If you work with electric motors and variable frequency drives (VFDs), you may have encountered the term “skip frequencies.” But why are these settings important, and how do they affect motor performance? In this article, we’ll explore the science behind skip frequencies and why they’re essential for optimal motor operation.
Skip frequencies, also known as avoidance frequencies or jump frequencies, are specific points within an inverter’s operating range that the system is programmed to bypass. When a motor accelerates or decelerates, the VFD automatically skips these predetermined frequencies rather than operating at them continuously.
The primary reason for implementing skip frequency settings is to avoid a phenomenon known as mechanical resonance. Every motor and the equipment it powers has a unique natural vibration frequency determined by factors like:
- Physical dimensions and material properties
- Mounting configuration
- Load characteristics
- System rigidity
When a motor operates at or near this natural frequency, resonance occurs, creating a feedback loop that amplifies vibrations throughout the system.
Without proper skip frequency settings, resonance can lead to numerous problems:
- Excessive noise pollution in the workplace
- Accelerated wear on bearings, gears, and other mechanical components
- Premature equipment failure and increased maintenance costs
- Reduced precision in manufacturing processes
- Safety hazards from excessive vibration
- Potential damage to structural components
Certain types of machinery are particularly susceptible to resonance issues and therefore benefit greatly from skip frequency settings:
- Precision machine tools requiring tight tolerances
- Elevators and escalators where smooth operation is critical
- Centrifuges and high-speed rotating equipment
- Industrial fans and blowers
- Pumps and compressors
- Conveyor systems and material handling equipment
To establish proper skip frequency parameters, follow these steps:
- Conduct a frequency response analysis of the system
- Operate the motor across its full frequency range while monitoring vibration
- Use specialized equipment to detect resonance points
- Document frequencies where vibration amplitudes increase significantly
- Program these frequencies as skip points in the VFD
Modern VFDs typically offer advanced skip frequency capabilities:
- Multiple skip frequency bands can be programmed
- Each band includes a center frequency and a bandwidth
- The width of each skip band should be sufficient to avoid the entire resonance range
- Settings should be adjusted based on specific load conditions
When properly configured, skip frequency settings provide numerous benefits:
- Extended equipment lifespan by reducing mechanical stress
- Improved operational efficiency and energy savings
- Enhanced product quality through reduced vibration
- Lower maintenance costs and fewer unplanned downtime events
- Safer working environment with reduced noise and vibration hazards
Skip frequency settings represent a critical aspect of motor control system design. By understanding and implementing these parameters correctly, you can significantly improve equipment performance, reduce maintenance costs, and extend the operational life of your motor systems.
Whether you’re setting up a new VFD system or optimizing an existing installation, taking the time to properly configure skip frequencies will pay dividends in improved performance and reliability.
