Hoist motor burning out

[FoRmaT]

Hi Everyone,

 

We have 3 identical 5Ton cranes at work and in the past 5 years i have been there, we have burnt out about 15 hoist motors on the one crane. These cranes use a halfwave thyristor speed feeding a Siemens slip ring motor. Thr motor is rated as below:

 

MODEL: 1LT3 166 4AA11

KW: 14.5KW

V: 380V

I:32A

RPM: 1450 RPM

RI: 36A

 

I have checked the resistance backs and they read 6 ohms on all 3 cranes which seems fine to me. I have also taken current readings of the motor at full load and their readings are around or lower than the rated values, on the stator and slip rings. I was told that i can only get accurate readings on the resistance banks using a wheatstone Bridge rather that my DMM. Is this true ? I have been through everything i can think of along with other people and no-one seems to know what the problem is. Any help would be much appreciated.

[GGOSS]

Hello FoRmaT,

 

I would be interested to know what type of damage these motors have suffered, and whether it is consistent accross all failed motors.

 

That sort of information could assist us to guide you further.

 

Regards,

GGOSS