The underwater special propulsion motor is a highly technical electric propulsion device, whose technical content includes fluid design, motor design and material selection. It has a novel and unique structure with unique and superior characteristics, which makes it ideal for use as a propulsion system for underwater robots and also for torpedo propulsion.
Figure 1 shows the structure of the integrated motor thruster. The integrated motor thruster consists of an integrated motor, a deflector shield, rotor blades and a static blade grille. The motor is used to drive the propeller directly. The rotor of the motor and the rotor of the pump jet thruster are designed as one piece, and the motor magnet is mounted on the rotor rim, and the rotor is mounted on the mounting shaft by bearings, which can rotate freely around the shaft. The stator of the motor and the propeller guide cover are designed as one unit, and the armature core of the motor is mounted and fixed in the guide cover. The rotor and stator of the motor need to be insulated and sealed separately, and the whole motor works in water, so the heat dissipation problem of the motor can be well solved. The static leaf grille of the thruster is mounted behind the rotor to eliminate the rotational motion of the fluid flow and to balance the torque. The Integrated Motor Propulsion Module (IMP) can be mounted relatively easily on the underwater vehicle by means of a mounting shaft. The motor cable is connected to the motor control unit by means of a channel in the deflector, the hydrostatic grille and the mounting shaft. The control unit is installed inside the underwater vehicle and is controlled by a computer. 
The choice of motor type has a large impact on the power system as well as on the overall performance of the vehicle and requires a comparison and analysis of different motors and their control methods. The four main types of small and medium power drive motors being applied or developed are DC motors, induction motors, permanent magnet brushless motors, and switched reluctance motors. The overall comparison of the drive systems composed by these four types of motors is shown in the table.
DC motor has the characteristics of simple structure and excellent electromagnetic torque control, so it is still the main object of research and development at home and abroad until the mid 1980s, and the DC drive system is still widely used. However, the mechanical commutation structure of ordinary DC motor is easy to produce electric sparks, which is not suitable for use in dusty, humid, flammable and explosive environment, and its commutator is difficult to maintain, so it is difficult to develop to large capacity and high speed. In addition, the electromagnetic interference generated by electric sparks will be fatal to the highly electronic underwater vehicles. In addition, DC motors are more expensive, large in size and weight. With the development of control theory and power electronics, DC drive system is at a disadvantage compared with other drive systems.
Induction motors are the most widely used motors in factories and mines, transportation, entertainment, research and agricultural production, and daily life. There are three major types of induction motors: squirrel-cage asynchronous motor, wire-wound asynchronous motor and various control motors. The general induction motors have the disadvantages of small starting torque, low overload capacity, large size, low efficiency and unsatisfactory speed regulation, which often cannot meet the requirements in the demanding situations.
The permanent magnet brushless motor system can be divided into two categories, one is the brushless DC motor system driven by square wave (BDCM) and the other is the permanent magnet synchronous motor system (PMSM), also called the brushless DC motor system driven by sinusoidal wave. the BDCM system does not need absolute position sensor, generally using Hall element or incremental code plate, also can change phase by detecting the counter-electromotive force waveform. PMSM systems generally require absolute code plates or rotor position sensors such as resolvers. From the structure of the magnet in different positions, the permanent magnet brushless motor can be divided into surface type, inlay type, deep buried type and other structure types.
The switched reluctance motor (SRM) is now in practical use. SRM is a brushless motor without any form of rotor conductor and permanent magnet, which has convex stator and rotor poles. SRM has the advantages of simplicity and reliability, efficient operation in a wide speed and torque range, flexible control, four-quadrant operation, fast response time, and low cost. However, SRM has disadvantages such as high torque fluctuation, high noise, need for position detector, and non-linear characteristics of the system. Therefore, the applications are still limited. 
It can be seen that relatively brushless DC motor combines the advantages of DC motor and AC synchronous motor with small size, long life, simple control, high speed regulation accuracy, and no loss of step. Considering the special motor used in the integrated motor propulsion system is greatly restricted in structure, and the propulsion system requirements for motor speed regulation performance, the integrated motor propulsion system using permanent magnet DC brushless motor is more suitable.
Brushless DC motor can be divided into sine wave motor and square wave motor, the flux distribution of sine wave motor is in the form of sine wave. Square wave motors have a flux distribution of 120. flat-topped square wave, tile permanent magnets are magnetized in the radial excitation direction, the pole arc width is greater than 120′ electrical angle, if the whole distance concentration winding is used, the trapezoidal counter potential can be obtained and the armature current is rectangular wave. The square wave motor is selected in the propulsion system, which has the advantages of simple motor and power electronic control circuit structure, reliable work, high material utilization in the same situation, high output and simple control method compared with the sine wave motor.
Development and Applications Editor Podcast
The IMP was first developed jointly by the U.S. Naval Underwater Warfare Center and the Applied Research Laboratory at Penn State University, and was originally used for unmanned submersibles by placing the motor inside the outer housing of the submersible. This arrangement eliminates the need for a conventional motor cooling water jacket, motor auxiliary cooling system, drive shaft and coupling joints. This increases the payload of the submersible and the arrangement of the internal space is very flexible and of great benefit. The saved empty space can be used to install auxiliary sensors or store energy, thus increasing the ability to perform the mission and endurance. The IMP’s motor selection of permanent magnet synchronous motor, the advantages of this motor is high power density, high efficiency and excellent acoustic performance.
It is due to the outstanding advantages of IMP, some developed countries in the West attach great importance to the development of IMP. IMP module has a very broad development prospects, as he has obvious advantages over traditional electric propulsion devices, its emergence is likely to bring a revolution in underwater propulsion. revolution.
However, in the domestic research on the integrated motor propulsion device can be said to be a gap, so far, no reports on the product or experimental prototype, there are only a few domestic related information.