Effective sizing of contactors in control of industrial electric motors 1

[Awordtunny]

Please fellow engineers, i'm facing d problem of effective sizing of contactor to be used in connection of motor controls. Please i need the mathematical relation. Thank you all

 

[Skogsgurra]

Do you need to do it any better than the standard AC1, AC3 etcetera?

Mathematical relation? What input variables - kW, voltage, LRA, inertia, motor type, operation type, duty cycle, altitude, environment, load's time constant and characteristics and much more?

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[LionelHutz]

Motor HP <= Contactor HP Rating @ Motor Rated Voltage

 

[oldfieldguy]

Every manufacturer of contactors has this question reduced to a table, making it pretty easy. The only departure that I've had to deal with were some super high efficiency motors that had inrush currents far above the high efficiency motors. Had contact welding issues.

old field guy

 

[ScottyUK]

If you are using IEC contactors rather than NEMA types and you want a long trouble-free life I would over-size it a little. Certain European brands are very good - Siemens for example - while others are rather less generous with the copper and iron in their designs resulting in contactors which run blistering hot and then fail early. I usually go up a frame size.

 

[DRWeig]

Awordtunny, I am guessing that English is not your first language, though you do seem to have a good grasp of it. So I'll ask you for a clarification:

Are you referring to the selection of a contactor for a given motor load, or are you actually designing a contactor -- including contact size, geometry, material, contact pressure, arc quenching, coil, case, and all else?

Best to you,

Goober Dave

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[Skogsgurra]

He is asking for "the mathematical relation" - I don't get it. Do you?

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.