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!

 

[BrianE22]

Contacts for relays are de-rated for inductive loads because the inductance will create a large EMF across the contacts when they start to open (as the current quickly goes to 0 amps).

 

[waross]

You may be confusing two effects here.
One effect is the derating of the contacts for an inductive load. When the contacts open in an inductive circuit, the voltage across the contacts may go extremely high. The greater the current, the higher the voltage transient.
A second effect is pulling in the contactor coil. The current is limited by the impedance of the coil. The greater the air gap in a magnetic circuit, the lower the impedance. When the armature or clapper of a contactor or relay is at rest there is a *fairly large air gap and a corresponding low impedance. When the contactor "pulls in" much of the air gap is eliminated, the impedance increases and the current drops.
*fairly large air gap: With no air gap there is a tendancy for residual magnetism to "stick" a relay armature, so that it does not drop out. Given the inverse square relationship between magnetic force and air gap distance, it does not take much air gap to mitigate any residual magnetism. Early contactors used a brass rivet in one of the pole faces to create a small air gap. Early practical Engineering texts discussed this and warned that in time the rivet could be flattened or broken and must be replaced. A sticking contactor was often an indication of a faulty brass rivet. Then someone realized that an air gap anywhere in a magnetic circuit would reduce the flux density in the entire magnetic circuit. The air gap was moved away from the moving armature and often placed at the back of the core and at right angles to the direction of impact of the armature. A close examination of some contactors will reveal a thin line of non-magnetic material in the back part of the core.
Try it at home: With a horseshoe magnet and a keeper, place one or more pieces of paper between one magnet pole and the keeper, so that one side of the keeper contacts the magnet pole and the other side of the keeper is separated from the pole by the paper spacers.
You should find that the force required to lift the keeper from the magnet pole is reduced on both sides, that is, on the side with a gap and on the side without a gap.


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

 

[Jk1996]

Thanks guys so I understand resistive loads can’t cause overloads only faults can cause excessive current to flow but with just an inductive coil going being brought on through a relay contact cause overload. So say Under normal conditions the coil of the relay draws under and amp and the relay contact is rated 3amp but the whole circuit is protected by a 6 amp is this a problem as the rating of the contact is below 6 amps? Thanks again guys.

 

[Jk1996]

???

 

[electricpete]

> Thanks guys so I understand resistive loads can’t cause overloads only faults can cause excessive current to flow but with just an inductive coil going being brought on through a relay contact cause overload.

That's not really what they said. It has nothing to do with overload or load-carrying ability. The difference between inductive and resistive loads is how they challenge the contacts during interruption.

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(2B)+(2B)' ?

 

[Jk1996]

Yes I understand that sorry should be more to the point with my question. Is having a 6amp fuse in a control circuit where there is a relay contact rated at 3amps wrong? The relay contact is only bringing another relay in 110v control circuit?

 

[BrianE22]

JK -

I think what you are asking is, for an inductive load (another relay coil) is there a good reason to use a fuse rated higher (than the first relay's contact rating) than the contact rating (6 Amps fuse versus 3 Amps contacts)? Is that the info you are looking for?

If so, I would say no. If the current runs high then the contact life will be shortened. But, as Waross talked about above, the impedance of the second relay's coil will be low on starting which might draw more than the coil's rated amps for a very short time (milliseconds I'd guess). So maybe, if the second relay's coil is rated 3 amps, it might draw something greater than 3 amps for a very short time. I would think a fuse rated 3 amps would have no problem with that.

 

[Jk1996]

Thank you Brian yeah that’s what I was trying to get at sorry it was so unclear. So are you saying there is an issue using a 6 amp fuse protecting that 3 amp contact?

 

[waross]

Your concepts may be badly mixed. The point is not about induction as much as about inductive reactance.
The inductive reactance of a relay coil may change by a large factor between the de-energized state and the energized state.
Will the 6 Amp fuse hold the energization surge of the relay?
That depends.
Will the initial current be more than 3 Amps.
Yes.
How much more?
That depends.
On what factors does it depend?
Roughly on the ratio of the 'at rest' air gap and the energized air gap.
What type of fuse is used.
A fast acting fuse will probably clear. It depends.
A code fuse may hold in, and it may hold in for quite a large number of operations and then fail. It depends.
A dual element fuse AKA Slo-Blow fuse will hold in for unlimited operations.

And even the experts don't always get it right.
I remember a brand new GE motor starter with an integral control transformer.
This was used on an HVAC motor that started automatically.
When the brand new starter was energized, the 2 Amp control fuse blew. No problem, must be a faulty fuse. After two more fuses blew, it became apparent that GE had screwed up on their fuse sizing. A 4 Amp fuse worked well.

Back to fusing. Contrary to popular belief, the fuse will not protect the downstream relay*. If the relay fails it fails. The fuse protects the wiring and components ahead of any failure. What gauge of wire are you using and what amperage of fuse may be safely used for that gauge wire?

*In a motor circuit, the fuse or breaker does not protect the motor. They protect the wiring in the event that the motor fails.
The overload relay protects the motor.

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

 

[Jk1996]

Okay speaking in general terms I’m trying to keep it fairly simple as my electrical knowledge is not as in-depth as you guys. I know I’m basic circuit design we rate the fuse to protect the cable. I guess the question I’m trying to ask is would a bigger fuse rated for the cable be a problem if the contact isn’t the same rating? Will it reduce the life of the contact or overheat and cause a fire?

 

[waross]

No.


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

 

[Jk1996]

Thank you waross. What would I have to consider when selecting contact current then?

 

[electricpete]

As far as I know, contacts are rated by their interrupting capacity.
I assume their steady state current carrying capacity is somewhere higher than that, but you wouldn't want to carry more than that because it would challenge the contacts if they happened to open.

So the contacts should have an interrupting rating higher than the current expected in the circuit at any time.
The contact rating often includes qualifiers as you noted resistive, inductive also sometimes ac or dc because these aspects affect the ability of the contact to interrupt current.

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

 

[waross]

What Pete said.
I remember a coupling exploding on a 400 HP wound rotor motor direct drive fan.
The electrical foreman questioned the engineer about the wisdom of using a 250 HP rated starter for a 400 HP motor.
The reply;
"I have a Phd in engineering,and we have tested this contactor under load and found that it is capable of carrying the current without overheating. Furthermore, sir, you should know youe place and stay in your place.'
The motor ran for a few days, until the day that the contactor flashed over, phase to phase, interrupting full load motor current.
At this point, the motor became an induction generator feeding into a short circuit. With the wound rotor, the motor circuit had more resistance than an induction motor and generated more kW. Feeding the kW into a short circuit, the motor decelerated violently.
The one inch key rolled out of the keyway and split the coupling. Parts were found in all corners of the room. The motor shaft was bent as was the fan shaft.
The electrician foreman's place became the leader of a crew working on overtime over a long weekend replacing the motor and controller, while a crew of millwrights replaced the fan.

What Pete said. The interrupting ability is often more important than the carrying capacity.
If the 6 Amp fuse blows, use a higher rating, up to the capacity of the conductors.


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

 

[BrianE22]

I'd also make sure the coil on your second relay is protected. Higher than rated amps (for the coil) for too long a period will heat up the coil.

There is situation where the steady state current is the determining factor for the contacts. If the contacts are closed for a relatively long period of time then the over current passing through the contact resistance can raise the temperature of the contacts.

 

[electricpete]

> There is situation where the steady state current is the determining factor for the contacts

There could be but it wouldn't be common as far as I know.

https://www.panasonic-electric-works.com/pew/cz/downloads/technical_information_relay_en.pdf

11. Maximum Carrying Current
The maximum current which after closing or prior to opening, the contacts can safely pass without being subject to temperature rise in excess of their design limit, or the design limit of other temperature sensitive components in the relay (coil, springs, insulation, etc.). This value is usually in excess of the maximum switching current. [emphasis added]

If a relay contact was intended to carry current in excess of its switching limit, there would have to be provisions in the surrounding circuit to ensure the relay did not change state to cause contact opening while carrying that high current. Maybe there are some oddball relays built to suit oddball applications like this somewhere.

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

 

[waross]

See my anecdote about the engineer who sized a contactor based on contact heating and ignored the breaking capacity of the device.


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

 

[electricpete]

I enjoyed your anecdote Bill. It sounds like a good thing to avoid.

I had to think for a moment about how the motor stopped so abruptly that it damaged the coupling. It sounds strange at first, but I can believe it, the fault current can act like dc dynamic braking on steroids. We had a similar experience where motor stopped rapidly during a fault described here thread237-67490 (although the circumstances of the fault aren't entirely understood) The coupling was a rigid coupling which screwed together. The direction of the screw threads was such that normal-direction torque tightens the coupling assembly. But the coupling unscrewed during the fault as the motor stopped and the compressor inertia tried to keep going (reversing the direction of torque in the coupling). To op - my anecdote has nothing to do with your question (unlike Bill's)... just an interesting thing to talk about. It's also a walk down memory lane to go back thru that thread with responses from busbar, shortstub, peebee, DanDel, GusD, rbulsara.. all great contributors that have moved on from eng-tips for one reason or another. (btw does anyone know why busbar left?)

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

 

[Jk1996]

So the contact rating is more to do with the breaking ability of the contact rather than the continuous rating?