Abstract: A drive system is provided for driving a rotatable seed drum of a seed planter which includes a fan for supplying air under pressure to the drum. The drum drive system includes an alternator which is mechanically driven, preferably from a drive for the planter fan, and which has output terminals connected to a motor mechanically connected to the seed drum. A ground speed signal generator is mechanically connected to a ground-engaging wheel to develop a signal which has an amplitude proportional to ground speed and which is applied through an adjustable resistance to a field winding of the alternator after passing through a control box. A drum speed signal generator arrangement is provided for developing a drum speed signal having an amplitude inversely proportional to the speed of rotation of the seed drum. This signal is also directed to the control box. Preferably, the signal generators include reluctance generator devices.