Relays coils

[Jako13!]

Bit of a random question but I’ve got a practice reason for asking. I know with a lot of control circuit timers and relays they state on them what there contacts can carry for example 2amp inductive 5amp resistive. My question is if we are pulling in a relay through these contacts is it possible for the relay to cause an overload for example the coil usually only draws 1 amp. The reason I’m asking is there’s a control circuit protected by a 6amp fuse and most the contacts are rated at 5 amps. the science of this is a bit beyond me.

 

[marconi]

File this away in your knowledge folder after reading it carefully:

Relay Applications | Relay Basics 1-3 | OMRON - Americas - https://components.omron.com/relay-basics/general-app

Omron is a good reliable brand.

 

[Lucien Nunes]

is it possible for the relay to cause an overload for example the coil usually only draws 1 amp

With 48V-230V coils it is almost impossible for any kind of fault to create a significant overload. Other than for large contactors the normal working current is usually tens of milliamps and if it increased to an amp or more as a result of shorted turns, the ends of the very fine winding wire would simply blow open-circuit like a fuse. What tends to happen is that a subtle increase in current occurs and the coil burns out with increasing numbers of turns shorting until it blows open.

With lower voltage relays, especially large ones, operating currents can be in the order of 1A at 12V. Given the much heavier wire from which their coils are wound, yes in theory there could be a situation where the coil will briefly draw a current beyond the rating of the switching contacts controlling it. Ideally the control supply fuse would be within the contact rating but a contact that can switch 5A on and off all day, will usually handle 10A for a while. Again the excess heat dissipation in the coil will usually burn it to a crisp sooner rather than later, before the control contacts come to any harm. And coil faults in low-voltage relays are very rare indeed.

 

[static zap]

Inrush currents . Do take an interst in fuse types (it makes a big difference )
In my day there were Quick blows / standard / Anti-surge=Time delay .
F - Fast
T - Slow(time delay)
Just leaves me to wonder if a Relay with welded contacts acts as a stalled motor ?

 

[Lucien Nunes]

As in with an armature that's jammed in the open position? On DC it makes no difference to the steady current although there is less back-EMF when switching off as the inductance is lower. On AC the reduced inductance increases the steady coil current and makes it run hotter.

 

[Jako13!]

So in theory if there is not much chance of overload is it wrong to assume that 6amp fuse for a control circuit and someone has fitted a relay with contacts of say 3 amp inductive load will cause any major issues other then reducing the life of the contacts if they did overload?

 

[Lucien Nunes]

A faulty coil with shorted turns will be less inductive than a properly working one, and if the overload is significant it will be mainly resistive. So it is the resistive load rating, not the inductive one, that matters when considering the impact of a faulty coil on the contacts controlling it.

Coils really won't last long at significant overload before self-destructing. A relay that is supposed to draw 0.1A at 24V but has so much shorted out that it draws 1A, will not operate because there won't be enough coil remaning intact to produce enough magnetic flux. If left energised it is likely to burn out completely in a few minutes. It probably couldn't take 5A without the winding immediately fusing open at the leadout, so will never present that amount of overload to the control circuit.

Contact ratings include an absolute maximum current carrying capacity that is governed by their resistance and the resulting temperature rise. This thermal limit applies to any type of load even if the contacts never switch it. Then there is a resistive load rating for one of a number of switching duties, which is a lifetime-imposed rating. The higher the current, the more wear occurs to the contacts at each switching cycle, so for a given duty and expected life, the resistive load limit might be lower than the thermal current-carrying limit. Then the inductive load limit is lower still, also depending on the specified duty category, because of the greater erosion of the contacts cause by the arc dissipating energy stored by the inductance. There is a certain trade-off between current and voltage, higher currents might be permissible at lower voltages because of the lower energy in the arc. Always subject to the thermal limit of course.

 

[Jako13!]

A faulty coil with shorted turns will be less inductive than a properly working one, and if the overload is significant it will be mainly resistive. So it is the resistive load rating, not the inductive one, that matters when considering the impact of a faulty coil on the contacts controlling it.

Coils really won't last long at significant overload before self-destructing. A relay that is supposed to draw 0.1A at 24V but has so much shorted out that it draws 1A, will not operate because there won't be enough coil remaning intact to produce enough magnetic flux. If left energised it is likely to burn out completely in a few minutes. It probably couldn't take 5A without the winding immediately fusing open at the leadout, so will never present that amount of overload to the control circuit.

Contact ratings include an absolute maximum current carrying capacity that is governed by their resistance and the resulting temperature rise. This thermal limit applies to any type of load even if the contacts never switch it. Then there is a resistive load rating for one of a number of switching duties, which is a lifetime-imposed rating. The higher the current, the more wear occurs to the contacts at each switching cycle, so for a given duty and expected life, the resistive load limit might be lower than the thermal current-carrying limit. Then the inductive load limit is lower still, also depending on the specified duty category, because of the greater erosion of the contacts cause by the arc dissipating energy stored by the inductance. There is a certain trade-off between current and voltage, higher currents might be permissible at lower voltages because of the lower energy in the arc. Always subject to the thermal limit of course.

Thank you for your in-depth knowledge really got the brain working. So when a relay says 230V 3amp inductive for its contacts we can in theory let them run at 6 amps at 110v right?

 

[Lucien Nunes]

You can't make that assumption, there is no direct relationship between current and voltage ratings of a given contact. In general it is true that the higher the voltage (up to the maximum rated voltage) the further the current must be reduced below the thermal rated maximum, to prevent the inductive discharge wearing the contacts out too quickly. But exactly what the limits are, and how the current / voltage tradeoff is made, is up to the manufacturer because it depends on the construction of the relay and especially the exact metal alloy used for the contacts. There are many different alloys in use for different applications - some give very low contact resistance, others are more wear-resistant, others are chosen for economy or ease of construction.

Also, one has to consider the switching duty. To inch the metal stock forward in our automatic cutoff saw, you jab at the inching button and a contactor puts the 3/4hp feed motor direct on line. You might nudge the button half a dozen times - that's half a dozen DOL starts and stops of an induction motor in just a few seconds. Each start and stop vaporises a tiny bit of metal and over time, those thousands of starts wear it away. The same contactor might find application in a shop lighting installation where it only switches on in the morning and off in the evening, but the load is long line of LED drivers with vicious inrush that nearly welds the contacts every time it closes, causing a different type of wear. Or, in a heating installation where again it only does one switching cycle a day and there's no inrush or inductive kick, but it runs at maximum thermal rating and although the contacts would last for centuries on the basis of wear, any rise in their resistance even for a short time would trigger rapid overheating and burnout.

Contact design and specification is an interesting subject that I tend to get involved in because I specify and use switches and relays for a wide range of applications, and when studying the history of electromechanical equipment one has to study contacts in detail because they are the Achilles heel of many systems.

 

[Darkwood]

@Lucien Nunes and that is why I love VSD's .... direct control limited current spikes and better inch control.