Albert,
You can observe following simple steps to diagnose condition of a motor. If motor is not work at all, check connectivity between winding terminals. If connectivity between windings is shown, then windings are OK. Then check connectivity between winding terminals and motor body. Connectivity should not be shown between terminal and motor body. if motor trip after working some time , following matters may be reason,1. Wearing of motor bearings (friction of bearings have been increased and need to change)2. Low voltage (This will lead to draw high current by motor. Major reason may be lack of kVA of power source)
Thanks & Regards,Kaushika