Soft Start System with Autotransformer and VFD
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Solution Overview
Problem
Existing soft start systems for motors, particularly in remote locations with weak electrical supplies, face challenges in minimizing inrush current and providing sufficient breakaway torque, leading to high costs and inefficiencies in starting large compressor motors.
Innovation Solution
A soft start system comprising a variable frequency drive connected to an autotransformer, which increases voltage and allows for controlled starting by synchronizing motor frequency with the power supply, reducing inrush current and enhancing breakaway torque through a capacitor filter and resistor configuration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a motor is started by simply applying utility voltage by turning on a switch, then the motor starts immediately, but high inrush current occurs (at least 500% of rated current)
Solution Approach 1:
The system performs preliminary actions by first connecting the motor to the power supply through an autotransformer at reduced voltage before full voltage operation. The controller prepares the motor for starting by initially applying lower voltage, then gradually increasing it to full utility voltage, thereby avoiding the sudden inrush current that would occur with direct full-voltage switching.
2Use of energy by moving object
If a variable frequency drive applies low frequency and voltage to the motor during starting, then inrush current is reduced, but the motor requires higher voltage for sufficient breakaway torque
Solution Approach 1:
The system dynamically adjusts the voltage transformation ratio of the autotransformer based on the motor's operating conditions. The controller monitors motor current and power supply conditions, then dynamically changes the autotransformer tap position to provide the optimal voltage boost for breakaway torque while maintaining reduced inrush current. This dynamic adaptation allows the system to provide high torque when needed without sustaining high current draw.
Solution Approach 2:
The autotransformer acts as an intermediary device between the power supply and the motor. It transforms the utility voltage to an appropriate level for the VFD and motor combination, providing voltage boosting capability when high breakaway torque is required while maintaining the ability to limit inrush current. The autotransformer mediates between the conflicting requirements of high starting torque and low inrush current by providing conditional voltage transformation.
3Force
If an autotransformer is used to increase voltage for breakaway torque, then sufficient starting torque is provided, but the system complexity increases
Solution Approach 1:
The controller serves multiple functions: it controls the VFD operation, monitors motor parameters, manages the autotransformer tap switching, and coordinates the transition between starting and running modes. By making the controller multi-functional, the system reduces overall complexity despite adding the autotransformer, as the controller's additional responsibilities are integrated into its existing control architecture rather than requiring separate dedicated components.
Solution Approach 2:
The system merges the voltage transformation function into the existing soft-start configuration by integrating the autotransformer with the VFD and controller. Rather than treating these as separate independent systems, they are combined into a coordinated starting system where the autotransformer works in conjunction with the VFD's control capabilities, creating a unified solution that achieves high breakaway torque without proportionally increasing system complexity.
4Ease of operation
If a fully rated variable frequency drive is used to start large compressor motors, then starting control is achieved, but the cost is exceedingly high
Solution Approach 1:
Instead of using a fully rated VFD that would be oversized for the application, the system uses a smaller, lower-cost VFD combined with an autotransformer. The autotransformer provides the necessary voltage boosting capability that would otherwise require a much larger, more expensive VFD. This partial action approach uses just enough VFD capacity needed for control, with the autotransformer handling the voltage transformation, thereby significantly reducing system cost while maintaining starting control capability.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The system achieves a significant reduction in starting current, up to twenty-fold, while providing over 60% breakaway torque with less than 10% starting current, enabling efficient motor acceleration and allowing for multiple starts per hour without the need for oversized generators or lengthy connections.
Implementation Method 1
an autotransformer connected between the variable frequency drive and the motor for passing power to the motor
Implementation Method 2
enhancing breakaway torque through a capacitor filter and resistor configuration
Data Source
AI summary
A soft start system for starting large motors has a power supply suitable for passing power to the motor, a variable frequency drive connected to the power supply and an autotransformer connected between the variable frequency drive and the motor for passing power of an elevated current to the motor. A switch is connected between the motor and power supply for connecting the motor to the power supply when the motor reaches a desired speed. The autotransformer includes a single core with a winding extending over the single core. The autotransformer includes a first terminal connected the variable frequency drive, a second terminal connected the motor and a grounded neutral.