A method for controlling a rotation rate of an electric motor includes the s following steps: determining if an absolute value of a difference between an objective rotation rate of the electric motor and an actual rotation rate of the electric motor is greater than or equal to a predetermined value, and if yes, compensating a q axis current of the electric motor to adjust the rotation rate. A first and second conducting wires extend in parallel with an axis of the shaft, and are equidistantly positioned relative to the axis, on a virtual circle that has a center point on the axis of the shaft. In one embodiment, the multijunction solar cell structure includes a substrate, a bottom cell, a middle cell, a top cell, a bypass diode, a lateral conduction layer, and a shunt. The kit may further include a carry case with a rotatable disc remotely controlled simultaneously with the front and rear discs.