Frequency-controlled drives (drives, VFDs, inverters) are indispensable in modern industries and buildings. They provide energy savings and process optimisation. However, these power electronics simultaneously represent one of the biggest sources of electromagnetic pollution in your electrical installation.
Without proper measures, inverters can lead to unexplained failures, control system failures and accelerated equipment wear and tear. As Power Quality Doctor, we see daily how these 'innocuous' components compromise operational reliability. In this article, we dissect the technical cause of EMC problems in drives and offer a concrete path to a solution.
The Source: Inverters 'chop' voltage into pieces (PWM), leading to high-frequency interference (EMC) and low-frequency pollution (harmonics).
The Risk: This can lead to interference with PLCs, sensor signals, bearing damage in motors and overheating of transformers.
The Solution: A combination of correct cabling, earthing, filters and isolation transformers is often necessary.
The Approach: Always start with a baseline measurement to determine whether the problem is conducted or radiated.
This information is crucial for:
To understand why drives cause failures, we need to look at how they work. A frequency converter converts the sinusoidal AC voltage from the mains (50Hz) to a direct current (DC), and then back to a variable AC voltage to drive the motor.
The latter process is done via Pulse Width Modulation (PWM). The inverter switches the voltage on and off at lightning speed (switching frequencies from 2 kHz to as high as 16 kHz or higher).
The equation: Imagine a water tap. Instead of turning the tap half-open for a gentle stream (linear), turn the tap fully open and close 100 times a second. On average, less water comes out, but the pipes clatter and you create pressure waves (transients) throughout the pipe system.
In electricity, the same thing happens. The fast switching moments of the IGBTs in the inverter cause steep voltage slopes (high dU/dt). This results in two main problems:
Why is this important for your installation?
The impact of poor EMC (Electromagnetic Compatibility) is often underestimated because the effects are not always immediately visible.
EMC problems rarely manifest themselves as an obvious label on a display. They are often 'ghost faults'. Be alert to the following symptoms:
Nuance: Not every failure is an EMC problem. Poor connections or faulty hardware can look like interference. Therefore, measurement is essential for correct diagnosis.
The problem with inverters is twofold: harmonics (low frequency) and EMI (high frequency). It is crucial to make this distinction because the solutions are completely different.
On the input (mains) side, the inverter behaves as a non-linear load. The rectifier draws the current not in a nice sine wave form, but in short pulses. This creates harmonics (multiples of 50Hz, such as 250Hz, 350Hz).
At the output side and through internal circuitry, frequencies arise in the kHz and MHz range.
Common Mode Currents: This is a common phenomenon in drives. Due to the parasitic capacitance of the motor cable (especially on long cables), high-frequency currents flow from the phases to earth. If the earthing is not high-frequency capable, these currents seek another path, for example through the shielding of data cables or the bearings of the motor.
Solving EMC problems requires a structured approach. We start with the basics (the installation itself) and then look at hardware additions.
Many problems can be traced back to installation methods.
If the basic installation is in order but problems persist, there are components to improve Power Quality.
For installations with many drives and significant harmonic contamination:
Are you experiencing vague faults and suspect the drives? Follow this roadmap:
The 'Pigtail': The cable's shield is twisted together into a wire and plugged into a terminal block. As a result, the shield loses its high-frequency effect almost completely.
Wrong filter: A standard EMC filter does not solve harmonic problems, and a harmonic filter does not solve EMC disturbances (MHz range).
Assumptions about standards: "The drive has a CE mark, so it does not interfere." A drive is a component, not an end product. The method of installation determines whether the overall installation complies with the EMC directive.
Long cables: Underestimating cable length. With long motor cables (>50-100m), the cable acts as a capacitor, which can lead to huge peak current spikes and drive shutdown.
There is no need to immediately call in an external party for every incident. Simple wiring faults or loose earthing can often be solved by your own technical department. However, there is a tipping point where 'trying it yourself' turns into irresponsible risk.
Call in a Power Quality specialist when the following signals occur:
Advice: Are you going to expand with new LED lighting, EV chargers or solar panels? Have a simulation or measurement carried out preventively. The addition of these power electronics can suddenly make an existing, stable installation unstable.
Delve further into the subject matter via these related pages:
Do you suspect drives are causing failures in your installation, but can't put your finger on it? Don't experiment with filters without a diagnosis. With specialised measurements, our engineers can visualise exactly where the contamination is coming from and which solution - from earthing to active filter - is most cost-effective.
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