590,576. Automatic control systems. ENGLISH ELECTRIC CO., Ltd., CARNEGIE, H. S., and DOLAN, H. Jan. 29, 1945, No. 2241. [Class 38 (iv)] [Also in Group XXXVII] Current speed.-The speed of an electric motor is controlled by means including an electromagnetic induction type brake which is mechanically coupled tothe motor and which is automatically excited in response to any increase of motor speed above a desired value, thereby tending to restore the speed to that value. The wound rotor of a threephase alternating current motor 4 is connected through slip-rings and brushes to a resistance 7 which may be a liquid resistance actuated by the handle 10. An electromagnetic brake includes a motor-driven field system 11 and a stationary secondary system 12 which may be fluidcooled. The field system 11 is supplied with direct-current through slip-rings and brushes by an exciter 13 driven by a motor 14 which also drives a substantially constantvoltage exciter 16. A rectifier 18 is included in the brake field winding circuit. A field winding 13b of the exciter 13 is supplied from the exciter 16 through a rheostat 32 which is adjusted by the handle 10 so as to reduce the field resistance when the secondary resistance of the motor 4 is reduced. A field winding 13a opposes the field winding 13b and is supplied by a tachometer generator 33. A bridge rectifier 34 ensures uni-directional current in the field winding 13a irrespective of motor direction, alternatively a reversing contactor in the field winding circuit may operate with the reversing contactors for the main motor. Due to the rectifier 18, the brake field is only excited if the field winding 13a overcomes the winding 13b. The exciter 13 may have an additional self-excited field winding 13c. The regulator 32 may be placed in the circuit of the field winding 13a in which case it will increase the field resistance as resistance is removed from the secondary circuit of the motor 4. The regulator 32 can further be placed in the field winding circuit of the tachometer generator 33. In Fig. 3, a winding 13a of the exciter 13 is supplied through rectifiers 19 from current transformers 20 in the supply to the motor 4. Opposing excitation is supplied by the field winding 13b fed through a regulator 21 from an exciter 16. The effect of the rectifier 18 is such that the brake field is only excited if the field winding 13b overcomes the winding 13a. An economy resistance 23 shunted by contacts 24, a resistance 25 shunted by contacts 26, or a resistance 27 shunted by contacts 28 are provided so that when the mechanical brake is applied these contacts are opened and are reclosed when the brake is taken off. The current transformer can alternatively be placed in the main motor secondary circuit. Automatic reversing means, comprising a potentiometer 29, the brushes of which are reversed by a torque motor 31 which is supplied through a reversing switch 35 from current transformers in the main motor secondary circuit, are provided. On phase reversal in the secondary circuit of the motor 4 when it is re-generating, the relative positions of the brushes of the potentiometer 29 are reversed, to reverse the field winding 13a and increase the resultant excitation of the exciter 13. Alternatively, a biassed exciter driven by the motor may supply a further component of the excitation to the main exciter only above a predetermined speed. The operation of the switch 35 with the main motor switch prevents reversal of the torque motor 31 with reversal of the main motor. The main exciter 13 may be of the type described in Specifications 560,531 or 579,570.