A defective drive can definitely cause fuses of breakers to trip. Since the drive conducts the same amount of current as the motor, it is one of the most stressed components of a drive system. In most cases, drives do not simply go bad but rather are stressed to the point of failure. Locating the source of the stress is the key to correcting the problem. Grounded motor: Check to make sure the motor has not become grounded by checking the resistance from each terminal of the motor to earth ground. If using an ohm meter, the resistance to ground should be in the Megs of ohm's or essentially an open. A motor's internals can either permanently or momentarily short to the case. A grounded motor will almost instantly cause permanent drive damage giving you the false impression that the drive is the source of the problem. Line Power: Make sure line power is clean. Certain drives are more susceptible to problems caused by “dirty” line voltage as they may use certain portions of the 60Hz line as a clock. Line power can be distorted by the cycling of large machines, motors, pumps or welding operations. AC line filters can help maintain a clean AC waveform. Wiring: If using a field or shunt wound motor, connecting the armature winding of the motor to the field output of the drive will permanently damage the drive and give you the false impression that the drive is the problem. Even if the motor is disconnected, a damaged drive will continue to blow fuses or trip breakers. Speed command signal: If using an external command signal to control motor speed, make sure the signal is isolated or make sure the drive has input isolation. Connecting two non-isolated devices together will cause damage to the drive and the device providing the signal. Overloading: Exceeding the drives current rating or ambient operating temperature can stress the drive to the point of failure. Monitor motor current to make sure it is within the level you expect and not exceeding the rating of the drive. If the drive is in an enclosure, adding forced air will help insure the drive is operating within the ambient rating.