3-Phase Motor Protector

[mcmsat13]

3-Phase Motor Protector
I have done this with discrete and passive components, it works but there are so many parts and thus complexity. I want to know if MCUs can deliver more accurate result.
This picture is only the block diagram of the proposed design. The components here are only for reference. You can choose any component that work best for the purpose.
If any of the line CURRENT SAMPLE is greater than 1V, the controller will delay 3seconds and if the v sample is not 1 or less than 1 after 3seconds, the will switch off the relay and light up short circuit LED and it must be reseted manually before it will work again

If the Voltage sample from either of the lines is less than 5V or greater than 10V, the controller will delay 3seconds and if it does not come back between 5V & 10v after the 3seconds delay, the controller will switch off the relay and it will automatically came back to work when the v sample of all the 3 lines are between 5v & 10v. This voltage sample monitor will be automatic reset unlike current sample monitor which is manual reset.

JP1 is power on MODE OPTION I can use pc motherboard jumper to select this in the PCB. Example;
= '0' means always in standby mode on power on
= '1' means start working on power on
The block diagram speaks clearly about this design. If anything is confusing please ask me to clear the area.
Thanks in advance.

 

[pic.programmer]

This can be done using a PIC or AVR which has atleast 6 ADC channels. 6 ADC channels are used to monitor the 3 phase currents and 3 phase voltages. Based on the adc readings the output circuit is controlled.

ADCs of PIC or AVR can only measure max 5V. So, your 5V to 10V input have to be scaled to 5V range using a voltage divider.

 

[betwixt]

Conceptually, I do not think it will work properly. Monitoring the phase voltages/currents alone will not give the protection you expect. The reason is that a missing supply phase will not result in zero voltage on that phase line, it will carry a voltage back from the motor which is derived from the remaining phase(s) and inductance of the motor windings. For proper protection you need to monitor the phase angles as well.

Brian.

 

[pic.programmer]

@betwixt

Do we have to use 3X ZCD circuits for detecting the ZC and then phase angles ?

 

[mcmsat13]

At pic.programmer, any voltage sample that can be OK for the MCU is OK for me. I can even sample 500mV so you can use 2v as the low limit and 4v as the upper limit. Obtaining any voltage sample that you use is not a problem for me. So the the voltage sample that you use is entirely what you choose from 0.5 to 4v, I can obtain them. Any chip that you use is OK as far as I can get it from aliexpress from where I easily buy components.

Thanks

 

[pic.programmer]

I am waiting for betwixt's response. If he tells how to find the phase angles and based on that how to decide the failure of one or more phases then I will make a code for you.

 

[mcmsat13]

@betwixt
In my analogue circuit I used 3x 555 circuits as the voltage sample monitors.I also use them with optocouplers as the current monitors. The device performed well, only that the components count is ridiculous!
It gave me the protection that I want. Remember that the voltage sample is monitored both high and low. In the analogue circuit, when any of the 555 detects below the lower threshold, the whole three relays will go off
And this same circuits serve as the phase gsiku cut-off, that is when any of the line is lost, the whole three relays will also go off. This function does not require a separate command as far as losing a line is Even far less than a line going below low limit threshold.
So in MCU, though I don't do programming, but I think the command will be:
Switch off relay when the voltage at port x is '<1', '>4', it '0'.
I think it will work. Remember, in this design, one transistor controls the three relays. This is very important so that all the relays work once or fail once.
Thanks.

 

[pic.programmer]

@betwixt

Have a look at these two documents. Is these enough for a 3 phase motor protection project ?

http://www.ijareeie.com/upload/2014/november/2_DESIGN.pdf

**broken link removed**

 

[betwixt]

I note that both of the papers use only voltage measurement. The second paper seems to consist mostly of copies from data sheets and explanations of basic components, little time is spent on explaining the actual operation. It also uses quite a long time constant on the regulated supplies from each phase so it would be slow to react to a failure.

I am not saying that voltage measurement will not work, what I am saying is a break in power on one phase doesn't leave zero volts after that break. There will be residual voltage passed from the remaining two phases and how much that voltage is will depend upon the kind of motor, it's size and the load on it. The voltage measured after the break may not be the best way to detect a phase failure, the design in the second paper is particularly vulnerable to missing a fault condition because the voltage is passed through a regulator which may still produce the same output when the incoming voltage has dropped considerably.

The most reliable method is to use the phase difference as the fail indicator. Under normal operation, the phases will be 0 degrees (your reference phase), 120 degrees and 240 degrees apart. During a fail, one of the phases will become a composite of the remaining two with some additional shift due to the inductance and rotation of the motor but it is unlikely to follow the 120 degree delay closely. You can use the phase error as a fairly reliable method of detecting a line break, if you ALSO measure voltage and in particular the current you can be sure of detecting a fault.

There is no need to use a ZCD although nothing stops you from doing so. If your circuit has to be electrically isolated from all the incoming phases, the use of an opto-isolator makes a ZCD an attractive solution. If isolation isn't needed, a microcontroller can be used by simply monitoring the AC voltage through a rectifier and suitable voltage divider. As all the inputs will have (almost) the same logic thresholds, there is no need to detect the zero crossing, the rise or fall of voltage passing the threshold will be the same on all phases and it is only the relative delay between them you need to be concerned with.

Probably the most reliable method has not been mentioned, a broken phase cannot pass any current from source, even if a voltage exists at both sides of the break. A small current monitor in each phase, maybe a small transformer, will always give zero output if the phase is disconnected.

Another factor to consider is how the contactor is powered. Either it has to be engaged (C - NO position) from the circuits own power source or it can be used as a powered breaker (C - NC position). If the contactor coil is used to break the power, the circuit is more efficient because no current is needed during normal operation but care has to be taken as to where the contactor coil supply comes from. It needs either a completely different supply or to be fed from a source derived from more than one phase. If it was powered from a single phase supply and that phase failed, it would work in reverse and switch the remaining phases to the motor ON!

I found it interesting that the phases are called R, Y and B which I presume refer to Red, Yellow and Blue. For some unfathomable reason, in Europe the specifications were changed so all three phases are now Brown! In most circumstances it makes no difference but when it comes to motors it decides the direction of rotation. Changing the colors was a backward step in my opinion.

Brian.

 

[pic.programmer]

@betwixt

So, I use a PIC with there INTx pins and interface the three ZCD circuits to them and then use a timer and calculate the angle between the phases and based on that decide whether a phase has failed or not ?

 

[betwixt]

That is one method. As I said, you may not need ZCD circuits, consider that the input pins have a threshold where the voltage is seen as low or high, as long as you ensure the voltage is safe (polarity and maximum) and scaled to stay within VSS and VDD, you can connect it directly. I suggest one phase (any) is connected to create an interrupt, that becomes your 0 degree reference then as long as the other two inputs change state within the allowed time windows they can be deemed to be present.

The alternative, using curent transformers in the incoming three phase lines is a more certain way of doing it as a broken line can never pass current. You can simply say "if all three transformers produce voltage it's OK" so you can do it without measuring angles or voltages using only some simple analog circuits. Just rectify the CT outputs and use a summing circuit or 3-input AND gate to detect they are all present.

Brian.

 

[mcmsat13]

.
@ Moderator (betwixt)
Sir,
Look at the picture above, this is what I want. The first picture may look confusing.
As the MCU watches port Vs1, Vs2, Vs3 and switch off the relay when the value of any of the ports is less than 1 or = 0, I think this function has also served as phase failure. Please just look at it. As far as the MCU switch off the relay at any sample less than 1v and and switch the relay back on when the sample is greater thanv 1v I think the system does not need more protocol for phase failure.

 

[FvM]

Electromechanical motor protection relays and motor circuit brakers are availiable as off-the-shelf industry standard products. A design project for an electronical motor protection device should at least tell at which specific features it's targetting and possible involving an improvement compared to industry standard products.

Or it's a pure academical project saying "I know, people are doing things usually differently, but I'm charged to use a microcontroller". In the latter case, you might still end up reinventing the square wheel, but you have no choice.

 

[pic.programmer]

@mcmsat13

We can measure the three phase currents using CTs. The CTs output will be converted to V for input to ADC. If phase is open then no current will flow and hence that ADC will give 0V. From this we can detect phase failure and it is the easiest method. If you can make the 3 CT interface to PICs ADC then I can make a code for you. You have to make the CT interfacing circuit so that the ADC inputs will be in the range of 0 to 5V for your phase current range.


See the attached image. I thought about doing like this. Detect the deltaT between each ZC and if deltaT1 and deltaT2 are approximately near to 6.667ms (which is the time between each phase) then it means that phase exist. So, there will be no angle measurement or any complex calculation. Just the time value obtained is compared with a range of values like

if((deltaT1 >= 6.662) && (deltaT1 <= 6.669))

if deltaT1 and deltaT2 lies within the range then it means that all the phases exist.

I still need experts advice. A 3X ZCD circuit will be cheaper compared to 3X CT circuit.

 

[mcmsat13]

Yes, actually everything we discuss in this edaboard is available as off the shelf product but it gives one a happy feelings knowing "this device on that serves me, I designed it". Another thing it shows the field in which you play around. And it also flares our own intelligence.
I have completed this project using couple of 555 and some other components. It works very well.
I have given to my close friend, as a free-will gift, to mount on his 9.9kw 3-phase sand mixer in his block industry. Since that my own device was mounted, the motor has not been damaged unlike before when he use to rewind the motor from phase fail damage and under voltage. He has been using off the shelf device. When I designed the analogue one I never looked into any existing already made. I just think of what the motor needs and what it doesn't need. My design based on that and it was a success.
Now, I want the same function of that design to be achieved using MCU because of the complexity of the analogue circuit. My son has seen the inside of the one we use in our milling machine and laughed at the device.
He has laughed
" Dad! Had you designed a mobile phone, it will be less thing longer than limousine ".
Believe me, that was a ridiculous joke, an expensive one that made me feel somehow.
Now that I don't move around freely due to auto accident which affect my waist, I want to automate some certain things around my home.
I will appreciate any help from anyone here who can help.
Use the second picture I posted above in post #12 as a reference.
Thanks.

- - - Updated - - -

@mcmsat13

We can measure the three phase currents using CTs. The CTs output will be converted to V for input to ADC. If phase is open then no current will flow and hence that ADC will give 0V. From this we can detect phase failure and it is the easiest method. If you can make the 3 CT interface to PICs ADC then I can make a code for you. You have to make the CT interfacing circuit so that the ADC inputs will be in the range of 0 to 5V for your phase current range.


See the attached image. I thought about doing like this. Detect the deltaT between each ZC and if deltaT1 and deltaT2 are approximately near to 6.667ms (which is the time between each phase) then it means that phase exist. So, there will be no angle measurement or any complex calculation. Just the time value obtained is compared with a range of values like

if((deltaT1 >= 6.662) && (deltaT1 <= 6.669))

if deltaT1 and deltaT2 lies within the range then it means that all the phases exist.

I still need experts advice. A 3X ZCD circuit will be cheaper compared to 3X CT circuit.

Yes I can make the 3x CT interface from 0-4.5v

 

[betwixt]

We can measure the three phase currents using CTs. The CTs output will be converted to V for input to ADC. If phase is open then no current will flow and hence that ADC will give 0V. From this we can detect phase failure and it is the easiest method. If you can make the 3 CT interface to PICs ADC then I can make a code for you. You have to make the CT interfacing circuit so that the ADC inputs will be in the range of 0 to 5V for your phase current range.

This is 'partly' what I described but not exactly. Why use the ADC? Putting the phase voltages aside, consider how much current flows along each phase wire in normal operation and then in the situation when one phase has gone open circuit. Taking an example, suppose 1A flows through of R, Y and B when all phases are present, if the Y phase failed (assuming the motor continued to run) R and B would rise to ~1.7A but Y would drop to zero. There is no need to measure the current though, as long as all of the phases are carrying current it should be safe to assume they have voltage on them. In other words, if any CT (or other current monitoring device) reads zero, a fault has occurred.

Looking at the circuit in post #12 - ask yourself what would happen if you disconnected Vs1 and instead wired the controller input through inductors to Vs2 and Vs3. Would it still work? What would the controller see instead of the real Vs1 and by measuring voltage alone could it tell if the real phase was being measured or conduction from the other two phases?

Small current transformers, (CT1005A for example) are not expensive and are non-invasive on the line supply so they are inherently giving you isolation from the phase voltages. You can make your own quite easily. The problem with ZCD circuits is they generally monitor the voltage so they require connection to the line and they make it necessary to check the phase angles. There is an additional risk that under rare circumstances, the ZC point may still be roughly where it should be even under fault conditions.

Brian.