(OT) Need inductive load

Why does the load need to be inductive?

Use the run winding of an ordinary induction motor. I just measured the inductance of a 1/3 HP 110-volt Dayton motor to be about 5.4 mH. You might need a bit of series resistance to keep the DC current down to 12 amps. The equivalent circuit for any inductive load is a pure inductor in series with it's own resistance, so adding series resistance to get the current you want is a valid approach.

?????

But when working with DC, other than "moving" stuff (Motors, speakers, relays - something that "taps" energy from the magnetic field in some manner, rather than leaving it stand there idle) an inductor is effectively nothing but a dead short. That's one of the reasons transformers are worthless for changing voltage up or down in DC systems. Or so I've been led to believe my for entire "electronics life"... ?

If enough energy is being pumped into it, heat starts building, and in surprisingly (to those who haven't seen it happen before...) short order, the whole shebang has transformed itself into a (puddle of melted)/(cloud of vaporized) conductor, an open circuit, and an optional, variably sized, cloud of smoke.

You've succeeded in confusing me...

Should I be reading that last quoted sentence of yours as saying that an inductive load is the same thing as a coil and resistor (of arbitrary value) wired in series, or are you saying that an inductive load is the same thing as a coil wired to a resistor matching the resistance of the coil? I'm definitely hazy as to which was your intent. Your (seeming) misuse of "it's" doesn't help clarify things any...

(Hint: "its" = "belonging to it" - "it's" = "it is")

Just a quick back-of-an-envelop calculation shows you'd need about 500 feet of 16 gauge wire for a dc resistance of 2.3 ohms in your coil. I'd not want to use anything less than

16 gauge and even then for just a short time. It's going to have to dissipate 336 watts once the core is saturated.

That's a big coil, either to wind or to find in an existing device.

Could you tell us why you need to do this. We might be able to come up with a better approach.

I might try a microwave transformer from a junk oven, the secondary might be close enough or you could series/parallel primaries/secondaries from several to get what you want. Very inductive and will stand a lot of watts for a short time, and cheap.

Real coils have real resistance, some more than others. Aside from superconductivity, there's no such thing as a "dead short". Coils wound with many turns of fine wire, as used in DC solenoid valves and relays, have considerable resistance.

That's why I suggested adding series resistance as necessary to make the steady-state current be 12 amps when excited with 28 VDC. A 1/3 HP 110-volt induction motor can certainly take 12 amps of DC with none of the dramatic effects you describe.

You're right, Don's error. I shouldn't have typed an apostrophe in "it's". I intended posessive case, not contraction of "it is".

In your paragraph above, the second statement is right. An inductive load may be viewed as being comprised of a pure inductance (no resistance) in series with a resistor whose value is the DC resistance of the coil, admittedly neglecting AC effects like eddycurrent loss and hysteresis loss.

The induction motor winding will have some resistance. The rest can be made up with 1/16" dia stainless TIG rod. I measured one such 36" piece to be about 0.313 ohms. I don't think it'd overheat at 12 amps. It's probably settle out at about 315C. .

I knew I was too close to the problem, and that there had to be a simple solution. Your solution is likely what we will use.

As for the why, we are testing a switch to ensure it won't set off an explosive atmosphere. The test method isn't too clear other than we need to test the switch at maximum current into an inductive load. Rather than try to justify why this really means "a partially inductive load", we'd rather worst-case the test conditions.

#! rnews 1609 Xref: xyzzy rec.crafts.metalworking:615226 Newsgroups: rec.crafts.metalworking Path: xyzzy!nntp From: "Ken Finney" Subject: Re: 17th Century Biodiesel? X-Nntp-Posting-Host: e244847.nw.nos.boeing.com Message-ID: X-Mimeole: Produced By Microsoft MimeOLE V6.00.2800.1441 X-Priority: 3 X-Msmail-Priority: Normal Lines: 27 Sender: snipped-for-privacy@news.boeing.com (Boeing NNTP News Access) Organization: The Boeing Company X-Newsreader: Microsoft Outlook Express 6.00.2800.1437 References: Date: Wed, 30 Mar 2005 23:34:21 GMT

I believe Rudolph Diesel used peanut oil when he developed his engine.

Unless I missed something, Ken.....

I'm sitting here scratching my head wondering how the thread got this far along without anyone asking you if you've got any idea what value of inductance you need to create the right test setup and how much saturation of that inductor would be acceptable at 12 amps dc.

Without that information, I can't imagine how you can expect to run a very meaningful test.

If you'd tell us more about what the thing you are testing has to do, and what kind of inductive device it normally controls, there's lot's of guys here who could give you a better handle on what to use.

Jeff

Now, even more than I posted to this thread an hour ago, I think you really need to find out just what inductance the authority governing the certification on your "switch" requires for that test, and also the composition and temperature/pressure conditions of the "explosive atmosphere".

It sounds like a job for UL, Factory Mutual or CSA to me.

Years ago I used to design "intrinsically safe" monitoring circuits for use in hazardous locations like the inside of gasoline storage tanks and had to get my stuff certified by actual tests at those three agencies.

They had pretty involved standards and procedures for testing such things and used carefully controlled mixtures of gasses and air they tried to get our stuff to set off. The same kind of gas test cells along with standardized loads were used for testing "explosion proof" equipment such as switches.

So forgive me if I'm misunderstanding your requirements, but the answer to your question may not be as simple as you hope it is, particularly in today's litigeous world.

Jeff

Inductors as a load *is* sort of a non-sequiter. They don't absorb energy, at least in the steady state.

Switching a large current into an inductor won't absorb any energy, to start. The current flow is zero at the instant the switch is thrown.

I'm also at a bit of a loss as to the application.

Jim

I am curious if a capacitor run in series with a resistor, would constitute an inductive load. I have a few high voltage 10,000 mf capacitors and wonder if they could be used for that purpose.

I am quite rusty in electrodynamics and will appreciate some clarification.

i

The flaw with that theory is that while a "real inductor" is indeed inductance in series with the resistance of the winding, putting a resistor external to the winding takes away from the amount of inductance you would have if that was all in the coil -- taken to the extreme, you could say 1 turn of wire and a 2.3 ohm resistor is the same as the 500 feet of 16 gauge wire -- from a DC viewpoint, that would be true, but for the purposes of an inductive load with a changing field, they are definitely NOT the same circuit. Of course it also depends if it is wound on an iron core or air core or ... Your best bet to meet your requirements as stated would probably be to find a large transformer with 2-2.5 ohms dc resistance and use that. There is still the part about liability that you need to make sure you are covered there -- as another poster said, probably UL, or others.

mikey

I'm wondering whether you may have it BACKWARDS. Could it be that they want you to run 12 amps through an inductive load, and then OPEN the circuit? That condition will tend to make the switch arc, which could set off an explosive atmosphere. If that's the case, the amount of inductance needs to be specified, since this will determine the amount of arcing.

snip

Your original post didn't turn up on my server so I may have missed some of the earlier comments.

For specifying switching conditions an inductive load can be accurately defined as a pure inductance in series with a resistance.

On switching "break" or contact bounce the inductive stored energy release is 1/2 L x I squared. This means that energy storage is directly proportional to inductance so that worst case testing requires an infinite inductance !!

Testing is meaningless unless the value of the inductance of the 12A load is specified or switching is tested with a worst case sample of the device(s) that are to be switched.

If you need a larger design margin it's better to test at increased current (energy is I squared) rather than mess about with non representative inductive components.

Jim

No.

Caps and coils are two *COMPLETELY* different animals that *CAN* achieve a similar result *UNDER SOME CIRCUMSTANCES*, but are a long, long, LONG way from being interchangable with each other.

I specified a lumped inductance able to handle 12 amps without saturating: an induction motor. Whatever the value of L is, the circuit here must have 2.3 ohms of resistance for steady-state current to be the specified 12 amps at 28 volts. It doesn't matter where this resistance is located in the circuit., and adding series R does not diminish inductance.

I'm not making this up. You'll find it in any first course in AC circuits for undergraduate EE candidates.

If you're dealing with explosive atmospheres, you'd better hire someone who knows what they're doing to do the testing and certify the results -- or perhaps tell you to walk away from this one.

12 amps and 28 volts are way far out of intrinsically safe territory with any switch or other device.

Please have a look at figure four in the following, which addresses circuits with no inductance but only resistance:

I strongly agree with Jeff Wisnia here. You should definitely get competent professional counsel rather than rely on opinions from guys on a newsgroup who are not accountable if/when your switch blows something up.

Bob

I just read your post below and missed the original post by Ken. If ken is looking for info on the current draw of a 24 volt starter solenoid, he has to identify which of the two winding in the solenoid he wants to know the current draw of. The "over the road, 18 wheeler" type starter solenoids are *not* difficult to get alot of data on. The diesel starter (Delco 40 MT type) will draw alot more current from the start button than a Chevy starter (Delco 10 MT).

I wonder how difficult it would be to locate an old 12 volt alternator rotor that draws about 6 amps field current with 12 volts applied.. I think an old (1980) Chevy alternator draws about 6 or 8 amps That would probably handle 24 volts for a while, depending on how well it gets cooled.

I sure wouldnt use a truck starter solenoid as an inductive load. They have two windings with the same number of turns but much different wire size. If Ken wants to phone Marks One, in Phoenix Arizona, he can get all the automotive solenoid current draw info he'll ever need. Or Ken can contact me. I do alot of work with Marks One (solenoid builders)

Jerry