In our last blog, we discussed AC power, frequency, and sine waves. Sine wave filters are critical when running VFD pumps and provide valuable benefits such as eliminating heating, audible noise, and problems with motor/cable insulation failures.
Sine Wave Filters and dV/dt
Sine wave filters also eliminate high dV/dt, the phenomenon of delta (Δ) Voltage / delta (Δ) time, where delta represents “change.” dV/dt represents the distortion and amplification of voltage spikes over the lengths of the leads. The longer your lead lengths are from the VFD to the motor, the more you need to worry about dV/dt.
How Motor Drives Work
Motors operating on alternating (AC) current require special power supplies to provide power to smoothly start and stop the motor. If the motor were forced to transition from full “on” to full “off” in a quick time period, this would put undue stress on the equipment’s mechanical parts. The motor requires a power supply to start and stop the motor gradually. Additionally, when a motor is running, the application may require an adjustment of torque and speed to accommodate for various mechanical loads.
Variable Frequency Drive (VFD)
Variable frequency drives (VFDs) deliver the power supply that provides a gradual start and stop along with variable speed and torque. VFDs are electronic converters that control the speed by adjusting the frequency of the current, the voltage, that is sent to the motor. VFDs are becoming more common to control the speed and torque of electric motors, compared to mechanical and hydraulic adjustable speed drives. VFDs are increasingly becoming the preferred method due to a decrease in cost and size, which is particularly true where energy efficiency is critical in high-power applications.
Sine Wave Filter Benefits
AC motors operate at peak efficiency when running on clean sinusoidal power. Variable Frequency Drives, however, produce modulated square waves, with the pulse width of the square waves being variable, which is recognized as pulse width modulation, PWM. VFDs control the amplitude of each square wave. Installing a sine wave filter between the VFD power source and the motor smooths out the VFD modulated square wave pulse width to convert it into a nearly perfect sine wave power waveform.
It may sound like a poor idea to run a motor with square waves, although VFDs maintain such precise control over the amplitude and width of the square waves that they are viewed on an oscilloscope to have quite good approximations of sine waves. VFDs alone, however, fail to match the purity of a true sine wave and can introduce harmonic distortion that may cause damage to the motor.
Potential Problems of Running VFD Without a Sine Wave Filter
A combination of VFD and a sine wave filter allow the motor to operate at adaptable torque and continually adjustable speeds to meet varying mechanical loads. Running a motor without a sine wave filter can create a multitude of problems, such as:
- Voltage spikes – when a VFD converts clean 60 Hz sinusoidal waveform power into square wave pulses, the leading edge of each square wave contains a proportion of voltage overshoot. These spikes in voltage may cause damage to the motor.
- Excessive stress – the final VFD modulation output also introduces harmonic distortion, which corrupts the shape of the pulse width modulation, PWM, output. Without filtering this harmonic distortion, it will be burnt up as heat, causing excessive stress in the motor’s windings and creating decreased efficiency.
- Bearing currents – common mode noise relative to ground can permit stray currents to flow through the motor’s bearings. This results in a phenomenon known as fluting or bearing currents. As low common mode current flows through the bearing, tiny pits are melted into the bearing. Over time, the bearing can break, causing the high-speed motor to effectively self-destruct.
Sinewave filters provide solutions to all of these problems, allowing the safe operation of motors. Illinois Process Equipment, IPE, is a leading pump and process equipment system provider offering quality pumps and components, including VFDs and sinewave filters, along with complete pumping system design.
IPE is your full-service pump provider, differing from other pump suppliers with our deep technical knowledge of pumps and process equipment. We offer comprehensive services, including full system design, installation, testing, maintenance, repair, and more. Contact us to learn more about improving your pump process with VFD pumps and sinewave filters.