Setting a thermal OL to its maximum position

[Jgreagori]

Red stop button

Red stop button

Pushing and releasing the red stop button on the OL unit if wired correctly should stop and not restart the motor. The OL contact (NC) n the motor starter OL unit should be wired in series with the coil oof the starter. Once the contact is manually opened by pressing the red button it should not allow the starter to re energize when button is released. Take a look at the wiring to see where that contact (the NC on the OL) is placed in the circuit.
Happy hunting..

 

[Jraef]

Aha. Well that makes sense now. Can't quite picture the mechanism as all that comes to mind is a current restraint but I'll see if I can find a diagram.
I can see a thermal residual given the design but what I actually had in mind was along the lines of zero sequence or residual current. Completely off the reservation I guess.

Yep, off the reservation, at least as far as basic simple thermal overload relays. All they are responding to is the heat created by current flowing through their own internal resistance.

The mechanism is that the three heating elements bend (due to dissimilar metal expansion rates) until they make contact with a release latch on a spring loaded pawl that is keeping the aux contacts closed. If any one of the bi-metal strips hits the release bar, it trips. On the single phase sensitivity version (not all b-imetal OLs do this by the way), the release bar is on a pivot held in place by another spring and the bi-metal strips are exerting force against that spring to equalize it and keep in true to the trip set point. If one of the bi-metal strips is not flexing, the counter balancing force it provided is reduced, so the trip bar advances a little toward the trip point, meaning the OTHER two that ARE flexing have less travel distance to go before causing a trip. So it doesn't really cause a faster trip, although that ends up being the EFFECT, what it really is doing is biasing the trip point to take place at lower amount of current on the other remaining phases. The PURPOSE of this is to help account for the added heating effect of negative sequence current due to unbalanced phase currents, but it does not actually measure then negative sequence current itself. Most good solid state overloads will take it to that next level however.

 

[mivey]

On the single phase sensitivity version (not all b-imetal OLs do this by the way), the release bar is on a pivot held in place by another spring and the bi-metal strips are exerting force against that spring to equalize it and keep in true to the trip set point. If one of the bi-metal strips is not flexing, the counter balancing force it provided is reduced, so the trip bar advances a little toward the trip point, meaning the OTHER two that ARE flexing have less travel distance to go before causing a trip. So it doesn't really cause a faster trip, although that ends up being the EFFECT, what it really is doing is biasing the trip point to take place at lower amount of current on the other remaining phases.

:thumbsup: Great explanation. I can picture that. :thumbsup:

 

[GoldDigger]

On the single phase sensitivity version (not all b-imetal OLs do this by the way), the release bar is on a pivot held in place by another spring and the bi-metal strips are exerting force against that spring to equalize it and keep in true to the trip set point. ....

So you suspect (or know?) that the "single-phase sensitivity" design was done deliberately to provide better performance when a three phase OL was wired to a single phase motor without looping the circuit through all three bimetals?

I imagine you would still have to use two of the three connections to get close to the desired performance, yes?

 

[Jraef]

So you suspect (or know?) that the "single-phase sensitivity" design was done deliberately to provide better performance when a three phase OL was wired to a single phase motor without looping the circuit through all three bimetals?

I imagine you would still have to use two of the three connections to get close to the desired performance, yes?

The "single phase sensitivity" design was intended to protect 3 phase motors from phase loss, especially when used with fuses in the distribution system instead of 3 pole circuit breakers that opened all three poles together. At the time they were developed in the early 1970s, single phase motor's in IEC countries were usually small and residential, therefore self protected and not needing external OL relays. It wasn't until IEC mfrs starter exporting their products en mass to North America that they ran into this issue of needing an OL relay for a larger single phase motor, so the "loop through" routine was the work-around. It does not really improve performance, it was necessary to make it useable at all.