VFD trends and solutions for drive systems

Harmonics mitigation, filtering voltage spikes are two key issues.

06/04/2015


The increase in variable frequency drive (VFD) installations across the globe has meant that drives are being applied in more challenging system environments.Not only is the environment challenging, but drives also have a few inherited issues that can limit the overall system performance.

International standards have helped guide system integrators to make the correct choices for equipment inside their systems. These choices help to eliminate problems such as electromechanical interference and harmonic noise at the input of the drive; as well as voltage spikes and short-circuit and interference currents at the output. Each of these issues should be taken into consideration during the design phase to increase overall system efficiency during operation.

When many electrical systems are installed in the same area, the equipment in the network begins to interfere with each other. This is often referred to as electromechanical interference, or EMI. This can lead to many problems within the network, including eventual equipment damage. These are risks that many applications such as datasystems, hospitals, and schools cannot afford.

Many integrators are turning to the simplest and cheapest solution, an EMI filter. These filters have a small footprint and are easy to install. They are installed on the line side of the drive system to help suppress the unwanted high frequencies associated with crowded networks. International standards such as International Electrotechnical Commission (IEC) 61000-6-4 have set guidelines for the acceptable levels of EMI in industrial systems. Integrators need to be aware of these guidelines and select components appropriately.

The problem of harmonics

A chart comparing harmonics. All images courtesy: REO-USAHarmonics are present in every drives application. Harmonics are created by nonlinear loads like VFDs. The non sinusoidal currents present in all drives using diodes or switching devices on the input may lead to harmonic currents in the system. Harmonic currents can cause a number of issues in systems such as component overheating, reduced power factor, and equipment malfunction.

There are several different component solutions that system integrators use to mitigate harmonic current content in today’s installations. One of the most common methods is a line reactor. As the name implies, line reactors are installed on the line side of the drive system. A simple 3% impedance line reactor can decrease system harmonic current content by more than 30%.

The reactor itself is just a simple inductor with a specific amount of impedance. Adding impedance to any system will help to mitigate harmonics in every situation. International standards such as the IEEE 519 and IEC 610003-2 should be referenced before installing any industrial or commercial VFD system. The simple addition of a line reactor will improve the overall system efficiency, decrease component maintenance cost, and increase the system equipment lifetime.

The output of VFDs sometimes causes issues that can lead to increased motor stress and increases in system downtime. A common problem at the output is voltage spikes. The switching devices inside the drive turn on and off very quickly, causing resultant voltage spikes greater than the rated voltage of the motor.

Depending on the cable length between the drive and motor, the peak voltage can often be greater than 1,200 V in a 480V system. The longer the cable length, the higher the voltage spikes. These spikes will eventually cause deterioration of the motor insulation and motor winding failures. Certain applications require long cable lengths due to the environment of the system installation.

Filtering voltage spikes

The formula dV/dt represents the instantaneous rate of voltage change. System integrators are turning to dV/dt filters to help mitigate voltage spikes in their VFD systems. These filters are installed at the output of the VFD, in front of the motor in the system. The dV/dt filters suppress the voltage spikes to limits that the motor can better manage. The increased motor protection will lead to an increase in overall motor lifetime, and a decrease in equipment downtime caused by the damage from the voltage spikes.

Like the line reactor and EMI filters on the line side, dV/dt filters are simple to install and are inexpensive, especially when compared to the cost of replacing damaged motors. Most commonly, these filters are used in applications with harsher environments such as oil and gas, mining, and marine.

Another common problem found at the output of the VFD system is the presence of differential-mode and common-mode currents. These are distortions in the output current of the VFD, often referred to as asymmetric and symmetric, respectively. Like voltage spikes, the current distortions exist because of the fast switching devices in the VFD. Current distortion has a negative effect on the motor, causing audible noise, higher losses in heat, as well as overheating when it is ignored.


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