Sensorless Motor Control With Smooth Back-EMF Transition
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Solution Overview
Problem
Existing sensorless motor control techniques for permanent magnet electric motors face challenges during start-up, as they require the motor to reach sufficient speed to induce a usable back EMF, leading to abrupt rotor angle changes and potential mechanical instability during transitions from open loop to closed loop control.
Innovation Solution
The method involves a controller that progressively transitions from open loop to closed loop control by determining the rotor angle as a weighted average of the open loop and closed loop angles during a defined frequency range, thereby smoothing the transition and reducing mechanical stress.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of stationary object
If sensorless control is used to avoid position sensors, then weight and bulk are reduced, but control accuracy deteriorates at low speeds because sufficient back EMF is not induced
Solution Approach 1:
The system performs preliminary open-loop operation to accelerate the motor to a speed where sufficient back EMF is induced, preparing the conditions for accurate sensorless control to take over. This preliminary action ensures that when sensorless control begins, the necessary voltage signals are available for accurate rotor position determination.
Solution Approach 2:
A transition period is introduced as an intermediary phase between open-loop and closed-loop control. During this period, the system uses a weighted combination of both control methods, allowing smooth handover while maintaining control accuracy. The transition period acts as a mediator that bridges the gap between the two control modes.
2Device complexity
If abrupt transition from open loop to closed loop control is made, then control mode switching is simplified, but mechanical stability deteriorates due to sudden rotor angle changes
Solution Approach 1:
The control system dynamically adjusts the weighting between open-loop and closed-loop control methods during the transition period. The weighting factor changes continuously based on the operating conditions, allowing the system to adapt smoothly between control modes. This dynamic adjustment prevents abrupt rotor angle changes while maintaining relatively simple control logic.
3Measurement precision
If motor speed is increased to induce sufficient back EMF, then sensorless control accuracy is improved, but mechanical stress increases during the speed increase phase
Solution Approach 1:
The system performs preliminary acceleration in open-loop mode, preparing the motor for sensorless control operation. This preliminary phase allows the motor to reach the necessary speed for accurate sensorless control while using simple voltage-frequency relationships, avoiding the need for high-stress closed-loop control during the acceleration phase.
Solution Approach 2:
The transition period serves as a cushioning phase that gradually introduces closed-loop control. By using a weighted combination of control methods during transition, the system softens the impact of switching control modes, reducing mechanical stress on the motor and connected machinery during the speed increase phase.
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
This approach ensures a smoother and more stable transition from open loop to closed loop control, reducing mechanical stress and improving system stability, thereby enhancing the reliability and efficiency of motor operation.
Implementation Method 1
monitoring an induced voltage in the stator that opposes the change in magnetic flux caused by the spinning rotor that caused the induced voltage (i.e. the counter or 'back' electromotive force (back EMF))
Data Source
AI summary
Methods of operating electric motor systems that comprise electric motors comprising a rotor having a magnet mounted thereto. The electric motor is initially operated in an open loop mode in which the rotor angle is estimated based on the input voltage frequency. Once the motor is running at sufficient speed, a transition to closed loop mode operation is performed, wherein the rotor angle is determined using an observed back EMF. To provide a smoother open loop to closed loop transition, the rotor angle is determined during a transition period as a function of both the open loop rotor angle and the closed loop rotor angle.