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Ian B
12-27-2013, 09:46 AM
I'm making a control circuit for a 3 phase motor. I want to start & stop the motor using a Telemechanique LC1 D1210 contactor. It's fitted with a 24V 50/60Hz AC actuating coil (the '12' in the part number relates to the main contactor's rating of 12A, not the coil's operating voltage).

I have 24V DC available in the control panel, and I'm trying to use this to operate the contactor. I checked the coil's resistance, it was 6.6 ohms.

I thought that I'd need to drop the voltage across the coil by 0.7071 down to 17V, to account for the additional heating value of DC over AC. I did this by putting a 3.3 ohm, 30W resistor in series with the coil, and also by dropping the supply to 20V DC. In this configuration, I'm seeing 15V across the coil, and 5V across the resistor, about what I expected.

The contactor closes when the 15V is applied. However, after about 3 minutes, the coil begins to overheat and smoke. I changed to another (identical) contactor, same problem.

Anyone have any ideas on how I can fix it, without resorting to using 24V AC, or is it a no-hoper?

Thanks,

Ian

jlevie
12-27-2013, 10:00 AM
I think You will have to use 24vac to control the contactor. That or get a contactor that has a DC coil.

RandyZ
12-27-2013, 10:33 AM
You might be on the right track. If you can, vary the dc voltage to the point that the contactor just picks up. measure the current and then re-size you resister to allow for pick up current + 50%. This was our method at work for setting relay currents.
I'm thinking that your still way too high if the coil is over heating.

Doozer
12-27-2013, 10:42 AM
AC solenoids have a shaded pole.
Not sure if that affects putting DC in them.

--Doozer

Boucher
12-27-2013, 11:18 AM
If you have 24 V DC in the control panel you probably have 24 V AC also. Over here 24 V AC transformers are used in irrigation controls and are available as the plug in ( wall wart)
Three phase contactors normally have four sets of main contacts where the fourth set is used for the manual control. Using the correct contactor is generally better than the work arounds like you are attempting.

Ian B
12-27-2013, 11:31 AM
Thanks all,

I tried dropping the voltage even further, to the point where the contactor only just latched - the coil still overheated! At this point, I admitted defeat and switched to using a plain old 24 VAC output transformer, and- of course - it now all works fine.

I believe the problem lies in the difference between DC resistance of the coil, and AC impedance.

Byron, yes, this contactor has 4 sets of contacts - I'm using the 4th pair in parallel to the (normally open) start button, to hold the contactor on. The panel's completely homebrew, the only 24V supply was a bought for the job switching power supply, which was also being used to feed a DC step-down board to actuate some relays.

MaxHeadRoom
12-27-2013, 11:57 AM
Not at all practical to supply DC to an AC contactor coil, it relies on the inductive reactance produced by the AC.
It would be much easier to get a small transformer or swap the coil for a DC type.
Max.

lakeside53
12-27-2013, 12:12 PM
I'd just change the coil to the correct type. Most are available on Ebay for few $, but you'll need to figure you the part number and dig a little. You can of course just get them from an electrical supply house.

Jaakko Fagerlund
12-27-2013, 02:23 PM
Not worth the hassle, as a new contactor costs less than 10 EUR in almost whatever config you want.

MaxHeadRoom
12-27-2013, 05:26 PM
Not worth the hassle, as a new contactor costs less than 10 EUR in almost whatever config you want.

Current list price for Telemecanique LC1 D1210 is $330 although they are fairly cheap off eBAY.
MAX.

PStechPaul
12-27-2013, 06:44 PM
A test set I designed uses 24 VAC contactors on 24 VDC. AC coils have a low inductance when the magnetic gap is open, so the current at 24 VAC is much higher, as it needs to be to generate enough magnetic force to close the contactor. When it is closed, much less current is needed, and the higher inductance of the closed magnetic circuit provides this. Typically there is about 3:1 ratio of inductance from open to closed, so the current will drop from perhaps 1 amp to 330 mA or so.

What I did was add a capacitor and a resistor in parallel, in series with the relay coil and the 24 VDC control signal. When the voltage is first applied, the capacitor acts as a short circuit and full 24 VDC is applied to the coil, causing it to pull in strongly. The relay closes in approximately 30 mSec, so the capacitor is chosen so that the RC time constant with the contactor coil resistance is about that. You have 6.6 ohms, so:


TC=RC, C=TC/R, C=30/6.6=4500 uF

The resistor is chosen to supply the current needed to hold the relay in. This is typically about 20% of pull-in current, so you can use a resistor about 5 times the coil resistance. So 6.6*5=33 ohms, which will provide about 24/40 = 600mA at 24/6 = 4V or 2.4 watts. The resistor will dissipate 18*0.6 = 10.8 watts, so it should be 15-20 watts.

I have also designed a PWM circuit which applies full 24 VDC to the relay coil for about 50 mSec and then uses about 20% PWM to provide the holding current. This is much more efficient and virtually all the power is in the relay coil and the series MOSFET power is less than 1 watt. I can supply circuit details and code for the PIC if you are interested.

Some modern DC contactors have such "economizer" circuits built-in. Other contactors use an extra NC pole with delayed opening and an economizer resistor that is applied to the coil once it has pulled in.

J Tiers
12-28-2013, 11:04 PM
That is pretty much the deal.

You need to supply some other way to cut the DC current down to be equivalent in "pull" to what the AC impedance would reduce AC current to.

The R and C is commonly used with DC relays and contactors also, for similar economizing reasons.

One thing to be careful about is that the current rating of relays and contactors may be affected by the coil current and resulting magnetic pull. Weaker pull than is expected can reduce the rating.

This may be a matter of the aggressiveness of the "wipe" of the contacts when pulled-in. It can also be due to the contacting force itself, affecting the resistance of the contact.

Jaakko Fagerlund
12-29-2013, 03:21 AM
Current list price for Telemecanique LC1 D1210 is $330 although they are fairly cheap off eBAY.
MAX.
Just checked with Farnell and that particular Telemechanique contactor runs at 33 EUR give or take, not hundreds. But why on earth would one buy such an expensive contactor that is bulky too, when for example AEG mini contactors provide the same cheaper and in a smaller package.

MaxHeadRoom
12-29-2013, 10:12 AM
Just checked with Farnell and that particular Telemechanique contactor runs at 33 EUR give or take, not hundreds. But why on earth would one buy such an expensive contactor that is bulky too, when for example AEG mini contactors provide the same cheaper and in a smaller package.

I am involved in industrial design so I need something reliable, but the added attraction is the add on options that Telemecanique has both solid state and non.
Aux contacts, timers, over loads, coil snubbers etc.
The DIN style they offer I do not consider bulky compared to the pre DIN versions.
Max

Forrest Addy
12-29-2013, 11:55 AM
OK you have a 24 VDC power supply and a 24VAC starter coil. Small 24 volt cotrol trasformers are cheap, readily available and would exactly suit your application. So why not add one in the control enclosure?

Back in the day of my youthful basement shop experimentation (I used to make killer electro-magnets from old tube TV power supply stransformers) I found that I could apply X/4 VDC to an X VAC rated coil and it would perform about the same, pull-in with the same vigor, draw about the same magnetizing current etc. So shoot for that. I suggest you size your dropping resistor for 18 VDC across it to yield 6 VDC on the AC rated starter coil.

J Tiers
12-29-2013, 12:01 PM
Just checked with Farnell and that particular Telemechanique contactor runs at 33 EUR give or take, not hundreds. But why on earth would one buy such an expensive contactor that is bulky too, when for example AEG mini contactors provide the same cheaper and in a smaller package.

The ratings may be entirely different.

ALL IEC contactors are somewhat "definite purpose"..... they are rated for one or more of the several dozen different applications... and are not going to last in a different one. You really need an engineer to "design" each application for a target lifetime, if you do what is expected.

Folks can laugh at a NEMA contactor, but you put it in within its easily understood rating, and it will just work, and keep working,.

Jaakko Fagerlund
12-29-2013, 01:22 PM
The ratings may be entirely different.

ALL IEC contactors are somewhat "definite purpose"..... they are rated for one or more of the several dozen different applications... and are not going to last in a different one. You really need an engineer to "design" each application for a target lifetime, if you do what is expected.

Folks can laugh at a NEMA contactor, but you put it in within its easily understood rating, and it will just work, and keep working,.
As I said, the exact same model number. Or if you were referring to my AEG suggestion, I'm still standing behind it. It is a motor starter/stopper circuitry, nothing fancy, so anything with the rated power/voltage/current will work.

J Tiers
12-29-2013, 03:40 PM
As I said, the exact same model number. Or if you were referring to my AEG suggestion, I'm still standing behind it. It is a motor starter/stopper circuitry, nothing fancy, so anything with the rated power/voltage/current will work.

If same model number, obviously no difference in rating, we presume.....

Sources vary.... if you were to look at the list price for many electrical items, it could be several grand for a breaker box. Your contractor will get the same thing for a couple hundred. The list pricing is completely stupid on many electrical items.

And, in different places items may be exotic, or common. Exotic costs more. You would pay huge money for a NEMA contactor, if you wanted one, but we may pay more for an IEC type.

Jaakko Fagerlund
12-29-2013, 05:15 PM
If same model number, obviously no difference in rating, we presume.....

Sources vary.... if you were to look at the list price for many electrical items, it could be several grand for a breaker box. Your contractor will get the same thing for a couple hundred. The list pricing is completely stupid on many electrical items.

And, in different places items may be exotic, or common. Exotic costs more. You would pay huge money for a NEMA contactor, if you wanted one, but we may pay more for an IEC type.
So true every word. Especially when you are just buying one of something, the prices can be multiplied by pi.