A Case Study on EMI Noise adversely affecting the performance & reliability of Robotics & Automation

Published by J A Simon, Chief Executive, Waveforms, Email: waveformspower@gmail.com, Website: www.waveforms.co.in


Automation improves productivity but it can also derail the automation process due to the issue of harmonics & Emi (high-frequency noise). This case study is about how to improve the reliability, electrical efficiency, & power quality of robotics & automation.

This real-time case study was done at one of the leading automobile industries using robotics.

The issue faced frequent failures of robotic PCB boards, control boards, power supplies, VFD & consequently disruption in production.

SAMPLE READING OF UPS & ROBOT

60KVA UPS OUTPUT SIDE CONNECTED TO ROBOTIC MACHINE:

First: Table for UPS output side
Second: Graphs of current waveforms/THD
Third: Individual Harmonics Graphs

FIG1: Real Time Graphs; First: Table for UPS output side, Second: Graphs of current waveforms/THD, Third: Individual Harmonics Graphs

OBSERVATIONS STUDY AND ANALYSIS:

The plant is an automated one and the loads are predominantly non-linear loads such as Drives, SMPS, and UPS, hence harmonics from 2nd to the 50th harmonics were observed. The dominant frequencies noticed were the 3rd, 5th 7th, 11th and 13th (Refer to the enclosed harmonics table provided for drives). These harmonics are all the characteristics of drives and SMPS. The harmonics are abnormal and needs to be mitigated. These harmonics are generated at the input of drives and injected into the source of supply. In this case, the source is UPS. However, these harmonics can impact the source voltage and current waveform. 

 Besides these Harmonics high frequency, harmonics are likely to be generated due to the high frequency Harmonics above the 50th to 150hz. High-frequency harmonics are generated by the switching of Mosfet/IGBT. Typically, The VFD is switched between 2 kHz and 8 kHz. The chopping of DC output creates noise (EMI). If the Motor is at a distance (>300M) the inductance of the long cable and the capacitance of the winding of the motor (due to high frequency) they interact create high frequency noise and high dv/dt and di/dt, hence dv/dt chokes or Sine wave filters (for distances >500m) may be necessary. Since the distance of motor in this case is short shielding the cables and grounding will be sufficient to supress this noise.

We also Observed that the 60KVA UPS installed is not Galvanic Isolated, hence harmonics are like to circulate in all the electronic systems in the Plant, It is a well-known fact that these frequencies are superimposed on the 5Khz frequencies will cause disturbances and cause malfunction and damage to the digital circuits.

Grounding of the drives and motors are critical to mitigate high frequency Harmonics by diverting them to the ground and prevent the propagation and interaction of Harmonic signals. The ground forms a low impedance path for Harmonics. A technique commonly employed in all electronic equipment’s and devices. Shielded cables need to be used to prevent coupling of these frequencies in cables. We observed these precautions are not adequately taken. A typical grounding diagram is given below for your reference.

Fig2: Transformer, Variable frequency drive and motor cable grounding diagram.

We also observed the sizing of UPS is inappropriate and oversized and could result in avoidable Power losses. The current harmonics at input of UPS was 15%, these harmonics are likely to take the low impedance path to the input transformer. The output of the UPS the voltage Harmonics is as low as 2%, due to closed loop circuit for voltage correction. However, the Current Harmonics observed was 40% to 60% and the power factor was also low at 0.78 lag. This is due to current distortion caused by the discontinuous currents of load. The absence of shielded Isolation Transformer will cause high leakage current and may cause shock in certain cases. The sensitive electronic loads are exposed to common Mode noise and transients.

We also observed that no chokes or reactors has been employed, the minimum requirement to filter Noise and Transients.

 We also noticed that the cables from VFD to motors were rolled concentrically and kept in the cubicle, and connected to motors, this can cause noise coupling within cables, and hence avoided. Use minimum cable lengths only.

 Due to these multiple factors enlisted, we are of the opinion reliability of Operation, electrical efficiency and Power quality will be affected adversely.

ABOUT EMI/ EMC

The plant is installed with multiple nonlinear loads like Variable frequency drives, UPS, power supplies which are characterized by the fact that they draw non sinusoidal and discontinuous currents, which, according to Fourier analysis, cause additional superimposed currents with multiple higher mains frequencies. These harmonic oscillations (also known as Harmonic currents) can have negative and deleterious effects upto the 50th Harmonic oscillation.

In the frequency range between 2.5 Khz and 9Khz the interference levels are not yet regulated. Between 9Khz and 150 Khz the frequencies are only standardized for certain application.

High frequency Interference (also Known as Electro Magnetic interference (EMI) ) are divided as follows:

  1. Harmonic range: 50hz to 2500hz (1st harmonic to the 50th)
  2. Low frequency :  2.5khz -9khz
  3. Conducted frequency range: 9khz to 30Mhz
  4. Radiated : 30Mhz to 3Ghz.

STANDARDS:

Variable frequency drive:  IEC 61800-6-3

IEC Emission:61000-6-4, Interference immunity: IEC 61000-6-2

The EMI (Electro Magnetic Interference) in your case can be from the various Non Linear Loads (VFDs,UPS SMPS) these EMI can also be termed as electrical Noise and they can be sub divided as follows:

Differential Mode Noise; These conducted signals noise can occur between Phase (P) and neutral (N) in single phase system. In three phase sytems it can be between the three Phases (RYB). In DC system the noise can travel from Plus to minus. They can also be called as symmetrical Noise.

Differential Noise is a result of Parasitic components in a circuit such equivalent series Inductance (ESL) or Equivalent series resistance (ESR). The Differential Mode noise usually occurs at lower frequencies and is commonly associated with switching frequency of VFD and SMPS.

Common Mode Noise: This is the noise that conducts between any line or Phase to the ground. The main difference it travels in all lines in the same direction to the earth. Common mode noise results from stray capacitances in the system, often occurring between semiconductors and heat sinks.

Noise Propagation: Electro Magnetic Noises are generated in electrical and electronic systems and can then propagate inside the system or even outside. This propagation can work along the lines or through radiation.

Fig3 Differential & Common mode noise

LEAKAGE CURRENTS :

Leakage currents in frequency inverters arise through internal interference-suppression measures and all parasitic capacitances in the inverter and motor cables. The largest leakage currents, though, are caused by the method of operation of the inverter. It controls motor speed continuously using pulse-width modulation (PWM), which generates leakage currents far above the grid frequency of 50 Hz. For instance, the switching frequency of an inverter might be 4 kHz, and the associated harmonics can have very large amplitudes at higher frequencies. These frequencies then travel over the motor cables to the motor, and so the motor cables with their grounded shields act like a capacitor to ground. Current is then diverted to earth through this capacitance. As per IEC 61950 regulation the maximum permissible leakage current is 3.5 mA.

SOLUTIONS:

1.   Installation of Phase shifted Waveforms Minimizer with isolation at the output of the 60 KVA UPS. This will mitigate Harmonics >70% of the dominant Harmonics up to 25th. This Transformer besides mitigating Harmonics will also provide Galvanic Isolation, filter common Mode noise and also provide protection against Transients. The reliability of UPS will also improve dramatically. Ensure the isolation transformer is shielded on the primary and secondary winding to Shield EMI by diverting the HF noise to ground. This technique results in cancellation and diversion of Harmonics.

2.   EMI filters: By employing a EMI/ EMC filter of the appropriate capacity Differential Mode Noise can be filtered and attenuate high frequency noise to IEC/EN 60939. This EMI Filter must be installed closer to the VFDs on a metal Plane to ensure high frequencies are grounded. These filters are inexpensive and hugely improve the reliability of drives.

3.   Cables: ensure all cables from the VFD to drives are as short as possible with shielding and preferably braided. Ensure Power cables are away by minimum of 20 cm away from control cables. The shielding must be grounded to the common ground of the Waveforms Minimizer . Incase of Power cable crossing ensure they cross at 90 degree.

We are confident with these good practices of harmonic and EMI/EMC, the reliability of the operation will improve significantly.

NOTE: We have used High-frequency noise, EMI, Harmonics at several Places of the report they all mean the same.

Pic credits: 1.VFD.org, 2.Tech Web

Published by PQTBlog

Electrical Engineer

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