How to use limit switches in a project circuit?

Thread Starter

fiftyv8

Joined Jan 29, 2018
58
I am no guru in the field of electrical circuits so please keep your replies down to earth and excuse any failings in my wording, thanks.
I wish to build a machine that consists of a 3 phase a/c motor driving a very slow reduction gearbox.
The gearbox in turn will rotate a shaft with an arm attached to it.
The arm will never rotate more than 180 degrees.
I need for the arm to have a home base with a limit switch to stop at HOME.
I need a 2nd limit switch which can be moved to STOP the arm rotation at a selected point for degrees of movement.
I would like to think there is a circuit that will allow the arm to be initiated by me manually thru a switch mentioned in the next line.
I have visions of this switch being a reversible 3 phase barrel/tumbler style with a lever arm.
I also envisage that when the arm reaches the predetermined limit and the circuit is switched OFF, then by me moving the barrel lever to the reserve setting that the arm will automatically initiate and return to HOME unless I stop it manually before it reaches HOME.

The other question, can this be done cheaply?
I can source a tumbler reversible switch and already have the motor and reduction gearbox.
I figure that I will need a pair appropriate limit switches, but not sure what else maybe needed to achieve what I seek.

Oh, power supply is from mains power and I have 3 power wires, an earth and a neutral at my disposal...

Any input will be much appreciated.

Thanks, Russ.
 

LesJones

Joined Jan 8, 2017
3,078
If you want to use a tumbler reversing switch you will also need contactors to break the feed to the motor when the limit switch is reached so that two or preferably three conductors are broken. (Or two or three pole high current limit switches if such items exist.) I think it would be better just use a light current three position switch to control relays and contactors. This wiring could be at low voltage such as 24 volts for safety. You could also use three push buttons for the controls. (Home, stop and the other limit position.

Les.
 

Thread Starter

fiftyv8

Joined Jan 29, 2018
58
Thanks Les. I guess I am hoping that somebody will post a circuit diagram that I can read and potentially source components to start building what I envisage.
Les, I have limited imagination in this field so if there is an alternative to a tumbler switch that I need to know about I am all ears...
 

LesJones

Joined Jan 8, 2017
3,078
Hi fiftyv8, The schematic I was planning to suggest would require two contactors each with three NO (Normally open.) main contacts and one NC (Normally closed.) auxiliary contact for interlocking. If you chose push button type controls they would also require a NO auxiliary contact. I have not been able to find any contactors that meet this specification. The nearest I have found are reversing contactors that are really two contactors. I have not managed to find a datasheet on these contactors showing the internal contact layout at terminal designations. Without that information I could not give you a simple diagram of how to connect them. Also the new price of these is quite high. (Between £150 and £200) Id did find this one on ebay for £65 There are other members of the forum that will probably know better suppliers for contactors and be able to tell you with confidence how to wire the reversing contactor set. (It is over 50 years since I was working with this sort of thing.)

Les.
 

Thread Starter

fiftyv8

Joined Jan 29, 2018
58
No problem Les, very good of you to offer your thoughts.
As you can see so far, I have not attracted much interest yet...
I hoping somebody will offer a simple and in expensive means of solving this puzzle.
 

GetDeviceInfo

Joined Jun 7, 2009
1,806
M1&2 are your 3 pole reversing contactor similar to the previous post. They each need an NO and an NC aux contactor, unless they are mechanically interlocked, which you could then omit the NC contacts. Regardless, the contactor needs to be horsepower rated.

Both forward and reverse momentary push buttons initiate a latching circuit when activated, and will run until a NC stop button activation interrupts control power, breaking the latch. A limit switch installed into the circuit, immediately prior to the respective contactor coil, will also break that respective latch.

The operation would be, start (forward) press, motor latches forward, runs to limit switch and stops. Reverse (home) press, motor latches reverse and runs to home limit switch. You can stop anytime with stop button.

Control and coil voltage needs to be determined to rate those components. What is your source 3 phase?

Just an idea.
 

Thread Starter

fiftyv8

Joined Jan 29, 2018
58
Make no mistake, I appreciate every bit of what you are offering here.
I just dont have a high level of electrical skills in my DNA like may on here.
I will digest what you have said and I am getting the feel for it now.
I guess the cowards way out would be to just run the forward and reverse tumbler switch and approximate STOP's at both ends, but I guess that just ain't the way I like to do a job...
 

GetDeviceInfo

Joined Jun 7, 2009
1,806
Momentary. The main part is the reversing power contactor. Take note that the previously referred contactor has Over current and Overload (OL designated on schematic) protection built in. If you went with a plain contactor, you’ll have to provide those functions separately.
 

LesJones

Joined Jan 8, 2017
3,078
While thinking about alternative solution (After seeing the price of reversing contactors.) I thought that using a VFD with all the switching at low voltage may be a cheaper solution. (As a few years ago I bought a 0.75KVA VFD for my lathe on ebay for about £50.) I then thought as I know nothing about your background that do you know you need a three phase supply available to run a three phase motor. Also do you realise that the motor will not stop instantly when it actuates the limit switches. As I don't know your application this may not matter. But in some cases such as using it to mill radial slots in something it would be important for it to stop in exactly the same place each time.
A couple more points. If the reversing contactor I linked to on ebay does have thermal overloads then it is unlikely that they would be correctly rated for your motor. There are many products sold that do not have thermal overload protection such as hobby lathes milling machines and saws. Your idea in post #11 would work but you would need double pole limit switches rated for the FULL MOTOR CURRENT. I would NOT recommend that solution.

Les.
 

Thread Starter

fiftyv8

Joined Jan 29, 2018
58
Les, thanks for your thoughts you are indeed a gentleman.
Funny thing is on a previous project several years ago, I was advised that a VFD was the way to go.
I did acquire one and in my naivety purchased what seemed to be the best fit and looking back it was.
However, the brand I purchased was a Chinese item and I had nothing but trouble programming it to the point that even contacting the maker in China was problematic due to language and internet/email restrictions on what could be sent or received due to Gov't controls on every darn thing.

By luck, I eventually received assistance from several very keen and helpful members over several Forums which I joined seek assistance and after a long period of time (months) I got it working and it is great.
A VFD will as you say do all that I require, but I remain unsure as to how to employ limit switches thru it.
Both this and the previous project are geared so low that there is little chance of over run when power is cut, especially when under load.

On my previous machine build, variable speed was a key component of its function, however this item does not have such a requirement, as I will mechanically build in a speed and that will be the end of it.
The machine I am building is known as a mandrel bender and is a method or drawing and bending pipe/tube in a set set curve/radius according to how many degrees of curvature that are required.
The limit switches really only aid and ensure accuracy for repetitive and multiple bends of the same specification.

I note that my VFD does utilise dynamic braking which is comforting and the unit is simply reversed and is controlled by a stop/start foot pedal if required.

I must say that my inner expectation (not being strong in the electrical field), was to anticipate receiving directions utilising my neutral wire and one power wire which I had guessed would go to some sort of power supply of lower voltage which in turn would be attached to a gizmo or ready made circuit kit that could be setup to operate a pair of limit switches that could also initiate a reversing of the motor with ease.
However, I must say I feel way off in my expectations I think...
 

LesJones

Joined Jan 8, 2017
3,078
VFDs control inputs can be configured to behave in different ways. In non latching mode a voltage (Normally 12 or 24 volts) is connected to one input for forward and another input for reverse, Some have a latching option where only a pulse is required and a pulse to a third input to stop. (The 12 or 24 volt control supply is normally available from the VFD.) So for your application all you would require would be a three position center off switch and two limit micro switches (Probably V3 type micro switches would be suitable.) The 12 / 24 volt supply would go to the common connection on the switch. One side of the switch would go via a limit switch to the forward input terminal of the VFD. The other side of the switch would go via a limit switch to the reverse input terminal of the VFD. The VFD would be configured for non latching mode. I agree with Yaakov's comments about safety. For that I would choose a biased center off direction switch (So you had to keep one hand on that switch and another push button switch in series with the 12/24 volt supply. This way releasing either switch would stop the motor. You could even add a latching emergency stop switch in the mains feed to the VFD. If that had to be used (If for example the VFD would not stop when the control inputs were removed) then this could damage the VFD but that is better than a serious accident.

Les.
 

MaxHeadRoom

Joined Jul 18, 2013
22,641
All the pipe benders I know of use servo's, either via servo motor or hydraulic valve.
The dual switches referred to are known as Palm buttons, you could use simple P.B.s, the industrial versions have detectors built in to avoid an operator faking it by sitting somthing on one switch while operating the other single handed.
As to Reversing Contactors, I have commonly used the compact DIN style Telemecanique/Schnieder models, these will also take an add/clip on aux contact for other purposes, Often come up on Ebay, cheap.
You can also rig up a E-stop in the common line to the VFD inputs, as disconnecting the power without the motor being at stop, can damage the VFD.
I have also used a circuit which detects when the VFD is at Stop before dropping the contactor, this uses a programmed output on the VFD which only operates when the VFD is At-Zero-Speed.
Max.
 

Thread Starter

fiftyv8

Joined Jan 29, 2018
58
Safety has always been in the fore front of any project I build especially since I am generally the likely victim should something go wrong.
I always include as a minimum an emergency STOP switch and any other secondary device that I see has playing a part.
Honestly, I thought I was going to be able to avoid using a VFD on this occasion.

Max is correct there are most hydraulic driven mandrel benders out there, but some well geared straight electric units also exist. I being more mechanical have decided to go that route on this occasion.

Hydraulics can be messy and expensive also and again not my forte so much...
I have a complete hydraulic setup here that I want to save and employ later in a horizontal bender/press which will work out just right in that role.
 
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