OT Series"ed" switches for higher voltage

On another web site (for electric vehicles) a person asked "can I install 3 relays with 120 V contacts in series to reach a bus voltage of 312V ?? The idea here is to use these to have an emergency motor disconnect.

I have of course seen switches in series say in control circuits for refer compressors (hi discharge, low suction, low oil) or redundancy say for E stops to allow for one contact welding or sticking shut.

Never have I seen them seriesed to gain higher voltage. My past experience has dictated if you have 120VDC you use 120VDC rated contacts - same for 250 or 312 etc etc.

However I can sorta see the theory but would not do it myself. I don't know everything and with electrical there is always some exception it seems.

So are they correct??

Dan Bentler

I don't think so.

If two relays are closed, and one is open, what is the voltage across those contacts. Obviously more then 120volts. So if one opens or closes, it is getting full voltage.

regards.....kc

One important thing I forgot

The relay coils will be in parallel so in theory they are supposed to open simultaneously - which I doubt will happen with electro mechanic relays due to friction etc. Thus the firts opening contact will have the "full voltage" across it.

Dan

You left out some details

Are we talking RMS or DC voltage? If so it "may be" possible to sum the voltages together to create a desired voltage. The series relays would be used to "add" the voltages together.

To make this simple lets say we have 3-12vdc power supplies. We could use 3 relays in series as a safety ckt to connect the 3 voltages together....ie 12+12+12=36. Break any of the relays and all power is lost.

The issue involved is each relay will need to have rating for the voltage across the contacts..ie 12 then 24 then 36
BUT
Place a resistor across the relay contact, a voltage divider, and each will be at 12v but total series voltage will be 36v with a slightly lower current capability.

As some have said, there is more than one way to skin a cat.

IF you are talking AC voltage this would be much more difficult.

Ron


I did use VDC but not on every value. It is all DC.

Dan

I made one statement that could be misinterpreted

To make this simple lets say we have 3-12vdc power supplies. We could use 3 relays in series as a safety ckt to connect the 3 voltages together....ie 12+12+12=36. Break any of the relays and all power is lost.

Click to expand...

The relays would be powered in parallel but the accumulative voltage would be in series.

The issue is controlling the "discharge" from the relays when they are opened....if you control this then it will be the same thing as opening simultaneously.

The short answer is, it depends.

It is possible, and actually practical to do this for many instances, with a caveat... you MUST put an external balancing network in parallel with each set of contacts. Similar to the balancing networks used across 'half' banks of capacitors in an AC Drive, or across the multiple series diodes in a high-voltage rectifier stack (think CRT Anode Voltage rectifier assemblies).

I'd rather, in this case, use an appropriately rated contact though. KUP type relays can withstand 480VAC (though most KUP sockets aren't rated for that, go figure)

leitmotif said:

"can I install 3 relays with 120 V contacts in series to reach a bus voltage of 312V

Click to expand...

I don't think this is at all feasible or safe. Contact ratings are based on allowable current and allowable voltage. Here is my understanding:

Current rating identifies the amperage that can be switched without premature failure from I²R heating (contacts do have some resistance) and arcing (especially with inductive loads).

Voltage rating is determined by the maximum voltage that can be expected to not jump the air gap and arc between fully open contacts (air conducts electricity - they call it lightning sometimes). Voltage rating is also a function of the dielectric resistance between the current carrying parts of the relay and ground, other panel components, or your finger. If you exceed the voltage rating you stand a chance of the electricity getting from where you want it to someplace you don't want it - such as the aforementioned finger!

I would never, ever do this.

Well it sure sounded goofy and unsafe to me. I think what gets them in this mind set is to increase DC battery voltage add more cells. Therefore you should be able to do the same (in reverse ??) with switches.

I think the purpose of this was to act as an emergency disconnect for the motor drive. 312 VDC is commonly used to drive inverter then 3 phase motor (Ford did this, Prius also - I think - Siemens systems are 312 VDC also). I think they are forgetting a couple things - dieelectric resistance, the volts current and wattage across the switch, and surges when the switch opens.

I surely would not do it using 3 or so relays in series, but then it is not my equipment and I don't have to fix it.

Yeah I want to go electric vehicle, use 3 phase motor BUT I hope to do it with drive and motor adapted out of forklift which operate on 72 VDC (at least in this era). We will see in a couple years.

Dan

Tom Jenkins said:

I don't think this is at all feasible or safe. Contact ratings are based on allowable current and allowable voltage. Here is my understanding:

Current rating identifies the amperage that can be switched without premature failure from I²R heating (contacts do have some resistance) and arcing (especially with inductive loads).

Voltage rating is determined by the maximum voltage that can be expected to not jump the air gap and arc between fully open contacts (air conducts electricity - they call it lightning sometimes). Voltage rating is also a function of the dielectric resistance between the current carrying parts of the relay and ground, other panel components, or your finger. If you exceed the voltage rating you stand a chance of the electricity getting from where you want it to someplace you don't want it - such as the aforementioned finger!

I would never, ever do this.

Click to expand...

Wow i cannot believe that Ron D said you could do this..I am just a simple sparky but if a customer asked me to do this i would say NO!!! as tom said the contacts are rated for a creatain voltage at a certain current..what you asking Dan is like saing is it o.k. if i put 10 12 volt fuses in line to pritect my 120 volt load..will it work..possibly..untill you have the mother of all sohorst and it welds those contacts together..

I wouldnt do it..just spend the extra bit of cash and buy a contact rated for it..

I was 99% certain and am now 100% certain this is a screwed up
bad
- REAL BAD --
idea.
I posted the question to ensure there was not something or an exception I did not know.

I saw this on a web page for electric vehicles by the way. These are the kind of guys who buy surplus and adapt it to their need. Great idea when you know what you are doing, but in their case - once again I am glad I dont have to fix their problems. Think I would charge
25 per hour for electrial work
10 surcharge for their ignorance
30 surcharge to have to explain it to them
50 surcharge to argue.

I have seen people use automotive fuses (rated for 30 V ?) in 120V appliations
- must be OK after all they fit OK
and it worked for the last year.
Why did it blow up and melt down?
Well gee maybe you ought to consider using the right rating fuse ????

Last week it was "the breaker keeps tripping - all we have to do is replace it - right? So I explained in my experience that 99% of the time the breaker is just fine and doing exactly what it is supposed to. The circuit had a dead short by the way -- after 5 hours of crawiling thru attic and tracing wire in finished walls it turned out the problem was extension cord in puddle of water in yard.

Put in six fuses and it still keeps blowing.
ahhh the hell with it just stick a penny in the fuse
-- gotta be bad fuses.

Dan Bentler