Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Explain and calculate for autotransformer starters.
• Describe the operation and advantages of Variable Frequency Drives (VFDs).
• Analyze PLC-based motor control circuits.
• Differentiate between AC motor braking methods.
• Evaluate the impact of starting methods on the power grid.

Lesson summary

This week, we delve into advanced motor control and starting methods. Understanding these concepts is crucial for your future as electrical technicians or engineers. South Africa's industrial sector, from mining to manufacturing, relies heavily on electric motors. Efficient motor starting and control minimize energy waste, reduce equipment wear and tear, and enhance productivity – all contributing to a more sustainable and prosperous South Africa. Imagine working on a large mining operation, ensuring the powerful conveyor belt motors start smoothly and operate reliably. This is the kind of impact you can have.

Lesson notes

2.1 Autotransformer Starters Autotransformer starters utilize an autotransformer to reduce the voltage applied to the motor during starting. This reduced voltage limits the inrush current, preventing voltage dips in the supply network and reducing mechanical stress on the motor and driven equipment.

Principle of Operation: The autotransformer is connected between the power supply and the motor. Taps on the autotransformer allow for selecting a percentage of the line voltage to be applied to the motor during starting (typically 50%, 65%, or 80%). After a pre-set time delay (depending on the motor's load), the autotransformer is disconnected, and the full line voltage is applied to the motor.

Advantages: Effective inrush current reduction. Higher starting torque compared to star-delta starters for the same line current.

Disadvantages: More expensive than other starters. Larger and more complex than DOL or star-delta starters. Open circuit transition during switching from start to run, potentially causing a current surge (though mitigated in modern designs).

Worked example

A 100kW, 400V, 50Hz induction motor has a full-load current of 180A and a locked-rotor current of 600

A. It is started using an autotransformer starter with a 65% tapping.

Calculate:

a) The line current during starting.

b) The motor current during starting.

Solution:

a) Motor Current (I_motor): Since the voltage applied to the motor is 65% of the line voltage, the motor current during starting will be 65% of the locked-rotor current: