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Derating a switch's load rating with inductive load

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I am selecting a switch to energize a 3/4 hp AC motor. I am concerned about the prospect of contact arcing with such an inductive load.

If a switch is rated at 25A @ 125 VAC, it will handle the continuous current, but might there be a reliability issue over time due to contact arcing at turn on?
 

ronsimpson

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I could not find a switch with a motor rating.
So I found relays with ratings. Many relays will say how many HP they can switch.
Here is an example of a 10A 120V/220V relay "switch" and how the switch part works.
They show 10A for AC resistive loads. 8A for a DC load. But 5A for a inductive load.
Note also if you push the edges the life of the part get shorter.
upload_2016-7-27_8-18-55.png
 

schmitt trigger

Well-Known Member
It is not only arcing what is at stake.
It is the much larger startup current as compared to the running current.

Unfortunately there are many types of induction motors: shaded pole, permanent split phase, capacitor start, capacitor start-capacitor run.
The starting current may be anywhere from 2 to 5 times the full load current.

Post a photo of the motor, including its plate.
Many times the plate shows the full load amps, and the startup (locked rotor) amps.
 

MaxHeadRoom78

Well-Known Member
If in doubt pick up a small Telemecanique contactor on Ebay, you might even get a set of O/L's with it!
Especially if it involves much cycling on/off during operation.
Or pick up a A/C. switch from H.D. etc.
Max.
 
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Tony Stewart

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Without the exacts specs for the switch and Motor locked Rotor Code , it is impossible to say with certainty.

Depending on motor, typical start current surges are 600-800% rated current of typical <5A full load for 3/4 HP at 120V thus Isurge=30-40A.

What cycle life expectancy do you need?
 

spec

Well-Known Member
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You could also consider using a solid state relay to switch the motor and use a mechanical switch to switch the solid state relay.

spec
 

dr pepper

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In industry motors almost always use a contactor.
 
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