Design and analysis of marine permanent magnet synchronous motor based on Motor-cad

In the marine electric propulsion system, the motor is an important part to ensure safe navigation, which should have the characteristics of low noise, low vibration, high performance protection level, etc. The permanent magnet synchronous motor (PMSM) has the advantages of rapid dynamic response, stable operation, strong overload capacity, high efficiency and small size, which is especially suitable for electric propulsion systems in marine environments. According to the performance requirements of marine motors and the permanent magnet synchronous motor structural characteristics, an electromagnetic design and simulation of a 120kW marine permanent magnet synchronous motor is completed in this paper. Meanwhile, the stator skewed slot is used to reduce the amplitude of harmonic distortion and the additional torque caused by tooth harmonic. At same time, the rotor structure with U-shaped magnetic steel groove is used to enhance the density of power and overload capacity of the motor, expand the scope of motor constant power operation effectively. A prototype with high efficiency, reliability and a small volume has been developed. According to the simulation results, the marine permanent magnet synchronous motor designed in this paper fully meets the design requirements.


Introduction
Nearly a century has passed since the invention of the electric propulsion system for ships, the current common ship electric propulsion device generally consists of prime mover, generator, electric motor, propeller and control and adjustment equipment, etc., one of the essential part of the electric propulsion system is propulsion motor [1], the current common propulsion motor can be divided into DC motor, AC asynchronous motor, AC synchronous motor, permanent magnet synchronous motor, switched reluctance motor, multi-phase AC motor, high temperature superconducting motor, etc.Compared with other motors, permanent magnet synchronous motor has the characteristics of small size, light weight, supernal efficiency, small noise, easy to realize centralized remote control, smooth operation, strong overload capacity, high reliability, good maintainability, etc.It is applicable to electric propulsion systems in marine environments with large load torque variation and high space requirements, and is an ideal choice for marine propulsion motor [2].

Design requirements for marine motors
Different from the general propulsion motor the marine propulsion motor needs to be fully considered in the design of the ship's operating environment and work needs.In addition to the use requirements of anti-humid heat, anti-salt spray, anti-mold, electromagnetic compatibility, etc., the requirements of the space in the cabin and the dimensions of the external structure should also be considered.Marine propulsion motors are generally installed in the stern.In order to obtain better hydrodynamic and acoustic performance, the shrinkage of the ship's tail leads to a smaller cabin volume.Therefore, the requirements of small size, light weight and high specific power of propulsion motor are put forward.[3].Ship navigation requires that the propulsion motor must be able to smooth speed regulation in a wide range and it can work normally at any speed, and it also has high reliability requirements [4].In addition, the ship's water pump needs to run at a uniform speed for long periods, which requires high steady-state performance of the motor.

Selection of marine motors
Motors are divided into DC motors and AC motors, AC motors are further divided into asynchronous motors and synchronous motors.In contrast to DC motors, AC motors have unlimited single-machine capacity.The commutation capability of DC motor commutators limits the capacity and speed of the motor, while AC motors break through this limitation.The AC motor has a simple structure, small size, light weight, and smaller rotational inertia than the DC motor, which increases the response speed of the transmission system substantially.In addition, there is no restriction on the overload capacity of the reversing spark, the AC motor has greater dynamic acceleration current and better dynamic response characteristics.The excitation power of the asynchronous motor is obtained through the stator side and the stator current is always lagging, The synchronous motor has an independent rotor excitation regulation control, so, the synchronous motor's power factor is higher than the asynchronous motor's.Under the same capacity, synchronous motor has smaller volume, lighter weight, more diversified structure, and smaller rotational inertia and size than asynchronous motor.Therefore, when choosing marine motors, synchronous motor is a better choice than asynchronous motor.Permanent magnet synchronous motor uses permanent magnet to replace electric excitation, without excitation loss and corresponding heating problems, which has obvious energy-saving effect.Moreover, permanent magnet synchronous motor doesn't need excitation for its rotor is permanent, so the motor can maintain synchronous operation at a very low speed, meeting the requirements of marine motor for normal operation at any speed and has high reliability.And the permanent magnet synchronous motor can realize step-less speed regulation through the frequency conversion speed control equipment, and the speed regulation limit is small.In addition, the stator and rotor cores have no iron loss, and the stator current and copper loss are small, so the efficiency is higher.PMSM has a higher power to mass ratio and larger output torque, also has good braking performance.PMSM also has the advantages of fast dynamic response, stable operation, strong overload capacity and low noise, which is more suitable for marine electric propulsion system.

Parameter requirements of marine permanent magnet synchronous motor
The following Table 1 shows the performance index of the motor: Table 1.Performance index of marine PMSM.

Design flow chart of marine permanent magnet synchronous motor
The design flow chart of marine PMSM is shown in Figure 1:

Electromagnetic design of marine permanent magnet synchronous motor
In motor design, the speed, frequency, pole numbers, stator length, internal and external diameters, rotor slot number of the motor will affect the running effect of the motor.Therefore, we need to consider copper loss, back electromotive force, iron loss, self-inductance and mutual inductance, magnetic flux and other factors in the design.

Selection of main dimensions of motor
In the design of PMSM, the determination of the main dimensions of the motor has a great impact on the motor characteristics manufacturing cost.Generally, the permanent magnet synchronous motor's main dimensions refer to the armature core diameter and core length , which can be chosen according to the given rating.The main dimensions are calculated by the following formula: In above, ' is the calculated power, P N is the rated power； ' calculates the pole-arc coefficient; is the waveform coefficient of the air gap magnetic field; is the winding coefficient of the armature.The product of armature core diameter and core length can be obtained by selecting appropriate winding load A and magnetic load [8].
In above, p is the polar logarithm; τ is pole distance; λ is the main size ratio.If the main dimension ratio is selected, the armature diameter D a and core length L ef can be determined according to the formula.For large motors, usually λ =1~3.5, and the more poles, the higher the value, therefore, according to preliminary calculation, the diameter of the armature is about 175mm and the length of the core is about 112mm.The size of stator slot can be determined by the comprehensive performance index.The outer diameter of the stator can be calculated as 250mm based on the slot size and inner diameter of the stator.After calculation, the air gap length is 1mm and the outer diameter of the rotor core is 173mm.

The choice of motor pole number and slot number
Different slot and pole fit will show different electrical and mechanical characteristics in the motor design.The more slots in the stator, the ability to weaken the harmonic generation in the air gap will stronger , the motor loss will smaller, and the corresponding motor efficiency will increase.The increase of stator slot number will also reduce the conductor number in each slot, so the heat dissipation capacity and the reliability of the motor can be improved effectively.When the phase and pole number of the motor are fixed, the number of stator slots depends on the number of slots q in each phase of each pole.Since the number of poles of the propulsion motor is large and the pole distance is relatively small, q should not be too large.After comprehensive consideration, q is selected as 2, which can not only effectively reduce the cogging torque, but also raise the utilization ratio of stator slot.Therefore, in order to optimize the motor manufacturing process and control motor polar arc coefficient, this paper selects the slot machine with the stator slot number of 48 and the pole pair of 4 to cooperate.

Design of stator winding
Stator windings are divided into centralized and distributed types according to different coil winding shapes and embedded wiring mode.Distributed winding can reduce the ripple in the back EMF, so that the back potential of the motor has good sinusoidal characteristics and the output of the motor is more stable.Therefore, the distributed winding is used in this design.Distributed winding can also be separated into two kinds: single-layer winding and double-layer winding.Single-layer winding only has one layer of wire in each slot, which does not need to be insulated in the slot, so the utilization rate of the winding is high, and the process is relatively simple.The chain connection of winding can also reduce the harmonics of the motor magnetomotive force.Therefore, the stator windings of marine motor designed in this paper are single-layer distributed chain windings.The spatial arrangement and linear wiring diagram of stator windings are shown in Figure 2 and Figure 3:

Rotor selection
For the sake of improving the reliability of marine PMSM, the rotor adopts a interior U-shaped hybrid permanent magnet structure.The hybrid rotor structure includes the advantages of radial and tangential rotor structures, allowing more permanent magnets to be placed in the rotor.The increase in the number of permanent magnets increases the flux density, while the magnetic leakage of the motor decreases.The hybrid rotor structure has good mechanical performance, and the permanent magnet is easier to be fixed on the rotor.It can be used in the situations where the motor rotates at high speed, and is suitable for ship navigation [10].

Design of Permanent Magnet
The mainly size of permanent magnet includes the axial length , magnetization direction length h M and width of permanent magnet .When designing, we generally believe that equal to the axial length of motor iron core, so in fact only two dimensions of permanent magnet need to be considered.The estimation formula of permanent magnet size of built-in tangential rotor magnetic circuit structure is as follows: The stator and rotor structure diagram of the motor is shown in Figure 4:

The Choice of permanent magnet materials
Unlike traditional motors, the permanent magnet motor rotor has permanent magnet which can generate magnetic field.Different permanent magnet materials have different effects on the performance and structure of the motor.The selection of permanent magnet materials for marine motors requires consideration of these principles: ensuring sufficient magnetic field in motor air gap and specified motor performance index; the stability of magnetic properties and good mechanical properties can be guaranteed under specified environmental conditions, working temperature and service conditions.The propulsion motor generally uses NdFeB and SmCo as the main permanent magnet materials, among which NdFeB is known to have the highest comprehensive performance of permanent magnet materials.SmCo has a good magnetic energy product performance, and its magnetism is less affected by temperature, it also has good reliability and stability.But its price is expensive, as a mass used motor material, the cost performance is not high so its applications are limited.While the magnetism of NdFeB is better, and the magnetic induction coercive force is higher.Although the temperature stability is insufficient, it is rich in resources and cheap, the overall performance of NdFeB is relatively stronger than that of SmCo.With the emergence of the third generation of rare earth permanent magnetic materials NdFeB and the increase of temperature and relatively low price, NdFeB is still widely used [11].This design also chooses NdFeB as permanent magnet material for marine motor.

Cooling way
PMSM use a permanent magnet as rotor excitation.Since there is no excitation winding that generates heat, the rotor does not need to be cooled, only the stator winding needs to be cooled.Stator cooling can be divided into two ways: stator water internal cooling and water external cooling.Stator water internal cooling can directly cool the stator winding, with good cooling effect, but complex structure.
Stator water external cooling removes heat from the stator windings by cooling the stator core, its structure is relatively simple and the operation is pretty safe and reliable.Therefore, this paper adopts the stator water external cooling mode.

Motor parameters
After calculation, parameters of the marine permanent magnet synchronous motor designed with the above analysis are shown in Table 2

Motor simulation and results
The marine PMSM designed above is simulated by Motor-Cad.Motor-Cad is an electromagnetic, thermal and magnetic thermal mutual coupling design software for motor, which is used to optimize the design of magnetic and thermal characteristics of motors.It can efficiently and accurately calculate the electromagnetic and thermal performance of the motor in the design stage, and the output results are rich and intuitive.
As can be seen from the torque-speed diagram shown in Figure 5, the motor's maximum torque is 212Nm.One of the key points of motor design is efficiency, the efficiency of the motor will be different under different speeds and torques.Figure 6 shows that the maximum efficiency is as high as 96% in the triangular region with a speed of 2500-10000rpm.It can be seen from the data and curves of the simulation results that the motor designed in this paper can meet the performance characteristics of marine motor and meet the design requirements.

Conclusion
According to the working conditions, performance requirements of the motor, combined with the structural characteristics and index requirements of PMSM, a rated power of 120kW of marine PMSM is designed in this paper and motor simulation model is established by using Motor-Cad simulation software.Due to the particularity of marine motor operating environment, there are higher demands on the size and working efficiency of the motor, which greatly affects the selection of stator structure of the motor.After several parameter adjustments and optimization, we can see from the simulation results of the prototype that the model designed in this paper can meet the requirements of marine motor, and the working efficiency of the prototype can maintain a high efficiency within a certain speed range, reaching 96%, which means that the motor has better working performance than than the traditional motor.Although the motor designed in this paper can meet the design requirements of marine motor, there are still some areas to be improved.The next direction of work will be to effectively reduce the amplitude of harmonic distortion and motor noise in real motor operation.