Soft Starter Back-EMF Speed Estimation for Induction Motors
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing methods for estimating the speed of AC motors driven by thyristor-based soft starters are either computationally intensive, require knowledge of machine parameters, or induce potentially destructive transients, making them impractical for real-time speed estimation and control.
Innovation Solution
A system and method that disconnects the AC motor from the power source for a predetermined period to measure back-emf voltage, allowing for speed estimation and controlled reconnection using a multi-step firing sequence of thyristors, avoiding transient currents and relying on zero crossings of the mains phase voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If high-frequency current injection is used for speed estimation, then speed estimation can be achieved, but it is not feasible with thyristor-based soft starters
Solution Approach 1:
The patent replaces the mechanical/electrical current injection method with a measurement-based approach using back-emf voltage detection. Instead of injecting current to estimate speed, the system measures the back-emf voltage that naturally occurs in the motor windings during normal operation, eliminating the need for additional active components and making it compatible with soft starter systems.
2Measurement precision
If model-reference adaptive system or FFT analysis is used, then speed estimation can be achieved, but computational intensity increases
Solution Approach 1:
The patent extracts only the essential information needed for speed estimation from the motor's operational signals. By focusing solely on measuring back-emf voltage and using a simplified relationship between back-emf and speed, the system eliminates the need for complex computational algorithms while maintaining adequate estimation accuracy for control purposes.
3Loss of information
If SCR triggering is interrupted to estimate speed, then speed information can be obtained, but dangerous current transients are induced
Solution Approach 1:
The patent uses back-emf voltage as an intermediary measurement that can be obtained without interrupting the normal SCR triggering sequence. By measuring the voltage that naturally appears in the motor windings during normal operation, the system obtains speed information without the need to disrupt the power flow or create transient conditions.
Solution Approach 2:
The system utilizes the back-emf voltage that is naturally generated by the motor during normal operation as the measurement signal. This self-generated signal provides speed information without requiring external excitation or interruption of the normal operating cycle, making the measurement process non-intrusive and safe.
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
Enables accurate and safe estimation of AC motor speed without requiring machine parameter knowledge, reducing computational intensity and preventing destructive transient currents during reconnection.
Implementation Method 1
measuring a back-emf voltage of the AC motor during the disconnection period
Data Source
Figure 1
Figure 2
Figure 3
AI summary
A system and method for estimating operating characteristics of an induction motor is disclosed. The system includes a motor control device that is electrically connectable between a motor and a power source. The motor control device includes a plurality of switching devices comprising at least one thyristor corresponding to a respective phase of the motor. The motor control device also includes a controller programmed to disconnect the power source from the motor for a predetermined time period following a first plurality of cycles of a mains phase voltage of the power source. The controller is further programmed to measure a back-emf voltage during the predetermined time period, estimate an operating characteristic of the motor from the measured back-emf voltage, and trigger the plurality of switching devices to reconnect the power source to the motor following estimation of the operating characteristic.