overcurrent in dc motor

[cranedude]

Hi ,first time posting

looking for advice on a current problem we have with a rail mounted gantry crane.

2 long travel motors ( 40hp ) were found to be beyond repair recently..we changed both and carried out a stall test ...i found the current values exceed far beyond normal working and this no doubt is the initial cause of the motors failing.

These 2 motors are in parallel with a balancing cable between the interpole coils and armature connections to keep the motors running at the same speed due to rail irregularities...fields are kept consistent..with voltage changing from the generator/resistance circuit feeding the armatures...there are also ballast resistors to absorb the heat.

variable voltage is from a ward Leonard generator with the speed control changed with resistance changing as demanded.

All resistances are of the correct value....and check out for I/R faults.also cables are not earthed/shorted.

speed 1 seems ok..however speed 2 is around 180A on both motors ( should be around 120A)..at this point we can hear the genny struggling ...speed 3 -4 is causing flashing and for the overload to trip and motion to cut out..

Any advice on where to start this fault greatly appreciated

 

[Darkwood]

Welcome to the forum cranedude

 

[cranedude]

Welcome to the forum cranedude

thanks.

thinking on this fault....i should post a pic of the relevant circuit...im on nightshift ..n with respect to my dayshift colleagues i expect to be landed with this disaster..ha

 

[Bobster]

Ahhh Ward Lenard systems. Dealt with a few in my time.

I'll await some more info if you can before advising.

We did have an expert on here, until he went an got banned.

 

[Marvo]

I've given you full member permissions so feel free to post photos or circuit dwgs etc. There's a filesize limit so I'd suggest keep them < 500kb.

 

[cranedude]

Thx Marvo.

no need to post diagrams at the moment as the dayshift came through and found the fault.

Turns out that the marking on the connections poles for A and AA were reversed ( refurbed motor)...which in turn means that the interpoles are incorrectly wired in one motor...and reversing the fields just wouldnt cut it..

The most difficult of faults oftentime have the simplest of answers..kicking myself..ha.

No doubt the ward leonard diagrams will return soon..it does often throw the odd spanner at us...this was just senilty on this occasion

 

[Marvo]

Ahh, I doubt you'd have got you answer in this case via the forum without mentioning the fact it's just been rewound. Glad you got it sorted anyway.