Home Forums General Discussion Technical Questions suspect otis generator problem

gidday. just lately encountered a possible otis generator problem circa 1954 in melb victoria. not sure of the part numbers but would have been state of the art at the time, full large gearless mg set. the problem is 3p keeps popping when the lift sits at standby with the genny sounding as its loaded down. have isolated loop ccts to the point where only the ga/ma plus the series field is connected with also the series diverter round spring as well and we get a dc output to earth.(5 to 25vdc) there should be no output at all at this point. if we disconnect the series field the fault goes away. the problem is the motor rewinders say the genny is ok, they have split it down found not fault apart from some insolation cracks which they have fixed. your thoughts?

I know you say you isolated some circuits, and does that include disconnecting the hoist motor? Hope this isn't a stupid question, but if you take the hoist motor brushes out, does the genny still sound loaded down? The 3P overload is in the loop, so if it keeps popping, then there must needs be current flowing through it, and through the loop. The problem might not be on the genny at all? I had a loop wire disintegrate once, where it was snaking through the control cabinet, taking the loop to ground.

If the hoist motor has already been disconnected in your isolation, then how about check the series field windings with a megomoeter? Might as well check the field windings, too.

It would not sound loaded down, unless current was flowing. How about take your hall effect DC Amprobe, and check all conductors, to see where the current is flowing? That might help isolate the issue. Suicide circuit operational?

thanks vic for replying. yes when the motor brushes are lifted you open the loop cct so yes the fault clears as well.we have also checked the motor armature as well. just had a update that the motor rewinders have rechecked and meggered the genny and they think it is ok. i have to say that you might be right in that we opened the contacts on h relay n/c to isolate the loop cct but did not disconnect them, so it could be in the controller wiring. it also has dawned on me that the fact that the brake wasnt lifting that could also increase current flow so you're right might have to check the suicide cct for leakage.

thanks vic much app.

did this problem just start occuring while in service, or was the generator removed and checked out then reinstalled? If you have had the generator rewound, or removed, it is imperative that the generator field and the series field are hooked up with the correct polarity. For example, if you were to reverse the generator field wiring on a good unit, and turned on the generator, the loop circuit would immediately begin to build until it pulled through the brake or popped the overload. this is due to the fact that the suicide circuit is designed to put residual loop current through the generator field to negate the magnetic field produced by the series field. If either field is not in the correct polarity, then the suicide circuit will not decrease the magnetic field, but will increase it, causing the generator field to increase its output and cause the above problem. I have been on many jobs where the fields were not hooked up correctly, causing the car to run in the opposite direction commanded. to fix this the mechanics will sometimes reverse the motor field to get the car to run in the right direction. the problem is that after the car stops, the loop circuit will continue to build up either slow or fast, depending on the strength of the series field. If this problem just started out of the blue, then you must check the suicide circuit to make sure it is intact as Vic stated. usually a grounded loop circuit will cause compounding problems, not the problem you are encountering. Also, why are you measuring the generator output to ground? you should be measuring across the brush riggings of the motor or generator, as this will give you the correct output voltage.

thanks scott for the reply. yes we have been testing the output from the brush gear. this fault has just appeared not after a repair. i have just found out we found a high resistance with the controller wiring within the suicide cct which was not letting the magnetic field bleed out. my fault that i didnt actually disconnect wiring instead of opening contacts to isolate the fault. you live and learn everyday! thanks for all your help.

cheers.

uppo72
Glad you got out of that tight spot. It always helps to talk to someone.

Scott, thanks, that's more than I remembered about the suicide circuit!

cheers vic thanx for all the help lads.