ATPL · Navigation · Electrics And Electronics Atpl Ground… lesson — Page 96, Lesson 95

I want to walk you through what happens when a generator in a parallel system has a low output, and how the system is designed to protect against that and to keep the generators sharing the load properly. Let's start with the problem. If one generator in a parallel system produces a lower output voltage than the others, that low-output generator will actually be attempting to turn the generator into a motor. That's a critical point: instead of being driven by the engine to produce electricity, the generator would try to act like an electric motor and rotate on its own. The direction of rotation of that motor action would be in opposition to the direction of rotation of the engine. So you'd have a generator fighting the engine's rotation, which is mechanically undesirable and could cause damage. Because flow of current to the low-output generator is undesirable, parallel systems are fitted with reverse current relays to protect against this fault in the event of a failure of the load sharing circuit. So the reverse current relay is a safety device: if the load sharing circuit fails and current tries to flow backward into a low-output generator, the reverse current relay will open the circuit and stop that reverse current. Now, let's look at the load sharing circuit itself. The load sharing circuit consists of equalizing coils in the voltage regulators. These equalizing coils finely adjust each generator's field current to ensure that the output voltages of the paralleled generators are equal. So the whole purpose of the load sharing circuit is to keep the voltages matched so that no generator tries to motor. Inside each voltage regulator, the equalizing coil is positioned such that it affects the magnetic field produced by the control coil. That magnetic field from the control coil affects the value of the variable resistance. That variable resistance, in turn, affects the current through the shunt field coil, and that regulates the output voltage of the generator. So it's a chain: equalizing coil influences control coil's magnetic field, which changes the variable resistance, which changes shunt field current, which changes output voltage. The direction of flow of current through the equalizing coil will determine whether the voltage output of the generator is increased or decreased. So if the equalizing coil senses that this generator's voltage is too low compared to the others, current flows one way through the equalizing coil, and that causes the voltage regulator to increase the output. If this generator's voltage is too high, current flows the opposite way, and the regulator decreases the output. That's how the system keeps them balanced.