Discrete frequency operation for unit capacity control
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Solution Overview
Problem
Variable speed electric motors in refrigerant systems often encounter undesirable conditions such as mechanical and acoustic resonance, leading to noise, excessive vibration, and potential damage due to operational frequencies that pass through resonance zones during start-up, shutdown, and adjustments to meet thermal load demands.
Innovation Solution
The motor controller alternates the operating frequency between multiple discrete frequencies to maintain an average desired frequency, thereby avoiding undesirable resonance conditions, ensuring continuous variable speed operation and efficient system performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the motor frequency is continuously varied from zero to the desired operational frequency during start-up and shutdown, then the motor can achieve variable speed operation, but it passes through resonance frequencies causing excessive vibration, noise and mechanical stress
Solution Approach 1:
The patent segments the continuous frequency spectrum into discrete frequency bands, identifying specific resonance frequencies that should be avoided. The controller divides the frequency range from zero to the desired operational frequency into multiple discrete levels, selecting only those frequencies that do not coincide with system resonance frequencies. This segmentation allows the motor to achieve variable speed operation while systematically avoiding harmful resonance conditions.
Solution Approach 2:
The patent implements periodic action by using pulse width modulation (PWM) to switch the motor between different discrete frequency levels. Rather than providing continuous frequency variation, the system periodically switches between approved discrete frequencies, using the temporal averaging effect to achieve the desired average speed while never actually operating at harmful resonance frequencies. This periodic switching between discrete frequency states resolves the contradiction between continuous speed control and avoidance of resonance.
2Reliability
If stepless control is used to operate through undesirable frequencies for a very limited time, then the system can minimize exposure to resonance, but it cannot entirely avoid the undesirable frequencies and their associated harmful effects
Solution Approach 1:
The patent applies preliminary action by pre-identifying and mapping all resonance frequencies within the operating range before actual motor operation begins. The controller is programmed with a lookup table of forbidden frequency zones determined during system commissioning or design phase. This preliminary identification allows the system to proactively select only safe discrete frequencies during operation, completely avoiding resonance rather than merely minimizing exposure through rapid passage.
Solution Approach 2:
The patent changes the fundamental parameter of frequency control from continuous analog variation to discrete digital selection. By transforming the frequency parameter into a set of discrete, pre-approved values, the system fundamentally alters how speed control is achieved. This parameter transformation enables complete avoidance of resonance frequencies while maintaining effective variable speed control through intelligent selection and temporal combination of discrete frequency levels.
Data Source
AI summary
A variable speed electric drive for use in refrigerant systems includes an electric motor for driving an associated component at a variable speed that is a function of an operating frequency of the motor; and a control for supplying alternating discrete drive frequencies to the electric motor to provide a continuously variable speed drive of the associated component. The control cycles the drive frequency to the electric motor among the at least two discrete frequencies so that the variable average resultant speed at which the associated component is driven is a function of a combination of the selected at least two discrete frequencies.