Variable Frequency Drives & Harmonics: Cause, Problems, and Solutions

Variable Frequency Drives (VFDs) are widely used in industrial applications to control the speed of electric motors. VFDs offer several advantages over traditional motor control methods, including energy efficiency, improved process control, and reduced wear and tear on equipment. However, VFDs can also produce harmonics, which can cause problems for the power system and the equipment connected to it. In this article, we will explore the technical causes of harmonics, the problems they can cause, and the solutions available for mitigating harmonics from VFDs.

Picture Credit: Sentinel Power Quality FZC

Technical Causes of Harmonics

Harmonics are currents or voltages at frequencies that are multiples of the fundamental frequency of the power system (usually 50 or 60 Hz). Harmonics are caused by non-linear loads, which draw current in a non-sinusoidal pattern.

VFDs are a common source of non-linear loads in industrial applications. The switching action of the VFD creates a pulse-width modulated (PWM) waveform, which contains high-frequency harmonics. The higher the switching frequency of the VFD, the higher the order of harmonics produced.

Problems Caused by Harmonics

Harmonics can cause several problems for the power system and the equipment connected to it. Some of the most common problems include:

Picture Credit: EEP - Electrical Engineering Portal

1. Overheating: Harmonic currents can cause overheating in transformers, motors, and other equipment, leading to reduced lifespan and increased maintenance costs.
2. Overheating and premature failure of VFD components like capacitors and transistors
3. Voltage distortion: Harmonic currents can distort the voltage waveform, leading to malfunctions in sensitive equipment, such as computers and control systems.
4. Resonance: Harmonics can cause resonance in the power system, leading to voltage surges and equipment failures.
5. Power factor reduction: Harmonics can reduce the system's power factor, leading to increased energy costs and penalties.
6. Interference with communications systems like Wi-Fi, cellular, and radio frequency devices
7. Malfunction of sensitive electronic equipment like adjustable speed drives, programmable logic controllers, etc.
8. Excessive neutral current leading to voltage drops, transformer overheating, and nuisance tripping of circuit breakers
9. Torque pulsations in motors reducing efficiency and mechanical wear

*Harmonics Penalty: Other than this, many governments are now comping with harmonics penalties to be levied if harmonic levels are not within the permitted levels in IEEE 519 standards.

Variable Frequency Drives

There are several solutions available for mitigating harmonics from VFDs:

1. Line reactors: Line reactors can be installed between the VFD and the power supply to filter out harmonics. Line reactors are passive devices that use inductance to smooth out the current waveform.
2. Passive harmonic filters: Passive harmonic filters are the most common type of harmonic filter used with VFDs. They are installed between the VFD and the power supply to filter out harmonics. Passive harmonic filters are passive devices that use inductance and capacitance to smooth out the current waveform. They are effective at reducing low-order harmonics but can be less effective at higher frequencies.
3. Active harmonic filters: Active harmonic filters are electronic devices that use active power electronics to filter out harmonics. They are more expensive than passive filters but offer better performance at higher frequencies. Active harmonic filters can be programmed to filter out specific harmonics and adjust to load and output frequency changes.
4. Multi-pulse VFDs: Multi-pulse VFDs use multiple sets of power semiconductors to reduce the order of harmonics produced. For example, a 12-pulse VFD uses two sets of six-pulse rectifiers, which cancel out the 5th and 7th-order harmonics.
5. Active front-end VFDs: Active front-end VFDs use active power electronics to shape the current waveform and filter out harmonics. Active front-end VFDs are more expensive than other solutions but offer the highest level of harmonic mitigation.
6. Choose a motor with lower susceptibility to harmonics and a high power factor.
7. Ensure the VFD is properly grounded and shielded.
8. Keep the VFD wiring short to minimize interference.
9. Load the VFD to the optimal level for minimum harmonics.

Choosing the Right Solution

The best solution for mitigating harmonics from VFDs depends on the specific requirements of the application. Passive harmonic filters are generally the most cost-effective solution for low-order harmonics, while active harmonic filters are better suited for higher-order harmonics and applications with varying loads. Line reactors and multi-pulse VFDs can also be effective in certain applications.

Conclusion

Harmonics from #vfds can cause a range of problems for the power system and the equipment connected to it. Fortunately, there are several solutions available for mitigating harmonics, including passive and active harmonic filters, line reactors, and multi-pulse VFDs. Choosing the right solution depends on the specific needs of the application.

If variable frequency drive #harmonics are disrupting your industrial operations, let's work together to eliminate these power supply disturbances. Contact me on LinkedIn (Follow Rohit Sinwer & click 🔔) or book a call to explore how we can harmonize your VFD systems and boost your plant's productivity.

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