12V DC remote control of 240V AC system

Thread Starter

James Hockney

Joined Oct 23, 2018
11
I want to add a low voltage remote control to a 415V Motor starter. The actual switches etc are all run currently off a single phase at 240V but I want to operate it with 12V. Will this suffice? Thanks.

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Alec_t

Joined Sep 17, 2013
10,350
I haven't worked through the relay logic but note that your transformer outputs are labelled 12vDC. A typo? Or are you intending to use 12VDC adapters rather than standard transformers?
In the interests of safety and EMI reduction, have you considered using SSRs instead of electromagnetic relays?
What is the 'M' component?
 

MisterBill2

Joined Jan 23, 2018
3,847
First of all, you will not get 12 volts DC from the transformer shown. You will need s rectifier also, or you can use a 12 volt DC power supply.
Now some questions, the first is if this is intended to replace the current arrangement or to be in addition to the present controls arrangement? The second question is will there be the possibility of unsafe operation if the stop button fails to switch off the motor power? The circuit shown has both controls connected.
The circuit arrangement shown will not work if the motor control starter coil is that symbol with the "M" in it, because that is in series with the 12 volt relay coil, and it is not safe because the stop portion will not function if a wire becomes disconnected.
And it looks like the circuit is made with single-pole automotive relays and door bell buttons, which are really not OK for controlling a 480 volt motor.
A good motor control will use a normally closed button as the stop control. And in this case the circuit would be simpler.
 

Thread Starter

James Hockney

Joined Oct 23, 2018
11
First of all, you will not get 12 volts DC from the transformer shown. You will need s rectifier also, or you can use a 12 volt DC power supply.
Now some questions, the first is if this is intended to replace the current arrangement or to be in addition to the present controls arrangement? The second question is will there be the possibility of unsafe operation if the stop button fails to switch off the motor power? The circuit shown has both controls connected.
The circuit arrangement shown will not work if the motor control starter coil is that symbol with the "M" in it, because that is in series with the 12 volt relay coil, and it is not safe because the stop portion will not function if a wire becomes disconnected.
And it looks like the circuit is made with single-pole automotive relays and door bell buttons, which are really not OK for controlling a 480 volt motor.
A good motor control will use a normally closed button as the stop control. And in this case the circuit would be simpler.
OK Guys Thanks &Sorry for the slightly confusing diagram.
OK Thanks. Unfortunately there are very limited components available on the circuit drawing app that I used, So I had to improvise somewhat.
Starting Comment - Yes I am expecting to use a 12 (or possibly 24) Volt DC power supply in place of each of the Transformers in that diagram, I.E. they will be powered on when that section of the 240V control system is active.
First Question - This is an addition to the current panel mounted 240V control switches which will remain in place at the panel.
Second - We have a separate E-stop on the main power into the panel, and the stop on the panel front could be used in addition to the remote, so there is little risk of un-intended operation on circuit failure.
On your third point - The central Relay and M symbol (motor) are representative of the Main Contactor and Auxilliary switch operated by such in the real system, but as I had no way to draw those with any degree of correctness I drew the equivalent (or so I thought) if the Motor was driven on the 240V circuit.
In reality the 240V system triggers a 415V contactor which does the motor starting, then the flowing current in the 415V system induces enough current back to the 240V system to hold the Auxilliary contact (My Relay's contacts) closed, which is wired in parallel with the main Start Button, Stopping is achieved by pressing the Normally Closed Stop button which is wired in series with the start button and Aux contact.

In order for the system to remain as is an additional start buttons (or relay contacts) must be wired in parallel with the existing Panel one, and all stop buttons must be wired in series with the original one.
The remotes are to be used outside and need to be able to move around and there are actually 3 of them, they can't be 240V as they may well get wet and there is a possibility that the wires become damaged hence the use of 12 V for the remote.

The main system control at 240V runs with a 1A breaker so just about any relay which can handle the 240V will be able to make or break connections in the circuit (I believe). I'm pretty sure that I can operate the remote running LED with a just a relay from the 240V system rather than a second 12v PSU and 12V relay - The reason that links into the other 12V system is to limit the number of cable cores required for the remote as SY cable is expensive to obtain here with 6 cores.
 

MisterBill2

Joined Jan 23, 2018
3,847
OK, and thanks a whole lot for the explanation. Really, you only will need one power supply, which will make the system simpler. Depending on how important the motor running indicator is, you might even get away with only 4 wires per remote position. You will need two sets of contacts for the control relay, one set for the 12 volt latching function and another set to power the motor starter relay. I suggest that those contacts be in parallel with a "jog" button position in the 240 volt section in order to avoid a possible turn off problem.
For the low voltage system each remote will need a normally closed button and a normally open button, for the start function. To simplify the description of the circuit all of the remote control boxes are the same, each with w 3 wire cord, one black, one white, and one red. The black wire connects to one terminal of the stop button, the white wire connects to both the other terminal of the NC stop button and also one terminal of the start button. The red wire connects to the other terminal of the normally open start button. So now there are three remotes all wired the same. At the control box the black wire from the first remote connects to the power supply negative terminal, the white wire connects to the black wire from the second remote, and the white wire from the second remote connects to the black wire from the third remote. The white wire from the third remote connects to one normally open contact of the first relay. The other normally open contact of the first relay connects to both one side of the first relay coil and also to the junction of all three red wires. The other coil terminal of the first relay connects to the positive terminal of the power supply. So now pressing any start button switches the relay on.
 

MisterBill2

Joined Jan 23, 2018
3,847
(Continued because my computer went goofy on me)
And any off button releases the first relay. A second relay , or another set of contacts on the first relay, can control the motor starter.
Hopefully folks can get through my extensive text description of the circuit, and those with impaired vision will be able to picture it. Adding an indicator to each remote without adding another wire is a bit complex, but if low current indicators are OK, ones drawing less current than the holding current of the relays, they could be connected between the red and white wire of each remote. This circuit is about as simple as it can be, I think.
 

MaxHeadRoom

Joined Jul 18, 2013
18,995
From my experience, you would be better off to fit a small bridge rectifier off of the 12vac and use DC relays, also it is far better and clearer to use ladder logic schematic rather than the form you are using.
As @Alec_t asks, what is the 'M' in series with a relay coil?
You already show 12vdc, just the bridge is missing.
Max.
 

MisterBill2

Joined Jan 23, 2018
3,847
From my experience, you would be better off to fit a small bridge rectifier off of the 12vac and use DC relays, also it is far better and clearer to use ladder logic schematic rather than the form you are using.
As @Alec_t asks, what is the 'M' in series with a relay coil?
You already show 12vdc, just the bridge is missing.
Max.
Often AC powered relays run cooler, which can be a benefit. And there are fewer components to fail. High voltage lie spikes that destroy diodes cause no problems with AC circuits, usually.
 

MaxHeadRoom

Joined Jul 18, 2013
18,995
The only time AC solenoids/relays/contactor coils have the edge is at pull in time, any other time they can not only be noisy but burn out if the armature of any of them does not move over completely for some reason.
In my experience the AC runs hotter if anything.
As I have mentioned before, I got a bit of negative reaction when first spec'ing in DC solenoids in here in N.A., my reasoning was backed up by manuf. plants having to stock a number of AC solenoid coils for maintenance spares.
After upgrading systems to PLC and using the DC version coils, the coil inventory became redundant.
See Relay Coil p6.
Max.
 

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MisterBill2

Joined Jan 23, 2018
3,847
Solenoids are for some reason far different from relays designed for industrial applications. My main experience with DC solenoids is with hydraulic valves, where the power supplies to energize large banks of high pressure valves were large and warm. Of course I was following up after those who created that package had their money and were long departed. And in one appalling instance where I had added a relay from a customer's stock, not realizing that it had an internal protection diode, which I had not noticed that the whole thing was polarity sensitive, it caused a poor tech to struggle for quite a while before discovering the problem. Polarity sensitive devices should be very well marked!!
So DC solenoids can be fine for solenoid applications, much different from relay applications.
 

MaxHeadRoom

Joined Jul 18, 2013
18,995
So DC solenoids can be fine for solenoid applications, much different from relay applications.
But basic electronic/magnetic principles still apply.
I have spec'ed in DC for relays and solenoids since the mid-80's, also now that industrial control circuitry has primarily moved away from 120vac to 24VDC, it is easier to implement the DC versions, also many P.B. indicator devices etc, are LED illuminated which also goes better with a 24vdc control circuitry.
For one custom machine that was made in the USA for a customer of mine here who had asked me to specify electrical recommendations in the design, I had stipulated the 24vdc rule for all magnetic devices and control, resulted in some protesting in the form of "We have always done it this way"! (120vac).
The customer preferred the lower down times in production!;)
Max.
.
 

MisterBill2

Joined Jan 23, 2018
3,847
But basic electronic/magnetic principles still apply.
I have spec'ed in DC for relays and solenoids since the mid-80's, also now that industrial control circuitry has primarily moved away from 120vac to 24VDC, it is easier to implement the DC versions, also many P.B. indicator devices etc, are LED illuminated which also goes better with a 24vdc control circuitry.
For one custom machine that was made in the USA for a customer of mine here who had asked me to specify electrical recommendations in the design, I had stipulated the 24vdc rule for all magnetic devices and control, resulted in some protesting in the form of "We have always done it this way"! (120vac).
The customer preferred the lower down times in production!;)
Max.
.
ZERO down time from any AC powered relay or contactor in the past 35 years tends to indicate that the quality of the components matters quite a bit. Allen Bradley 700N series control relays seem to have been good choices. Not cheap, but they do have a tendency to keep working.
 

Thread Starter

James Hockney

Joined Oct 23, 2018
11
OK Thanks,

The reason that I have used NO Swiches for the stop circuit is so that if a remote is unplugged the system will still work. I could, if I remove the running light and make the system require all switches to be plugged in at all times, simply wire a series of 3 NC switches to a relay (NO) coil and have that break the 240V system in series with the integral Stop Button. Then have the 3 Parallel wired NO starter switches activate another (NO) relay Coil to switch on the 240V in parallel with the starter button.
 

MisterBill2

Joined Jan 23, 2018
3,847
OK Thanks,

The reason that I have used NO Swiches for the stop circuit is so that if a remote is unplugged the system will still work. I could, if I remove the running light and make the system require all switches to be plugged in at all times, simply wire a series of 3 NC switches to a relay (NO) coil and have that break the 240V system in series with the integral Stop Button. Then have the 3 Parallel wired NO starter switches activate another (NO) relay Coil to switch on the 240V in parallel with the starter button.
OK, I had not considered that the remotes would be plug-in types. My design was for wired in remotes, mostly because I have seen that anything that can be unplugged will vanish eventually, at least in the areas that I have worked.
 
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