PLC components and Ladder Logic.
• Apply knowledge of protection devices.
• Develop strategic exam-taking techniques.

Lesson summary

This week is dedicated to solidifying your understanding of crucial Electrical Technology concepts in preparation for upcoming assessments and ultimately, the final Matric examination. Electrical Technology skills are vital in South Africa, impacting areas from household electricity supply to large-scale industrial automation, and contributing to economic growth and job creation. A strong foundation in these principles will open doors to various career paths within the electrical engineering field. We'll be focusing on a comprehensive review, problem-solving strategies, and exam technique.

Lesson notes

2.1 Motor Control Direct-On-Line (DOL)

Starters: DOL starters apply full voltage to the motor windings at start-up. Simple and cost-effective for small motors.

However, they draw a very high starting current (typically 6-8 times the full load current), which can cause voltage dips in the supply.

Star-Delta Starters: Star-Delta starters reduce the starting current by initially connecting the motor windings in a star configuration, then switching to a delta configuration once the motor reaches a certain speed (typically 70-80% of synchronous speed). The starting current is reduced to about 1/3rd of the DOL starting current. Requires a motor with six terminals brought out.

Auto-Transformer Starters: Auto-transformer starters use a tapped auto-transformer to reduce the voltage applied to the motor during starting. This reduces the starting current proportionally. They provide more flexibility in controlling the starting current compared to star-delta starters but are generally more expensive and complex.

Important Considerations: Overload Protection: Essential to protect the motor from overheating due to excessive current draw. Overload relays (thermal or electronic) are used.

Short-Circuit Protection: Provided by circuit breakers or fuses to protect against high fault currents.

Motor Selection: Correct motor selection is critical for the application and must match the load requirements.