tim_callinan
Member
OP
First of all I wanted to say I really appreciate this group. Its made my life as an instructor alot easier over the years. Appreciate all the advice.
Tim
Tim
Fuses Help please
- Thread starter tim_callinan
- Start date
jraef
Member
Nobody seems to have caught this issue. The mechanical brake for a 1/3HP motor should NOT be taking 5A of current! Something is likely wrong with your brake coil. A mechanical brake for a motor that small should not be pulling more than 50W of power, usually a lot less than that. Assuming your brake is connected to the motor power terminals in the peckerhead, there might be a short in the rectifier for it. If the brake was intended to be powered externally, why would your motor current go up at all? Are you sure the brake is functioning correctly? It almost sounds as though the brake is NOT disengaging, so the motor is going into "locked rotor" condition.
ndzied1
Lifetime Supporting Member
I got the impression that the OP is talking about the motor current, not just the current to operate the brake. So if I have a motor running and then stop the shaft by applying a break, I would expect to see the locked rotor current of the motor.
Hopefully the OP can chime back in to clear this up.
jraef
Member
Motor brakes are intended to stop the shaft when the motor is NOT running... Applying the brake while the motor is running would be counterproductive. It would be like driving your car and slamming on the brakes while simultaneously flooring the accelerator pedal.
Use an Overload on the bottom of the motor contactor, the NC contacts for the overload are in series with the coil for the motor starter so it will turn off if tripped.
You could use fuses, but fuses just get replaced typically. it's much easier to spec for a motor starter than it is to ensure you have the right type of fuses for your applications.
In your setup you probably have a main incoming power distribution with circuit breakers, and then a circuit breaker is going to feed to a panel with fuses. the fuses in the panel serve the same purpose as the breaker, they protect the wires, not your motors.
Then from that panel you feed to your motor contactor and have another set of fuss to protect that wire (which will be smaller typically than the incoming power). and then the motor itself is protected by an overload that can reset since it is going to be (or at least better be) the first thing to trip on that circuit that needs protection.
you could either do resettable circuit breakers instead of fuses if you really wanted, or a motor starter and overload would be best and standard. If you have a set of fuses only, then you also want to have a phase loss/phase detection circuit in there because of the possibility of losing a fuse while running and then single phasing the motor to death.
By the way.... what is your motor overload set to and how is it wired? It's odd you can pull 8 amps on a 1.6 fla motor and not trip. they'll run something like 200% current for 10 seconds before tripping, and even less when its 4x (from the factory, as long as you don't set the switch to 20 or 30 seconds)
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I believe he is 1) saying the motor current is driven up by the brake, not that the brake current is going up. and 2) he is using that brake to put a load on the motor for demonstration purposes, so as counter-intuitive as it sounds, it's exactly what he's doing.
right?
There is certainly a lot of good information here to help the OP out so I won’t add my 2 cents worth.
What concerns me is that the OP is asking pretty basic motor control and fusing/overcurrent issue and he (she?) is the TEACHER? Note: I do applaud him for trying to get the correct info and not just winging it.
I this where we are at in our educational system?
What concerns me is that the OP is asking pretty basic motor control and fusing/overcurrent issue and he (she?) is the TEACHER? Note: I do applaud him for trying to get the correct info and not just winging it.
I this where we are at in our educational system?
PLCDontUQuitOnMe
Member
I didn't want to say it because I thought it would be kind of mean, but yes, how do you teach without knowing these things? Maybe it's not his core course and he's filling in for someone else.
A fuse or a breaker will accomplish the same goal, but the difference is in how fast it will clear the fault. Speaking general terms, a fuse will clear a ground or phase to phase fault much faster than a breaker. And the closer to the load the fuse or breaker is placed, the better. The reason this matters is that when you do arc flash calculations, the time needed to clear a fault will greatly effect the level of protection needed when working on the equipment. This is probably well beyond the scope of your class, but it would be a good idea to at least touch on it with your students so they are better prepared for what they will encounter in the real world.
There are a lot of variables involved when choosing what to use and where to place protective devices, and there is always disagreements on what is the "best" way to arrange things. Arc flash, personal in protection, and equipment safety are all indepth classes on their own, so don't get caught up going too deep in the weeds on them with a beginner class. Just hit the highlights so your students know they exit.
Bubba.
Yes, sadly it is. Who would want that job? I thought I would and I tried the teaching route for a year or so. I made just under 45k / year for full time. I ended up leaving and getting a job that paid twice that for less work.
(truthfully, I just hate people and I have a job where I don't have to deal with too many people - but seriously I would strongly discourage anyone from going into education right now)
If memory serves me Bussman came out with LPJ fuses years ago and one of their claims was that they could be sized low enough to provide motor overload protection. I don't think it quite caught on. I was raised that you provided fuses for short circuit protection and thermal overloads (heaters) for overload protection. Then overload relays came along to replace heater elements. Then motor protection circuit breakers came along and replaced the fuses and overload relays.
OP, as you found out fuses of any size are expensive and in bigger sizes I prefer to design around circuit breakers. There might be reasons here and there for fuses but finding that $150 fuse late at night on the outskirts of Timbuktu will make a circuit breaker disciple out of ya'.
OP, as you found out fuses of any size are expensive and in bigger sizes I prefer to design around circuit breakers. There might be reasons here and there for fuses but finding that $150 fuse late at night on the outskirts of Timbuktu will make a circuit breaker disciple out of ya'.
that is unfortunately the way it goes. you can either earn 50k a year as a teacher teaching how to do motor controls/programming/industrial technology. or you can actually go and do it for 2 or 3 times as much depending on where you live.
the salary is dictated by the necessity of it. most people learn controls and industrial technology by OJT, so there isn't a large difference in the number of people wanting to pay a college for classes on it vs the number of people who want to teach it.
The other issue is always that a college isn't going to charge a lot or even bother to elevate an industrial technology course in the way they do even a programming course. as far as most colleges are concerned, you're either a hammer mechanic or a rubber stamp engineer.
Wages for educators in general are abysmal currently (here in the US at least).
I agree with Rson that right now education is simply not a career worth pursuing.
Not to derail the thread from the original topic. But if there isn't much difference in later pay from people who went to school and those that didn't, then the teaching position may not be as necessary as some think. Too much "Bachelors required" positions that used to be a year of OJT.
jaden
Member
Because of the shortage of technical people, including Instructors, some schools are settling for instructors without practical experience. The Community College that I teach continuing Ed at (SE Nebr) has one of the best Associate Degrees in Electromechanical and Electronic Systems in the country because of the high standards and combined experience of the staff. The instructors that I know don't do it for the money, but rather the satisfaction and a sense of calling. Graduates from the good schools do earn more money and advance faster. They may have hundreds of job opportunities from several states to choose from when they graduate.
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