Hey there... We are working on one DC motor after it was overhauled. DC Boom Motor 100 KW for Portrek Crane We have performed Servicing, Re varnishing,Commutator skimming undercutting Setting of Brush.and test the motor on no load.Motor was running perfect without sparking. When motor was fixed on crane just couple without load it was giving heavy sparking. We have opened three times motor to find the fault but Motor is perfectly OK. Even Drop test/armature windings/field interpole megger as well as ohms are also perfect. Even we changed carbon brush original Morgan. We have check all the possibilities for Sparking: Following Check/test we have carried out. 1. Defective Carbon Holders. - in good condition. 2. Carbon brushes – Replaced. 3. Uneven condition of Commutator - Surface is even. 4. Armature windings - Windings are in good condition 5. Field/Interpole windings - Good condition 6. We also looked for shorts or opens in the armature circuit and for grounds shorts or opens in the field winding circuits. – No Shorts/open found. 7. Drop Test - OK Can any body help me out on this matter what could be the reason ? One motor thing they swept one similar motor in place of this motor and it was running perfectly fine.that was also overhauled by us. I have doubt on Compensated windings which comes below commutator ? But not sure this DC Motor contain the same without opening armature wires. is that anyway to check compensated winding without opening armature windings. We do not carry any load testing facilities.only No load. Can any body help me out on this matter what could be the reason ?