Concerning these Cutler-Hammer motor starters for supply/exhaust air fans - I looked at three starters including the one that we replaced the transformer in. I checked the VA, current on the secondary of the transformer, fuse size on the secondary and checked operating temp with a FLIR IR scanner.
The current on the secondary was 0.4 amps for the two comparison starters and 1 amp on the starter with the new transformer (I measured 1.8 amps prior but that was on the load side of the fuse - need to double check that).
The fuse size varied from 2A-3A @ 250V.
The transformer size varied from 100VA to 125VA.
The operating temp for the new 100VA transformer that I put in a week ago was 50 degrees Celcius. in one of the comparison starters there was an identical 100VA transformer, age unknown, that measured 82 degrees Celcius (plus I noticed that some of the various conductors insulation was cracked to the point you could see the copper showing through). The third control transformer was the 125VA and it had temp of 75 degrees Celcius.
After a little research I now understand that these starters are mid 70's vintage. I am suggesting that we start going through all of these starters and do some upgrading hopefully before another one fails.
I tend to agree with Greg on fuses providing better protection for certain loads.
And, yes by all means someone is supposed to check to determine why the fuse blew, or the breaker tripped. This is dependent on way too many variables to discuss now. Back in the days of Edison base plug fuses, you would learn really quick to check the loads before you screwed in a new fuse. The same for <30 amp 250 volt cartridge fuses with the line side still 'hot'. Today we have 'finger safe' fuseholders, and plug fuses are mostly artifacts.
I once showed an apprentice class that I was doing a slide of the various fuse amperage, and fuse types. There were some heads spinning. Not just the standard 'sizes' within the NEC, but the 'stock' sizes from the mfg.
#218607 - 07/08/1708:14 PMRe: Control transformer - burn up from excess current?
When a contactor is first energized, before it pulls in, it draws many times it's running current. When it is fully pulled in and the magnetic parts are in contact, the completed magnetic circuit limits current to the coil.
If movement of the contactor is restricted or the voltage drops below the level that will pull the contactor in, the coil draws too much power and the fuse will blow, or the coil will burn out or, maybe, both.
Perhaps someone increased the size of the fuse because the contactor wasn't pulling in fast enough and the fuse was blowing. Another good reason to increase the size of a fuse is because the electrician ran out of smaller fuses and forgot to go back to correct the temporary fix.
I also agree with Greg and John about getting better protection with proper fusing selection. My pet peeve on many time - current curves, is that they run out of time. I like to see curves where the fuse will actually blow near its rating. For instance, I have an FRN-R 60A fuse. When will it blow??? I pull out the Bussmann curves and page 4 tells me that it should blow in a second at over 750 amps. Hey look, it should blow in 10 minutes at around 90 amps. So why don't they tell me when it should blow at anything less than 150% of its rating?
Looking at different families of curves, you see how you can easily get into more trouble with the same amperage rating of a different type fuse, than with a close rating of the same type of fuse. Joe
#218609 - 07/09/1704:07 PMRe: Control transformer - burn up from excess current?
After doing a fair bit of leg work I inquired with my boss about the possibility of upgrading these old C/H starters with new combination starters. They gave the green light and right now I am looking at a quote for two Eaton combination starters with electronic overloads. I do not have experience with electronic overloads so I've been researching the subject. It appears to me that there are several advantages over bimetallic overload relays. Beyond these advantages I started looking into the additional communication module that was not a part of the original quote. I like this option but I wonder how far to take this upgrade. The existing starters that I would like to replace are all for 600V building supply/exhaust fan motors. Obviously these are critical to the function of the building and fire safety. So if you have the option to exit the dark ages of monitoring and control and can now enter the modern era why would you not proceed that direction? Basically what I am asking is can anyone give some of their own thoughts on the pitfalls of going high tech for this application?
A malfunction at the junction
#218647 - 07/20/1709:09 AMRe: Control transformer - burn up from excess current?
First you need to figure out if you have any need to continuously monitor whatever the starter is doing. Then you need to know if you have the capabilities to monitor it. Then you need to see if you have a way to connect everything up.
If the salesman talks you into buying the communications node he won't tell you that you now need to buy a SCADA system...even if there's only 1 thing to monitor. Then after you have the new starters and the new SCADA system you need to run wiring (or fiber) to connect things together.
My first question is "since we have never monitored this before why do we need to now"? If there's no good reason to monitor it in real time the communications option is just a waste of money.
#218649 - 07/20/1709:20 AMRe: Control transformer - burn up from excess current?