DC Electrics - Generators and Alternators — Page 102, Lesson 96

I want to walk you through the load-sharing circuit for DC generators — this is how two generators on an aircraft are made to share the electrical load equally, so one doesn't end up doing all the work while the other loafs. Let’s start with the condition where both generators are “off line.” That means neither generator is connected to the aircraft’s electrical distribution system — the bus bar. In this state, there is no output from either generator, and two key components are open: the Equalizing Relays and the Line Contactors. I need to define what a line contactor is, because it’s central to everything that follows. A line contactor is a large solenoid-operated contact. Its job is to connect the output line of the generator to the bus bar — but only after the generator’s output voltage has been checked and found to be acceptable. That contactor can be closed either automatically or manually from the cockpit. Now, let’s bring generator number 1 “on line.” When that happens, the No. 1 generator line contactor closes. The output from generator 1, regulated by its own voltage regulator, is now supplied to the aircraft bus bar. At the same time, the No. 1 Equalizing Relay closes. That relay is actually part of the generator line contactor — it’s not a separate standalone component; it’s built into the contactor assembly. Next, generator number 2 is brought “on line.” The No. 2 generator line contactor closes, and its output — regulated by its own voltage regulator — is also supplied to the bus bar. The No. 2 Equalizing Relay closes as well. And here’s the critical step: with both equalizing relays closed, the two generator voltage regulators are now connected into what’s called the Equalizing circuit. So what happens if the two generators aren’t putting out exactly the same voltage? If there is any potential difference — any voltage difference — between the output of generator 1 and generator 2, a current will flow through the equalizing coils. That current flow applies correcting values to each voltage regulator. Specifically, it increases the voltage of the lower-voltage generator and reduces the voltage of the higher-voltage generator. This correction continues until the two generator voltages are exactly the same. At that point, they equally share the total aircraft load. Let me be precise about the components in this circuit, because Figure 6.18 shows them clearly. Each generator has a field coil — that’s the coil that creates the magnetic field inside the generator. Each has a voltage control coil, which is part of the voltage regulator. And each has an equalizing coil — that’s the coil that senses the voltage difference and applies the correction. The line contactors are shown connecting each generator to the bus bar, and the equalizing contacts are the contacts within the equalizing relays that close to connect the regulators into the equalizing circuit. There’s also a variable resistor in the circuit, which allows adjustment of the equalizing sensitivity or balance. The diagram shows a 14-volt reference — that’s the nominal system voltage for this example. So in summary: the load-sharing circuit uses equalizing relays to connect both voltage regulators together. If one generator tries to produce a higher voltage than the other, current flows through the equalizing coils, which then command the regulators to raise the low one and lower the high one until they match. The result is equal load sharing between the two generators.