Temperature Limit Switch Failure

[JSong]

Hi all,

We had a temperature limit switch failure problem in a heat exchange fan control circuit. The fan is a 1/6 HP single phase motor, and it is controlled by a Honeywell 'BM-2R-A2' temperature limit switch. This limit switch is rated for 1/2 HP that shouldn't have any problem in a 1/6 HP case, but it kept failing and the contactor got welded. Does anyone have some insight regarding this? Would you think that adding a surge suppressor would help?

Thanks,
JS

 

[wayne440]

The only data I can find for the BM-2R-A2 indicates that it is a mechanical limit switch, not a "temperature limit switch".

Obviously, you could use it as such with added hardware, and that may be the case. Is the switch rated for temperatures
in your application?

 

[JSong]

Thanks a lot for replying. You can find all the spec via:

http://sensing.honeywell.com/honeyw...-bz-large-basic-product-sheet-004955-5-en.pdf

and

http://sensing.honeywell.com/nomenclature-micro-switch-bz-ba-bm-be-004970-3-en.pdf

The operating range is : -55 °C to 85 °C [-67 °F to 185 °F], and that is sufficient for our case (125 °F).

 

[waross]

Although Honeywell does make a 22 Amp, HP rated swtch, I cannot verify that your switch is HP rated.
NOTE: There is no correlation between current rating and HP ratings of switches.
Is there a possibility that the electrical damage is precipitated by mechanical damage due to possible over travel of the operating mechanism?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[JSong]

Bill, if you look at this tech spec:

https://stevenengineering.com/tech_support/PDFs/31BASW.pdf.

It says that it is HP rated. Once the contacts close, the moving contact stops, and the over-travel shouldn't have further impacts. Am I understanding it right?

 

[controlnovice]

Water or condensation getting to the switch?

______________________________________________________________________________
This is normally the space where people post something insightful.

 

[itsmoked]

I would think that if the switch appears to be rated for your application and is frying then your application is having a problem you might not realize. For instance the motor might be greatly overloaded causing a much much larger current than the switch can handle when the interruption occurs.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[wayne440]

The referenced literature reads "... Designed with a hysteresis as small as 0,008 mm [0.0003 in]...". I claim no expertise, but wonder if contact chatter might be a factor. It might not take much vibration in the external mechanism to bounce the contacts.

 

[lukin1977]

temperature limit switch?

Are you trying to limit temperature or what? Sorry, I dont understand. I see only a micro switch

lukin1977

 

[waross]

Some specs on the complete temperature limit switch may be helpful.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[JSong]

Thank you all for the inputs. It is a limit switch that is triggered by a temperature sensor/actuator. I will look into the possible contact chatter.

We also measured the load current. It runs at ~4.5A, with a ~11A inrush on startup.