Synchronous Motor Speed Estimation Using Low-Frequency Signal Injection
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Solution Overview
Problem
Existing methods for estimating the speed of a synchronous electric motor at zero torque are challenging due to zero current at the stator, requiring high-frequency injections and motor parameter knowledge, which complicates accurate speed estimation.
Innovation Solution
A control system in a variable speed drive that uses a second speed estimator and a signal-generating module to apply a low-frequency, non-torque-generating sinusoidal current signal to estimate motor speed without motor parameter knowledge, allowing for accurate speed deduction even at zero torque.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If high-frequency injections are used to estimate motor speed at zero torque, then speed estimation becomes possible, but the system complexity increases and requires sufficient decoupling of time scales
Solution Approach 1:
The patent changes the frequency parameter of the injected signal from high-frequency to low-frequency (below motor frequency, e.g., by a factor of 10). This parameter change simplifies the control system by eliminating the requirement for high-frequency injection while still enabling speed estimation through the response of the motor phases.
Solution Approach 2:
The patent employs periodic low-frequency sinusoidal current injections into the motor phases. This periodic action creates measurable current responses that allow speed estimation without requiring high-frequency decoupling, thus reducing system complexity while maintaining measurement capability.
2Measurement precision
If high-frequency injections are used for speed estimation, then speed can be determined, but current ripples must be sufficient to disregard nonlinear sensor effects
Solution Approach 1:
The patent changes the frequency parameter from high-frequency to low-frequency injection. This parameter change modifies the current ripple characteristics to be sufficient for measurement while avoiding the nonlinear effects of current sensors that occur at high frequencies, thereby maintaining measurement precision without requiring excessive current ripples.
3Speed
If zero torque condition exists in synchronous motor, then motor can rotate with non-zero speed, but current at stator becomes zero making speed estimation difficult
Solution Approach 1:
The patent applies preliminary action by injecting low-frequency sinusoidal current signals into the motor phases before speed estimation is performed. This preliminary injection creates measurable current responses even when the motor operates at zero torque, enabling speed estimation without relying on stator current that would otherwise be zero.
Solution Approach 2:
The patent introduces an intermediary low-frequency current signal as a mediator between the zero-torque operating condition and the speed estimation requirement. This intermediary signal creates measurable effects in the motor phases that allow speed determination without directly relying on the absent stator current.
Data Source
AI summary
A control system included in a speed selector connected by output phases to a synchronous electric motor, the synchronous electric motor being controlled according to a control law implemented by the speed selector. A first speed of the synchronous electric motor is determined by a first speed estimator. A second speed estimator is used to determine a second speed of the synchronous electric motor. The system includes a signal generator module configured to apply, to the output phases, voltages taking account of a non-constant current signal. The second speed estimator is configured to recover the current response on the output phases, to deduce therefrom the second speed of the synchronous electric motor.