Variable Frequency Drive PLL Feedforward Control for AC Motor Startup
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Large AC motors experience inrush current, thermal, and mechanical stress during direct startup, causing voltage drops and potential equipment damage, and existing phase-locked loop (PLL) control systems are inadequate for rapidly changing frequencies.
Innovation Solution
A control system for AC motors using a PLL circuit with a proportional integral (PI) regulator and a feedforward generator that tracks stator flux position to provide a dynamically adapted output signal, improving dynamic accuracy and reducing errors during rapid frequency changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If direct connection to utility power source is used for starting large AC motors, then the motor can start immediately, but large inrush current is drawn causing voltage drops and mechanical stress
Solution Approach 1:
The VFD performs preliminary action by gradually increasing the frequency and voltage before full power connection. The motor is accelerated through a controlled ramp-up process where frequency increases from 0 to rated frequency over time, preventing sudden inrush current while achieving the desired startup speed.
Solution Approach 2:
The VFD changes operating parameters dynamically during startup. Frequency is increased gradually from 0 Hz to rated frequency (e.g., 60 Hz), and voltage is adjusted proportionally to maintain constant flux. This parameter transformation allows the motor to start smoothly without drawing excessive inrush current.
2Measurement precision
If VFD with PLL control is used for precision speed control, then speed control accuracy is improved, but dynamic accuracy deteriorates during rapid frequency changes
Solution Approach 1:
The PLL control system uses feedback from the motor's back-EMF signals to continuously adjust and lock onto the rotor position and speed. During rapid frequency changes, the PLL dynamically tracks the changing frequency by comparing the reference frequency with the actual motor frequency and adjusting the control signals to maintain synchronization and precision.
Data Source
AI summary
Systems and method for controlling an alternating current (AC) electrodynamic machine (390) with a variable frequency drive (VFD) (380) include a control system (300) with a phase-locked-loop (PLL) circuit (382) for providing a stator flux angle signal (338) to the VFD (380), the PLL circuit (382) comprising a proportional integral (PI) regulator (332) providing an output signal (334); and a feedforward generator (350) in communication with the PLL circuit (382), wherein the feedforward generator (350) tracks a stator flux position of the AC electrodynamic machine (390) such that the feedforward generator (350) determines a stator frequency signal (352) based on stator flux signals (308, 310, 312) and supplies the stator frequency signal (352) downstream of the PI regulator (332), and wherein the stator frequency signal (352) is summed with the output signal (334) of the PI regulator (332) to provide a dynamically adapted output signal (335) of the PI regulator (332), and wherein the adapted output signal (335) is used to determine the stator flux angle signal (338).