Electric chain hoist: need help with control circuitry

[GTC]

TL;DR Summary

Relay circuit, which inhibits hoist operation, is a mystery to me.

I have been given an electrical chain hoist to repair. The symptom is that it suddenly stopped working.

I have checked the motor's windings and they each show similar DC resistance of 0.0018 ohms.

I have attached the schematic (dunno why it was rotated 90 degrees during the upload).

The hoist operates from 3-phase mains. Its up and down motion is controlled via push buttons which are supplied with 24 volts AC from a step-down transformer connected across 2 phases. The push buttons operate a reversing contactor which, via the up/down limit switch, swaps one of the motor's phases to effect forward and reverse modes. I have ascertained that the 24 volt circuit is working correctly as is the reversing contactor.

I have also ascertained that the reason the hoist is not operating is that the normally-closed relay that I have highlighted in the schematic is energized as soon as power is applied to the hoist, the effect of which is that the 24 volt supply is interrupted thus rendering the up/down control function inactive.

I don't understand the highlighted area of this circuit and would appreciate help with it. Specifically, which condition would leave the relay in its normally-closed position (and thus allow the hoist to operate) as opposed to the condition that causes it to go to normally-open (and thus inhibit hoist operation)?

Note that all 3 phases are connected to it. The coil is rated at 220V AC, i.e. the voltage of one of the phases.

 

[Dullard]

I don't really understand the way the highlighted relay is drawn. If it was a 'black box' I'd be pretty confident that it is a 'phase loss' and/or 'phase rotation' device. It protects the motor from being 'single-phased' and the operator from unexpected 'backward operation.' In spite of the way it's drawn, I suspect that's what it is. Best guess: You're missing a phase or your rotation is swapped.

Edit/add: Or the highlighted relay is bad.

 

[Baluncore]

Welcome to PF.

Are you testing it in place, or has it been moved to another supply ?

Maybe someone swapped two of the 3PH supply lines, so rotation is being disabled by the non-reverse relay. Try swapping two lines to see if it works again.

 

[GTC]

It is being tested within the workshop -- where it normally lives hanging above a CNC machine -- but is now on a bench for this purpose and is plugged into a different socket.

I could try swapping around the phase connections, although I don't see how this relay circuit could detect a phase swap. As per the thread theme, I do not understand how this relay circuit works and am seeking an explanation.

The fact that it is labeled 'nonreverse' seems to be an industry thing with hoist components to distinguish it from 'reversing' relays (which in fact are contactors).

I have attached a sketch of the board's layout and a photo of it. Note the resistors perched above the PCB's surface for ventilation and the scorch marks on the board, so they have been running hot at some stage of this hoist's 27 year lifetime. The 39k resistors still measure a tad less than 39k.

 

[Baluncore]

It is being tested within the workshop -- where it normally lives hanging above a CNC machine -- but is now on a bench for this purpose and is plugged into a different socket.

Since the non-reverse relay contains a capacitor and an inductor, it is possible that it detects the direction of phase rotation, which may be different for the new socket.

Maybe transformer power was lost. Have you checked that all lines were active on the original socket?

 

[GTC]

Since the non-reverse relay contains a capacitor and an inductor, it is possible that it detects the direction of phase rotation, which may be different for the new socket.

Maybe transformer power was lost. Have you checked that all lines were active on the original socket?

The original socket is fine and being used for a temporary replacement hoist.

The hoist in question was operating fine until it suddenly stopped working, so whatever the cause it appears to be related to the relay in question.

 

[Baluncore]

The hoist in question was operating fine until it suddenly stopped working, so whatever the cause it appears to be related to the relay in question.

You are building a house of cards in your head. You must perform some physical tests to get more information, to build a more solid platform of understanding. Only then can you safely advance and narrow your view.

Does the state of the non-reverse relay change when you reverse the direction of rotation by swapping two of the 3PH lines ?
That tests the relay, and the possibility that it is a distraction.

 

[GTC]

You are building a house of cards in your head. You must perform some physical tests to get more information, to build a more solid platform of understanding. Only then can you safely advance and narrow your view.

Frankly, I find the personal tone of the above offensive.

Does the state of the non-reverse relay change when you reverse the direction of rotation by swapping two of the 3PH lines ?
That tests the relay, and the possibility that it is a distraction.

No it doesn't.

My question is how does that relay circuit work. So far, no one has answered that question.

 

[Baluncore]

Frankly, I find the personal tone of the above offensive.

No offence intended. To fix faulty equipment requires critical thinking. You must analyse and discipline the way you think and gather the evidence.

My question is how does that relay circuit work. So far, no one has answered that question.

There are probably internal components and connections that are not shown. You have not yet identified the manufacturer, nor provided a part number for the relay. Collect the data sheets on that and similar products.

I would test it immediately. See what the relay does when you flip the rotation direction. How can you be so certain that the new socket is wired to rotate correctly?

No it doesn't.

No it doesn't what? That the relay detects rotation direction, or that the socket is wired correctly.

Don't overthink the possibilities. Do the test.

 

[Averagesupernova]

@Baluncore is correct. You have no way of knowing if there was something changed upstream of the hoist. That relay in question is designed to detect such a thing. It is entirely possible you are completely ignoring the message that it is trying to send you. It could be that the state of the relay is the effect of a cause upstream and not the root cause of the inoperable hoist.

 

[Averagesupernova]

As per the thread theme, I do not understand how this relay circuit works and am seeking an explanation.

Basic troubleshooting would suggest you switch the phase rotation to observe the behavior. Assuming that the circuit does in fact do what is suspected, failure to observe any change in the behavior suggests it is defective. Now, it's a pretty basic setup. You have four resistors that have been rather hot. Assuming you know what the values are supposed to be based on color bands, test them. Components that have been hot for extended periods have a tendency to develope poor connections to the board. Check that as well as damage to the tracks on the board itself. It seems the relay does pull in. Can't be an open coil. Only thing left is the capacitor has failed or for some reason one of the phase conductors to that little board is open. It's not that difficult.
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Way back in school many years ago I was told it would be very likely that someday someone would bring me something that did not work and I would be expected to repair it not knowing exactly how it was supposed to work or even what it was. This would be such an example in your case. With the correct mindset, you can do this.

 

[GTC]

Basic troubleshooting would suggest you switch the phase rotation to observe the behavior. Assuming that the circuit does in fact do what is suspected, failure to observe any change in the behavior suggests it is defective.

Yes, and before I proceed I want to know how that circuit works, hence this thread.

Now, it's a pretty basic setup. You have four resistors that have been rather hot. Assuming you know what the values are supposed to be based on color bands, test them.

Have done that. As per my post #4 "The 39k resistors still measure a tad less than 39k."

Components that have been hot for extended periods have a tendency to develope poor connections to the board. Check that as well as damage to the tracks on the board itself.

Yep, and the solder joints are still in very good condition.

It seems the relay does pull in. Can't be an open coil. Only thing left is the capacitor has failed or for some reason one of the phase conductors to that little board is open. It's not that difficult.

Tested all of that already.

Again, my question is how does the relay circuit work.

So far no one has yet attempted to explain it. The reason I posted under Electrical Engineering is that I had hoped that qualified electrical engineers might respond.

 

[GTC]

There are probably internal components and connections that are not shown. You have not yet identified the manufacturer, nor provided a part number for the relay. Collect the data sheets on that and similar products.

There are no internal components. The hand drawn diagram I provided shows exactly what is on that board.

I would test it immediately. See what the relay does when you flip the rotation direction. How can you be so certain that the new socket is wired to rotate correctly?

As I said earlier, I have already flipped the phases.

Whatever is triggering that relay to operate has nothing to with phase connections.

No it doesn't what? That the relay detects rotation direction, or that the socket is wired correctly.

Doesn't change the fault situation.

The socket is not wired incorrectly. Again, as I have already said above, the hoist was operating fine above the CNC machine connected to its usual socket until it suddenly stopped operating. It has zero to do with the socket.

Don't overthink the possibilities. Do the test.

@Baluncore: explain the theory of operation of that relay circuit to me.

 

[GTC]

That relay in question is designed to detect such a thing.

@Averagesupernova: explain the theory of operation of that circuit to me.

 

[Baluncore]

So far no one has yet attempted to explain it.

"Phase sequence detectors" operate in several different ways. Most also detect loss of a line, and high or low deviations in line voltage. Some contain phase-shift networks and diodes, others use voltage comparators and flip-flop logic to determine the sequence, but now I think they will be cheaper and use a microcontroller to analyse the three line waveforms.

The symbol used on your 25-year-old schematic is for a primitive phase-shift network, but it is only a symbol and there are several internal parts not shown. Your particular unit may actually use flip-flop logic to analyse the sequence. But it really does not matter how it works internally, you test it just the same, by reversing the sequence.

You can google the different types available and study how they work internally, or you might identify the manufacturer and the part number, so we can download a datasheet.

 

[GTC]

The symbol used on your 25-year-old schematic is for a primitive phase-shift network, but it is only a symbol and there are several internal parts not shown. Your particular unit may actually use flip-flop logic to analyse the sequence. But it really does not matter how it works internally, you test it just the same, by reversing the sequence.

Yet again, there are no "internal parts not shown". What I have sketched is exactly what is on that board.

There is nothing mysterious about the relay itself. It's simply a common or garden SPDT (Form C) with a 220 volt coil.

It matters to me how the circuit works, which is why I created this thread.

I live in hope that a qualified engineer might respond with the answer.

 

[Baluncore]

Did that relay change state when you reversed the sequence?

 

[Baluncore]

What are the values of the resistor and the capacitor shown inside the sequence detector?

 

[Tom.G]

A fair possibility is that the Capacitor in the phase-detect circuit has failed. It could be either shorted, open, or possibly changed in value due to a hard life.

I suggest that if you can readily get a replacement capacitor that you do so and just replace it as part of troubleshooting. Be sure to get a capacitor of with at least the same, or higher, voltage rating and also a similar style and physical size. The physical size can be important to dissipate any internal heat it generates.

If the marking on the capacitor are visible, please post a closeup photo of same. If the markings have disappeared over time, I would start looking in the catalogs for a Sprague "Orange Drop" capacitor with a voltage rating At Least 3 times the line-to-line supply voltage.

Cheers,
Tom

p.s. please keep us posted on your results!

 

[Dullard]

This link might help you understand the basic circuit. Inductors or capacitors (or both) may be used.

https://www.watelectrical.com/phase-sequence-meter-and-its-working-principle/

 

[Averagesupernova]

I had hoped that qualified electrical engineers might respond.

It would seem they have. The most we get from you is: "I already checked that." Etc...
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If you truly believe the purpose of the little board is to detect phase sequence then it should be obvious that if all of your checks and tests were accurate then obviously there's a problem with something on the board. My previous posts should have spelled it out pretty well. Just about has to be the capacitor.
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As far as how it works, the link provided explains it pretty well. I really didn't think it was that hard to grasp. To me it was like asking what a capacitor is. Had you redrawn the hand drawn schematic you may have seen the light.

 

[anorlunda]

I live in hope that a qualified engineer might respond with the answer.

Statements like that make you look bad. The qualified engineers have been schooled on the dos and do nots of troubleshooting. At the top of the list of do nots is, "Do not jump to conclusions too early, or without evidence." They
are trying to steer you get get more evidence before favoring a theory of what it must be.

 

[Averagesupernova]

@GTC have you read the link provided?

I don't see how this relay circuit could detect a phase swap.

Do you believe that the board in question is in fact able to detect a phase swap? I am very very very confident that is it's purpose. Assuming you have accepted this, here are some pitfalls:
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You say you have all three phases present on the board. Be cautious here. You still may be measuring voltage but it may be from another leg feeding your voltmeter from a different path if you have a broken conductor en route to the board. The volt meter tells you voltage, not phase. Phase is what it's all about on this board.
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Think of the circuit as a summing circuit. The capacitor shifts the phase in such a way as to be either additive or subtractive to act on the relay all dependent upon incoming connection.

 

[GTC]

A fair possibility is that the Capacitor in the phase-detect circuit has failed. It could be either shorted, open, or possibly changed in value due to a hard life.

I suggest that if you can readily get a replacement capacitor that you do so and just replace it as part of troubleshooting. Be sure to get a capacitor of with at least the same, or higher, voltage rating and also a similar style and physical size. The physical size can be important to dissipate any internal heat it generates.

If the marking on the capacitor are visible, please post a closeup photo of same. If the markings have disappeared over time, I would start looking in the catalogs for a Sprague "Orange Drop" capacitor with a voltage rating At Least 3 times the line-to-line supply voltage.

Cheers,
Tom

p.s. please keep us posted on your results!

Thanks Tom. You are likely on the money given that the other components seem to check out and the resistors have clearly been running hot at some period of the hoist's life.

I won't get access to the hoist again until the weekend.

If push comes to shove I can probably find out the original spec by ringing around some of the repair shops.

Statements like that make you look bad. The qualified engineers have been schooled on the dos and do nots of troubleshooting. At the top of the list of do nots is, "Do not jump to conclusions too early, or without evidence." They
are trying to steer you get get more evidence before favoring a theory of what it must be.

I asked a simple 'how does this circuit work' question. A qualified engineer who knows the answer would have answered.

 

[Baluncore]

By summing the three 120° phasors through in phase R, delayed L and advanced C paths, the sequence will influence the relay current.

Here is an LTspice simulation. The three-phase supply voltage is plotted in red, blue and yellow, scale on the left. The relay current is plotted in magenta for normal sequence, green for reversed sequence, scale on right.

This is a primitive sequence detector. I do not know why they put the resistor in series with the relay. If the inductance of the AC relay had been used to delay relay current, then double the sequence discrimination should be possible.

As it is, the AC current needed to pull-in the relay flows continuously, yet it is only needed once, when the equipment is first turned on. That explains the cooking of the phenolic PCB.

The components used look reliable and should not fail. But there are much more reliable sequence discriminators available. If it fails to operate correctly when the sequence is reversed, then replace it with a new digital module.
https://www.ebay.com.au/itm/174535260665?

 

[henzelmen]

@ Baluncore; You hit the nail on the head for this issue, the LTspice simulation tells it all.
Today i get exactly the same chain hoist to repair, It soon turned out that the problem was in the same PCB as described here. The relay was active and interrupted the supply voltage for the rest of the circuit. It was clearly some kind of security, but I didn't yet understand how and what.
I too was initially surprised by the strange circuit and took it home to study further.
After reading this article it turned out to indeed be a phase reverse protection, in the diagram it is also called "NONREVERSE RELAY". So there is clearly a problem with the phase sequence or... a defective component on the PCB.
Given the extremely simple and (in my opinion) unreliable circuit (a lot of heat development), I will install the digital module you indicated.
Thank you for your contribution, it helped me.
Sorry for my English, translated by Google.