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.

 

[jraef]

I still think that what you are describing as "stiction" in the first few mm is the added mechanical burden of pulling in the bellows of the pneumatic timer head.


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

 

[waross]

Jeff; most all of the timer heads that I have seen were not directly connected to the armature. The armature pulled away from the bellows and the bellows then followed at its own speed driven by its own spring.
When the armature dropped out, it compressed a spring which pushed the bellows up and the timing started.

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

 

[che12345]

Dear All,
1. Mr. waross :
"... most all of the timer heads that I have seen were not directly connected to the armature. The armature pulled away from the bellows and the bellows then followed at its own speed driven by its own spring.
When the armature dropped out, it compressed a spring which pushed the bellows up and the timing
started. "
1.1 There could be different designs/working principles. The basic [pneumatic timer block] on the market:
1.2 it is [clipped] onto the top of the (block contactor).
1.2.1 it is [hocked] onto the contactor (plunger) which is [mechanically linked] to the (moving armature).
1.3 there are two different types, Type a) time delay on energization or Type b) time delay on de-energization.
1.3.1 for Type a), the timer block contacts [do NOT change status] when the contactor is de-energized.
When the contactor is energized, the timer contacts [change status] after set time delay.
1.3.2 for Type b), the timer block contacts [do NOT change status] when the contactor is energized.
When the contactor is de-energized, the timer contacts [change status] after set time delay.

Che Kuan Yau (Singapore)

 

[waross]

Those relays/contactors have been produced in the thousands without operational issues. Likewise the timing head add-ons.
(In that size and with that construction, the naming is somewhat dependent on the application.)
Given the number of problematic devices that you have encountered, re-check the rated voltage of the coil. You may be consistently getting devices with the wrong voltage coil installed.
By the way, the minimum coil operating voltage is 16.8 Volts.
The minimum voltage of 17 Volts and minimum current of 5 ma is the switched current. If the device is used to switch circuits of less than 17 Volts or 5 ma, the contact film may not be reliably penetrated.
Look at the specs for the same device with a 220 Volt coil. You will see the same 17 Volt, 5 ma minimum for the switched circuit.

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

 

[waross]

One other possible issue.
If you are field installing the timer blocks, try the device WITHOUT the timer installed. If it functions well without the timer installed, make sure that you are installing the timer block correctly.

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

 

[SilverfoxUK]

Gents

We seem to have over complicated this.

This small contactor is sat on my desk. No wires to it and the armature is extremely difficult to push down. Once past the first few millimetres it travels easily. The timer block is still in the box. This stiction has been the same for all 4 Schneider contactors and although to a much less extent the Siemens contactors have a similar problem.

Thanks to Che12345 for pointing out this is a relay. In my world any device with the 3 or 4 main heavy duty contacts mechanically linked together making and breaking the supply at 3 phase 415V / 690V is a contactor. Relays are small control devices. I do understand the need in this PC world to call everything by the same name.

A dead ABB A95-30 contactor (55kW) pushes easily with 1 finger, much easier to push than the Schneider units.

Gents - the homework question is this stiction usual and if so why?

 

[waross]

In my world any device with the 3 or 4 main heavy duty contacts mechanically linked together making and breaking the supply at 3 phase 415V / 690V is a contactor.

In my world also.
The device is HP rated.
Its construction is common with many contactors. (Not all relays are share construction patterns with contactors.)
It is suitable for use as a motor starter and for use switching other loads.
In this PC world, it is a contactor when it is used as a contactor to power a load.
It is a relay when it is used to switch control signals.

WHAT IS THE ACTUAL VOLTAGE RATING ON THE COIL?

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

 

[3DDave]

Stiction is when an item won't move -at all- until a higher than expected force breaks it loose. It is short for "static friction", where "static" is the main defining characteristic. If it moves, then it is dynamic friction.

If the item requires a large amount of force to move then it could just be broken.

Take it apart and look.

 

[SilverfoxUK]

The coil is 24Vdc. Doesn't matter really because this is sat on my desk without wires.
Stiction appears to be the correct term. I haven't taken the contactor apart, all 4 were equally difficult to close so from the responses so far it would appear I've received a batch of 4 faulty contactors.

 

[stevenal]

I agree with IRStuff regarding PLC transistor vs. relay outputs. We've seen some odd behaviors with transistor outputs driving inductive circuits that were fixed when we changed to a relay output card. Itsmoked also has a good point about PLC outputs interrupting inductive loads. Another option may be to ensure the timing is such that the output turns off after the coil current has been interrupted elsewhere by a device that can do it properly.

 

[itsmoked]

This is not a contactor because it has no motor starting specifications and is specifically called a "Control RELAY" by the manufacturer and is specified as a relay.

I don't believe any of "us" are willing to believe Schneider provided you with four (4) bad relays.

I could believe that they've all been damaged by misuse being mounted in the wrong position, over driven, in an overtemp ambient, or subjected to a chemical attack causing swelling of the nonmetallic armature bore.

Continued repetition of stiction when it is not specifically stiction doesn't help the discussion either.

Disassembly of one of them or having Schnieder examine one is likely the only way to resolve this.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[3DDave]

I'm willing to bet these are e-bay or chinese market knock-offs, bought for a few dollars and are not the over $100 each originals, hence not immediately contacting Schneider about the defect. I bet the seller is gone in the wind.

Not sure what the point of complaining about the performance at this point serves. It's not as if the OEM refused to handle or acknowledge the problem; they don't appear to have been involved. It's known how they should work; they don't seem to, and the OP doesn't care to investigate on their own.

Maybe send one to Big Clive. He enjoys taking fakes apart.

 

[SilverfoxUK]

Gents

Happy to take 3DDaves bet regarding dodgy imports. I work on a tier 1 COMAH site so fakes do not happen.

In my opinion a contactor has a row of 3 or more contacts barred together able to switch typically 415V 3 phase. Typically these contacts are of a higher rating than the auxiliary control contacts.
In my opinion Schneider are wrong to call this a relay.

The contactors are fresh out of the box and brand new. Not suffered any electrical abuse.

Stiction, look to my earlier posts, it appears jammed with a lot of force it releases and moves freely.

My next contact will be Schneider and I will report back on the findings.

 

[3DDave]

I'm constrained by evidence supplied. Calling Schneider should have been the very first step. If that was no use, then I would take one apart. Only if those failed would I have asked for external opinions.

You can see how it looks to do things the entire 100% opposite order.

If you think fakes do not happen, explain how the aerospace industry has found fake ICs, fake fasteners, fake aircraft parts that have made it into the supply chain. If there is money to be made, and $100 a shot has plenty of room, there's a fake to be made.

 

[waross]

I apologize and retract my previous statement. I must have gotten on a page for the wrong device when I saw the HP ratings.
You are correct Keith. No HP rating. Another lack is a fourth normally open contact.
A contactor has four normally open contacts. Three power poles and one normally open auxiliary sealing contact.
On a ten Amp contactor the pole may be identical. Larger sizes will have progressively larger power contacts and the same 10 Amp sealing contact
Yes this is a relay.

On the stiction issue and a possible cause.
The force seen when manually closing may not be the same force encountered by the magnetic action.
This contactor may be restrained in the off position to prevent mechanical shock from jarring the contacts closed momentarily.
I have had this happen in a sawmill where a mechanical shock caused a relay to close momentarily.
Unfortunately the relay closure changed the position of the log on the carriage, in the middle of a cut. It got expensive right after that.
When these mechanisms exist, the first movement of the armature is to release the restraint. Closing manually is very difficult until the armature is forced past the restraint.
Given the very low closing power, and the inverse square law for magnetic attraction this must have quite weak return springs.
An intentional restraint is a possibility.

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

 

[3DDave]

It might be intentional to resist external actuation, but the current and voltage excess is likely not intentional.

I don't believe the minimum switching voltage is the coil voltage; it's the voltage this can control, which is why the switching minimum current is 5mA while the power consumption of this is 5.4Watts which requires over 200 mA at 24 volts.

 

[che12345]

Dear All,
Depending on the usage, a very low current small [contactor] can also be used as a [relay] but in most cases a [relay] (is not intended to be used as a contactor).
On the market, a certain brand Axx named their smallest ALx [3-pole contactor] rated IEC 400V: AC3 3kW AC1 25A and UL/CSA: 480V 5ph 600V 21A on the (3 main poles). They can be fitted with a 24Vdc coil.
The said manufacturer further states that: the ALx [contactor] are mainly used for (controlling 3-phase motors) and more generally for controlling power circuits up to 690Vac...
Note: a) These [contactors] has (double-break) main poles,
b The main poles switching voltage has nothing to do with the coil voltage.

Che Kuan Yau (Singapore)

 

[waross]

2[/sup]]I don't believe the minimum switching voltage is the coil voltage; it's the voltage this can control, which is why the switching minimum current is 5mA while the power consumption of this is 5.4Watts which requires over 200 mA at 24 volts.

You are correct, Dave. The same relay with a 230 Volt coil has the same 17 Volt minimum voltage limit for the switched circuit.
The minimum coil voltage for that relay is 16.8 volts.
There are a lot of devices such as limit switches and push buttons that are exremely reliable switching 120 Volt control circuits that are NOT reliable if used to switch low voltage logic inputs.

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

 

[3DDave]

Oh, that's where it is. Would it kill them to put the range in terms of voltage rather than a ratio? Just right there at control voltage?

Now it has me wondering if someone wired these to 120V and melted parts of it and then just went - Oops - and quietly put them back in the box. Might not even have the fresh-baked smell if they were quick enough.