Motor Circuit Surge Surpression

[JSong]

Dear all,

We have a temperature limit switch that controls a 1/6 HP single phase motor/fun. This limit switch is 1/2HP rated according to the product sheet, however, it failed (welded and shut) due to arcs across the contacts caused by inductive kicks. I am wondering if there is a practical solution to avoid it happen? Would a MOV across the main circuit help?

Thanks a lot,
JJ

 

[waross]

Stuff happens. More likely there was a short circuit somewhere in the loop. Possibly a faulty temperature switch closed or opened slowly and lit up an arc. This is a common circuit and there are probably a couple of million similar circuits working happily with no attempt at surge suppression.
It this in a very wet or damp location?
Any special environmental concerns such as trace amounts of ammonia, SO₂, H₂S or other nasty gasses?
Is this in a dusty or dirty location?

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

 

[aap2]

An inductive kick can usually be prevented with a diode.

 

[waross]

On an AC motor?

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

 

[JSong]

Thanks for your inputs! The environment is all right and we don't think that is the key reason causing this problem. We have a few of units with the same setups in different locations, and this failure happens in several of them. It has to be due to the circuit.

Diode type surge suppression is often for the coils. I want to find something good for this main circuit. Any thoughts?

Thanks.

 

[JSong]

Yes, it is an 1/6HP AC motor.

 

[waross]

Can you give us the make and model of the switch?
A good quality 2 HP rated switch should have no issues with a 1/6 HP motor.
The switch may be poor quality and reaching the end of a short life span. Normally a temperature switch that is HP rated will have some type of snap action. Temperature switches are slow acting and need some type of snap action mechanism. If the snap action becomes inoperative the switch may open or close too slowly and the motor current may start an arc that destroys or damages the switch.
I doubt very much that there will be any kind of destructive surge associated with 1/6 HP motor.


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

 

[JSong]

Sorry Bill, I miss-typed on the rating. It is 1/2HP rated. Here are information about the original limit switch (MICROSWITCH BZ-2R-A2) and the replaced limit switch later on (MICROSWITCH BM-1R-A2) within the temperature switches. The original one is 1/8HP rated at 125VAC and is not sufficient for the 1/6HP motor. We didn't know it until we opened the temperature switch and failures make sense there. we replaced all with a similar limit switch but 1/2HP rated. We thought that should solve it, but the same failures came again. We tend to think that the weld and shut of contact is due to arcs caused by voltage surge during switching. Do you agree? That is why I am looking for some surge protection for the main circuit.

Thanks a lot.

http://files.engineering.com/getfil...d69d-4686-9232-d0782f17993a&file=bz-2r-a2.pdf

http://files.engineering.com/getfil...57e7-40d2-9be6-70d84e90b4b3&file=bm-1r-a2.pdf

 

[itsmoked]

JSong; You do not have a "surge issue", you have a mechanical switch problem that is causing the switch to close THEN open with a sub-second time between them. This causes the switch, which has no capability for that situation what-so-ever, to fail - exactly as you're seeing it fail.

The switch is closed.
There is the standard 6~8 times inrush to start the motor.
The switch opens again during the inrush.
Instant weld.

Because the switch is trying to interrupt the starting current it thinks it's trying to interrupt a much larger motor.

That is an abnormal function occurring because the mechanism that is actuating your micro-switches is screwed up in some manner. Sometimes it's because the mechanical jolt of starting is mechanically feeding back to the structure or mechanism. Sometimes it's because of a bad adjustment. Sometimes the thermal sensing medium has leaked out somewhere. Often fools have set the open and close temperatures way too close or the hysteresis of the switch mechanism is dead or missing.

The solution is to repair or flat-out replace the exiting temp switch system with something new.

Google: "refrigeration temperature switch" and pick one that makes sense for your application. You might need knob adjustable or screw adjustable.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Thanks for flat out saying that, itsmoked. Suggesting it was going nowhere.

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

 

[Watthour]

Consider installing a PTC thermistor current limiter (Ametherm) in the circuit to limit inrush. Several are available suitable for that current range at 120V (or higher). I've been installing them routinely in larger control transformer applications (up to 5.0kVA and 460V) with good success to limit inrush in sensitive locations.