Motor power device of wheelchairs for disabled users, wherein the wheelchair comprises a bearing frame that supports a seat of the wheelchair, device comprising: - a motor unit comprising a head tube wherein a steering column is inserted, supporting a handlebar in upper position and associated in lower position with a fork to which a wheel is pivoted, being provided with electrical motor and brake, both of them being actuated by means of controls mounted on said handlebar, characterized in that the device further comprises: - an intermediate frame removably mounted on said motor unit and adapted to be removably attached on the bearing frame of the wheelchair, in such manner that the intermediate frame is disposed in intermediate position between said motor unit and said wheelchair, - first inclination adjusting means disposed between the motor unit and the intermediate frame for adjusting the inclination of the head tube of the motor unit with respect to the intermediate frame, and - second inclination adjusting means disposed between the intermediate frame and the bearing frame of the wheelchair for adjusting the inclination of the intermediate frame with respect to the bearing frame of the wheelchair; wherein said intermediate frame has a substantially "U" shape comprising a crosspiece and a pair of arms substantially orthogonal to the crosspiece; said first inclination adjusting means are mounted in said crosspiece of the intermediate frame and said second inclination adjusting means are mounted in end sections of said arms of the intermediate frame, said second inclination adjusting means comprise a joint comprising: - a cylindrical sleeve adapted to tighten an upright of the bearing frame of the wheelchair; - a tubular element having a cylindrical duct adapted to house said end sections of the arms of the intermediate frame, - a first disk mounted on the tubular element; and - a second disk mounted on the cylindrical sleeve; wherein said first disk and second disk are revolvingly coupled in order to adjust the inclination of the intermediate frame with respect to the bearing frame of the wheelchair. - a motor unit comprising a head tube wherein a steering column is inserted, supporting a handlebar in upper position and associated in lower position with a fork to which a wheel is pivoted, being provided with electrical motor and brake, both of them being actuated by means of controls mounted on said handlebar, - an intermediate frame removably mounted on said motor unit and adapted to be removably attached on the bearing frame of the wheelchair, in such manner that the intermediate frame is disposed in intermediate position between said motor unit and said wheelchair, - first inclination adjusting means disposed between the motor unit and the intermediate frame for adjusting the inclination of the head tube of the motor unit with respect to the intermediate frame, and - second inclination adjusting means disposed between the intermediate frame and the bearing frame of the wheelchair for adjusting the inclination of the intermediate frame with respect to the bearing frame of the wheelchair; - a cylindrical sleeve adapted to tighten an upright of the bearing frame of the wheelchair; - a tubular element having a cylindrical duct adapted to house said end sections of the arms of the intermediate frame, - a first disk mounted on the tubular element; and - a second disk mounted on the cylindrical sleeve;