Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

This page supports the lesson note with a companion video and a short classroom-ready summary.

For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.

Performance objectives

• Explain the principles of Variable Frequency Drives (VFDs).
• Calculate reduced starting current and torque for star-delta starters.
• Describe the operation of autotransformer starters.
• Compare different motor starting methods.
• Apply safety procedures for motor control systems.

Lesson summary

Advanced motor control and starting methods are crucial for operating electrical motors efficiently and safely. In South Africa, where industries such as mining, manufacturing, and agriculture heavily rely on electric motors, understanding these methods is essential for reducing energy consumption, minimizing motor wear and tear, and preventing costly downtime. From the water pumps supplying our communities to the machinery powering our factories, the effective control of electric motors is integral to our nation's infrastructure and economy. Inefficient motor starting methods lead to voltage dips that can affect the entire power grid and damage sensitive electronic equipment.

Lesson notes

2.1 Variable Frequency Drives (VFDs) A Variable Frequency Drive (VFD), also known as an adjustable speed drive (ASD) or inverter, is a type of motor controller that drives an electric motor by varying the frequency and voltage supplied to the motor. The AC motor's speed is directly proportional to the frequency of the supplied voltage.

How it Works: A VFD consists of three main sections: Rectifier: Converts the incoming AC power to DC power using diodes or thyristors.

DC Bus: Stores the DC power using capacitors. This smooths out the rectified DC voltage.

Inverter: Converts the DC power back to AC power at the desired frequency and voltage. This is achieved using insulated-gate bipolar transistors (IGBTs) that switch on and off rapidly. The inverter outputs a Pulse Width Modulated (PWM) waveform, which approximates a sine wave.

Advantages of VFDs: Precise Speed Control: Allows for infinitely variable speed control, improving process control and efficiency.

Energy Savings: By adjusting the motor speed to match the load requirements, VFDs can significantly reduce energy consumption, especially in variable torque applications such as pumps and fans.

Reduced Mechanical Stress: Provides soft starting and stopping, reducing mechanical stress on the motor and driven equipment.

Improved Power Factor: Some VFDs include power factor correction, improving the overall power factor of the electrical system.

Example: Imagine a water pump used to irrigate a farm in the Free State. Without a VFD, the pump runs at a fixed speed, delivering the same amount of water regardless of the actual irrigation needs. With a VFD, the pump speed can be adjusted based on soil moisture levels and weather conditions, reducing water and energy waste. 2.2 Star-Delta Starters The star-delta starter is a reduced-voltage starting method used for AC induction motors. It reduces the starting current by initially connecting the motor windings in a star configuration and then switching to a delta configuration once the motor has reached a certain speed.

How it Works: Star Connection: In the star configuration, the voltage across each winding is reduced to 1/√3 (approximately 57.7%) of the line voltage. This reduces the starting current to 1/3 of the direct-on-line (DOL) starting current.

Delta Connection: Once the motor reaches approximately 80% of its rated speed, the starter switches the windings to the delta configuration, applying the full line voltage to the windings.

Calculations: Starting Current Reduction: I start (star) = I start (DOL) / 3 Torque Reduction: T start (star) = T start (DOL) / 3

Example: A 15kW, 400V AC induction motor has a DOL starting current of 6 times its rated current. If a star-delta starter is used, the starting current will be reduced to 6 / 3 = 2 times the rated current. This significantly reduces the voltage dip in the electrical system.

Worked example

A 22kW, 400V, 50Hz, 4-pole induction motor has a full-load current of 40A and a starting current of 5 times the full-load current when started direct-on-line. Calculate the starting current and starting torque as a percentage of the DOL values when the motor is started using a star-delta starter.

Solution:

DOL starting current = 5 40A = 200A

Star-delta starting current = 200A / 3 = 66.67A

Star-delta starting current as a percentage of DOL = (66.67A / 200A) 100% = 33.33%

Star-delta starting torque as a percentage of DOL = 33.33% (torque is also reduced by a factor of 3).

Why it works: By reducing the voltage applied to the motor windings during the starting period, the initial surge of current is reduced. This protects the motor windings and minimizes voltage dips in the power supply.

2.3 Autotransformer Starters