Compare and contrast Static Frequency Converters versus Motor Generators.

Introduction

1. This paper will compare and contrast Static Frequency Converters (SFC) to Motor Generators (MG) within a naval defence environment. This paper will consider the function of each, their reliability, the reasoning behind their use within the Royal Navy and their use within a wider defence environment. It is important to initially provide context and background to both the SFC and the MG. This paper will achieve this by detailing the physics and mathematics associated to both systems as well as some of the more intricate technical details.
2. It is important to provide information about the physics and mathematics around both systems, in order for the reader to better understand the implications of each systems advantages and disadvantages, and each system’s essential differences. This will be done by looking at the work of scientists such as Lorentz and Maxwell, as well as the physical make up of both systems. The MG focuses its operation around turning DC input, into mechanical energy, before transforming that energy into a pure sine wave output. The SFC however operates statically using a combination of electrical components to produce a ‘dirty’ sinewave.

Motor generator

3. In order to compare and contrast both systems this paper must elaborate on how both of these systems operate. The MG system primarily operates around a rotor, power from an external source provides a current which causes a wire commutator to spin within a magnetic field. This is due to the Ampere – Maxwell law: [1]

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The circulating magnetic field is produced by any electrical current and by an electric field that changes with time. The motor is connected mechanically to the generator, the power flows between the two components as mechanical torque. This motion of the rotor through a magnetic field produces and induced EMF. The motor generator takes a DC input and transforms it into a pure AC sine wave. This AC sine wave is then applied to equipment at 400Hz and 1000Hz.

4. Whilst motor generators are widely used, they have considerable advantages and disadvantages. Within a DC motor two types of magnetic flux are present, the armature flux and the main field flux. The effect that the armature flux has on the main field flux is called armature reaction. This occurs when a motor generator is running, both magnetic fluxes will be present at the same time. The armature flux interferes with the main field flux and therefore disturbs it. This Armature reaction can have adverse effects on the motor generator system. Most crucially any weakening of the main flux therefore weakens the generated voltage outputted by the system.     In order to counteract this in the large MG’s most commonly used within the RN, compensating windings are used on the pole faces to counteract the armature flux. This is achieved by placing additional windings in close proximity to the armature windings. Whilst it carries the same current, it carries this current in the opposite direction to that of the armature current. This considerably

reduces the effect of the armature field, and in turn reduces the armature reaction.

5. A significant advantage of the motor generator system is the ability to support heavy starting and stopping loads. As a motor converts electrical energy into mechanical energy it will require an electrical input. There is a potential difference within the motor, resulting from magnetic forces exerted on the currents in the conductors of the rotor as the rotor spins within a magnetic field. The electromotive force is called induced emf, it is more specifically in this case called back emf as it is in opposition to the current flow.
6. When no load is applied to the system the motor can spin at high speed as the back emf is also high. However, when the motor is spinning at lower speeds, as it is under load, the back emf is small. As loads are added to the system this causes the speed of rotation to reduce. This is because magnetic force is directly proportional to velocity, and therefore proportional to the speed of the rotor. This slowing of the rotor can be controlled by increasing the supply to the system as it is required. This means that multiple systems that rely on the supply being provided by the MG can be started at the same time, particularly when the rotor is already spinning. This is crucial in a maritime warfare environment, particularly when multiple essential heavy load systems require start up after power failure.
7. Another significant advantage of the motor generator centres around the rotor itself. Once the motor has started up and the system has begun spinning, even with the loss of input power the system will continue to run for a period before we start to see a decline in output power. This decline in output power will be steady due to the inertia of the spinning rotor. This is known as the flywheel effect. The flywheel effect is particularly beneficial within the RN. This effect can be used to maintain power supply to essential equipment until such a time that equipment can be switched over to another continuous supply. This is essential when effecting battle damage repair within a combat environment.
8. Crucially the motor generator has a simple design. A single rotating part that is easy to replace and maintain. This can be crucial for use in front line operational units, replacement of parts and easy maintenance are essential to the continued operation and effectiveness of combat units. For the Royal Navy this is essential due to the overseas nature of that operation. Parts can be flown into various locations or stored and manufactured on board the unit. More complicated systems often require outside help and more specialist equipment that can often be difficult physically as well as logistically to obtain.