PLC Damage

PLC is at the heart of SCADA and control system in the manufacturing plants. If the factory power distribution is designed poorly or if the factory runs large VFDs or DC drives motors, PLCs can burn out frequently or will lose memory quite often.

To understand this, we have to look at the power supply mechanism of PLC. Typically it is 24V DC or 220/110V AC to DC SMPS. In either of the cases, if AC input power is corrupt beyond a certain limit, electronics components of the PLC can be damaged easily. Permanent memory can be reset.

Typically, 24V or SMPS supply is robust against voltage and current surge. Also, all the power supply will have built-in protection. But if a large VFD is switched on in a close by area, two detrimental phenomena take place simultaneously, and their orchestration can damage PLC electronics.

First, during the start-up of large VFD motors, a large inrush current is generated which flow to the PLC power supply. However, the most damaging part is the high harmonic content of that current. Vector drives generate 40-70% current harmonics. Although power supply will filter major part of the harmonics, higher order harmonics of Inrush current will sneak through and instantaneously heat up the PLC board.

This can also lead to voltage harmonic surge which in turn can damage PLC capacitors easily. RAM/EPROM of the PLC is highly susceptible to an unwanted surge in harmonics in current and voltage.

To deal with this kind of a nuisance, one needs to understand the surges which lead to frequent failure of PLC. Therefore, a high-quality recorder ( Such as MachineSense PA ) is needed which will capture voltage currents and its harmonics in each second for 24×7 operation. Also, the system should have built-in alarms so that the surges will be recorded and conveyed via SMS and email with a time stamp so that a correlation between surge and occurrence of the event of PLC destruction can be correlated.

About 80 per cent of PLC failures are a result of power supply issues, I/O module failure or field devices failure. These failure results in sudden process stop or irregularity of performances.

To summarize, PLC can fail due to the following reasons:

a. Presence of VFD/DC drive nearby
b. Poor grounding
c. EMI/EMC interference due to high current harmonics

Fig. 1: PLC power input current due the presence of VFD nearby

• Fig. 1 shows the PLC power input current waveform which directly goes to the PLC SMPS. It creates unnecessary heat to the SMPS and leads to faliure. Apart from this, it also generates interference which may cause unusal behaviour and PLC faiure.

Impact of improper grounding:

Proper grounding is important in protecting both the PLC and maintenance personnel. A good grounded enclosure can also act as an insulator to an electrical noise. If grounding is not done properly, power line noise gets conducted to the control ground and PLC starts mal-functioning.

Till now, someone has to go there with a multi-meter to check the ground resistivity using offline meter available and that too is a costly device. MachineSense PAbrings that cost down significantly and helps user to monitor ground health 24x7 online.

Effect of EMI/EMC interference due to high current harmonics:

It is basically the electromagnetic interference that causes mal-operation of the PLC or even can damage PLC sometime. This EMI/EMC can be generated either by starting of the large induction motor or large VFD drive or anything from radio transmitters used by some other working personnel in that location. Sometimes high current harmonics carried by the power line running nearby PLC system can create interference or harmonics get conducted to PLC input from where the PLC supply is taken.

• Use MachineSense Power Analyzer (PA) to have a closer look at this to take preventive action. MachineSense PAwill monitor the abnormalities 24x7 and send you sms/email whenever any alarming condition is detected.
• You can also contact ( info@machinesense.com) MachineSense PowerAnalyzer team to consult and get free guidance to select proper low pass filter to reduce harmonics.

Solution

PLC drive

• Isolate power source from VFD/DC drive system if they are running nearby. To do so, a transformer with proper KVA rating can be installed. To estimate the KVA rating install MachineSense PA and look for the peak usage in the Real Power.
• Replace ordinary VFD/DC drive with IGBT based digital drive which has inbuilt PFC (Power Factor Corrector) at drive input. IGBT allows bidirectional current flow and reduces harmonics significantly and thus improves power factor at input.
  
  You can contact MachineSense Power Analyzer team to consult and get free guidance to select better quality drive to solve your factory problem.
  
  Use MachineSense Power Analyzer (PA) to measure the current harmonics which is passing through the PLC SMPS. MachineSense PAwill monitor the abnormalities 24x7 and if it crosses the safe threshold, will warn you via email/SMS.
• If you have already measured the harmonics please verify with MachineSense PAas it maintains IEC standard for each quality parameter.