Help with IEC Util. Category AC-3 description

[jraef]

I am involved in a project wherein I must educate an American company on the specific details of how contactors are selected via IEC standards and I have come across a statement in several manufacturer's descriptions for the AC-3 Utilization Category system that has me puzzled.

AC-3
This category applies to squirrel cage motors where the breaking of the power contacts would occur while the motor is running. On closing, the contactor experiences an inrush which is 5 to 8 times the nominal motor current and at this instant, the voltage at the terminals is approximately 20% of the line voltage. Breaking remains easy.

This statement is verbatim identical in several web sites where I was looking for information, so I am assuming that it must be copied from something within the IEC standard (I do not have access to a copy).

The part that puzzles me is the 20% of line voltage issue.

What the heck? Are they perhaps saying that the rating is assuming that the contactor will close at a specific point in the sine wave?

 

[1x1x1x]

jraef

I think it means that the motor is still rotating and the motor terminal voltage is approx 20% of the line voltage due to the trappad flux in the rotor circuit.

 

[jraef]

So then the rating is intended to be only for motors that are already rotating? Why would that be?

 

[1x1x1x]

Occasional inching and plugging could be the reason.

 

[ScottyUK]

1x1x1x,

AC3 rated contactors are not rated for plugging duty, that requires an AC4 minimum.

Jeff,

Have a look at the following document, specifically page 5. There is a really good summary of both AC and DC duties. The summaries appear to be directly lifted from the IEC 947-4 standard.

http://www.lpt-electro.ru/pdf/bremas/tex.pdf

Your quote takes on a slightly different sense when it is read in the fuller context. There are a few words missing from the one you posted.


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Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[jraef]

Thanks Scotty, although I still don't get it. maybe I'm just being a bit thick tonight...

AC-3
... The switching conditions are easy, as the voltage at the motor terminals is approximately 20 ÷ 30% of the supply voltage.

Assuming the "division" symbol was supposed to be a dash (it consistently appears that way throughout that page), I am still puzzled as to why the motor terminal voltage would ever be 20 - 30% of the supply voltage on a DOL starter?

The difference between that and the AC-4 rating is that under AC-4, it says the voltage at the motor terminals would be 100% of the supply at the time of switching. Why would it be anything else but that for AC-3 either?

 

[Marke]

Hi Jraef

Have a look at AC4 as well as AC3.
The major difference between AC3 and AC4 is that AC4 is rated for inching and plugging. In the standard, it refers to the motor terminal voltage being equal to the line voltage.
Essentially, if you open a contactor and then reclose it, you are reclosing onto a generator. If you leave the time for long enough, the generated voltage will have dropped to zero. (motor stopped or rotor field decayed to zero)
If you switch too soon, the generator could be running close to line voltage and you will get a very high current transient to resynchronize the field.

So, the major difference between AC3 and AC4 in the standard, is that AC3 is designed for controlling motors that have been off for long enough for the generated voltage to decay to less than 20%-30% of line voltage. AC4 is designed for almost immediate reclose on a rotating motor.
Note, there are still overriding figures often quoted \by the manufacturer for nomber of operations etc.
General rule of thumb, is if you are starting a stationary motor, use AC3. If you are closing onto a rotating motor, use AC4.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jraef]

Excellent, that is perfectly clear now. It has to do with switching frequency, or more precisely, the minimum time between switching cycles. Now it all makes sense, even 1x1x1x's statement. I was just a little slow on the uptake there.

I knew could count on you guys. Thanks to all 3 of you.

The application I am working on is a shredding machine which reverses to clear jams, 500ms between reversing cycles but 6 reverses over a 2 minute period once every 4-6 hours. So technically I might "get away with" AC-3 but I am going to recommend his using AC-4, or at least use the calculation for "mixed use" since most of the time it will just be used in an AC-3 mode. My predecessors have used just AC-3 ratings and he has smoked contactors in short order, thinking it was poor quality control. I think they have all been undersizing the contactors and intend to prove it to him, hence the report I am writing.

Excellent help as usual from one of the best engineering sites out there (Marke's being the other ).

 

[Marke]

Have a look at the total "start" time during that two minute fault clearing cycle. It can add up to quite a long time at locked rotor current, and a lot of temperature rise on the contacts. You may need to oversize the contactors to limit the maximum contact temperature.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[1x1x1x]

ScottyUK,

The following excerpt from IEC 947-4-1

"5.3.5.5 Starting and stopping characteristics of starters

c) one direction of rotation, or two directions of rotation as in item b), but with the possibility
of infrequent inching (jogging). For this service condition, direct-on-line starters are
usually employed (utilization category AC-3);"

 

[ScottyUK]

The key word being 'infrequent', with adequate time for the contacts to cool down between operations, and AC3 makes no reference to for plugging duty which is much tougher on the contactor than inching. Paraphrasing IEC 947-4's commentary on AC4 duty: 'Starting, plugging and inching of squirrel cage motors'.

I would not push my luck an use an AC3 contactor for inching when I can buy the right device for a couple of pounds more, but I'm in an industry where downtime is very costly. Maybe the saving is worth it in some applications where reliability takes second place to installed cost. Many IEC contactors are designed right on the edge anyway so I almost always go up a frame size and I'm fussy what brands I buy. The Siemens range is pretty good in my experience. I'd rather pay a bit more up front and reap the benefit in improved availability, a policy which is solidly supported by the very low failure rates we see.



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Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[Marke]

Many years ago, I did a survey on contactors and what I found was that they were far from being equal.
If you looked at the AC3 ratings and the number of operations, there was a wide divergence on the criteria.
In those days, there were derating curves published to allow you to adjust the ratings based on the number of operations etc. Some contactors were rated at 1 million operation, others at as low as one hundred thousand. When you equalized the number of operations, the expensive contactors were no longer expensive. The rating of some of the contactors dropped significantly to get them on to the same criteria as the more expensive ones. If you have a relatively arduous operation, look closely at the specs. With some designs you may be advised to up size, and others may not need it.

Best regards,

Mark Empson

http://www.lmphotonics.com