motor control question

[Clayton79]

Heres the situation,

there is a little oil pump,motor, resevoir, for a test bench GE 15hp,3ph 440v 1170 rpm motor

the other day it was brought to my attention that when you tried to shut it off it didnt, until one shut off the main 480 safety switch,

so i replaced the start stop combo switch ( the one were internaly you have your jumper from stop to start)

now it does shut off but you have to hold in the stop button for a few seconds.

i didnt like that so i wired in temp. a maintained e-stop button, removed the float and temperature switch, so only thing in the circuit was, e-stop, start stop combo, and the result was after hitting the e-stop you could still count to two onethousand before
the 709 motor starter dropped out.

normal?? i'm used to starters dropping out fast with no control power. this one has me baffeled, any suggestions? thoughts?

 

[RLCL Trekker]

RLCLtrekker

RLCLtrekker

With the start/ stop wired properly as a given, the coil assembly on the starter could have picked up a magnetic tendency. If you have a demag table, take the starter plunger and stationary piece from the starter and demag it. With the control power removed you should drop out as you say immediately. Hope this helps

 

[Clayton79]

thank you, that would seem to be a very good explanation, since this machine is older than the hills, i dont know what a demag table is, (degaussing?) but i can prehaps get a replacement coil for the starter. i will try this and see what happens


thanks again.

 

[jbelectric777]

trekkers right i wanna add one more thing, make sure someone didnt put any kind of time delay relay in that the stop is controlled thru. e stop is exactly what it says EMERGENCY STOP and it could be the difference of someone getting hurt bad, be sure, i cant make this clear enough. if ya have to color the e-stop red and the normal stop another color so the operators know the difference.

 

[kwired]

Is the stop button in series with all control elements or just in series with the holding circuit? If pressing start and stop at same time nothing should operate if it is in series with all control elements.

Also if the stop would happen to be installed on the grounded conductor any unintended ground faults could cause undesired operation.

 

[Clayton79]

it is the simplest of control circuits. 120v control transformer, power goes thru a float sw, a variable temp switch, and then thru the start stop sw and the overloads.

for testing purposes, i unwired the float and temp control, put a maintained e stop in series ahead of the start stop and the result was the same.

 

[Jraef]

No way a 709 contactor armature became magnetized unless it is seriously damaged. Much more likely the coil was over heated and it has swollen and is not letting go of the armature as it should. The one drawback of A-B's famous "vertical lift gravity dropout" system was that if the coil swells, as it does when it over heats, it hangs up on the armature. This was a common problem with the 709 Series.

A-B fixed that in the 509 Series that came out in the late 1970s; they put in a thermal cutout device embedded inside the epoxy of the coil that opens before the thermoplastic would swell. So you probably just have to replace the coil and maybe clean up the goo that may have oozed out of it if it actually melted. The next thing would be to determine why the coil over heated, which most likely is because something got into it and prevented the armature pole faces from coming together completely. That could have been a piece of debris like wire insulation or even a bug that took up residence inside the unit.

But your 709 is at least 30 years old, might be time to consider scrapping it. It also had only 2 overload heater elements, something that is now illegal (but grandfathered).

 

[augie47]

Folks. Little "tricks" like that is what makes Jraef a valuable asset to this Forum.
(and the Forum valuable to those who join)
Gentlemen, You don't learn that in the books !
We thank you, Sir !

 

[Jraef]

Folks. Little "tricks" like that is what makes Jraef a valuable asset to this Forum.
(and the Forum valuable to those who join)
Gentlemen, You don't learn that in the books !
We thank you, Sir !

My wife says I am a fountain of useless information...

 

[augie47]

well, at least yours talks to you which may be a good thing :grin:
(mine does not as I am "always on that d*** Holt thing" -- can you relate, Dennis ? )

 

[Clayton79]

Update,

checked the freedom of movement of the contactor armature, it fell very readily
then i noticed the secondary of the control transformer was not grounded, there was 40v ac from the x2 side to ground, i grounded it and now it is working as i would expect, near imediate drop out.

thanks
clayton

 

[don_resqcapt19]

Update,

checked the freedom of movement of the contactor armature, it fell very readily
then i noticed the secondary of the control transformer was not grounded, there was 40v ac from the x2 side to ground, i grounded it and now it is working as i would expect, near imediate drop out.

thanks
clayton

I don't see how changing the control circuit from an ungrounded circuit to a grounded circuit would make it work differently.

 

[hillbilly1]

I thought Jraef had it nailed, but it looks like you may have a ground fault, or another induced voltage there. When the stop is opening, there may still be some voltage on the coil, but not enough to keep it sealed. I would check voltage there as the stop button is pressed to see if there is still some voltage on the coil.