I rewired a 3-bay pump Stand and I added individual Transformers for each meter. They are a 100 VA Transformers and I have them wired up 208. Functions will not work unless the right common is going to the right terminal strip. Is it possible to tie all the commons together on all three Transformers will this hurt anything? Or how can I overcome this problem?
Can multiple Transformers share commons?
- Thread starter APW
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JGinther
Zip-tie engineer
You can tie them together. I would also ground the same leg and fuse the secondary on the transformer. Not having a floating neutral makes troubleshooting easier, and when something grounds out, you will know by a blown fuse instead of smoke.
MEP001
Well-known member
I would NOT ground the commons. There's nothing wrong with a floating common, but assuming you've got two hot leads into the transformer you'll also have two hot leads out, each one 12V. You don't want to ground your voltage legs.
JGinther
Zip-tie engineer
Why not? Its still 24V difference between the two legs. Equipment won't care.
Ontario Car Wash
Member
mine are together just put a inline fuse its cheaper than smoke
MEP001
Well-known member
I don't know why you'd want to ground one leg of your voltage. I can tell you for sure it can make diagnosing electrical issues much harder when you can't get proper voltage readings between the two legs. And there's no advantage to grounding the common even if it's a true neutral.
MEP001
Well-known member
Yes, there should always be a fuse or breaker on the power side.
JGinther
Zip-tie engineer
I can tell you why you would want to. Its for the same reason neutrals are grounded at the panel in services. Coming out of the power transformer, one leg is grounded - that is what makes it have 0 volts. 0 volts is a good thing when troubleshooting: if you use a meter, you can see if there is potential between a wire and ground, not just the return path. When you see that there isn't potential (voltage), or that a wire is continuous with the ground, you obviously have a ground fault. This is especially helpful when a device fails (internally shorts to case) which is mounted to a gounded item. An example in a car wash would be this: If your meterbox was grounded and bonded to the pumprack where the transformer is, and the transformer neutral (or leg 2 or whatever you want to call it) is gounded, and you have a problem where a keypad or rotary swith fails internally putting power to the meterbox door, you would have a blown fuse. Hook up a voltmeter to the power supply and see its grounded. Remove one path at a time until you find the circuit open again. Solved - bad keypad or switch or whatever. If it was left floating - you have a hot meterbox which can cause all sorts of electronic problems... The examples are many, but it is normal practice with power mains for safety reasons, but mainly so that a breaker or fuse has a good reason to trip when something goes wrong.
Also, even if you have 208 or 240 feeding the transformer, and have 24vac out, there's not really ever a "neutral". You make a neutral by bonding whichever leg of the transformer that is "common to the loads" to the ground. By doing so, it becomes "neutral" (0 volts to ground). Since a transformer is by definition an isolation device, There is no reference to ground on the secondary (outlet). The exception is an internally grounded transformer where they just bonded it for you.
MEP001
Well-known member
Then I would deliberately choose a 120V transformer so the entire right input AND output is neutral and can be safely grounded.
I will have to do some real-world tests to see if grounding a live 12V AC leg of power from a transformer causes a change in voltage or an increased load on the transformer.
I will have to do some real-world tests to see if grounding a live 12V AC leg of power from a transformer causes a change in voltage or an increased load on the transformer.
JGinther
Zip-tie engineer
It won't have any issue unless you use a transformer that already has a dedicated neutral (0 volt out). In that case, it is already internally grounded, so you can't ground out another leg. Most Transformers are not set up like that, but some are. You are right that in that case you would have a problem. The problem is that they have already bonded half the winding to ground. Also, in that case, you don't have the floating neutral problem that i was referring to above... You just have 12V showing as potential to ground on both the "power" side and the "common" side. But, the common is still "hot" (12VAC potential to ground) which is confusing to most people.
JGinther
Zip-tie engineer
Ok, i just looked into this a little... Pretty much 0 transformers (exception would have to be specialty made) have internal grounding. Its up to the installer to create the ground. In the example above, where there is a 12V, 0V, and "-12V" terminals, as long as you leave 0 (or x0) unused, you can bond the -12V to ground and then you have a 24VAC system as I mentioned above. First leg is 'hot' with 24VAC potential to the other leg which is now same potential as ground (since it is grounded).
mjwalsh
6 bay SS w/laundromat
I have had the best reliability & ability to troubleshoot with common grounds ... ever since an electrical control engineer got me in that direction. Potentially troublesome ground loops ??? are more likely it seems with floating grounds because instead of 0V it could be slightly off at a terminal. For most car wash equipment ... it may not make any difference. Based on one transformer that we have for use outside of the car wash ... it actually allows for either 12VAC output or 24VAC output. I don't think it is utilizing a floating ground to accomplish that though.
Since we have been involved in amateur prototype Arduino board (vdc only) testing of embedded logic ... I notice on some of the boards there are 3 separate ground or 0V ports. I read on an engineering website-forum that the 3 separate 0V ports has something to do with a STAR ???topology & it supposedly helps distribute the sensitive voltages better ... not sure ... others may be able to clarify better. That would supposedly be an example of not wanting to have 100% bonded ground wires.
JGinther
Zip-tie engineer
Its not. A floating ground means that the ground wire is not at the same potential as the actual ground (dirt) that most people are standing on. The transformer is an isolation device. That means the secondary has nothing in common with the power supply except that it is "using" its power to make "new power". All connections after that are up to the installer.
I'm pretty sure the 3 terminals referenced on the arduino are there so you can hook wires up to them. That's all. Star and Delta terminology has to do with different ways to hook up a power main transformer or motor for different voltage possibilities (240 vs 208) and different starting torques in the case of motors.
MEP001
Well-known member
Just to make sure you don't have any issues down the road, you might want to check power between the bays. If you take a voltmeter and put one probe on the transformer "hot" output for bay 1 and the other to the "hot" output on bay 2, it should read no voltage; same between bays 2 and 3.
Grounding the neutral off of a 24 vac transformer is commonly done. Some pieces, such as Etowah's credit card system require it.
