Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Explain reduced-voltage starting methods (Star-Delta, Autotransformer, Reactor).
• Calculate starting current and torque for different starters.
• Describe the operation of electronic soft starters and VSDs.
• Select an appropriate starting method for a given application.

Lesson summary

This week, we delve into advanced motor control and starting methods, building upon the fundamental motor principles you learned previously. Understanding these advanced techniques is crucial for becoming proficient in electrical technology. In South Africa, where industries like mining, manufacturing, and agriculture rely heavily on electric motors, knowing how to efficiently and reliably control these motors is a highly sought-after skill. Improper motor starting and control can lead to significant energy wastage, equipment damage, and costly downtime, impacting productivity and profitability.

Lesson notes

2.1 Reduced-Voltage Starting Methods Large induction motors draw a high inrush current (typically 5-8 times the full-load current) when started directly online (DOL). This inrush current can cause voltage dips in the supply network, potentially affecting other equipment connected to the same line. Reduced-voltage starting methods aim to limit this inrush current.

Star-Delta Starting: This method is applicable to motors designed to run with a delta-connected stator winding at normal voltage but can be started in a star configuration.

Principle: During starting, the motor windings are connected in star. This reduces the voltage across each winding by a factor of √3 (approximately 1.732). Consequently, the starting current is also reduced by a factor of 3 (√3 √3). Once the motor reaches a certain speed (typically 70-80% of synchronous speed), the windings are switched to the delta configuration for normal operation.

Advantages: Relatively simple and inexpensive.

Disadvantages: Reduced starting torque (only 1/3 of DOL starting torque). Not suitable for high-inertia loads requiring high starting torque. Switching from star to delta can cause a transient current surge.