Inductors 5

[Jk1996]

I have a question on inductors I’m trying to get my head round. I have a rough understanding how they work as the current through the coil produces a magnetic field and it’s the back EMF that limits the current through the inductor. I also understand with motors when you get a locked rotor the current draw goes up due to the back EMF not limiting the current. But my question is when it’s just an inductor such as a relay coil can these ever cause a similar overload to what we’d get with an induction motor when it has locked rotor?

The reason why I ask is I have seen relay contacts with the current of the contacts stated a 3amp inductive and 8amp resistive which then again got me thinking as the relay was bringing on one relay but is only being backed up by a 6amp fuse so my thinking was if it could overload the contacts would melt before the fuse?

Thanks for your help in advance guys!

 

[electricpete]

The relay contact continuous rating is irrelevant for most purposes:
[ul]
[li]from a relay standpoint, most relay contacts have continuous ratings at/above breaking rating.[/li]
[li]from an application standpoint, most relay contacts should not be used where they would see any current (continuous or otherwise) above their breaking rating[/li]
[/ul]

Assuming the above are true for your relay and application, the contact continuous rating wouldn't play any role in relay selection (ensuring the contact breaking rating is higher than highest expected contact current would automatically ensure that contact continuous rating is higher than highest expected current).
It's a generality. Maybe there are exceptions somewhere, but it applies to every control relay I know of.


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(2B)+(2B)' ?

 

[waross]

Usually the breaking rating predominates.
That is also why there wil be different AC and DC ratings on a relay. Despite the AC peak rating being higher than the RMS or DC equivalent rating, the zero crossings of an AC current are a significant help in interrupting current.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Jk1996]

Thanks again guys I really appreciate the in-depth answers. Found some details of a electronic level monitoring relay I see used a fair amount. Details state 5amp max breaking current doesn’t specify inductive or resistive but I take it this is what you mean we need to consider.

 

[electricpete]

> Maximum switching current 5 A AC/DC
> Maximum switching voltage 250 V AC/DC
> Switching capacity in VA 1250 VA [1250=5*250]

I'm not sure. The fact that it's 5A AC and 5A DC makes it sound more like it arose as a steady state heating limit. Either way there's only one current limit given on the sheet and the person applying it would not apply it to switch more than 5A or carry more than 5A.

I'm not sure what to make of that, maybe someone else will comment.

=====================================
(2B)+(2B)' ?

 

[davidbeach]

Most relay contacts used to trip breakers have a higher carry rating than the break rating. Making and carrying is what they do, breaking is done by the breaker 52A contact when the breaker opens. Not uncommon at all to have a carry rating higher than the break rating.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[electricpete]

> Most relay contacts used to trip breakers have a higher carry rating than the break rating

That's what I said before, including the Panasonic link.

What sounded strange to me was the example posted where the switching rating is given as 5A AC / DC (same for ac and dc), with nothing stated about the type of load interrupted. Aside from the fact that they call it a switching rating, it sounds suspiciously like steady state rating. (but either way there's only one current limit given on the sheet and the person applying it would not apply it to switch more than 5A nor carry more than 5A.)

=====================================
(2B)+(2B)' ?

 

[waross]

I had a set of destroyed relay contacts that I kept around for years as a novelty.
I was called out to look at a machine with the complaint that the electric brake was not engaging.
Each pole of the relay had two fixed contacts and a movable bridging contact. Two poles were wired in series so that the current was broken in four places.
This is not the same relay, but is the same type of relay:

The brake coil had both high inductance and high resistance.
The current was just a fraction of the rating, but it was not DC rated.
When the relay opened, the arcs were not extinguished. There were four, small but persistent arcs in series. The damage may have progressed over several contact openings. The relay did not fail immediately but lasted for a week or more.
The arcs melted small portions of the silver alloy contacts, which flowed together and extinguished the arc, but left the current flowing and the brake held off.
A couple of the shorted contacts were messy but one or two formed nice little silver alloy hourglass shapes.
There was very little heat damage to the plastic.
The steady state current was about 1A or 2A on a 20 Amp rated relay. However it was not capable of breaking a 120 Volt inductive DC current.
Th relay was replaced with a suitably DC rated relay.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

Dear Mr. Jk1996 (Electrical)
"...I have a question on inductors.... they work as the current through the coil produces a magnetic field and it’s the back EMF that limits the current through the inductor... But my question is when it’s just an inductor such as a relay coil can these ever cause a similar overload to what we’d get with an induction motor when it has locked rotor? ..."
1. I noticed that your question is not explicit. It resulted to valuable advice focused on "contactor" instead .I think what you wish to clarify is:
a) the contact current and voltage ratings. Its protection etc.,
b) the operation coil. How can it be over-loaded when connected with the rated voltage? How to protect the coil/control circuit? etc...
2. In the IEC world, IEC 60947-4-1 covers "contactor" and IEC 60947-5-1 on "contactor relay".
3. IEC 60947-4-1 covers:
a) i) the current ratings under AC-1..AC-8b, DC-1..DC-8, AC-12..AC-15 and DC-12..DC-14. That is, a contactor would be rated with different (current value) when used under different [utilization category],
ii) the (short-circuit) protection current rating for the contacts by fuse or breaker is dependent on the [Type-1 or Type-2] coordination.
b) An AC contactor or relay coil:
i)For example, a certain contactor with [average pull-in value] 800VA, but 44VA/15W [average holding]. Therefore, the (coil control) fuse or breaker must take this in consideration,
ii) the coil when connected to the rated voltage would be (over-loaded/over-heated) when the contact/magnetic iron gap [failed to close] due to any mechanical reasons or dirt/rust etc.
Che Kuan Yau (Singapore)

 

[Jk1996]

Yes che12345 I guess you put it clearer I guess them questions are what I was trying to get my head round.
1. I noticed that your question is not explicit. It resulted to valuable advice focused on "contactor" instead .I think what you wish to clarify is:
a) the contact current and voltage ratings. Its protection etc.,
b) the operation coil. How can it be over-loaded when connected with the rated voltage? How to protect the coil/control circuit? etc...

 

[che12345]

Dear Mr. Jk1996 (Electrical)
1. I am glad that my observation is what you are looking for.
2. My suggestion:
a) buy a copy of IEC 60947-4-1. All the technical terms are therein. (note: it is rather costly!),
b) borrow it from the public library in your location (FOC),
c) request the "contactor" catalogue from any contactor manufacturer agent/sales office in your location. Most of the information are in print therein (FOC).
3. You are welcome to raise any further questions, but please be "explicit".
Che Kuan Yau (Singapore)

 

[Jk1996]

So to clear things up if I had the previous level control relay contact pulling in a relay similar to the one here I shouldn’t have any concerns if the control circuit is fused at 6amps and my contact rating is 5amps (or even 3 amps) and only bringing in one relay would the only problem be when I start bringing big contractors into the equation? The relay contact should only be breaking a small amount of current but if we had large amounts over the 3 amps resistive that’s where we would have an issue. Am I right in saying the circuit fuse has no correlation with contact rating?

 

[che12345]

Dear Mr. Jk1996 (Electrical)the contact rating
" .... Am I right in saying the circuit fuse has no correlation with contact rating? ..."
1. No. You are Wrong !. The circuit fuse [shall] be rated i)suitable for the load and ii) to protect the contact; in order to achieve Type-1 or 2 coordination. It [shall] protect the contact from being blown into pieces=(fire); when a short-circuit occurred.
2. It is the (control circuit coil fuse) has [no correlation with contact or load rating].
3. Take note that the (circuit fuse) is a [separate/different] fuse from that of the (control circuit coil) fuse.
Che Kuan Yau (Singapore)

 

[Jk1996]

Maybe I’m getting my terminology wrong I’m on about the fuses protecting the control circuit.

So the control side of this circuit say a 6amp fuse and say K2 contact is rated for 3amps inductive.

 

[che12345]

Dear Mr. Jk1996 (Electrical)
"...Maybe I’m getting my terminology wrong I’m on about the fuses protecting the control circuit.. So the control side of this circuit say a 6amp fuse and say K2 contact is rated for 3amps inductive..."

Che. 1. There are/is:
a) three F1 shown in Fig.(a). These I name them (circuit) fuse which shall be rated for the load ... etc ., see my earlier post,
b) one F1 shown in Fig.(b). This I name it (control circuit coil fuse) which has [no correlation with contact or load rating]....see my earlier post.
2. Fig. 7.2 shows a [star-delta (SD)] starter. For a SD AC-3 starter, usually the contactors K1,3 are sized 0.58 Ie and K2 (star contactor) is sized 0.33 Ie , where Ie= motor rated current.
3. i) Fig.(a) 3off F1 shall take the (contact rating) of K1,2,3 into consideration,
ii) Fig. (b) 1off F1 has (no correlation with) K1,2,3 [contact rating].
Che Kuan Yau (Singapore)

 

[Jk1996]

So I’m right in saying the control
Circuit coil fuse is just protecting the control cables. So my 6amp control circuit coil fuse has no correlation with my 3amp contacts? The 3 amps is just the continuous current the contacts can take? How do we size the relay contacts accordingly?

 

[che12345]

Dear Mr. Jk1996 (Electrical)
"...i)So I’m right in saying the control Circuit coil fuse is just protecting the control cables. ii) So my 6amp control circuit coil fuse has no correlation with my 3amp contacts? iii)The 3 amps is just the continuous current the contacts can take? iV)How do we size the relay contacts accordingly?..."
Che. 1. i) correct. ii)correct. iii) correct.
iV) Refer to any manufacturer's catalogue on (star-delta) starter [contactor selection]. Look for the rated voltage, current/wattage rating, the table will show the recommended KM1,2, 3 sizing.
2. Attention:
i) the three fuses F1 shown in Fig.(a) shall be rated > motor full-load current rating,
ii) but, the (F2 thermal overload relay) shown in Fig.(a) [shall be set at 0.58 motor full-load rating].
Che Kuan Yau (Singapore)

 

[Jk1996]

Thank you che so my contact ratings are sized on the load they are making and breaking? I know the contacts I’ve seen for control relays are 3amps inductive so if I’m drawing 3 amps to pull on a coil the contacts are rated for this?

 

[waross]

Contacts rated for 3 Amps inductive will handle the inrush of a three Amp inductive load and will break the current to a three Amp inductive load.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

Note: If the relay contacts are not rated for an inductive load don't use the relay for inductive loads.
Note 2: There are exceptions, more for strong box solenoids and brake coils.
Even a relay rated for inductive loads may fail to interrupt current to highly inductive loads, particularly on DC.
Breaking is often more difficult than making. Consider snubbers or diodes across all inductive loads.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[davidbeach]

Or use something else to break the current. I doubt there’s a protective relay out there that can actually interrupt the current of a breaker trip coil with a normal output contact. But all they have to do is make and then hold on until the breaker ‘a’ contact clears the circuit.

Can’t pull an example out of the air, but I can imagine a situation where a given contact only needs to carry; something else does the make and a different something else does the break.

All three ratings actually matter, even if the vast majority are applied where they need to do all three functions.

I’ll see your silver lining and raise you two black clouds. - Protection Operations