Lesson Notes By Weeks and Term v5 - Grade 12
Advanced motor control and starting methods – Week 7 focus
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Advanced motor control and starting methods – Week 7 focus
Download the Lessonotes Mobile South Africa app for faster lesson access on Android and iPhone.
Subject: Electrical Technology
Class: Grade 12
Term: 1st Term
Week: 7
Theme: General lesson support
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Advanced motor control and starting methods are crucial for efficient and reliable operation of electric motors, which are the workhorses of industry and essential for powering many aspects of modern life in South Africa. From pumps providing clean water in rural communities to conveyor belts in mines extracting valuable resources and automated manufacturing processes in factories creating jobs, understanding these advanced techniques is vital. These methods allow us to reduce the initial high current draw that motors exhibit during starting, preventing voltage dips that can disrupt other equipment and protecting the motor itself from damage.
2.1 Autotransformer Starting: Autotransformer starting reduces the voltage applied to the motor during starting, thereby reducing the starting current. An autotransformer acts as a voltage divider. A tapping on the autotransformer is selected to apply, typically, 50%, 65%, or 80% of the line voltage to the motor. Once the motor reaches a certain speed, the autotransformer is switched out, and the motor runs at full voltage.
Advantages: Higher starting torque per amp compared to star-delta. Suitable for motors driving high-inertia loads. Less voltage dip on the supply network.
Disadvantages: More expensive and complex than star-delta. Open circuit transition when switching to full voltage can cause a transient voltage surge if not managed correctly.
How it Works: The autotransformer lowers the voltage during startup, thus decreasing the current. The current drawn from the supply is further reduced by the square of the voltage reduction factor. After a set time (or when the motor reaches a specific speed), the full voltage is applied.
Example: A 400V, 50Hz induction motor has a full-load current of 50A and a starting current of 6 times the full-load current with direct-on-line (DOL) starting. If an autotransformer starter is used with a 65% tap, calculate the line current at starting. Motor starting current (DOL) = 6 50A = 300A Voltage reduction factor = 0.65 Motor current with autotransformer = 0.65 300A = 195A Line current = 0.65 195A = 126.75A (The line current is reduced further by the voltage reduction factor). 2.2 Star-Delta Starting: Star-delta starting is a common and relatively inexpensive method for reducing the starting current of induction motors. During starting, the motor windings are connected in a star configuration, which reduces the voltage applied to each winding by a factor of √3 (approximately 1.732). After the motor reaches a certain speed (typically around 75-80% of synchronous speed), the windings are reconnected in a delta configuration for normal operation.
Advantages: Simple, relatively inexpensive, and widely available.
Disadvantages: Reduced starting torque (typically one-third of DOL torque). Not suitable for motors driving high-inertia loads that require high starting torque.
How it Works: Star connection reduces the voltage across each winding, hence reducing current. When switched to delta, the voltage across each winding increases to rated voltage.
Example: A 400V, 50Hz, delta-connected induction motor has a full-load current of 30A and a starting current of 6 times the full-load current with DOL starting. Calculate the line current during star starting and the starting torque as a percentage of DOL torque. DOL Starting Current = 6 30A = 180A Line current during star starting = 180A / √3 = 103.92A Starting torque is proportional to the square of the voltage. Since the voltage in star is 1/√3 times the delta voltage, the torque is (1/√3)² = 1/3 of the DOL torque.
Therefore, starting torque is 33.33% of DOL torque. 2.3 Variable Frequency Drives (VFDs): VFDs (also known as Adjustable Speed Drives - ASDs) are electronic devices that control the speed of an AC motor by varying the frequency and voltage of the power supplied to the motor. They consist of a rectifier (to convert AC to DC), a DC link (to smooth the DC voltage), and an inverter (to convert DC back to AC at the desired frequency and voltage).
Advantages: Precise speed control, energy savings (especially for variable torque loads like pumps and fans), soft starting capability, reduced mechanical stress on the motor and driven equipment, improved power factor.
Disadvantages: More expensive and complex than other starting methods. Can introduce harmonic distortion into the power system if not properly filtered. Requires specialized knowledge for programming and maintenance.
How it Works: A VFD takes the incoming AC power, converts it to DC, then recreates AC power at a different frequency. By changing the frequency, the synchronous speed of the motor is altered (Ns = 120f/p, where Ns is synchronous speed, f is frequency, and p is the number of poles). The voltage is also adjusted proportionally to maintain a constant volts/hertz ratio, which is crucial for optimal motor performance.
Example: Consider a pump motor used in a water distribution system in a township. Using a VFD allows the operator to adjust the pump speed to match the actual water demand, reducing energy consumption during periods of low demand. Without a VFD, the pump would run at full speed continuously, wasting energy and potentially causing excessive wear and tear. 2.4 Programmable Logic Controllers (PLCs) in Motor Control: PLCs are specialized industrial computers that are used to automate industrial processes, including motor control. They receive input signals from sensors, process the signals according to a programmed logic, and generate output signals to control actuators, such as motor starters, VFDs, and other devices.