Heater Sizing Charts

[adamscb]

Forum,

I'm using the sizing chart via the link below to size eutectic alloy overloads in MCC motor starter buckets. From my understanding, the horizontal columns (Size 0, Size 1, Size 2, Size 3, etc.) represent the size of the contactor inside the bucket, is this correct? My question is then, how can the trip setting of a eutectic alloy overload change based on starter size, when the alloy overloads themselves are the same? Why does the trip setting of let's say a W61 overload increase going from a Size 1 to a Size 2 starter? Or am I completely misunderstanding how to use these charts? Please advise, thanks

http://literature.rockwellautomation.com/idc/groups/literature/documents/td/592-td010_-en-p.pdf

 

[Ingenieur]

look at pg 5
first starter bulletin
then enclosure type
then size
this gives you the tables 112 is single ph, 110 is 1 or 3 ph

 

[adamscb]

look at pg 5
one is for single ph only, the other single or three

I believe that is telling you which table to use, I'm talking about the columns on, let's say for example, Page 10, Table 144. Under "Size 1" it says a W61 will eventually trip at or above 21.3 amps, but with a "Size 2" it will trip at or above 21.7 amps. A small difference, but a difference nonetheless. I was taught that the "Sizes" are the size of the contactor in the starter bucket. Is this correct? Also, what makes these amp ratings change from one size to the next, when the heater element itself remains the same?

 

[Jraef]

Forum,

I'm using the sizing chart via the link below to size eutectic alloy overloads in MCC motor starter buckets. From my understanding, the horizontal columns (Size 0, Size 1, Size 2, Size 3, etc.) represent the size of the contactor inside the bucket, is this correct? My question is then, how can the trip setting of a eutectic alloy overload change based on starter size, when the alloy overloads themselves are the same? Why does the trip setting of let's say a W61 overload increase going from a Size 1 to a Size 2 starter? Or am I completely misunderstanding how to use these charts? Please advise, thanks

http://literature.rockwellautomation.com/idc/groups/literature/documents/td/592-td010_-en-p.pdf

No, that's how you use them.

In this type of "heater", the element is the solder pot and pawl of the ratchet. The solder melts at a specific temperature, allowing the pawl to rotate and trip the relay contacts. The "HEATER" portion is the little U-shaped bent piece of metal that spans the gap in the OL block itself. When the starters get larger, the block gets larger too, so the heat build-up inside of the block is different and the solder melts at a different amount of current going through it. In other words the same amount of current through the heater block of a Size 2 OL relay produces a more heat in the solder pot than the same amount of current flowing through the heater block of a Size 3 OL relay, because the area around the heater element is allowing more air flow around it. So a W61 trips at 21.9A if mounted in a Size 2 OL block, but it takes 24.8A of current flowing through a Size 3 OL block to melt it, because the block is bigger and there is more space around the heater element allowing it to dissipate rather than melt the solder.

 

[Ingenieur]

larger enclosure (more surface area, cooling) and larger ol block (more thermal mass)
likely takes more current to trip/heat up the same thermal mass element to trip point

 

[kwired]

Did not look at the table you posted link to, but number of overload elements is often a factor as well, goes along with what others have already said. The amount of heat necessary to melt the solder changes as the environment around it changes, so the heater necessary to get the precision you need changes.

Something else you run into that many don't seem to be aware of is needing to make adjustments if motor service factor is not 1.15 or 1.25.

 

[adamscb]

Did not look at the table you posted link to, but number of overload elements is often a factor as well, goes along with what others have already said. The amount of heat necessary to melt the solder changes as the environment around it changes, so the heater necessary to get the precision you need changes.

Something else you run into that many don't seem to be aware of is needing to make adjustments if motor service factor is not 1.15 or 1.25.

From what I've picked up my first few years, is that if the S.F. is above 1.0 you can round up to the next size of overload, if the motor FLA is between two different sizes. if the S.F. is 1.0, then you have to round down in that case. Would you agree with that?

 

[kwired]

From what I've picked up my first few years, is that if the S.F. is above 1.0 you can round up to the next size of overload, if the motor FLA is between two different sizes. if the S.F. is 1.0, then you have to round down in that case. Would you agree with that?

with tables that just give you one value per overload unit, yes. Many tables have a range per overload unit, then you pick the one that your FLA falls within the range. Such table values already factor in 1.25% of FLA, so you don't need to calculate anything when motor SF is 1.15, but usually need to apply an adjustment to FLA before selecting overload unit for a 1.0 SF motor.

Table that is supplied with the motor starter is usually the correct table for the application, has already considered how many thermal units are in the enclosure, size of starter if same units work on multiple starters, size of enclosure, whether other heat producing components are in the enclosure, etc. If you are looking up information in a catalog, you usually need to determine all factors of the application and use that information to select the correct table to use.