Patent GB-1017065-A (GB)
Title: Improvements relating to electrical control circuits
IPC: H02P23/00
Filed: 1962-08-30
Abstract: 1,017,065. Automatic speed control. GARRETT CORPORATION. Aug. 30, 1962 [Aug. 31, 1961], No. 33349/62. Heading G3R. [Also in Division H3] A circuit for controlling or limiting the speed of a turbine comprises a frequency discriminator network 11 supplied with speed-responsive current, a silicon controlled rectifier 37 being actuated by the output of the network to control the fuel supply. The A. C. signal input is applied between terminals 20, 21, so that, when diode 33 conducts, capacitor 34 is charged until it triggers the controlled rectifier 37 to energize valve solenoid 40 and limit the speed. At low frequencies of the input signal, inductor 27 offers low impedance and capacitors 25, 26, 30, high impedance, the voltage across capacitor 30 exceeding that across the inductor to prevent the capacitor 34 from being charged through rectifier 33. As the frequency increases, the voltage of resonant circuit 24 exceeds that of capacitor 30 so that capacitor 34 is charged through diode 33 while diode 31 is non-conductive. The capacitor 34 is charged at a greater rate with increase of frequency. A resistor 23 having a negative temperature coefficient compensates for temperature variations. A capacitor 43 produces a holding current in the controlled rectifier 37 when the load is highly inductive. In a modification, the rectifier 37 is operated at a low power level by the inclusion of a cathode resistor which ensures that the rectifier is shut off after being triggered. This provides a pulse for triggering a succeeding controlled rectifier, Fig. 3 (not shown). An additional component of the signal for triggering the silicon controlled rectifier may be provided to allow for a greater degree of over-acceleration. A circuit for detecting the rate of change of frequency is associated with the input terminals 20, 21, and, when the output of this circuit is sufficient to cause breakdown of a Zener diode, an additional signal is conveyed to the trigger of the controlled rectifier, Fig. 4 (not shown). In another arrangement, a controlled rectifier has its trigger electrode connected to the frequency discriminator and its anode connected to the turbine control. When the rectifier is fired, a pulse generated across a resistor appears at the trigger of a second controlled rectifier after a short delay period. A capacitor then passes current to block the first rectifier, the second rectifier ceasing to conduct when the capacitor becomes charged. When the apparatus is used for speed control, the time constant of a delay network is selected to provide the required on-off periods of power for turbine. In further embodiment, a diode is shunted by a resistor to provide both positive and negative output pulses from the frequency discriminator. A controlled rectifier conducts when a positive output from the discriminator appears on a line due to the pre-set speed of the turbine being exceeded. As soon as the speed decreases below the pre-set value, a negative pulse on the'line fires a second rectifier so that the first rectifier ceases to conduct and the speed increases again. The turbine speed will therefore fluctuate about the preset value.