DC Electrics - DC Motors — Page 110, Lesson 111

I want to walk you through how a shunt wound DC motor behaves under load, and then we’ll move into starter-generator systems used on turbine-powered aircraft. Let’s start with the speed characteristic. If the load on a DC motor decreases, the motor speeds up. As it speeds up, the back EMF — that’s the voltage the armature generates opposing the supply — increases. A higher back EMF means less voltage is available to drive current through the armature, so the armature current decreases. Less armature current means less torque developed. With less torque than the load demands, the motor slows back down. So there’s a self-regulating balance. Now specifically for a shunt wound motor: the variation in speed from ‘no-load’ to normal or ‘full’ load is only 10% of the ‘no-load’ speed. That means if the motor runs at 1000 RPM with no load, at full load it might drop to about 900 RPM — only a 10% change. Because of this small speed variation, shunt wound motors are considered constant speed motors. Where are they used? Shunt wound motors are normally used where constant speeds under varying loads are required, and where the motor can start under light or no-load conditions. Typical examples include fans, centrifugal pumps, and motor generator units. Those applications don’t need high starting torque, and they benefit from stable speed even as the load changes. Now let’s move to a different topic: starter-generator systems. Several types of turbine-powered aircraft are equipped with starter systems that use a starter generator. This is a single unit with a dual function: it starts the engine, and once the engine is running, it supplies DC power to the aircraft’s electrical system. A starter-generator unit is basically a compound-wound machine. That means it has two sets of field windings — a shunt field and a series field — plus one armature winding and a commutator. The unit is permanently coupled with the appropriate engine via a drive shaft and a gear train. For starting purposes, the unit functions as a fully compounded motor. During start, the shunt field winding is supplied with current through a field changeover relay. Once the engine is running and the starter motor circuit is isolated from the power supply, the changeover relay is automatically de-energized. When it de-energizes, its contacts do two things: first, they connect the shunt field winding to a voltage regulator. Second, they permit DC to flow through the shunt winding to provide initial excitation of the field — that’s the small current needed to build up the magnetic field so the generator can start producing voltage. After that, the machine functions as a conventional DC generator. Its output is connected to the bus bar when it reaches the regulated level — meaning the voltage has built up to the correct value for the aircraft’s electrical system. So in summary: a shunt wound motor gives you nearly constant speed with only a 10% variation from no-load to full load, and it’s used in fans, pumps, and motor generators. A starter-generator is a compound-wound machine that starts as a motor, then switches to generator mode via a changeover relay, supplying DC power to the aircraft bus.