Apprentice needs clarification on Relays

mscott52

Member
Location
Washington
Occupation
Apprentice-High Voltage
Hi all, I'm a 2nd year apprentice and this question is really more for my own understanding. I understand HOW an electromagnetic relay works. I know that control and power circuits are electrically isolated. I understand the design config of core, winding, applied I/P voltage, magnetic field, max voltage/current at contacts, max/min pull in and fall out. I know that by design you can have differing coil and contact sources (DC in/ AC out), as well as differing coil and contact rating voltage (24VDC coil and 125VDC power out). It's this last bit I don't understand. How are we able to get 125VDC to our contactor when A) our relay circuits are isolated, and B) our supply voltage is only 24VDC? is it just the result of current flow in the second circuit (power); is it contact design (air gap, materials).. I'm sure I'm making it harder than it has to be, since operational data is provided with the relay. I would love a real explanation on what happens to allow this to ensure I have the right handle on theory. Thank you in advance.
 

paulengr

Senior Member
Hi all, I'm a 2nd year apprentice and this question is really more for my own understanding. I understand HOW an electromagnetic relay works. I know that control and power circuits are electrically isolated. I understand the design config of core, winding, applied I/P voltage, magnetic field, max voltage/current at contacts, max/min pull in and fall out. I know that by design you can have differing coil and contact sources (DC in/ AC out), as well as differing coil and contact rating voltage (24VDC coil and 125VDC power out). It's this last bit I don't understand. How are we able to get 125VDC to our contactor when A) our relay circuits are isolated, and B) our supply voltage is only 24VDC? is it just the result of current flow in the second circuit (power); is it contact design (air gap, materials).. I'm sure I'm making it harder than it has to be, since operational data is provided with the relay. I would love a real explanation on what happens to allow this to ensure I have the right handle on theory. Thank you in advance.
125 VDC comes from another source if you have one. Within reason the maximum rating on the relay is just that. 125 VDC power comes from either a battery string (95 NiCd cells or 60 lead acid cells) or a capacitive trip device or a rectifier. You find it in switchgear and power plants where it is sort of a standard for backup power.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
How are we able to get 125VDC to our contactor when A) our relay circuits are isolated, and B) our supply voltage is only 24VDC? is it just the result of current flow in the second circuit (power); is it contact design (air gap, materials).. I'm sure I'm making it harder than it has to be, since operational data is provided with the relay. I would love a real explanation on what happens to allow this to ensure I have the right handle on theory. Thank you in advance.
As Paul said that requires a separate source of power. A relay is just an electrically operated switch. Think of a single pole relay as simple toggle switch where instead of you walking over and flipping the switch to open and close the switch you use electricity to control the switch. Applying voltage to the coil is the same as you manually moving the switch from one position to the other. Neither of those actions will provide power to the switch they will just change whether it's opened or closed.
 

mscott52

Member
Location
Washington
Occupation
Apprentice-High Voltage
125 VDC comes from another source if you have one. Within reason the maximum rating on the relay is just that. 125 VDC power comes from either a battery string (95 NiCd cells or 60 lead acid cells) or a capacitive trip device or a rectifier. You find it in switchgear and power plants where it is sort of a standard for backup power.
Hmmm. My question arises from a motor control discussion. Does what you're saying still apply? I have:
24vdc supply voltage;
125 vdc contactor:
(1) 8pin ice cube relay rated for 24vdc on the coil and 125vdc. If the 125vdc comes from a separate source, why do I need a relay? Couldn't I wire straight to my motor contactor coil?
125 VDC comes from another source if you have one. Within reason the maximum rating on the relay is just that. 125 VDC power comes from either a battery string (95 NiCd cells or 60 lead acid cells) or a capacitive trip device or a rectifier. You find it in switchgear and power plants where it is sort of a standard for backup power.
Thank you
 

mscott52

Member
Location
Washington
Occupation
Apprentice-High Voltage
As Paul said that requires a separate source of power. A relay is just an electrically operated switch. Think of a single pole relay as simple toggle switch where instead of you walking over and flipping the switch to open and close the switch you use electricity to control the switch. Applying voltage to the coil is the same as you manually moving the switch from one position to the other. Neither of those actions will provide power to the switch they will just change whether it's opened or closed.
Thank you
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Hmmm. My question arises from a motor control discussion. Does what you're saying still apply? I have:
24vdc supply voltage;
125 vdc contactor:
(1) 8pin ice cube relay rated for 24vdc on the coil and 125vdc. If the 125vdc comes from a separate source, why do I need a relay? Couldn't I wire straight to my motor contactor coil?

Thank you
What are you using to turn the motor on and off? Sounds like you have a 24 volt control device. Can that be eliminated?
 

mscott52

Member
Location
Washington
Occupation
Apprentice-High Voltage
Now you see my confusion! It is a control circuit for a 480vac motor. This was a hypothetical question my lead asked me. "How do you make this control circuit work-24vdc supply, 125vdc contactor for a 480vac motor". I kind of guessed relay, which he liked, but electrically....I can't make it work in my head. If it does, how? If it doesn't, why? Thank you again for the help.
 

hillbilly1

Senior Member
Location
Atlanta,Ga
You would need a 125 volt dc supply for the contactor coil, which is broke though the relay. So you have three different voltages here, 24 vdc for the relay coil, 125 volts dc for the contactor/starter coil, and 480 volts broke through the contactor/starter to the motor.
 

kwired

Electron manager
Location
NE Nebraska
Now you see my confusion! It is a control circuit for a 480vac motor. This was a hypothetical question my lead asked me. "How do you make this control circuit work-24vdc supply, 125vdc contactor for a 480vac motor". I kind of guessed relay, which he liked, but electrically....I can't make it work in my head. If it does, how? If it doesn't, why? Thank you again for the help.
is contactor coil rated 125VDC or is the contacts rated 125VDC?

You obviously need contacts rated for 480VAC for the motor. That same contactor may also have a different DC rating and 125 seems very possible.

You then apparently have a 24VDC signal, where it comes from and what logic is controlling it can be another topic but bottom line is you have a 24VDC signal telling you to run this motor, so ideally you would want a 24VDC coil on the contactor. If you are for some reason stuck with a 125VDC coil you would need to find a way to convert that 24VDC signal to 125VDC, and there could be many ways to possibly do that, in many cases may cost more than just getting the right contactor with a 24VDC coil.
 

Besoeker3

Senior Member
Location
UK
is contactor coil rated 125VDC or is the contacts rated 125VDC?

You obviously need contacts rated for 480VAC for the motor. That same contactor may also have a different DC rating and 125 seems very possible.

You then apparently have a 24VDC signal, where it comes from and what logic is controlling it can be another topic but bottom line is you have a 24VDC signal telling you to run this motor, so ideally you would want a 24VDC coil on the contactor. If you are for some reason stuck with a 125VDC coil you would need to find a way to convert that 24VDC signal to 125VDC, and there could be many ways to possibly do that, in many cases may cost more than just getting the right contactor with a 24VDC coil.
Just separate contacts would be fine. The coil contact would be fine.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Just to restate:

Both contactors and relays are electrically controlled switches. They do not supply power to anything. They can control power from a source to a load. They are as similar and different as 'porn vs erotica'. https://www.springercontrols.com/news/contactors-vs-relays/

You have a contactor with a 125V DC coil and 480V contacts controlling the motor. But your 'control signal' is 24V. So you add _another_ device that takes the 24V and controls a 125V DC power supply. Thus the motor is being controlled through _two_ devices.

The 24V initial signal operates the coil of the relay, which then switches 125V power to the coil of the contactor, which then switches the power to the motor.

Another option would be to replace the contactor with a different one that has a 24V coil. But perhaps a large 480V rated contactor with a 24V coil would draw too much current for your 24V control circuit.

-Jon
 

kwired

Electron manager
Location
NE Nebraska
A simple coil.
What is not simple is OP mentions 24VDC that is presumed to be the control signal yet seems to be stating the simple coil is rated 125VDC. If that is correct my suggestion was either get the right coil or complete contactor with the right coil or use additional components to change the voltage.

I was also trying to get him to verify actual coil voltage as well as contact rating, he hasn't really made either one all that clear. I can see his contacts having both 125VDC and 480 VAC rating, maybe at different current rating for each voltage rating.
 

paulengr

Senior Member
Now we get to it.

Once in a while I see some small contactors that can run directly off 24 VDC but generally speaking there’s not enough power. A NEMA size 3 or so contactor usually uses around 300-500 VA to pull in although about 1/10th of that to hold. So at 100 VAC we would need 3-5 A (to make the math easy) but 4 times that amount at 24 VDC, or 12-20 A. This might not seem like much but most control relays have a 10 A max and PLC outputs are often1 A. So for practical purposes no way to drive the contactor directly off 24 VDC.

The solution is to come up with a higher control voltage. There are at least two ways to do this. The first way is just use the 480 VAC power directly with a 480 V coil in the contactor and a 480 V rated isolation relay with a 24 VDC coil or if the PLC relay card will take it use a 480 V relay card directly. This is very old school. A few decades ago it was common to even see push buttons with 480 V on the contacts. I would not be surprised with electronic coils in starters becoming more common if someone starts selling an electronic starter with a built in power supply running off line voltage but I haven’t seen one yet.

But there is a big problem with this...when 480 goes bad, it’s REALLY bad. It tends to kill people if they get shocked and instead of a little spark it blows components up. Push buttons literally explode if you try to gently push in the button and cause it to arc. Relay contacts tend to not just burn up but burn out (of the plastic) when a relay goes bad. So if you can even find a 480 V coil in a contactor (not common anymore) keep the isolation relay in the starter bucket please! That way not if but when it fails the damage in the switchgear where it belongs.

The alternative is add another voltage. You put a CPT (control power transformer) in the bucket. Usually these are 120 VAC, not DC. Everything in the starter runs on this. Ideally use TRIAC or relay output cards and 120 VAC input cards on the PLC with the IO next to the starter (remote IO) for easy troubleshooting but if not then use an isolation relay (24 VDC coil, 120 VAC contacts) to operate the starter.

I have played with 48 VAC. This has one big advantage. It is under 50 V so the safety idiots think it is “safer”. As in no shock or arc flash hazard. That’s the same voltage the phone companies use in some equipment, including POE (power over Ethernet) used in Cisco phones. The downside is that the coils are always special order, and they are physically quite large. And let’s face it you are only working with it in the starter bucket which already has 480 V in it so it’s not like we are making anything safer that matters. So just forget this whole idea.

I’m more on the fence with 230 and 208. This splits the middle between the destructive tendency of 480 and the power/convenience of 120 VAC. Coils are still special order but most relays are rated up to 250 VAC on the contacts. So it is a convenient alternative with plants running 230 or 208 V deltas for motor loads.

In medium voltage and a lot of breakers they use 125 VDC as control voltage. So in those cases you have 3 options. You can use a rectifier to convert AC to DC, use a substation battery system with a battery charger to provide 125 VDC (60 cells, usually split into 10 jars), or use a device called a CTD (capacitive trip device) which is a rectifier and a fairly large capacitor to maintain power for a couple minutes. If you don’t have to ride through a power outage the simple rectifier is fine. On small systems (2-4 breakers) CTDs are much cheaper. But eventually the cost and maintenance of the battery system wins over. How much? You can get a small 50 Ah system for under $10,000. On small systems they might weigh say 300-1000 lbs. and take up roughly 2 foot by 4 foot of floor space. And need monthly maintenance. In contrast a CTD is a few hundred dollars EACH (one per breaker or for a set of breakers) and almost no maintenance.
 
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