small contactors with DC coils - stiction 2

[SilverfoxUK]

Gents

I've had cause to use a couple of small contactors with timer blocks fitted to the top. I'm talking here 3 phase contactors with the main contacts rated at about 10A. The operating coil on the contactor is 24Vdc.

At FAT with a bench supply of closer 27Vdc and enough amps to run an arc welder the contactors pulled in nicely. Onto the site where the supply was a little more delicate and at 22V with 2A available the contactors would not pull in. 22V at 2A is still a handful of power to operate a small contactor but without human aid it wasn't going anywhere. 22V is well within spec and I believe the contactors were 5VA so well in.

What we found was that there an amazing amount of stiction in the contactor operation where to operate the contactor you really had to push hard on the top of the armature to get it to pull in. Once in the armature would sit happily on the yoke and if required release.

We dumped this particular manufacturer and opted for another. Again some stiction but no where near as much as the one that came out of the green and white box.

Homework question - is there supposed to be a lot of stiction in a DC contactor.

 

[waross]

You know that there is an inverse square law in regards to magnetic attraction.
Another issue is the air gap. Two of the variables in a simple formula for magnetic attraction are magnetizing force per inch and the equivalent length of the magnetic circuit.
The equivalent length of the magnetic circuit takes into account that an inch of air gap equals about 50,000 inches of iron core.
When you combine the effects of the air gap with the inverse square law, you don't have much force to start the contactor armature moving.
Either of the contactors should have pulled in with a solid 22 Volts.
The timers may be causing the stiction but timer blocks are often arranged so that the armature pulls away from the operator and they follow the armatire due to an internal spring.
Hence, no added forces on the armature.
You may have already changed the wrong part.
Throw out that wimpy power source and use something better.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[SilverfoxUK]

Hi
This was down to stiction. We had 3 contactors from one manufacturer (green and white box) and the action was just really stiff. Appreciate your comments about magnetism and what not but this was without power.

Sadly the 24V was coming from an electronics card rather than a couple of truck batteries.

In all cases the action was dog rough.

Just wondered whether there was a reason.

 

[djs]

Most of the contactors I have seen with DC coils use two different coils. The first is a low resistance high current coils which starts the contactor closing, and then a switch on the contactor opens that coils and a high resistance coil is used to maintain the contactor closed. It may be that the power supply you are using in the field is current limited, and the low resistance coli is causing the power supply to drop voltage and limit the current.

 

[IRstuff]

There ought not be stiction, per se, but there should be quite a bit of spring resistance to work against, since the contactor must open by itself, when the coil is de-energized. The question, which you've not answered, is what is the specification for the coil's operating voltage and current, and whether you've supplied enough. Nevertheless, based on

http://www.farnell.com/datasheets/2043973.pdf

it would appear that your contactor isn't operating to specifications, since the cited datasheet for a presumably comparable contactor requires only 3W to energize, so less than 150 mA, at your supplied voltage. That same contactor shows a considerably shorter de-energizing time than energizing, so the coil power is essentially holding the spring at bay.

TTFN (ta ta for now)
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[jraef]

I would think also that the addition of the timer head is adding to the force required. Some timer heads are electronic, so the only force required is that of moving another contact spring. But some are pneumatic, in which case you are compressing a bellows. Generally when they use a pneumatic timer head, you are told that the voltage tolerance decreases and closing current increases.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[ScottyUK]

On the positives for the pneumatic timers, they are considerably more reliable than the electronic types over the long term. Maybe not as good in absolute accuracy and repeatability from a timing perspective, but will work in 20 years time. I think Schneider and maybe A-B are the last to offer them, ABB stopped making theirs a few years ago

 

[che12345]

Dear Mr SilverfoxUK

1. Those [dc voltage coil] operated 10A block contactors may be used for ac or dc load switching.
2. For a 24Vdc coil, the minimum pull-in voltage shall be not lower than 0.8x 24=19.2V. The pull-in power would be around 3W; where 3W/24V=0.125A.
3. " the 24V was coming from an electronics card rather than a couple of truck batteries"
3.1 measure the [voltage (across the coil before energisation] and take note of the [voltage (on energization),[note: measurements to be taken at site location as installed].
3.2 try it on a truck 24V battery. It should be [snap action].
4. contactor of this small size has only [one dc coil], not two coils with resistor.

Che Kuan Yau (Singapore)

 

[waross]

4. contactor of this small size has only [one dc coil], not two coils with resistor.

I agree with Mr. Yau
Another thing to check is if the timer blocks are installed correctly. Remove the timers and try the contactors.
If the contactors pull in without the timers then the timers are probably installed incorrectly.
Side mount auxiliary blocks are particularly bad for jamming the armature in some makes of contactors if they are not installed correctly.
In some designs there is a tab that is meant to engage a slot. If these are misaligned and forced together they will cause jamming.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Have you measured the actual output of the PLC output? Your supply is 22V, you're almost certainly going to drop a little more in the semiconductor output, the panel wiring, etc.

Many 24V PSU's have a small range of adjustment available - can you increase the 22V to, say, 25V or 26V? That should be well within spec for anything powered by 24V but sitting high allows for the little volt-drops through the panel.

 

[SilverfoxUK]

Gents

In answer to the ScottyUK the PLC output is a healthy 24Vdc. The 22V we measured was in the field at the contactor. The Voltage changed between 22V and 23V dependent on the whether we had helped the contactor to close or whether it was open. Still well above the required 17V.

Removing a variable with the timer head removed it is very difficult to push the plastic pips down at the top to manually operate the contactor. The timer made little difference.

Moving onto the contactor this is the 4th one we have tried and all exhibit the same horrible actuation where it sticks at the top. The contactor is a Schneider CAD32 BD.

The alternative contactor which is operating nicely still exhibits some stiction but not to the same degree.

The choice of timer head was the requirement to have a delayed off as required by the emergency shutdown to ensure an orderly shutdown. The delayed off scuppers most electronic timers.

I'm still at a loss as to why this stiction is present.

 

[che12345]

Dear Mr. SilverfoxUK

1. Remove the dc supply from the PLC output terminals. Replace the PLC dc source by a couple of truck 24V batteries. It should be snap action.
2. If above works fine, try connecting a capacitor say around 20uf across the PLC output to see whether
any improvement.
3. Measure the [total (wiring resistance) including any switching (contact resistance)] from PLC to the contactor at site as installed. The resistance should be lower than say 0.1 Ohm.

Che Kuan Yau (Singapore)

 

[IRstuff]

Is the PLC output that you mention specifically a relay output? What's the actual spec for the PLC output? Unless the output is specifically specified to be a relay output, it might have insufficient current capacity to drive any sort of coil. The fact that the voltage droops tells you that the output you are using is underpowered. Another option, if you don't have relay outputs, is to parallel two or three outputs together, with some small resistors for shorting protection and see if two or three outputs in parallel can drive your contactor.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[SilverfoxUK]

Forget putting power onto this contactor.

All 4 contactors I've tried have been very hard to push in for the first couple of millimetres. Once past the intial stage they are fairly easy to push.
The bench test had lots of amps and some healthy volts and pulled the contactors in. The PLC supply after volt drop measured in the field still has 22V and a potential 2 amps behind it. This doesn't droop below 22V with the armature stuck. All 4 contactors have had this poor action. Could have put it down to a bad batch by Schneider but the Siemens contactors are not a whole lot better hence the question. The siemens contactors do work quite happily in the field.
I haven't had the contactors apart so original manufacture rather than me misaligning something.

 

[waross]

We are taking your word for it that the contactor has a 24 VDC coil, but have you checked the coil?
If you are ordering 24 Volt coils you may be getting AC coils instead of DC coils.
There is a difference.
A higher voltage AC coil may behave as you describe.
If someone in the order/supply chain is shortening your part number from CAD32 BD to CAD32 you will have a 230 Volt AC coil.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[3DDave]

If there is resistance for a couple of millimeters, that's not stiction. Stiction would be if they did not move at all and suddenly released. This sounds more like a detent is resisting motion.

 

[che12345]

Dear Mr SilverfoxUK

1. " The [bench test] had lots of amps and some healthy volts and pulled the contactors in"
This is evident that the contactor and the coil are in order. The problem could be the PLC capacity or the wiring resistance on the way from PLC to the contactor at site.
1.1 Try installing a 20uf capacitor across the PLC output and see whether any improvement.
2. " The PLC supply after volt drop measured in the field still has 22V and a potential 2 amps behind it. This doesn't droop below 22V with the armature stuck " If the measurement is carried out (at site) with a [digital] multi-meter, it may not respond fast enough? to catch the voltage drip.
3. FYI: It is unlikely that the [dc coil] being wrongly replaced by an [ac coil]. A (dc coil) would have [far higher winding resistance] than an (ac coil) of the same voltage rating. Note: an ac coil will draw very high pull-in (AV) while a dc coil needs low pull-in power (W).

Che Kuan Yau (Singapore)

 

[waross]

Note: an ac coil will draw very high pull-in (AV) while a dc coil needs low pull-in power (W).

More accurately: A coil with AC applied will draw very high pull-in current while a coil with DC applied will not have any inrush or extra current when energized.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

First; can we stop calling this RELAY a contactor.. It is NOT a contactor.

Second; Stop with the stiction. It's not stiction as 3DDave has explained.

Third; These things work. I've used dozens of all these relays old ones and new ones, Schnieders, Square D, Metasol, Fuji, and I have never had them fail to pull in when an appropriate supply is driving them. Your PLC has a problem supplying what they need. Perhaps the outputs are failing contacts or zorched transistors with reduced capacity.

Relay out PLC contacts crap-out driving relays due to the arcing they're subjected too while interrupting inductors.
Transistor PLC outputs also crap-out due to the dv/dt hammering they get from inductors.
For any kind of reliability you need to protect both kind from relay coils. Typically I use a flyback diode in DC applications. All DC relays that I run with PLCs get diodes. I've had enough 6 month output failures!

I would install a diode and then test your relay for pull-in directly across the power supply that supplies the PLC outputs (almost always different from the power supply running the PLC). If it/they pull-in then install single pole pilot relays relays to feed your big relay(s) directly with the supply. If they don't pull in across the power supply then you need to install a better suit one. Preferably an unregulated one with honking capacitor.

Hi che12345; I'm not sure I'd want to put a cap of uF size across a PLC output as it looks like a short initially.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[che12345]

Dear All,

1. " First; can we stop calling this RELAY a contactor.. It is NOT a contactor "
Many of these 10A 3-4 pole "block contactor" having AC3 DC3 600V ratings which are used as a [contactor] with (double breaks) for switching small ac 3-phase or dc motor loads. Usually DIN-rail or fixed mounting but also plug-in.
1.1 There are others 10A 3-4 pole "cube relays" with usually (single break) used as a "relay", not intended for motor load switching. These are usually plug-in type.
2. " Hi che12345; I'm not sure I'd want to put a cap of uF size across a PLC output as it looks like a short initially ". The PLC out-put power should have sufficient capacity to charge up the capacitor. This [external capacitor] is to assist the internal capacitors (if any?), to maintain the voltage during switching surge.
3. " Typically I use a flyback diode in DC applications ...." This flyback diode is to prevent the high back emf generated by the inductor (coil) on opening the circuit. It is irrelevant or would not solve the poor pull-in problem.
Noted: Mr. itsmoked did not mention that flyback diode will assist to pull-in.

Che Kuan Yau (Singapore)