Contactor Selection Expert Analysis Please!!

[noel0589]

Hi all,

If anyone's not tired of this one yet, here is my final question on the matter.

Question: Does anybody know what effects the force at which a contactor will make or break? Why would it close with so much force that it would bounce in one case causing heavy arc damage and close softly in another case under the same starting conditions? Do transients play a role in this?

I have a 200V 50HP AC induction motor operating with an open transition wye-delta starter and 208V power supply.
Our supplier assembles our controllers with starters rated for 40HP, 105A according to the manufacturer. So basically our motor is 50HP, but starter is for 40HP. Normally we have no problems with their starter selection, but in this case we get burning contactors. (We sell elevator systems)

Now, our controller supplier says that it is ok to use a smaller sized reversible contactor in the wye-delta application because of the idea that the current being switched in Delta mode will be a factor of 1/sqrt3. So if the line current of our system at full load runs at 120A,
then Delta contactors will switch at 70A. Since this 70A is well below the rated 105A capacity, our supplier says everything is OK! And it was ok until now.

Actual Observation:
Our delta contacts are burning out. About 1 in every 3 starts it seems like the delta run contacts will "make" or close with so much force that a bouncing effect occurs resulting in heavy arcing damage.
Question Repeated:
Why would delta contacts close with so much force that it would bounce in one case causing heavy arc damage and close softly(normally) on another start under the same starting conditions?
Basically, what affects the force at which a contactor will close?

Would a transient surge in current cause this? Could a surge in current during the wye to delta switch (which is normal in open transition wye delta applications) cause the delta contacts to close with a force that results in bouncing and arcing damage?

Or is it something else completely?

Any suggestions or advice would be appreciated!!

 

[GordS]

The arcing current when going into delta will be related to the motor speed at the time of switching. Too soon and the current being switched approaches the stall value which is very high and should cause greater heating damage.

Contact bouncing behavior is a function of contactor design. It should be related to the magnetic flux in the magnetic circuit. The only thing you can affect is the voltage and frequency applied. The latter should be constant. So potentially a high coil voltage could increase the closing force - but I've never seen this being such a problem the contactor would not reliably function.

 

[noel0589]

GordS

About the bouncing, its strange because when it does occur(about every 3 starts) you know it because such a loud noise is made presumably because the contacts are slamming into each other at a very strong force. and of course after they slam into each other a huge arc is generated.

 

[noel0589]

also,

I thought that actuation of a contactor was controlled by an electromagnet. And i just read that magnetic field strength is related to the current of the source and is measured in milliAmperes per meter, or gauss.
So does anyone think a transient current surge would be able to cause the electromagnet of the contactor to have too strong a pull resulting in excessive bouncing?

 

[Skogsgurra]

noel0589,

I think that you are looking for the wrong thing. The current of the delta contactor is first of all not lower than current for the wye contactor - it is the opposite. So if you change that, you will probably have a much better starting point for the rest of the analysis.

Next, wye/delta transitions are not as clean-cut as many think. There are subtle things going on like a phase shift forward or backward depending on how the starter is wired and connected. There is also considerable voltage induced in the stator winding the first hundreds of milliseconds before the rotor field collapses.

Do these things right before you start looking for transients in motor current coupling into the magnetic system of the contactor (it does not). If you experience bouncing - it could come from bad mounting practice (not enough screws or a not-so-stable mounting plate).

 

[noel0589]

skogs,

I have also suspected mounting and other mechanical issues and taking what you say into consideration. But when you say current in delta contacts is higher than in wye I think you are mistaken. In wye connection line current is phase current. In delta, that is not the case and phase current is 1/sqrt3 of line current.
Also, although i do not have a deep conceptual understanding of wye-delta open transition, I am aware of most issues.
But what does interest me is why you can say that transients which will be present in the motor circuit will not affect the contactor. How else would the contactor know when to make or break if it wasn't connected/related to the motor circuit?

 

[busbar]

 
You might obtain NEMA ICS2-2000 and review Part 2 §3.5.2, §4.1.7 and table 2-4-7 with corresponding figure 2-3-6 regarding wye-delta starter ratings, contactor combinations and capabilities. Compare listed contactor sizes for controller sizes 3YD and 4YD 200V horsepower ratings.

Elevator duty may vary compared to NEMA’s continuous, plugging and inching duties.

 

[skiier]

Sounds to me like something is the jamming the operation of the contactor armature. Maybe an intermittent thing where a piece of metal or something is jiggling around inside.
Just a thought. The sealing force of an ac contactor is much greater than the holding force. A chattering relay with even a minute airgap may appear to be slamming the relay in while at the same time producing high inductive voltages. Maybe the relay bounces until whatever it is that is jamming it is dislodged.

 

[jraef]

noel0589,
Transition current spikes that occurr during open transition in Wye Delta starters is a subject that has been discussed many times in this forum. A Keyword search should be very helpful for you. Remember to also seach Y Delta and Star Delta in order to cover all of the variations of nomenclature. Also do a Google search for information on Closed Transition Y-D, S-D etc., because often in the descriptiion of why Closed Transition is important, people will describe the problems with open transition.

Bottom line, it can very well be a big issue for you.

As to transients affecting the closing force of a coil, no. In addition to the transition current spike from the phase shift of Y to Delta, there can be a regeneration voltage spike if the motor slows down and is reconnected before the magnetic field colapses. This has been mathematically shown to be capable of being 2 - 3X the line voltage, but you will not get that spike until AFTER the Delta contactor has already closed!

However, if both that voltage AND current spikes occurr, some contactors use a magnetic blow-out contact design that may prove problematic. This is a horseshoe shaped current path that helps to open the contacts faster in the event of a fault because the magnetic fields around the components will oppose one another. Klockner Moeller invented that concept a long time ago, but lost their pattent a few years back and now several manufacturers have copied it. The solution however would be to solve the transition spike problems, not just change contactors, because there are plenty of other issues at play with those spikes.

Typically in Y-D starters, both of the main caontactors are sized the same, at 57% of the motor FLA minimum, and the Star (shorting) contactor can be 33%. I don't think component sizing is your problem, but keep in mind that if you are using IEC contactors, duty cycle MUST be taken into consideration, where in the old NEMA design you pretty much didn't need to think about it.



"Venditori de oleum-vipera non vigere excordis populi"

 

[Skogsgurra]

noel,

You are right about delta contactor carrying line current/sqrt(3). But on the other hand, the line current in wye is lower than in delta by about the same factor (that's why you use the wye as an intermediate stage in the first place). So let's meet half-way and say that equal size contactors is better than trying to optimize the design with different size contactors in wye and delta. The trouble you see can, in fact, be caused by the undersized delta contactor. Especially if the wye contactor was chosen for the short duty that a start is.

Motor current transients do not couple into the magnetic system of the contactor. The coil/core system is a closed one and even more so when the armature has pulled in, which it has when the contacts close. So any influence from motor current transíents can be ruled out. The motor current, by the way, does not flow until the contactor has closed - and the armature has done its full stroke then and cannot be influenced in any substantial way.

I do not understand what you mean by: "How else would the contactor know when to make or break if it wasn't connected/related to the motor circuit?" Does that mean that your delta contactor is controlled by motor current? It is probably a very elegant solution, but I think you should consider the time (yes, pun intended) proven time relay to switch between wye and delta.

The improved breaking capacity using magnetic arc extinction, mentioned by jraef, could actually be a problem. But it doesn't really have any effect (force proportional to current squared) until you are well into the short circuit region and that should not happen during a normal wye/delta start. And, honestly, I have never seen any of these on an AC contactor your size. If they are used on AC contactors, it must be on much bigger ones. But, i have to admit, I haven't seen them all...

I would hook up a fast memory recorder (transient recorder), some fast current clamps with sufficient current rating (at least ten times rated motor current) and get a picture of what is happening. Or just change the delta contactor to same size as wye.

 

[UKpete]

As mentioned at by skogsgurra, the delay between the star opening and the delta closing can be crucial. I am assuming that any mechanical interlocking between these two contactors is working properly, and that you have the main delay (between starting and the star-delta transistion, as mentioned by GordS) correctly set.

So the delay is typically set by the opening time of the auxiliary contact on the star relay plus the delay between energizing the delta relay and the closing of the contacts. This delay may be further increased by putting a thermistor in series with the delta coil, this would give a slightly softer closing of the delta.

Also is it possible that the contactor only sounds like it is closing more violently, but in actual fact it is the heavy current switching that is making the noise?

It's all a bit of trial and error and may not behave consistently. Maybe you should check all the differences between your correctly working systems and this higher power application.

 

[noel0589]

Pete,

Starting amps is about 270A and FLA is 130A.
Motor is 50HP in 208V system.

You think that current switching at this level will sound as loud as throwing a beer bottle on a concrete ground pretty hard?

 

[jraef]

Sounds like a rapid chatter if you ask me.

"Venditori de oleum-vipera non vigere excordis populi"

 

[Skogsgurra]

I cannot imagine that a contactor sounds like throwing a beer bottle on a concrete floor. Not under any circumstances - be the bottle empty or full. Came to think of it - I would never hit a concrete floor with a full bottle...


The sound you describe can be heard when there are higher frequencies involved. Heard someting similar to what you describe when an induction heater coil was disconnected from the MF generator. A whining and rattling sound combined. That was due to inductance and compensation capacitors interacting with the generator. But that doesn't describe your application very well. Or are there capacitors connected on the motor side?

 

[noel0589]

skogs,

haha, no, it doesn't really sound like a beer bottle.
i just meant it sounds pretty loud.
you have a lot of experience it seems!!!!

 

[UKpete]

Well it was only a suggestion, I guess you have to be there.

I have tested a lot of star-delta starters this year (nothing to boast about, I know) and I haven't heard one do that. I have noticed that the star-delta transition usually sounds more violent for the motor than the initial start, I have also had some which opened again the instant they came in, then closed again, then opened again ... i.e. a continuous very loud chatter (of the type where everyone in the factory looks round at you accusingly) because the auxiliary contacts were incorrectly wired. But not a single very loud bang, nor anything like contacts bouncing.

 

[skiier]

Lots of very educated technical replys.
Have you checked to see if the contactor is clean? Most seemingly complicated problems have a simple solution.

 

[UKpete]

I found quite a few dirty contactors too last year. Not only are they a bit annoying because they hum when energized, they can also draw a bit more current than normal so the coils get warmer than they should. They usually only need dismantling and wiping over the pole faces with a soft cloth. I'm seeing contactors in my sleep at the moment.

 

[mc5w]

A wye-delta starter is a similar application to jogging duty. Life of a contactor is based on breaking of full load current, not breaking of locked rotor current for jogging duty or 1/3 of locked rotor for wye-delta starters.

Telemecanique has some life versus current broken for their contactors that were published in older versions of the SquareD digest. The life for jogging duty is about 1/10th of normal start-stop duty which means that you need oversize contactors to get reasonable life.

Using a 40 HP contactor for a 50HP motor in a wye-delta starter only works for the contactor that stays closed curing the transition and only carries 4/7th of full load current. The other 2 contactors should be more like 60 HP in your application because of the frequent starting of an elevator.

Also, an elevator is an intermittent duty or periodic duty application which requires wires that are 140% of full load current, not 125%.