Ways to protect CNC Machine’s electronics from surges & avoiding EPROM corruption
- Ways to protect CNC Machine’s electronics from surges & avoiding EPROM corruption
The introduction of CNC machines during the 1950s has radically changed the manufacturing industry. Now, 3 D printing also has been introduced. These machines are far, more, complex.
Depending upon its’ application, CNC machines have different types of electronics ranging from - dedicated electronics /PLCs / Display units with or without touch screens , interface to PC etc. etc.
In a sense, these new age machines represent a special segment of industrial robot system, as they are programmable to perform many kinds of machining operations (within designed physical limits, like other robotic systems).
The basic function of CNC machines i.e, precise and automatic motion control, makes this sophisticated machine so popular.
Today we hear more about 24 X 7 everywhere. Due to international competition & commitments, every industry wants to increase the availability of their plant so that the production targets are achieved .On one hand ,the size of the components of CNC machine becomes smaller and smaller every day, from room size equipment to small size cards. Though it is convenient because of its’ small size, on the other hand, the energy level it can withstand has reduced drastically. For e.g. an IC can withstand energy only up to 1 micro joule , whereas a transistor can withstand up to 10 micro joules.
Failures in CNC machine happen due to two important disturbances (variables present) in low voltage network - steady state errors and transient errors. Steady state errors mainly comprise of over voltage which lasts several cycles (swell) , harmonics, RFI/EMI effects etc. Transient errors comprise of transient over voltages i.e, Lightning surges and switching surges due to operations like welding, capacitor bank switching etc.
The two important reasons for failures due to lightning –direct hit, (in which the damage is visible because what we see, we believe ). But direct lightning hit to an equipment is very rare because CNC machines are installed inside a structure & is fed by sub distribution boards, whereas only main DB bears the brunt of direct lightning strike in lightning prone areas. The other reason, often un-noticed is, distant lightning. In this case, the lightning current is carried by all types of cables except optic fiber cables. Hence many of us have experienced that, immediately after thunder storm & showers, our phone line stops functioning or our sophisticated machines do not work due to electronic card failure. The myth here is, we have general feeling that when the telephone or machine is not in use i.e, when they are in “switch off” condition, they are safe. But the fact is , they would have been safe if its’ connections are physically removed from the respective sockets. These safety tips are available in “installation manual” of TV, Refrigerator, washing machine etc, but we seldom read. In manufacturing plants, which work 24 hours a day, how do we achieve this? The only solution is to protect them by using SPDs –Surge Protection Devices.
Before discussing about SPDs, let us first understand what is surge and how it is going to affect our CNC machines, so that it will be easier to install appropriate SPDs for protection of CNC machine.
Put in simple terms,surge is an abnormal sudden increase in current and voltage for a very short period of time (a duration of 350 micro seconds during lightning and 20 micro seconds during switching ) .There are various sources of surges .The two major classifications are surges generated by mother nature (due to direct lightning hit which is very rare and distant lightning- indirect surges- which is very common) and manmade transients ( sags due to brown outs, black out , capacitor switching , frequent switching on/off of heavy duty equipment, welding, electrostatic discharge etc.)
A simple example is, think of a load cell signal, wires of which run along with 415 V AC, 3 phase power line or a CCTV signal cable, which is running very near to a 11 or 33 KV HT cable, which is common in industrial environments. Any small change in power line will affect the millivolts signal of load cell, due to electromagnetic induction. This will induce measurement errors in milli-volt signal as they operate in very low level signals and may cause a process upset.
The result will be unwanted measurement signal observed in the DCS (Distributed control System) or PLC (Programmable Logic Controller) or SCADA (supervisory control and data acquisition), which is not due to actual measurement signal itself.
Many feel that unless they see a charred PCB they won’t accept that the problem is due to lightning /switching surge effects. The more common problem we encounter in the normal plant life is corruption of EPROM ( Erasable Programmable Read only memory).
This is visible from the sudden junk data appearing in the MMI (Man Machine Interface) panel. In general, it is not a common practice to- put that loop of PLC or DCS in manual mode, remove the corrupted EPROM , erase it in UV eraser, reprogram it and put it back into operation as this is a long drawn & time consuming process, resulting in production loss. . If we examine why such corruption of EPROM suddenly takes place, the reason will be surges.
Because, what is actually done is while programming is in operation, a voltage higher than the normal working voltage is applied to particular pins of EPROM, (so that EPROM understands that it is in programming mode and not in normal operating mode).The same thing happens during a surge. A sudden high voltage appears to those pins of IC, hence it believes it is in programming mode and the ones as the extra high voltage appears in the pins. This is nothing but EPROM corruption. Hence the failure or corruption of programs in CNC machine is entirely due to intensity of surges. Actually the SMDs and electronic equipment of CNC machines are built to withstand a small increase in voltage levels which is well within the tolerance limit. This is called withstanding voltage of equipment and it is typically 1000 V AC RMS voltage for 1 minute duration for PLC and electronic equipment having input power supply voltage of 230V AC.. If surge of magnitude up to this level appears, nothing happens to equipment .The secondary effect of surges is called Degradation.
These levels are higher than the normal tolerance limit but over a period , sufficient to fail the equipment permanently. Experience tells that in more than 90% of the cases, it is this degradation that happens to the equipment to reach the permanent failure level. It can be a few days to several months. But this is the most unpredictable problem for the maintenance engineers because the problem is intermittent and will appear differently at different point of time.
Generally we feel that SPDs are not required as we have number of (other existing) protection arrangements viz. external lightning arrestor, MCB/ MCCB, good earthing , bonding, shielding , isolation transformers, UPS, Intrinsically safe barriers, isolators etc.
Unfortunately the purposes of all these equipment are for different reasons altogether and not for protecting the equipment from transient surges. Let us analyse the scope of each one of them.
External Lightning protection with good earthing is for protecting the building from direct lightning strike.
MCB Miniature circuit breaker or MCCB- Moulded case circuit breaker is for protecting the equipments from short circuit due to flow of power frequency fault currents .MCB or MCCB operate in millisecond duration which means before a MCB or MCCB detects surges, surges would have already passed through them and damaged the equipments.
Earthing is for both personnel safety and equipment safety.
Bonding is to reduce the earth resistance and maintain equipotential.
Shielding is done to protect equipments from RFI/EMI effects.
Isolation Transformer is mainly used to protect equipments from the faults that happen at the primary side. For example, in case of a short circuit in the primary side, the equipments connected to the secondary side will not be affected if they are connected through an isolation transformer.
UPS or uninterrupted power supply, as the very name suggests are used to provide continuous power through battery in case of mains power failure or to regulate the power within a narrow band.
Intrinsically Safe barriers or Isolators are used in hazardous plant areas (for example refineries, Petro- chemical plants, fertilizer plants etc) to restrict the electrical energy to very low levels, so that even in case of any short circuit the available power is too low to initiate a spark or fire.
Hence SPDs are the only protectors for the CNC machines against transient surges. Now, we can discuss the proper selection and installation of SPD for trouble free operation of a CNC machine.
Depending on the incoming supply of CNC machine i.e, 3 phase with neutral or without neutral , SPDs shall be installed with potential free contact, thermal disconnecter , including base element & pluggable arresters to be connected between Phase to Neutral (3 nos. for 3 phase to neutral ) and between neutral to protective earth. SPDs for three phase power supply (L-N), (N-E) at local or sub distribution board or branch panel should have l MOVs for L-N (WITH MCOV of 275 V to take care of power supply fluctuations & SPARK GAP for N-E. Requirement class is , Class I + II as per IEC 61643-11.
Model: DMD3-30 MAG
3 phase power supply with neutral (i.e R, Y, B, N & PE)
Model: DMD3-30 MA/3
3 phase power supply (without neutral- I.e R,Y,B & PE)
This is the first level of defence. For MMI having 24V DC power supply or PLC with DI/DO , suitable SPDs need to be selected, apart from having Class I + II SPD in main distribution board . This is an expert’s job. Please contact us for holistic solution which will ensure decades of trouble free operation of all of your sensitive electronic equipments.
We, JEF Techno Solutions Pvt Ltd., are pioneers in Lightning protection, surge protection, earthing, EMI/EMC Services and providing solutions for such kind of problems.