Back EMF

Hi.

This is in line with my previous post but I just made a new one to further discuss back EMF. I have been reading quite a number of articles about back EMF as one major reason of damaging PLC output, among others.

I have attached a pdf file on this thread. It is divided into 2 parts, 1st is the distribution panel diagram, the 2nd is the power & control diagram of compressor 1. As they say, inductive components like motor winding resist sudden changes in current. That's because the magnetic field caused by the current needs time to build up or decrease. That means that when current is flowing and this is suddenly cut off, the winding will try to maintain that current, and becomes a power source generating a voltage to be able to do so. It gets its power form the built up magnetic field.

In my diagram 1, I have drawn a distribution panel containing 2 3-phase compressors, 5 1-phase outlets & a PLC. Please take note that this is only an example & not exactly what is connected on site. On diagram 2, I have described the forward EMF when the compressor is still working. If ever they shutdown the main CB while compressor 1 is still on, there would be a back EMF coming form the its motor. I was just wondering how this will possible damage the PLC? and what will be the path of the back EMF since the MC is surely open?

You could "possibly damage the PLC" by connecting it's power supply without surge protection, but it's unlikely. Most PLC power supplies are capable of withstanding very poor quality power regulation.

You mention damaging PLC outputs, yet your drawings do not feature any PLC I/Os. If your concern is connecting a coil to an AC output, any large inductive load should be connected via an interposing relay. Snubber circuits can be used to reduce transients from contactor or motor starter coils, but I've seen plenty of applications that don't use them. PLC AC output damage is typically caused by either lightning (welding machines can sometimes be an adequate substitute), shorted wires or shorted turns in valve solenoids. I like to use interposing relays for solenoid valves... You may have noticed?

Bit_Bucket_07 said:

You mention damaging PLC outputs, yet your drawings do not feature any PLC I/Os.

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In my case, my PLC is not controlling any large inductive load but somehow I have many damaged outputs. This had happened after they shutdown the power source, which I suspect was not properly conducted. I was thinking, maybe they cut-off the power (main CB) while one or more of the large inductive loads (like compressor 1 or 2) were still running. I just cannot linked this suspicion to the damaged PLC output. The back EMF coming form these loads does not seem to make sense.

One technical person told me that the proper way of conducting a power shutdown is by shutting smaller loads (branch breakers) one at a time before shutting the main breaker down. This could be true. I just cannot explain how could this resulted to my damaged PLC outputs.

I hope I am making sense.

see if any of the ideas here are helpful ... in basic terms, any inductor (solenoid coil, transformer winding, motor winding, etc.) can generate many thousands of volts of "back EMF" when the power is suddenly turned off by simply opening the supply circuit ...

http://www.plctalk.net/qanda/showthread.php?p=498529&postcount=8

http://www.plctalk.net/qanda/showthread.php?p=402044&postcount=2

a perfectly good question to ask next:

but why don't more PLC outputs get damaged by "back EMF" when the PLC turns them off during normal day-to-day operation? ...

the reason is that most solid-state PLC outputs operate as "zero crossing" devices ... specifically, they only turn off when the AC waveform is at the "zero" part of its curve (technically when the polarity reverses) ... at that point in the curve there is little or no "flyback" effect - and so there is little or no "back EMF" to damage the PLC outputs ...

but ...

when the power is suddenly turned off by some OTHER means (other than by the PLC's normal control of the outputs) we don't know WHERE in the waveform we'll be ... it's an intermittent/random thing ...

theripley said:

One technical person told me that the proper way of conducting a power shutdown is by shutting smaller loads (branch breakers) one at a time before shutting the main breaker down. This could be true. I just cannot explain how could this resulted to my damaged PLC outputs.

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That is absolutely true. Power up should occur in the reverse order.

That said, unintended power outages will sometimes occur, and you should be able to withstand one without damage to your equipment.

Ron Beaufort said:

but why don't more PLC outputs get damaged by "back EMF" when the PLC turns them off during normal day-to-day operation? ...

the reason is that most solid-state PLC outputs operate as "zero crossing" devices ... specifically, they only turn off when the AC waveform is at the "zero" part of its curve (technically when the polarity reverses) ... at that point in the curve there is little or no "flyback" effect - and so there is little or no "back EMF" to damage the PLC outputs ...

but ...

when the power is suddenly turned off by some OTHER means (other than by the PLC's normal control of the outputs) we don't know WHERE in the waveform we'll be ... it's an intermittent/random thing ...

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Thank you for this very essential lecture. As always Ron Beaufort.

Bit_Bucket_07 said:

That is absolutely true. Power up should occur in the reverse order.

That said, unintended power outages will sometimes occur, and you should be able to withstand one without damage to your equipment.

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Could there be a relationship between improper shutdown conduction & my PLC output being damaged given the diagram I posted? I think the PLC output was damaged by something else, not just an improper shutdown conduction. Anyone?

What sort of loads have you got on your outputs? Is the problem only individual outputs or is it entire cards?

Zane_Terblanche said:

What sort of loads have you got on your outputs? Is the problem only individual outputs or is it entire cards?

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Only CFL lights, kindly check this thread: http://www.plctalk.net/qanda/showthread.php t=76594&highlight=Troubleshooting.

It damaged 10-15 out of 64 outputs.