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4.  Cabin Pressure Controller Internal Fuse Tests
A.  General
(1)  
Overcurrent protection is designed into Hamilton Standard CPC 21 and CPC 22 cabin pressure controllers. Three fuses are installed internal to the controller on the output drive signals to the outflow valve clutch coils and AC motor control windings. The purpose for the fuses is to prevent thermal damage caused by excessive current draw. Should resistance drop too low, as a result of aircraft wiring shorts or shorts in clutch coil or AC motor windings, the associated fuse will open. Corrective action requires not only isolation of the overcurrent failure, but removal and replacement of faulty controller. Should the controller be replaced prior to correcting the electrical short, the newly replaced controller will experience the same open fuse failure.

(2)  
Protective fuses are identified in controller electrical schematics as F1, F2 and F3. Theses fuses are associated with the AC clutch coil, DC clutch coil and AC motor windings, respectively.

(3)  
The outflow valve is driven either by an AC electrical actuator or a DC electrical actuator. Each actuator may be connected to the valve drive shaft by an electrically operated spring-loaded clutch. AUTO and AC MANUAL modes operate the AC actuator. STANDBY and DC MANUAL modes operate the DC actuator. When either actuator is in operation, the clutch to the other actuator is disengaged. With no electrical power to the clutches, the DC actuator clutch is spring-loaded disengaged and the AC actuator clutch is spring-loaded engaged. With electrical power to the clutches, the DC actuator clutch is engaged and the AC actuator clutch is disengaged.

(4)  
As described above, supply of DC voltage to the clutch coils is a function of mode selection. Current travels from the controller through the clutch coils to ground.

(5)  
Current flow through the AC motor control windings is also dependent on switch position, however, direction of current flow and source of electrical power will vary depending on the controller mode of operation. Starting at the controller, current flows through aircraft wiring to the AC motor then back through aircraft wiring and terminates at a ground provided in the controller. When operating in AUTO, voltage is supplied to the AC motor center tap. This electrical supply is protected by fuse F3. Current flow through the center tap and appropriate (open or closed) AC motor control winding is the result of a ground provided by switching logic within the controller. Thus, flow of current follows a loop path starting and ending within the controller.

(6)  
AC MANUAL operation also follows a loop path starting and ending within the controller. However, internal controller logic will cause switching to an alternate source of electrical power. As a result of this switching, current no longer flows through fuse F3. In addition, during AC MANUAL operation, electrical power is not provided to the motor center tap. Current flow in series through both control windings and direction of motor rotation is accomplished by toggling power and ground signals within the controller. Therefore, AC MANUAL will operate with an open F3 fuse.

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