Apprentice needs clarification on Relays

[Besoeker3]

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.

A coil IS a SIMPLE device.

 

[Besoeker3]

A coil is a simple device.

 

[Besoeker3]

Just ................

 

[Besoeker3]

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.

 

[kwired]

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.

480 volt contactor coil may not be so common inside plants but certainly is not something extinct either. They are still very common on individual equipment type applications. Pump panels in particular have coil voltage the same as intended motor voltage and control circuit is tapped directly from the supply circuit. We generally use a control relay when controlling that motor in "auto" position and the control signal is usually 120 VAC, sometimes 24 VAC. "hand" position it runs directly from 480 supply circuit.

 

[kwired]

A coil is a simple device.

Yes it is. But you can't power one rated 125 volts directly with 24 volts and expect it to work as intended.

 

[Besoeker3]

Yes it is. But you can't power one rated 125 volts directly with 24 volts and expect it to work as intended.

Of course not. Why would you?

 

[kwired]

Of course not. Why would you?

Exactly what I was trying to clarify with OP. The simple part is if he has 24 volt signal but a 125 volt coil, it won't work without more complexity being added to it. Still haven't confirmed if that is actually what he has though. OP seems to be possibly confused with what information he is providing us on what he actually has. The fact he was controlling a 480 volt motor didn't even come up until further into the discussion and all he mentioned before that was 24 VDC and 125 VDC and wasn't all that clear which voltage applied to what items.

 

[Besoeker3]

For what ever reason, my fault wasn't working...........any way......

Whatever, I the 480V should not control from that voltage.

 

[gar]

200614-1422 EDT

mscott52:

First, a relay or contactor are different names for what are basically the same kind of device.

What does the word relay mean?

From Merriam-Webster ---
"5: the act of passing along (something, such as a message or ball) by stages
also : one of such stages"

From www.thefreedictionary.com/relay ---

"5. (Mechanical Engineering) an automatic device that controls the setting of a valve,switch, etc, by means of an electric motor, solenoid, or pneumatic mechanism
6. (Electronics) electronics an electrical device in which a small change in current orvoltage controls the switching on or off of circuits or other devices
7. (Telecommunications) radio
a. a combination of a receiver and transmitter designed to receive radio signalsand retransmit them, in order to extend their range
b. (as modifier): a relay station.
vb (tr)"

Electrical relays come in many forms. Most often you probably think of electro-mechanical relays. These are usually made of a magnetic coil on a ferromagnetic core that can thru magnetic coupling influence the state of an electrically isolated binary electrical switch. You also might want to call a solenoid valve a relay, or a pneumatically actuated valve a relay.

A relay implies something that receives something, and then produces a retransmission of that first said something.

A very simple electromagnetic relay will usually consist of;

(1) A coil of wire wound on an insulating coil form,

This provides electrically conductive isolation of that coil from anything else. This coil is where an input signal is applied to the relay.

The number turns, coil; size, and associated magnetic circuit determine the input characteristics of the relay. The coil specifications determine the ultimate power or current capability of the relay. Only so much amplification can be provided by one relay.

(2) There is some form of mechanically operated switch, usually electrically isolated from anything else, that is magnetically coupled to the input coil. This switch's contact ratings (voltage, current, and breakdown voltage) determines what can be done with the relay output. Whether the output contact(s) is open or closed is determined by the magnetic state of the relay coil.

There is much more. Do your own study. Make measurements. Take a relay apart. Run experiments on different relays.
.

 

[paulengr]

A contactor is a heavy duty relay intended for driving large loads. All contactors are a type of relay but most relays are not contactors. A lot of relays fail prematurely with heavily inductive loads. A control relay doesn’t last long even with a small motor. That’s like taking a relay rated for an AC1 loaf and using AC3 on it using IEC terms. In NEMA terms it’s taking a DP contactor at maximum ratings and using it as a genersl purpose motor starter...it fails very quickly. Also you can’t use a contactor as a control power relay. It will oxidize and fail if you tried to wire up a contactor as a control relay, though I’ve seen it done in VFD conversions and it works for a little while.

Don’t get me wrong. Within reason you can use an old machine tool relay for almost anything. I’ve seen them used as contactors, control relays, and door stops. They are massive and take a beating.

 

[Besoeker3]

A contactor is a heavy duty relay intended for driving large loads. All contactors are a type of relay but most relays are not contactors. A lot of relays fail prematurely with heavily inductive loads. A control relay doesn’t last long even with a small motor. That’s like taking a relay rated for an AC1 loaf and using AC3 on it using IEC terms. In NEMA terms it’s taking a DP contactor at maximum ratings and using it as a genersl purpose motor starter...it fails very quickly. Also you can’t use a contactor as a control power relay. It will oxidize and fail if you tried to wire up a contactor as a control relay, though I’ve seen it done in VFD conversions and it works for a little while.

Don’t get me wrong. Within reason you can use an old machine tool relay for almost anything. I’ve seen them used as contactors, control relays, and door stops. They are massive and take a beating.

Generally, a relay and a contactor are quite different functions. For example, a 480Vdc can be that or any other voltage. On the other hand, a control relay is usually a fixed control relay.
It is just that simple.

 

[hbiss]

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,

Yeah you are. First off STOP thinking in terms of "in" and "out". What you are saying makes no sense. There is no in and out. There is a coil (simple electromagnet) that when you supply power to it it mechanically moves a set of contacts that can be used to control the source of power going to some device.

I think you have been given so much useless information that it just confuses you about the basics.

The voltage you supply to the coil is used to cause the relay contacts to open or close, and can come from anything that you want to use to control the device connected to the relay contacts. This is called the control or coil voltage.

Relay coils can be had to operate from a wide range of voltages. You use the voltage you think is appropriate for the control circuit you have controlling the relay. Coil voltage has nothing to do with the voltage on the contacts.

The contacts are just a simple switch or switches that are wired to control a device just like any other switch from a separate source of power.

And just like switches, relay contacts can be had to handle a range of currents and voltages. The voltage on the contacts has nothing to do with the coil.

-Hal

 

[Dibloafer911]

Run the line side of your 480 to a step down transformer 4801p1p) primary/120(or 208/1p) secondary, and get a contractor with coil rated for the aforementioned secondary(s). Use a 24v coil pilot relay for the 120(or 208v) contractor control ckt and =call it a day. All that should fit in the bucket of the MCC. the largest thing would be the transformer.
Keep it simple, babe

 

[paulengr]

Generally, a relay and a contactor are quite different functions. For example, a 480Vdc can be that or any other voltage. On the other hand, a control relay is usually a fixed control relay.
It is just that simple.

A 480 VDC contactor is very special. Those big old leaf contactors are long lead time and very expensive. An AC contactor just has to open wide enough to avoid a restrikes and survive the arc. Within a half cycle current will go through zero and the arc is extinguished. In contrast there is no current zero so a DC contactor has to physically pull the arc so long that it finally extinguishes. When you see a DC contactor it looks like something from the 1950s and has just one pole.

Right now Hubbell and an Italian company make a limited number of sizes. Lead times are long.

DC Contactors | Contactors | Industrial Controls | Electrical & Electronic | Products | Hubbell Industrial Controls

www.hubbell.com

So I’d agree in principle a DC contactor is “universal” but not in 99% of cases. In DC drives you might see 1 or 2 such as the starting contactors for synchronous motor fields.

 

[Besoeker3]

A 480 VDC contactor is very special. Those big old leaf contactors are long lead time and very expensive. An AC contactor just has to open wide enough to avoid a restrikes and survive the arc. Within a half cycle current will go through zero and the arc is extinguished. In contrast there is no current zero so a DC contactor has to physically pull the arc so long that it finally extinguishes. When you see a DC contactor it looks like something from the 1950s and has just one pole.

Right now Hubbell and an Italian company make a limited number of sizes. Lead times are long.

DC Contactors | Contactors | Industrial Controls | Electrical & Electronic | Products | Hubbell Industrial Controls

www.hubbell.com

So I’d agree in principle a DC contactor is “universal” but not in 99% of cases. In DC drives you might see 1 or 2 such as the starting contactors for synchronous motor fields.

Actually, we we rarely see contactors. We have used DC contactors on older machines for paper machine drives usually for Fourdrinier machines. My speciality.

 

[paulengr]

Actually, we we rarely see contactors. We have used DC contactors on older machines for paper machine drives usually for Fourdrinier machines. My speciality.

In a DC drive you don’t need one. You can just switch off the AC side to the SCR and you can change the firing angle on the SCRs to adjust voltage. You can even invert the voltage.

I see them in just one place. One method to start a synchronous motor is short the rotor with a small resistor, about 2 ohms. It starts like a wound rotor motor. At 85-90% speed engage the DC power supply in parallel then open the resistor circuit. Reverse to shut down. Never open circuit on a synchronous rotor so you need DC contactors. No amortisseur windings needed.

Also I have seen a few strange utility uses. Like a DC contactor for the oil pump or a water pump on the main generator fed from batteries. Just like a starter except minus two phases. If they are islanding (no power on the transmission side) this is truly the only way to start the plant. I haven’t ever seen such a system ever actually used. It’s more like an emergency backup system. Normally they have an auxiliary utility bus beside the GSU that they can power from when they take the generator offline and even lightning rarely takes down a transmission line. Often there is a backup or second bus and even a second transmission line with tie capabilities so even that system is redundant.

 

[Besoeker3]

In a DC drive you don’t need one. You can just switch off the AC side to the SCR and you can change the firing angle on the SCRs to adjust voltage. You can even invert the voltage.

Again, some drives feature them. The variable sectional speed drives don't feed the AC side.

 

[moonshineJ]

mscott52
What you achieve is controlling high power (e.g. 480 AC motor) with low-voltage circuitry. 24 VDC energizes the coil of the relay, which in turn provides 125 VDC to the motor contactor coil.
There can be multiple benefits in doing this. As an example, 24 VDC circuits may be used in wet locations, which is much safer than using 115 VAC or 125 VDC. I remember some old controllers using 480 VAC for control circuits within the cabinet (Ward-Leonard, I believe). Stop, start buttons, etc. would have 480 VAC sitting on them. That was very unsafe design, for sure.

 

[kwired]

mscott52
What you achieve is controlling high power (e.g. 480 AC motor) with low-voltage circuitry. 24 VDC energizes the coil of the relay, which in turn provides 125 VDC to the motor contactor coil.
There can be multiple benefits in doing this. As an example, 24 VDC circuits may be used in wet locations, which is much safer than using 115 VAC or 125 VDC. I remember some old controllers using 480 VAC for control circuits within the cabinet (Ward-Leonard, I believe). Stop, start buttons, etc. would have 480 VAC sitting on them. That was very unsafe design, for sure.

Nothing unsafe about using a switch rated for the voltage, but there is possibly advantages of lessened safety restrictions when working on controls while energized with the lower voltage. The compartment containing 480 volt line volts still has these safety issues even if there is a 24 volt control coil on the relay/contactor.