Bigger Fuse?

Status
Not open for further replies.
ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
Winters work, 2 months of it anyway, was to put control of small industrial process onto PLC & HMI. In the process we put control fuses in all the buckets. One is a 125 or 150 hp w autotransformer start. One of the operators jogs this motor on startup instead of cleaning the dies with air. He blows the control circuit fuses in the process.

Should I:

A. Increase the size of the fuse to increase the number of starts?
B. Decrease the size of the fuse to further limit the attempted starts?
C. Reshape a medium sized crowbar around his skull?
D. Other...your suggestion.
 
What is the purpose of the fuse you installed; to protect the PLC output, the control circuit, or the starter coils?
 
It is a 480v control circuit. Mostly to limit damage to timer contacts etc when the coils do go. The PLC is protected via interposing relays. The fuses could be bigger without a doubt. I was not expecting the jog protection they provided but then I wasn't expecting the jog either. I could limit the # of starts if this continues to be a problem or if management requests it.
 
An Autotransformer starter is not usually designed for as many starts-per-hour as the motor it is connected to. There is (or should be) a thermal cutout embedded in the transformer that would be wired into the control circuit to disable restarts if the transformer over heats. But you should NOT rely on that, it is a last resort backup to prevent a fire from starting in the transformer. You SERIOUSLY need to do something to prevent them from doing that. They are putting the motor and the transformer at serious risk.

Doing it with the control fuse is again a last ditch device, not a primary means of designing in safe operation. Limit the number of successive starts in the PLC program. A very typical RVAT starter duty cycle design criteria is as follows:

On time = 30 seconds
Off time = 30 seconds
Repeat = 2 times
Rest = 2 hours

You can order specially designed ATs for high inertia loads and/or higher duty cycles, but if nobody said they were willing to pay extra, they typically don't.
 
Last edited:
Without going off-topic, first, I think the OP should do "C". Nothing like setting an example to keep it from happening again.

With that said, why was it decided to use an autotransformer starter?
 
It may be possible that the fuse is doing it's job and the problem is the improper operation of the machine. (Choice C addresses that issue)

Fast forward to the point where you get the fuse not to blow and Mr. Jogstien manages to destroy some equipment that the fuse used to protect.

I say leave the fuse alone. If you have to make a change, make it so the machine can't be jogged by the operator.
 
Jraef said:
Doing it with the control fuse is again a last ditch device, not a primary means of designing in safe operation. Limit the number of successive starts in the PLC program. A very typical RVAT starter duty cycle design criteria is as follows:

On time = 30 seconds
Off time = 30 seconds
Repeat = 2 times
Rest = 2 hours
Click to expand...

I definitely would be looking at reducing the amount of starts, and if they have no access to the PLC program, this would be the best place. But you may need to inform management, because when the "it never did that before...." comes up, you know they will be looking for you. Been there and bought the T-shirt.
Good Luck
 
kingpb said:
I should have been more specific; why does the process need reduced voltage starting, i.e. can you switch to across the line starter?
Click to expand...

Not being there in the 60s I imagine the first service to the building could not handle the full current of an across the line start. Evidence points to that it may have even been an ungrounded Delta at the time. Sometime in the 70s the service was updated to 4 wire. A small POCO sub was probably installed about then as well.

There are enough issues with the current MCCs and the room they are installed in that the inspectors will allow no more changes or additions to it (inadequate working space and defeated GFP). Lucky I got to do what we did.
 
Status
Not open for further replies.
Top