Derating a switch's load rating with inductive load

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Robert Everest Johnson

New Member

• Jul 27, 2016

• #1

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

Well-Known Member

Most Helpful Member

• Jul 27, 2016

• #2

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.

 

MaxHeadRoom78

Well-Known Member

Most Helpful Member

• Jul 27, 2016

• #3

You can also place a R/C snubber across the contacts if concerned.
Max.

 

Attachments

• Snubbers.pdf

  1.5 MB · Views: 360

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schmitt trigger

Well-Known Member

Most Helpful Member

• Jul 27, 2016

• #4

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

Most Helpful Member

• Jul 27, 2016

• #5

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.

 

Last edited: Jul 27, 2016

Tony Stewart

Well-Known Member

Most Helpful Member

• Jul 27, 2016

• #6

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?

 

S

spec

Well-Known Member

Most Helpful Member

• Jul 27, 2016

• #7

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

Well-Known Member

Most Helpful Member

• Jul 28, 2016

• #8

In industry motors almost always use a contactor.

 

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spec

Well-Known Member

Most Helpful Member

• Jul 30, 2016

• #9

dr pepper said:

In industry motors almost always use a contactor.

Click to expand...

Thx, and with good reason I would think.

spec

 

MaxHeadRoom78

Well-Known Member

Most Helpful Member

• Jul 30, 2016

• #10

Frequent use applications such as HVAC and Plasma use a Special Purpose rated Contactor.
Max.