DC Electrics - Generators and Alternators — Page 96, Lesson 93

I want to walk you through how a vibrating-contact voltage regulator works in a DC generator system, and then we'll look at how multiple generators are connected together in an aircraft. Let's start with the regulator operation. When the generator is first started, both sets of spring-biased contacts are closed. That means the generator's output voltage is felt at both the shunt winding and the series winding of the voltage regulator. Current flows through the series winding and through the closed voltage regulator contact breaker, out to the field coil. This allows the generator's output voltage to build up. Now, as the regulated voltage is achieved — meaning the generator reaches its target output voltage — the current flowing through the shunt winding and the series winding creates an electromagnetic effect strong enough to open the contact breaker points. When those points open, they open-circuit the series winding. This forces the field current to pass through a fixed resistor instead. That resistor reduces the field current, which in turn reduces the generator's output voltage. As the voltage drops, the electromagnetic effect from the series winding is lost. The contact breaker then closes again under spring action. That restores field current, and the output voltage rises back up. This cycle repeats continuously. The frequency of operation of the contact depends on the load on the generator, but it's typically between 50 and 200 times per second. So the contacts are vibrating open and closed very rapidly — that's why it's called a vibrating-contact voltage regulator. There's also a current regulator, or current limiter. It limits the maximum output current in a similar fashion when the demand on the generator might exceed its maximum safe load. The current regulator contacts will open, switching in the resistor to reduce excitation current — just like the voltage regulator does, but triggered by excessive current instead of excessive voltage. Now let's move to the layout of a generator system in an aircraft. In an aircraft system, the generator, the load, and the battery are all connected in parallel with each other. The bus bar is the distribution point — it's the common connection where all these components meet. The generator's output voltage is maintained slightly higher than the battery voltage to keep the battery charged. Now, when an aircraft electrical system has two generators feeding one bus bar, that's called paralleling generators. The advantage of operating generators in parallel is much the same as having two batteries in parallel — you get double the capacity. It also allows the generators to share the total load of the aircraft, and it enables power to be maintained in the event of a generator failure. When you parallel generators, it's necessary for each generator to supply half of the total current demanded by the loads on the bus bar. This is called load sharing. To achieve load sharing, the output voltage of both generators must be exactly the same. If there is any potential difference — any voltage difference — between the generator outputs, then current will flow from the higher-potential generator to the lower-potential generator. This is called recirculating current. If that happens, the generator with the higher voltage output will be supplying all the current demanded by the bus bar loads, plus whatever current is demanded by the potential difference between the generator outputs. The generator with the lower voltage output will be supplying no current to the bus bar at all. There will be no load sharing. And the current flowing into the low-output generator will be attempting to turn that generator into a motor. The direction of rotation of that motor would be opposite to the generator's normal direction of rotation — which is a serious problem in an aircraft electrical system.