DC Electrics - DC Motors — Page 110, Lesson 113

I want to walk you through the next topic in DC Electrics: DC Motors. We've just looked at the starter-generator, and now I want to explain its advantage and disadvantage in more detail, then move into actuators. The starter-generator's big advantage is that a single device does two jobs — it works as both a starter motor and a generator. That saves weight and reduces complexity, which is always valuable on an aircraft. The disadvantage, though, is that it cannot maintain full output at low RPM. That is, if the engine is turning slowly, the starter-generator cannot deliver its rated power. Because of this limitation, starter-generators are typically used on turbine engines, which operate at a consistently high RPM. A typical starter-generator supplies 300 amps at 28 volts — so 300 amperes of current at a nominal 28-volt DC bus voltage. Now let's move to a new topic: actuators. In a modern aircraft, many components and pieces of equipment are installed in locations that the pilot or crew simply cannot reach to operate manually. To control those items remotely, we use electrical actuators. These actuators fall into two main groups: solenoid actuators and motor actuators. Let's start with solenoid actuators. A solenoid actuator is used to control selectors in hydraulic and pneumatic systems. When electrical power is applied to a solenoid — which is essentially a coil of wire — it creates a magnetic attraction that opens a valve. So the solenoid converts electrical energy into a linear mechanical movement to operate a valve. Now for motor actuator construction. The motor inside a motor actuator is a high-speed reversible motor. That means it can run in either direction — forward or reverse — and it runs at a high rotational speed. This type of actuator is widely used for electrically operating things like fuel valves, cooler shutters, and trimming tabs. Because the motor runs at high speed but the output movement needs to be slower and more powerful, a wide-ratio gear train is used to transmit the power and reduce the speed. The actuator itself can be either rotary — producing a spinning output — or linear — producing a straight-line push or pull movement. So to summarise: we have solenoid actuators for simple on/off valve control via magnetic attraction, and motor actuators with a high-speed reversible motor and a gear train for more complex positioning tasks like fuel valves or trim tabs.