AC Electrics - Alternators — Page 201, Lesson 207

I want to walk you through the world of AC alternators on aircraft, starting with a key distinction: frequency wild versus constant frequency systems. Let's begin with frequency wild alternators. These are alternators whose output frequency varies with engine speed — they don't try to hold a steady frequency. On aircraft, we typically use frequency wild alternators to power electrical de-icing systems. Why? Because the heater mats in those systems are made of resistive elements, and resistance isn't affected by changing frequencies. So the de-icing load doesn't care if the frequency drifts — it just draws power and produces heat. Now, what if you need a constant frequency supply but you only have a frequency wild system available? The answer is inverters. Here's the chain: you take the frequency wild AC, rectify it to DC, then feed that DC into a Static Inverter. That inverter converts the DC back into AC, but this time at a constant frequency. So you get a stable output from a wild input. That brings us to constant frequency alternators. The principle is simple: if you drive an alternator at a constant speed, its output frequency will be constant. But here's the practical problem — you can't just run the aircraft engine at a constant speed. That's not how you fly an airplane. So we need a device that keeps the alternator speed constant regardless of what the engine is doing. That device is the Constant Speed Drive Unit, or CSDU. Let me explain how it works. The CSDU consists of an engine-driven hydraulic pump. The output of that pump drives a hydraulic motor, and that hydraulic motor in turn drives the alternator. So the engine turns the pump, the pump pushes oil, the oil turns the motor, and the motor spins the alternator. The oil that makes this all work is contained in a reservoir that is entirely separate from the engine oil system. This oil serves three purposes: it's the fluid through which the mechanism operates, it provides lubrication, and it provides cooling. Now, how do we control the speed? The output of the hydraulic pump — and therefore the speed of the hydraulic motor — depends on the angle of a swash plate inside the pump. The swash plate angle is controlled by a device called a speed governor. And the speed governor itself is controlled by the load controller. The load controller senses the output frequency of the alternator and adjusts the torque output of the CSDU to the alternator drive — increasing or decreasing it as needed to hold the frequency steady. So what kind of accuracy are we talking about? Most CSDUs can maintain the alternator output frequency within 5% of 400 Hz. That means the frequency stays between 380 and 420 Hz. Now, what happens if the alternator itself suffers a mechanical failure? The CSDU is protected by something called a Quill Drive. Think of it as a weak link — a deliberately designed shear point that will break before any major damage can propagate back into the drive system. The CSDU operates in one of three modes. Let me walk through each: First, overdrive — this is when the engine speed is less than the generator speed. The CSDU is speeding up the input to reach the required output. Second, straight through drive — engine speed equals generator speed. The CSDU is just passing the drive straight through without changing it. Third, underdrive — engine speed is greater than generator speed. The CSDU is slowing the input down to match the required output. Some modern constant frequency generators combine the CSDU and the generator into a single unit. That combined unit is called either an Integrated Drive Unit, or IDU, or an Integrated Drive Generator, IDG. Same concept, just packaged together. Finally, let's look at what the pilot sees in the cockpit when something goes wrong with the CSDU. There are two main indications. First, low oil pressure warning lights. These illuminate when the CSDU oil pressure drops below a predetermined minimum value. Second, high oil temperature warning. This allows the CSDU oil outlet temperature to be monitored — if it gets too hot, you get a warning. Those figures show you the brushless alternator construction and the CSDU with its drive disconnect mechanism — take a look at them on screen as we go.