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Motor starter/contactor general specification


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Hello foxhoundme


Firstly, this is a rather involved question and very open ended!!

If you are specifying contactors for motor control, you should always specify them for AC3 or better. - AC3 allows for an overload of 600% durng start. If you use AC1, they will not last very long.

If you are using these contactors under a high frequency of operation, you may wish to go to a larger size in order to extend the contact life.


Motor starters are quite a specialised field, although many get away with no engineering and just use any starter.

If you have not already done so, have a look at the pages about motor control on this site and you will get a brief description of some of the options available.

The simplest starter is the Direct On Line or Across the Line starter. This is very effective and minimum cost, but it causes the motor to draw locked rotor current from the supply and produce maximum torque. If this is not acceptable, then you need to consider a reduced voltage starter.

The most common reduced voltage starter is the star/delta (wye/delta) starter, but this actually causes more problems than DOL so it should not be used. Beyond that you essentially have the options of AutoTransformer, Primary Resistance and soft starter. Each have their strengths and weaknesses, but in my opinion, the soft starter is the best option.

You could download our Electrical Calculation software and use the motor start current curve section to study the effects of the different forms of starter on your application.

If you are able to locate the motor starting curves and the machine speed torque curves, you can enter these into the software and plot out the reduced voltage curves for the different starters.


If you have further questions, please post them.

Rmember, for specific answers, it is best to post specific questions.


Best regards,

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Thanks marke for your input. I guess I was too general with my request and need to be more specific. I want to write a technical specification for a motor contactor but do not know enough about contactors to specify them properly. Is there someone in the forum who has a template or even a product specification for motor contactors/starters I can use to modify?
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  • 10 months later...


i am high voltage power engineer.

my email for your questions : eoots2000@yahoo.com

i am a new member in this site.this is a good site for power engineers.

your question is about a motor in range 186.42 Watt - 149139.97 Watt

motors in 4000 Watt have a step (small-power)

so motors in 150 kw have a step ( Lv-Mv 380-6kv)

Direct online is the most norm in small kind - this days 4kw-150kw is direct online too- but in big motors we have other kinds-

in addition contactors usually make methods like direct online and star-delta

star -delta have a surge and todays use this method belike.

today we have automatic starters- this Mcc kind devices have better performance and a little surge-


If you want a circuit wiring diagram of ( power - control - signal monitoring) contactor type starter email me. i send a file by email for you.

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  • 7 months later...

Howdy all, just found this site and it looks like a good one. Just a quick question to anyone who could answer it.

What is the derating factor on contactors used in reduced voltage starting?

That is will the stage 1 (and or stage 2) contactors in an auto-tx starter need to be the same size as the line/run contactor. I've seen some, and the look physically smaller, so i think that is the case, but my question is what is the derating factor?

An example is if you have a 40kw motor, is the run contactor rated for 40kw, and the stage 1 contactor rated at less, say 30kw approx. What is the go, anyone know?

my email is : aussiedan(at)hotmail(dot)com, or post reply in here.


Thanks in advance.;q

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Hello Foxhoundme,


Can help you with IEC but not NEMA. Will post something for you in the next few days.


Hello MotiveForce,


The general rule of thumb is as follows:


Line contactor 100% Motor Name Plate FLC - AC3

Transformer contactor 80% Motor Name Plate FLC - AC3

Star Point contactor (star connection) 33% Motor Name Plate FLC - AC3


This assumes a standard single stage start kornorfer arrangement. Needless to say the rules would be different for a multi-stage or rising star auto-transformer.


Hope that helps.




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The same ratings are technically true for NEMA contactors on RVAT starters, however some manufacturers do not have a mechanical interlock mechanism that works betweven dissimilar contactor sizes, so you end up with the Run and S1 contactor being the same size.


For foxhoindme,

I see you are in the US, so in general, you need to decide in advance which way you want to skew the specs, IEC or NEMA, since we have both available to us here (in other cointries they don't really have much choice because NEMA is not generally exported). The design philosophies are different.


IEC design is that the contactor is sized to do the job it is engineered to do, not necessarily anything else. So if you have a 10HP motor, use a 10HP contactor. If you have a 15HP motor, use a 15HP contactor. If you want to do inching or plug stopping, you look at a chart that tells you how to derate a larger contactor for the task at hand. If you want 10 million operations, you look at a different chart. The idea is that you only buy exactly what you need for the application, because someone is going to sit down and think it through every time (the Germans call it "rationalization"). Also, IEC contactors are not required to allow for changing out the contacts, and almost never are in small sizes. In fact, the concept of changing contacts is generally discouraged because there may be other parts that have worn out at the same time.


NEMA design philosophy is that the contactor is designed up-front to do the WORST thing you could possibly do to that motor, and it will last for 10 million operations minimum. If your application is never going to do inching or plugging, nor high duty cycle, or 10 million operations is relevant only to your great grand children, tough luck on a NEMA contactor. You are going to pay for it anyway. The issue here is, they are designed to be kept in inventory at a factory storeroom and can be used anywhere for anything in that factory without thinking about it. In addition, NEMA sizing has to do with reducing the number of components in inventory. So a NEMA size 1 is good for 10HP 460V, a size 2 is good for 25HP, but if you have a 15HP motor, you are going to have to use a size 2 contactor even though it is overkill. Overkill is really the name of the game for NEMA. Lastly, NEMA design always specifies that contacts can be replaced without replacing the entire contactor. the mechanical life of the contactor is supposed to be 10 times the electrical life of the contacts (I seriously doubt it is true for most, but that's a QC issue).


Once you decide which way to go, contact some of the major suppliers here in the US that offer both, such as Siemens, Schneider or Allen Bradley, and ask them for a "Specification Guide". This is a document they all have available for free that has their design specifications written so that theoretically only their product can be used. The reality is however that in the contactor world, there really are no significant differences in the major suppliers any longer so one's spec is a good as another's.

"He's not dead, he's just pinin' for the fjords!"
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