John Deere L120 burnt out new clutch?

Hi , i just fitted a brand new John Deere electric clutch,mowed fine for thirty minutes then blades stopped followed by engine, noticed some smoke coming from the clutch ,replaced blown 20 amp fuse,engine started but still no clutch,removed clutch to find the plastic insides have melted .The grass was dry and medium length (not heavy cutting), Volt meter showed clutch getting twelve volts.Has anyone else had this problem, can anyone tell me why a brand new clutch would do this,Thanks Dave