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Inductors
3

Inductors

Inductors

(OP)
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!

RE: Inductors

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).

RE: Inductors

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!

RE: Inductors

(OP)
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.

RE: Inductors

(OP)
???

RE: Inductors

> 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.

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

RE: Inductors

(OP)
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?

RE: Inductors

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.

RE: Inductors

(OP)
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?

RE: Inductors

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!

RE: Inductors

(OP)
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?

RE: Inductors

No.

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

RE: Inductors

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

RE: Inductors

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)' ?

RE: Inductors

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!

RE: Inductors

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.

RE: Inductors

> 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.

Quote (Panasonic - "Relay Technical Information")


https://www.panasonic-electric-works.com/pew/cz/do...
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)' ?

RE: Inductors

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!

RE: Inductors

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: motor stopping rapidly during a fault (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)' ?

RE: Inductors

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

RE: Inductors

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


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

RE: Inductors

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!

RE: Inductors

(OP)
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.

RE: Inductors

> 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)' ?

RE: Inductors

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

RE: Inductors

> 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)' ?

RE: Inductors

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!

RE: Inductors

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)


RE: Inductors

(OP)
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...

RE: Inductors

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)

RE: Inductors

(OP)
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?

RE: Inductors

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)

RE: Inductors

(OP)
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.

RE: Inductors

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)

RE: Inductors

(OP)
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?

RE: Inductors

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)


RE: Inductors

(OP)
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?

RE: Inductors

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!

RE: Inductors

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!

RE: Inductors

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

RE: Inductors

(OP)
This is me being naive now but how can you tell how much current a relay contact will be taking say for example this relay coil was being switched on by a relay contact rated for 3 amps

RE: Inductors

Check the spec sheet.
What I find: "Coil Power 1.2 VA"
1.2 VA at 110 Volts x 1000 = 10.9mA
Inrush limited by coil resistance of 3830 Ohms to less than 28.7 mA.
You don't suppose that you may be overthinking this?

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

RE: Inductors

Dear Mr. Jk1996 (Electrical)
1. There are numerous learned advice pertaining to AC contact rating and also pointed out to be cautious on contact rating on [DC switching].
2. Arc suppression is more difficult in d.c. than in a.c. To choose a d.c. contactor, it is necessary to know i) the current and the voltage to be broken and ii) the L/R time constant of the power circuit to be controlled.
3. Some "block type" d.c. contactor on the market designed for (d.c. switching) are fitted with arc chutes with permanent magnets [specially designed for d.c. breaking]. They can be 1,2,3 or 4-poles. The (coil voltage) can be designed for a.c. or d.c., differ from the [power circuit switching voltage].
4. e.g. a d.c. contactor rated 110Vdc,10A, switching with 1-pole on L/R < 1ms; is increased to 25A with 2-poles in series. In most cases, increase in number of poles in series increases the current rating, However, there can be cases that no current increment even connected 3 or 4-poles in series. On the other hand, the same d.c. contactor on 110Vdc is rated only 6A on 1-pole with L/R <2ms; and further lowered to 4A when L/R <7.5ms.
Che Kuan Yau (Singapore)

RE: Inductors

(OP)
Thank you guys waross I definitely have been overthinking it. Thanks Che

RE: Inductors

Contacts are rated according to VA product for breaking power because the motor contact opening instantly becomes a generator and that power turns in an arc burning the air between the contacts at over 5000'K. With AC the next current zero crossing might extinguish the flash, but with DC , it is sustained with a force proportional to current. Thus the gap of the contacts for DC determines the threshold for DC power that can be extinguished, and for reliability you want to be well below that curve.

An AC Relay when used with DC must use a flyback diode. For AC an RC Snubber is a good choice somewhat looks like a plastic Run cap, but with an embedded series R.

e.g. 6A "AC relay" rated for 1200 VA ac can only handle about 300 Watts DC Resistive breaking current.



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