DC Electrics - DC Motors — Page 117, Lesson 114

I want to walk you through the split field series actuator and then the two types of motor actuators used in aircraft systems. Let's start with the split field series actuator. This type of actuator has two differentially wound series field windings. "Differentially wound" means the two windings are arranged so that each one produces a magnetic flux in the opposite direction to the other. Only one winding can be energized at any one time, and the direction of rotation of the motor depends entirely on which winding is energized. So if you energize one winding, the motor turns one way; energize the other, it turns the opposite way. Limit switches are normally fitted in series with the field windings. These switches are operated by the mechanical load — the thing the actuator is moving. Their job is to stop the motor automatically when the load reaches the limits of its travel, so the actuator doesn't over-travel and damage itself or the mechanism. Now let's look at the operation. When you make an OPEN selection — say from a cockpit switch — a supply is fed to the armature via the limit switch, the open field winding, and the brake coils. Energizing the brake coils releases the brake, if one is fitted, allowing the motor to operate. So the brake is held off electrically while the motor runs. On completion of the actuator travel, the limit switches are tripped. There are two specific limit switches to know. First, the Open Limit Switch: this breaks the supply to the motor on completion of travel and simultaneously makes the circuit to the 'open' position indicator. Second, the Close Limit Switch: this sets up the 'close' circuit, ready for completion when a selection of 'close' is made on the control switch. So the close limit switch doesn't close the circuit yet — it just prepares it, so that when you later select close, the circuit is already armed. There's an important note here: the brake solenoid operates immediately the supply is broken, thus preventing over-runs or creep. So as soon as the limit switch cuts power, the brake snaps on to stop any further movement. A slipping clutch may also be fitted between the armature shaft and the gearing. This is a safety device — it prevents damage that could be caused by mechanical overload, for example if the actuator jams or hits an obstruction. The clutch slips instead of letting the motor force something to break. Now let's move to motor actuators in general. There are two types in use: rotary actuators and linear actuators. Rotary actuators are operated by small reversible motors which rotate an output shaft through a gearbox. The gearbox reduces the speed and increases the torque from the motor to the output shaft. They are used for the operation of fuel valves and air/oil shut-off valves — components that need a turning motion to open or close. Control is by means of an ON/OFF or OPEN/SHUT selector switch. Two limit microswitches control the extent and direction of travel and also operate the visual indicators in the cockpit. One limit switch is always closed, allowing current from the selector switch to flow to the actuator. The limit switches change over at the end of travel — meaning the closed one opens and the open one closes, reversing the circuit state. In the diagram you can see the selector switch feeding 28 V DC. The circuit goes through the close limit switch and the open limit switch, then to the motor. The motor drives through reduction gearing to the output drive. There are brake coils and a clutch in the system, and an open 'doll's eye' position indicator — that's a visual indicator in the cockpit that shows the position of the valve or actuator. So to summarize: the split field series actuator gives you two directions of rotation from two opposing field windings, with limit switches to stop travel and set up the opposite direction, plus a brake to prevent creep and a slipping clutch for overload protection. Rotary actuators use a reversible motor, gearbox, and limit microswitches to operate fuel and shut-off valves, with cockpit position indication.