In control circuits for controlling D.C. electric motors, the motor is supplied in pulses so that the mean power supplied to the motor is varied by varying the frequency and/or mark-space ratio of the generator pulses. In such arrangements electrical braking is obtained by reversing the field winding connections but the field current has to be kept relatively low even for heavy braking. In the present invention means are provided for ascertaining if the armature current exceeds the field current during each pulse of current and for restricting the range of current which can be applied to the motor if the armature current exceeds the field current by a predetermined amount.