How To Design Reversing Motor Control with One Float Switch

Draw a wiring diagram....

If you've already, "tried all possible solutions" then it must be impossible!

Now, why not tell us what you have really done.

This sounds like homework, an interview question or you are someone who has a job they don't know how to do.

The fact that you come here and ask the question you asked indicates that you have encountered something you do not understand and it is not just curiosity on your part.

Now, which one are you?

It cannot be done with the parts you've identified. You do not tell us what type of motor you have. For all we know, you might have a five phase stepping motor and no driver circuitry.

Thank you guys for the reply, no it's not a homework, I'm done with school years ago, one of my friends works in beverage factory asked to help him figure this out, so I've tried every possible arrangement of the components in ladder diagram but couldn't really satisfy all the requirements together. So I told my friend that it can't be done (or at least I can't ), he thinks there's a solution, so I just thought I post it in this forum to see if somebody would help with it. It's 3 phase induction motor. thanks again.

So, what's the process? Is the "motor" connected to a pump?

Describing the process involved is the first step in defining the parts of the system.

Describe the process and the desired goal.

Well to reverse the rotation of a three phase motor is simple. With power phases ABC connected to motor terminals 123 for one direction and power phases ABC connected to motor terminals 132 for the other. Now getting ladder logic of a PLC program to control this will be easy or mildly difficult depending on how the relays are wired that the PLC directly controls. I would first consider using one three pole NO relay for ON/OFF control and a second two pole relay with form C contacts for direction selection.

You may wish to consider dynamic braking and/or flow sensors in your logic to prevent inertia effects from damaging components when reversing flow.

Based on what info you have supplied, a simple reversing contactor installed in the circuit will likely do what you want.

There are two types of float switches, one is a simple float, like a mercury bulb in an air pocket, the other is a mechanically linked mechanism that operates levers. In the mechanical type, there is often a built-in hysteresis, a difference between On an Off cpnditions. THAT type could make this work.

For a description of the lever actuated type float switches, do a google search on "Allen Bradley 840". I'm on my iPad and this site doesn't work well with iOs browsers so I can't copy and paste links without having to retype everything, so you're on your own.

I don't really know the detail about the process but he did mention to me that he needs to reverse the rotation of a compressor mechanism by reversing the rotation of the drive motor (i think something to with capacity modulation, but I'm not sure), he just asked me if it's possible to draw the ladder diagram of the motor reversing control circuit based on the requirements he gave me with only the input devices mentioned in they previous message. So if you can help with ladder diagram, it would be appreciated. Thank you

Where is the float switch? Float switches sense liquid level. This sounds like a pressure switch is needed, maybe.

Heat pumps use a reversing valve, but the motor rotates in the same direction.

Reversing rotation of a compressor?

We need more information, but doesn't sound impossible.

The other three responders are all experts.

OK,

Compressor capacity modulation is starting to make sense.

Capacity modulation for scroll compressors - Emerson ...

Compressor Modulation Offers Efficiency, Reliability, and ...

I'm not sure it's going to be as simple as a couple of switches and a float switch????

Some of the larger brains may be able to do this, I turn lights on and off, that's it.

I am becoming more and more doubtful about the information being supplied here as it seems to be changing exponentially.

Capacity modulation is a term pretty much restricted to refrigeration technology, and I have never heard of (doesn't suggest that it's not possible) motor reversal as a method to achieve this.

The referral to a float switch tends to rule out refrigeration, but may support the proposition of a screw feed, as reversal of the flightling can be used to manage tank levels, but doesn't do so well with low viscosity liquids, and a beverage plant is mentioned, but this doesn't rule out the transport of solids or semi solids where a float switch may be of use. Or it could be just a simple pump of the positive displacement type, many of which will work as efficiently forward or backward. A flexible impeller pump could also be a candidate

Most beverage transport pumps tend to be of the positive displacement lobe or wave design, with Peristaltic ones often being used for more viscous fluids.

The mention of a compressor tends to introduce a furphy into the mix unless it refers to an axial type compressor which can be used to transport fluids, but would be unlikely to work so well for both filling and emptying as it would need to be below fluid level and is neither self priming nor self sealing.

More defined and accurate information needs to be forthcoming.

Most important thing has been left out.

What sort of motor?

If it's a simple DC motor then a two pole two way real wired to reverse it (outputs wired like an X ) will solve the problem. If it's an AC motor it's more complex but still do-able...

Del

3 phase induction motor

#4

And there was me thinking it would be in the original question

Stupid cat...

I don't kno nuffing abaht free phase motors innit?

Del

Does the motor already have a reversing starter? Is so, then does the starter have some spare auxiliary contacts or the ability to add some?

yes it does, two magnetic contractors and overload, each contractor has just one N.O. auxiliary contact and one N.C contact (interlock)

How many of the aux contacts are now being used, and how many are spares? If a correct combination is available, you won't need an additional relay.

None of them is being used, all are available

I think this can be done. I have a weird work schedule at the moment, so I may not be the first to figure it out. Stay tuned--somebody will get it.

Ok, thank you for your time.

I was too optimistic; the best I've come up with so far uses two extra relays.

If you have NO contact of float switch have a control contactor with NO and NC contacts this control contactor can be made to operate when float switch contact closes.

With this control contactor NO contact make forward power contactor ON for motor to run one direction.

With the NC contact of the control contactor make the reverse power contactor ON to run the motor in reverse direction.

But if a timer is introduced in operation of forward/reverse power contactor it would be better.

Since you specify 'ladder logic' does that imply there is already a PLC in place? This is a rather minor software problem. If there is no PLC in place, the software can be implemented on an Atmel 8 pin microcontroller ($1.87US) and with appropriate driver circuitry you can drive the existing relays.

RTFT. The puzzler stated that he ONLY has the electro-mechanical components described to work with, no PLC, no option for a PLC (per his rules).

And just FYI, the term "ladder logic" has been around for far far longer than PLCs have existed. In fact having a PLC be programmable in "ladder logic" was done (by Allen Bradley) specifically so that electricians who did not know computer programming languages or techniques could adapt a relay ladder logic schematic to work in a PLC.

Although Modicon is often credited with inventing the PLC, it was the result of a contest between them and Allen Bradley to get a large contract with General Motors. Modicon got there a few months ahead of Allen Bradley with their Sequential Machine Controller (SMC), but it programmed in something like FORTRAN. A-B then produced the Programmable Logic Controller a little later. But when A-B then released the ability to program it in ladder logic, they actually won that contract. For years, A-B owned the copyright to the term "PLC" as well by the way, but stopped fighting the tide sometime in the 80s once everyone used it generically.

Please help me, in which post does he say "no option for a PLC" or similar, I seem to have missed it completely......thanks in advance.

The added "compressor" terminology I have not understood yet either.....

There are MANY points to still be cleared up before a design can be envisaged......if the pump/compressor is allowed to be instantly reversed, its quite a simply operation just using a few relays....and a switch of course.....

But I personally am wary of reversing anything quite so quickly......assuming that what is attached can do it too of course......3 phase motors take large inrushes of current when just starting from stop, but reversing them in this manner will take even more, for even longer!!! As well as possible mechanical damage......

"I wanna design a motor control using: 1) Selector switch. 2) 2 Pushbuttons. 3) Float switch ( with only one N.O. contact, it doesn't have N.C contact)"

Now I see that differently to you as it doesn't say that nothing else is allowed.......

You just assumed that!!! I didn't!

You may be right and so might I!!

Nobody knows for CERTAIN!!!

Now I am right again!!!

Wunderbar!!

Deleted.

Already answered by JRaef.

"Please help me, in which post does he say "no option for a PLC" or similar, I seem to have missed it completely......thanks in advance. "

"... he just asked me if it's possible to draw the ladder diagram of the motor reversing control circuit based on the requirements he gave me with only the input devices mentioned in they previous message."

Thanks....

Sorry, guess I STFT, thanks for the history, I'm more familiar with conventional schematics and wiring charts. My only experience with a ladder logic compiler was on Siemens-Allis MC16 running an automated welding system. It was necessary to link assembly routines to process much of the positional information.

After reading the OP's posts I see a problem with the float switch, if it just switches at one point, it looks like it would 'short cycle' around the upper or lower point, depending on software or hardware wiring...there needs to be a definite high and low point indicator. And I see you have that covered in another post (I think it was #6....mechanically linked switch)

History of the PLC

Hi there, just joined this forum, so please forgive me if I don't get the protocol right to start with, I'm a pretty quick learner.

I hope that I may be of assistance, and maybe receive some from time to time.

I am a retired Elec. Engineer living down under (although, other than for those in the northern hemisphere claiming it to be so, there appears to be no proof as to which way is really up for the globe).

Anyway, getting to the question as I understand it. It would appear that the selector switch is required to set either continuous forward operation of the motor in one position, or automatic fwd/rev depending on float status in the other position.

If I have the above correct, then I would suggest that, if doing this with electro-mechanical devices alone, and wishing to have the stop start buttons as suggested, you are going to require 2 extra relays (something of the LY2 ilk would suffice).

I doubt that the required operation can be achieved satisfactorily using the existing hardware, but would be happy to be shown otherwise.

I would post up my take for a control circuit for your critique if anyone is interested, and if someone will kindly instruct me on how to achieve that.

Cheers.

A lot of needed detail is needed to be defined. For example you have not given details of the motor type you want to use which makes a design a bit more difficult (impossible till I know exactly!).

Furthermore, the type of pump needs to be defined as some pumps do not work well backwards, some get hot, some may get damaged......Some will work but at a far lower efficiency...Some types will need to be doubled up, two motors and two pumps for example if the type will only work in one direction....

How much fluid needs to be moved? Large, medium or small amounts?

Also, no mention has been made of electrical supply available - mains? DC? what voltage?

If the device fails, will people's lives or homes be in danger? That needs to be defined.

But other than that its actually quite easy to put in place using say a PIC as a controller, using the other hardware described....and probably a few relays, probably solid state, for example.

Also a power supply (probably tiny) will be needed for the PIC and any relays, Battery backup is a possible option, as is a mains failure system that must fail "Safe"....

You need to define the "safe" state fully as well......

Do also not forget that when reversing motors of any sort, enough time must be given without power to the motor, to allow the motor to fully stop before starting in the opposite direction. This is easy to implement using a PIC as a controller....

Once I know exactly the motor type and the pump type and the other design parameters and wishes, I might be persuaded to actually do some coding to get it working properly! Make me a valid offer!!!

Jeez!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! RTFT. T=thread, not manual.

Try to keep up!

This looks very straight forward,definitely can be done by you if you sure have electrical circuitry knowledge.Try working with a run relay ,float switch relay, a forward run contactor and a reverse run contactor if you think the pump can take the instant forward reverse switching, otherwise add delay on timers to the contactors.The run relay would be activated by your selector switch and press buttons to start up .

Is your "friend" expecting the pump to reverse flow? With many types of pumps he's going to be disappointed. There's also a possibility of the impellor becoming detached.

Until more information is forthcoming I'm staying out of this.

Me too. This train [thread] is running down the track without a brakeman or engineer in control.

Could be any number of types of positive displacement pumps and work fine in reverse. What confuses me now however is the newly introduced word "compressor", and what kind of a compressor would work in reverse AND be useful in a tank level operation system. You can use a compressor to pressurize a day tank based on tank level I suppose, but there would be no need to actually reverse the compressor to decrease pressure, you just vent to atmosphere. Unless this is some sort of closed gas system or nuclear waste system where the gas must remain captive? All of the design elements are unable to fit together if you ask me, so there is either some sort of gross misunderstanding going on here, a LOT of missing information, or we have all be subjected to troll bait.

Start and stop push buttons are instantaneous contacts which only change state when depressed so you need a relay to latch the start signal (2x N/O). The switch is a latching contact so you can either dispense with the switch or dispense with the push buttons and relay. If you leave it in circuit it duplicates the function of the push buttons and relay so it does not add to the functionality, but is sometimes required as an isolator to comply with regulations.

A single N/O contact float switch is unable to output two signals forward and reverse so you need a second relay with 1 N/O and 1 N/C contact.

I have described the circuit in the form of Boolean Equations

Key

SW = switch, ST = start PB, SP = stop PB, FL = float switch, O/L = overload relay, R1 = relay 1, R2 = relay 2, FOR = forward contactor, REV = reverse contactor. All contacts are normally open except where a mnemonic is underlined to represent a normally closed contact (Strict Boolean notation the line would be over the mnemonic but that feature is hard to represent in normal typing) I have used + for OR as standard but x for AND instead of a period mark.

SW x SP x (ST + R1) = R1 coil

R1 x ((FL = R2 coil) + ((O/L x (R2 x FOR = REV coil) + (R2 x REV = FOR coil)))

If you dispense with the switch just delete SW x from line 1. If you dispense with the push buttons, delete line 1 and substitute SW for R1 in line 2.

I have design the control circuit for you but I cant get the control circuit paste in this forum. I don't know why but please send me your email address to me and I will send the circuit to you with explaining how the circuit should be works.

With just one float switch to be the sensor, unless the switch and/or the process has a large hysterisis in it, you'll likely be burning out the motor out with the rapid switching:

* Tank is low, filling.

* Switch closes, start emptying tank.

* Switch opens, start filling tank.

* Switch closes start empty- switch opens start fill- switch closes, st- switch opens, start e- switch clo- switch op- swit- swi- sw- sw- s-s-s-s-s--s-s--s-s-s-s-s--s-s--ws-s--s-s-s-s--s-s-

What you need are at a minimum TWO float valves, one to open when the level drops below the LOW level, and one to close when the level rises above the HIGH level.

Once you have the float switches set up correctly, one DPDT Relay can use their info to create a 'forward/reverse' signal, and then two DPDT relays can take that signal and convert it into the line switching for the motor, if it is DC or 3-phase.

The start and stop buttons would be used with a single DPDT relay to create a standard start/stop latch, which would be used to control the power feed, either directly if the power is DC, or through a three-pole, single throw power relay if it is three-phase power being used for the motor.

From your description, the selector switch is only used in one position, is it an on/off switch to provide power to the system, or does something happen when the selector is in right or center positions?

I can provide more info if you refine your question.

Hope this helps.

Well put. The motor will be damaged quite quickly I feel as wished for by the OP it would seem.......by the rapid reversing, not forgetting the high currents through any switching contacts....

A second float to detect when the "tank"(?) needs refilling is a reliable method. Its not difficult in a range of ways. I prefer electronics as its easy to add timing and to effectively "reverse" the action of contacts, but our OP (according to Lyn anyway) does not want any other components....strange!

So assuming that a second float can be used, the simplest being to mount the bottom float upside down so that its contacts are reversed to the top float switch......simple!!

That way the top float closes only when the tank is full and the bottom float contact closes when the water level is below the float...

Feed that to a two position "latching" relay, that can be held mechanically in a position each time they are operated.

That way one float switch can pulse the latch relay when the tank is full and the other can pulse it again when empty.....that latching relay can thereby control the action of two interlocked (forward and reverse) contactors, from a single changeover contact.....

With the usage of a latching relay, there is some hysteresis between top and bottom level, but still the motor would be violently reversed each time....that I don't like.

I am not up to date with modern 3 phase contactors, but maybe there is one that can allow the motor to stop before opposite power is applied, maybe a dashpot system.....suitably adjusted.

We used to have reversing systems whereby when one contactor was activated, it mechanically prevented the other also closing, just in case for some reason, the electrics tried to close both together - a welded contact for example.....short circuit somewhere....

As we have not been told what currents are being used by the pump, it is difficult to be 100% exact.....but it may even be possible to have a contactor with center three fingers with the 3 phase supply and contacts in front and behind for the motor, one triple having a pair being crossed over. One contactor for forward and reverse!

Not forgetting a dashpot to slow things down would be really great!!

I feel the OP is restricting himself with the requirements as any company must be able to invest a little more for a good configuration I feel, but I could be wrong!!

If anyone did not fully understand the logic fully, no problem, simply drop me a line and I will try and unravel it for you.....

As to the "instant reversing" aspect of this, since we have no idea what the machine really is, we can't say that it is or is not a problem. But... a "chopper pump" used in the waste water industry has this kind of similar sequence, just not based on a float switch. In a chopper pump, if the impeller of the pump is blocked by "rags" or other tough fibrous materials that were not supposed to be there, the pump is immediately reversed. The design of the impellers is such that they are two sets of blades, like a meat chopper, so reversing them makes them cut the material so that it then can pass through. What they do is to monitor the motor kW and if it gets higher than the threshold, the reversing contactor bangs in to change the direction, 3 or 4 times in a row, then it resumes it's normal direction for flow. The motors are designed for this purpose in mind, meaning it can be done.

If that is the case then I can see it working without a need for a hysteresis, as it only reaches the 'high' level when there is a blockage, which triggers the blockage clear' function until the blockage is no longer there.

The more information we have about a problem to start, the easier it is to find a solution.

I'm reminded of an old Lateral Thinking puzzle, that is insanely hard if you don't have the right background:

"A man is painting his face on an egg. Why?"

Yes, that is all the information you are given, and you need to deduce the rest.

* There is a sane and logical reason the man is painting his face on an egg.

* He is not going to keep the egg when he is done, but will send it to someone else.

* That someone else will put it on a shelf and probably never look at it again.

* The shelf has several other eggs with faces painted on them, no two alike.

* The face the man paints on the egg is not the face he sees in the mirror when he shaves, yet it is still 'his face.'

* The man has often painted the face he is painting on the egg onto his own head in the past.

Or, for those who have the right background, The man is a clown, and is registering his 'face' (his makeup pattern) with Clowns International:

http://en.wikipedia.org/wiki/Clown#Clown_organisations

Once you know that one extra fact, all the leadup becomes redundant. Before that, it seems convoluted and confusing.

Get a long stick. Cut the cable off the float switch and duct tape the float part to one end. mount the stick vertically in two eye bolts so that it is free to move up and down. Tape another stick to the selector switch making sure it is a rotary or drum reversing switch without center off. Where the vertical stick and the drum switch stick intersect, have a rubber grommet mounted on the vertical stick, representing the reverse direction point, move the drum switch to the reverse position and remain until another grommet mounted on the vertical stick representing the forward direction point moves the drum switch. Incorporate this drum switch circuit into the load side of the motor starter.

The stop button on start/stop pushbutton station will stop all operation.

Please note, as mentioned by others, that you can expect very short life from motor and drum switch along with "nuisance" circuit breaker tripping.

You will wanna be "somewhere else" during tryout.

I do not understand why safety is seemingly unimportant to the OP, nor why there is such a restricted list of components.

If this is for a company, there needs to be a few more components "allowed".....and more money involved.

The problem is easily fixed with probably a 3 x larger budget.....with some "proper" components.

I shall watch from the sidelines for a bit longer.....But salsar1978, this needs fixing and fast!!!