Contactor for compressor: IEC or definite purpose? 1

[akrus]

Hello

I need to select contactors for some 220V single-phase compressors. I have experience with three-phase motor control but I’m not familiar with typical practices in single-phase applications.

I’m trying to decide between using an IEC contactor or a definite purpose contactor. I haven’t used a DP contactor before but my first impression of them is that they look cheap. The application could potentially be in relatively harsh environmental conditions so I am conscious of selecting quality components to reduce the probability of failure.

The problem is that I can’t find any 2 pole contactors that are not definite purpose. I know it is possible to use a 3 pole contactor by jumping terminal 4 to terminal 5, but that kind of seems like I’m trying to force the solution. I’m getting the impression that everyone must use definite purpose contactors for single phase because that is the only thing designed for it. I’m also aware that compressors are one of the intended applications for a definite purpose contactor.

I work at an OEM and the whole machine will be evaluated by UL so it is not an issue that the DP contactors are only UL registered.

Does anyone have some feedback about IEC v. DP contactors to be used for a single-phase compressor? Again I am just looking for a high quality solution that is not going to turn any heads.

Thanks in advance.

 

[ScottyUK]

A couple of observations from the IEC world:

- An IEC contactor is always much smaller and lighter than an equivalently-rated NEMA contactor.
- NEMA contactors last longer than IEC types.

If you jump to the next frame size up - which usually means a 20% - 30% increase in rating - you get an IEC contactor which is about the same size and weight as a NEMA contactor.

In your position I'd likely just use a 3-pole contactor and wire the outer poles only.

 

[jmbelectrical]

All credit for the following goes to jraef and was copied from [URL unfurl="true"]http://forums.mikeholt.com/showthread.php?t=85584&page=2[/url]:

NEMA starters were/are designed to take the absolute worst thing you can throw at them and survive with a reasonable amount of serviceable life that will likely exceed the equipment that it is controlling. The reasoning behind it was that auto makers would typically change out a complete production line in no more than 4 years, usually around 3 (now it's 5 or sure, but times have changed). That meant that they had to build a completely new line in parallel with the old one prior to the switchover, so to save their capital investment dollars they demanded starters that could be easily re-used without a lot of engineering effort every time. Engineers were expensive, electricians were cheap. That ended up being the NEMA design we love today because you can confidently re-use a NEMA starter for anything else in your plant without getting an engineer involved.

IEC starters were designed during the pre-WWII heydays of European expansion when engineering was a trade rather than a profession. So in Germany and France and England (who joined IEC much later actually), there were people with engineering expertise on every street corner, needing work. In fact, many governments in Europe after WWI had labor laws that essentially mandated that companies employ more people than were really necessary. So to save money on components (especially when most of their raw materials were imported), they used all that available engineering brainpower to decide on the exact amount of contactor necessary for each and every application; no more. They would "rationalize" every installation and process whereby they knew how many operations it was likely to see, how many times per hour or minute, starting current, likely voltage drop, etc. etc. etc. ad nauseum. The end result was that to properly select a contactor, it might take an engineer 1-2 hours on each machine application. If you had a machine with 4 or 5 motors in different uses, that added up to be a lot more engineering time than anyone would ever spend in the US. IEC contactors were also selected based on the expected life of the machine; there was no intent to re-use them somewhere else because you would still have an engineer rationalize it all over again anyway, so the contactor cost was relatively irrelevant. The "throw away" concept crept in in the late 70s only when automated production lines made them too inexpensive to bother fixing, but it should be noted that only the smallest of contactors are considered too cheap to fix. All large IEC contactors have replaceable contacts (at least those from reputable sources). IEC contactors today are still designed with that engineering intensive mentality in mind, although the selection process is a lot more streamlined now.

Definite Purpose contactors are a US concept that somewhat mirrors the IEC mentality, but is reserved for high volume OEM applications and requires even more engineering time because you must investigate and justify every tiny detailed aspect on your own, including short circuit calculations, wear time vs warranty of your end product etc. etc. So if you are designing a machine once and will make it thousands of times without redesigning it, DP made sense because the engineering expense was amortized by volume. DP contactors are terrible choices for one-off applications however because the ratings on the labels are even more application specific than IEC ratings are, so it's really easy to misapply them and end up with damaged equipment or a fire.

 

[jraef]

Couldn't have said it better myself...

I'll only add my running joke that "DP" should not really stand for "Definite Purpose". It should be read as "Don't Purchase". I would use IEC long before I would consider using DP even though you must take the extra step in wiring it. For me, having worked for several mfrs, DP contactors are like hot dogs. Now that I've seen how they're made, I cannot have anything to do with them.

By the way, the loop-back wiring issue is really only for the Over load Relay of the IEC motor starter because the internal sensors must see current on all 3 phases. If you are using a single phase motor with built-in thermal protection, as many compressor motors do, then you may not need an OLR at all, in which case there would be no need for the loop-back. If you do need the external OLR, you might consider using a Solid State OLR that has the ability to switch off Phase Loss protection. The slight additional cost of the SSOLR might be offset by less labor in wiring it (assuming a volume OEM project here)

"Will work for (the memory of) salami"

 

[jmbelectrical]

I'm ashamed to say that I've specified Definite Purpose contactors for motor control applications back when I didn't know any better. Learn from my mistakes and just go with NEMA, if at all possible.

 

[jraef]

I worked for a machine OEM a few (dozen) years ago and they used DP starters and contactors, controlled by cam programmed motor driven cycle timers made by a company well known at the time for being cheap. As complex and unreliable as those timers were, they were NOTHING compared to the trouble we had with contacts welding or coils failing on the DP contactors. Generally they barely outlasted the warranty on the equipment, but because we relied a lot on repeat business, we replaced most of them. It was completely backward thinking in my opinion. The ill will that created was not worth the few dollars they saved. But I was not in a position to advise anyone in those days.

"Will work for (the memory of) salami"

 

[akrus]

Thank you everyone. I’m glad I’m not the only one who doesn’t like the DP contactors.

Regarding the wiring, would using only two of the three poles have any effect on the life of the contactor? Even though this is for an OEM application, I think the volume is low enough that the slight reduction in labor doesn’t mean much. However if it truly doesn’t matter then there’s no reason to have one more potentially loose wire.

 

[jraef]

No, it would not have any effect on contactor life whatsoever.

"Will work for (the memory of) salami"

 

[jmbelectrical]

jraef,

This issue came up at just the right time and I'd like your input on this. I'm helping an engineer review the electrical contractor's proposed equipment submittals for a facility that we just designed. It contains numerous 120-volt, single-phase, fractional horsepower and a few 1 HP fans. Since we rarely ever know if these are provided with integral overloads, we always specify separate starters with overloads by default. The contractor is proposing to use two-pole NEMA-type starters with melting alloy thermal overloads. How must these be wired with respect to the second pole? Does it need to switch the neutral or can they usually be left unused?

 

[waross]

Melting alloy O/Ls MUST be reset manually. Bi-metal type O/Ls usually include the option to select manual or automatic reset.
Here is a single pole melting alloy type manual starter;

http://www.grainger.com/Grainger/SQUARE-D-Standard-Manual-Motor-Starter-1H385?Pid=search

Bi-metal O/L types are available in similar packages.


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

 

[jraef]

This issue came up at just the right time and I'd like your input on this. I'm helping an engineer review the electrical contractor's proposed equipment submittals for a facility that we just designed. It contains numerous 120-volt, single-phase, fractional horsepower and a few 1 HP fans. Since we rarely ever know if these are provided with integral overloads, we always specify separate starters with overloads by default. The contractor is proposing to use two-pole NEMA-type starters with melting alloy thermal overloads. How must these be wired with respect to the second pole? Does it need to switch the neutral or can they usually be left unused?

You do not need to switch or monitor the neutral current in a 120V motor, but you can if it's more convenient. The NEC requires that all ungrounded conductors be protected, but contrary to popular belief, it does NOT prohibit the switching of GROUNDED conductors, such a Neutrals, under certain conditions (article 240.22). One of those conditions is that you can if ALL conductors are opened simultaneously, as they are in a contactor od a motor starter. So no problem.

"Will work for (the memory of) salami"