Development of a Low-speed High-efficiency PMSM and Its Drive System for Electric Windlass and Mooring Winch

Permanent magnetization of electric windlass and mooring winch (EWMW) has become a research trend in the development of new generation deck machinery. The paper investigates the development of a low-speed and high-efficiency permanent magnet synchronous motor (PMSM) and its drive system based on the load characteristics and special requirements of an EWMW. Firstly, the design specifications and key points of the motor drive system are analyzed. Secondly, a design method of parameter matching is proposed to design a 110 kW 75 r/min PMSM based on finite element method, field circuit coupling, and co-simulation. Finally, a prototype is developed and applied to a 45 T EWMW. The test results show that the PMSM can attain an efficiency of 94.3% at the rated operation, good traction characteristics with high torque at low speed, and 3 times wide flux-weakening region at constant power. Therefore, the matching design method is effective in the design of PMSM and drive system for EWMW.


I. INTRODUCTION
Windlass is an important navigation device for ships. Windlass and mooring winches (WMW) are combined into one device at the bow to realize the dual use of one machine in most ships. Its operation performance directly affects the safety of ships. When any one of them has a fault, the ship is not allowed to sail. Therefore, they are vital to ships [1]- [3].
The load of the WMW is large. A multi-working condition under variable load requires that the drive system can start quickly and stop accurately. The traditional hydraulic-driven WMW has many problems, such as low efficiency, slow system response, complicated pipeline pavement (requiring independent pressure supply system), large space occupation, hydraulic oil leakage, and complex intelligent control, whereas EWMW has many advantages in terms of reliable operation, fast response, easy maintenance, no risk of oil pollution, and high efficiency. Therefore, EWMW has been widely used in ships. However, most EWMWs are driven by pole changing motor, winding motor based on the silicon-controlled rectifier, and asynchronous variable frequency motor [4]. Moreover, the high reliability, low noise and vibration, high efficiency, energy-saving, and intelligent integration of EWMWs are required with the development of the deck machinery. Therefore, low-speed, high torque, wide-speed regulation, and high-efficiency PMSMs have become the trend and research hotspot of the new generation of deck machinery.
Due to the advantages of high-power density, high power factor, and high efficiency, PMSMs have gained a longterm development in automotive traction, wind power generation, ship electric propulsion, electric vehicle, and rail transportation [5]- [8]. Most reported studies have focused on the analysis and optimization of the electrical parameters, thermal analysis of motor, vibration ,and noise, and influence of the PM materials [9]- [15]. For low-speed PMSM, it is mainly used in wind power generation, ship electric propulsion, and coal mine direct drive system [16]- [18]. Literature [19] investigated the design criteria for a high-efficiency PMSM. This method does not seek to simply reduce the electromagnetic load, but to optimize a set of motor design variables without increasing the overall size, which is usually imposed as a design constraint. Generally speaking, low speed and high efficiency are contradictory to permanent magnet motors. A lot of research on constant power demagnetization speed regulation has been applied to electric vehicles. Literature [20]- [22] focused on how to improve the speed range and power density, whereas there are few studies on the design of PMSM in deck machinery EWMW considering special load characteristics.
In this paper, according to the load characteristics and special requirements of the EWMW, the design difficulties and key points of the low-speed and high-efficiency permanent magnet motor and drive system are presented. A method of parameter matching is proposed to design a lowspeed and high-efficiency permanent magnet motor based on finite element method, field circuit coupling, and cosimulation. The correctness and effectiveness of the matching design method are verified through the experimental study of a 45 T EWMW.

II.DESIGN DIFFICULTIES AND KEY POINTS OF PMSM AND DRIVE SYSTEM OF EWMW
The duty type of the EWMW is S2, which has the characteristics of variable working conditions and multiple load spectrum. And the work conditions are divided into dynamic anchoring, free anchoring, anchor chain retraction, earth breaking, anchor bolt retraction, anchor bolt retraction to the outlet, and anchor bolt retraction to anchor lip. The work style of the winch is divided into the mooring, cable releasing, and drum constant tension. At the same time, the new generation of deck machinery is developing towards the direction of green, energy-saving, environmental protection, intelligence, stronger operation ability, and more complex structure and functional configuration. The power drive system requires a compact structure, high efficiency, strong overload capacity, good low-frequency characteristics, zero speed hover, constant tension control, fast response. Therefore, the design of the PMSM and drive system becomes more difficult.

1) LOW-SPEED AND HIGH-EFFICIENCY
The PMSM with low-speed and high-efficiency is designed to eliminate the multi-stage gear and improve the transmission efficiency of the drive system, which needs to reasonably allocate the loss, optimize the ratio between the effective length and the inner diameter of iron-core, and overcome the assembly process of the winding under the conditions of ultra-deep slot multi-winding and limited end length. At the same time, improving the machining and assembly accuracy of motor components is also an important way.

2) WIDE FLUX-WEAKENING REGION SPEED ADJUSTABILITY
The PMSM needs to have three times the speed adjusts the capacity of flux-weakening to respond quickly and improve the efficiency of retracting and unwinding anchor chains and cables. And this poses a challenge to the rotor magnetic circuit topology design of the PMSM and the control of the frequency converter.

3) HIGH IMPACT RESISTANCE
The PMSM of the EWMW needs to bear a high impact load due to the influence of the structure itself, sea conditions, and geology. Therefore, special requirements are put forward for the design of the bearing, the length of the air gap, the structural strength, and the type of selection of the rotor position signal.

4) HIGH TORQUE ELECTROMAGNETIC BRAKE WITH SHAFT
The EWMW needs to have the function of quick automatic braking and emergency braking. Therefore, the PMSM should have an electromagnetic brake with high torque. According to the specification, the general working model of electromagnetic braking is to turn on the power to release the brake and turn off the power to the brake. Thus, the special design of the electromagnetic brake and the reduction of the interference to the position detection signal is put forward as new requirements because the position accuracy error will lead to low-speed jitter and low-speed creep.
When the motor runs at high speed in the field weakening area, if it is cut off or stopped under fault, the terminal voltage of the motor will rise suddenly due to the existence of back EMF. At this time, the braking of the brake will also help to protect the power devices of variable frequency drive.

1) CONSTANT TENSION CONTROL ALGORITHM
Different from the torque control mode of the conventional PMSM, EWMW constant tension control needs to add a tension control ring outside the torque ring. The variable frequency driver receives the feedback analog signal of the tension sensor and the tension-given signal for closed-loop control. The PI parameters of the closed-loop control need to be set in combination with the parameters and the actual load of PMSM.

2) THERMAL DESIGN OF INVERTER UNDER EXTREME WORKING CONDITIONS
The thermal design of the frequency converter should be significant because the EWMW requires extreme working conditions such as 2.3 times of high overload capacity and zero speed hovering full torque, which involves the selection of IGBT and switching frequency.

3) ELIMINATION AND SUPPRESSION OF LOW-FREQUENCY TORQUE RIPPLE
The elimination of torque ripple is generally carried out from the design and control of the PMSM. Among them, the ripple torque is mainly suppressed by improving power supply high order harmonics and injecting harmonic method. VOLUME XX, 2017 10

III. A METHOD OF PARAMETER MATCHING DESIGN OF PMSM
It can be seen from the above analysis that the design of low-speed and high-efficiency permanent magnet motor for EWMW has its particularities and difficulties, which not only involves the motor itself but also is closely related to the control strategy of the driving system. Thus, the selection of parameters cannot be designed by conventional methods, and matching design must be combined with control synthesis. In this paper, a co-simulation of the method based on finite element and field-circuit coupling is presented. The specific method flow chart is shown in Fig.  1. From Fig.1, the key idea is the parameters with a high weight ratio match the special requirement of EWMW, and the initial scheme is completed by using the finite element method and field-circuit coupling, and when combined with the control requirement of EWMW. Motor Solve and Simulink are used to carry out the co-simulation, the second iteration is completed, and the motor parameter design and control parameters are finally determined. In addition, the adaptive and matching design for EWMW is also mainly related to the high weight ratio parameters and electromagnetic brake and signal anti-interference design, cooling, oil-proof seal, and protection class in the structural design.

1) COPPER LOSS OF STATOR
The iron loss of the low-speed and high-efficiency PMSM is very small because of the low operating frequency, lower yoke magnetic density, and the low loss of thin silicon steel sheet, whereas the stator copper loss is very large. Therefore, the key point of parameter design is to reduce stator copper consumption on the premise of ensuring the overall dimension of the stator. The optimization direction is derived according to the following estimation formula of stator copper consumption. The copper loss of stator can be approximated as where J is the current density of stator armature winding, A is specific electric loading, L K is length-inner diameter ratio of the iron core, 1 K is the winding end-inner diameter ratio of the iron core, y  is the winding pole pitch, s h is equivalent slot height,  is the winding short distance coefficient, cos is the cosine of the Angle between the end of the winding and the horizontal line, and p is the number of pole pairs of the rotor.
Substituting (2)-(5) to (1), the copper loss of stator is It is well known that the 2 1a i Dl is the constant value when the torque of the motor is fixed. Therefore, (6) can be rewritten as From (7), the copper loss of stator takes the minimum value when the L K is equal to 1 2K . Then, the minimum of the copper loss is 12 Based on the above equations, the major conclusions are listed as follows: (1) The copper loss of the stator is directly proportional to the thermal load. The copper loss of the motor can be reduced with the decrease of the thermal load.
(2) The copper loss of the stator can be reduced by selecting the aspect ratio L K that is equal to 1 2K or approximately equal.
(3) By choosing a larger pole pair p value, the end length can be reduced, then to reduce the copper loss of the motor. (4) The copper loss of stator can be reduced by controlling the ratio of groove height to inner diameter or increasing the full rate of groove. It can be shown that the length-inner diameter ratio of iron core and winding end, pole pairs, slot size, and heat load are the high weight ratio parameters affecting the design of low-speed and high-efficiency PMSM, which provides an optimization direction for the scheme matching design. At the same time, it can also be seen that the design difficulty and process realization of low-speed and highefficiency PMSM, especially under the condition of the limited overall size of the new generation of EWMW.

2)AIR GAP LENGTH AND POLE SLOT COORDINATION
To meet the requirements of anti-shock design, the motor air gap is designed to be a larger value under the premise of meeting the performance, which is chosen to be 3 mm in this paper. As mentioned above, the copper loss decreases as the number of pole pairs increases, thereby increasing the efficiency of the PMSM. In addition, the increase in the number of slots makes the stator slots tend to be long and narrow, which makes it difficult to embed and leads to an increase in slot leakage resistance. The number of stator slots is determined by the number of slots per phase for each pole q. Considering vibration and noise, q is generally an integer of 2-4. In this study, p is 12 and q is 2.

3)ROTOR MAGNETIC CIRCUIT TOPOLOGY
The rotor is usually equipped with an interior-type radial rotor topology to improve the wide-speed regulation and weak magnetic region. The general approach is to improve the weak magnetic effect by reducing the Quadrature-axis inductance Lq, increasing the Direct-axis synchronous inductance Ld, increasing the limit voltage and limit current of the motor, reducing the permanent magnet flux, and other measures. The distance between adjacent magnetic poles, the implantation depth of the magnetic pole center [23], and the design of the rated voltage point are the high weight ratio parameters that affect the effect of the weak magnetic field.
Literature [24] proposed and analyzed a kind of internal segmental rotor magnetic circuit structure that could effectively increase the direct-axis inductance, and the direct-axis inductance increased significantly, and the weak magnetic energy was improved. Considering the 3 times of weak magnetic field, the rotor topology adopted in this paper is interior PM type "V" and "Segmented -shape".

4)ADAPTIVE DESIGN
The adaptive design of the EWMW mainly includes the protection class of IP56, the oil-proof seal design at the transmission end, the large-torque electromagnetic brake and the torque transmission design of the shaft, the design of preventing the interference of the electric field of the brake on the position sensing signal, and the cooling design of the motor.

B. MAIN PARAMETERS AND STRUCTURE OF PMSM
The main parameters of 110 kW and 75 r/min PMSM for a 45 T EWMW are listed in TABLE I. The structure diagram of the PMSM is shown in Fig.2. The rotor adopts interior PM type "V" and " Segmented -shape"; The electromagnetic brake adopts the structure of polygon friction pair in the rectified high strength friction disc. Position signal monitoring using a large gap of the reluctance type rotary transformer and the installation of the transition plate using non-magnetic materials to reduce the brake on the signal interference; The motor adopts a natural cooling design.

1) MASTER TOPOLOGY SCHEME
The main loop topology of the inverter adopts a modular ACDC-AC topology scheme. The rectifier part of the converter adopts mature and reliable 6-pulse rectifier technology to reduce the loss of the converter. And the rectifier inlet line is equipped with a 2% input line reactor to reduce the influence of the uncontrolled rectifier device on the grid side. The output side of the frequency converter with a dv/dt filter is provided to reduce the output dv/dt of the frequency converter. The single-line diagram of the main circuit and the schematic diagram of the inverter VOLUME XX, 2017 10 component are shown in Fig. 3 and Fig. 4, respectively. The central unit controlling the inverter is adopted DSP28335+cpld(Complex programming logic device), and the dead time of transistors is about 2.1us. IGBT switching frequency is set to 2kHz.The signal of the rotary transformer is input into DSP28335 through the position sampling board, and the motor speed is calculated in realtime through the speed sampling function to achieve the closed-loop speed regulation function. The signal collected by the current sensor is input to DSP28335 through the current sampling sample, and the current is calculated in real-time.

2)CONTROL FLOWCHART
The EWMW winch can adjust the retracting and unwinding of the winch according to the tension of the cable. Therefore, the tension can be stabilized at an ideal value. The control process includes speed control, torque control, and constant tension control, as shown in Figure 5.

3)LOSS CALCULATION AND THERMAL ANALYSIS
The EWMW withstands extreme working conditions and the special requirement of 98% efficiency. And the loss calculation and thermal design calculation are carried out. The loss and temperature of frequency converter calculated by the thermal simulation analysis software are listed in TABLE II. As listed in TABLE II, the efficiency is greater than 98%, and the temperature rise of the device meets the requirements.

A. BASIC MODEL OF ELECTROMAGNETIC FIELD CALCULATION
The transient electromagnetic field calculation model of the PMSM can be described as follows: where 1  is the calculation region, S1 is Dirichlet boundary condition, v is the reluctivity, s J and m J are armature winding current density and permanent magnet boundary equivalent surface current density ,respectively, Z A is magnetic vector potential, n is normal unit vector outside the boundary of a permanent magnet, 1 V and 2 V are Non-permanent magnet region and permanent magnet region, respectively.
The three stator phase windings which are Y-connected with neutral are shown in Fig.6. Their induced voltages are given as ea, eb, and ec, respectively, ua, ub, and uc are the phase voltage, unis the potential of the neutral n, and their currents are given as ia, ib, and ic, respectively, where subscripts a, b and c represent the three stator phase windings. R1 and Lδ are the resistance and end-winding VOLUME XX, 2017 10 leakage inductance of the stator winding per phase, respectively. From Fig. 6, the voltage and current equations are as follows [25]: Three-phase windings are connected in a star shape, then the three-phase current satisfies the constraint that the sum of current is 0: For operation from a balanced three-phase system, ua+ub+uc=0 (12) n u is obtained by adding each side of (9) and then applying (10) and (11) a b c n e +e +e u =-3 (13) Substituting (13) into (10), the voltage and current equations are given by The electromotive force of the stator winding is the key parameter that combines the field and circuit in the stator region.
In the stator windings region, the field calculation model is as follows The stator current is unknown, and the stator electric density J can be expressed as follows where 1 N is turn of a coil, a is a number of parallel branches, b S is the area occupied by one coil edge, a  , b  and c  is Winding current coefficient of the unit, respectively. e.g., when the unit belongs to A-phase winding, the positive phase band a  value is 1, the negative phase band a  value is -1, and the value in other cases is 0.
Take the A-phase winding for example ， the electromotive force of the stator winding can be given as follows [26]   is the area of the unit, so the same goes for B-phase winding and C-phase winding.
Here em T is the electromagnetic torque, L T is the load torque, and  is the angular velocity, J is the moment of inertia.
Therefore, we can obtain the vector potential and currents at the same time by solving Eq. (9) and Eqs. (14), (16), (17), and (18) simultaneously. The above equations are discretized by the weighted residual method, and the coupled equations can be solved by the time-step Newton Raphson iterative method.
One simulation model based on field-circuit coupling analysis is shown in Fig.7.In this study, the equivalent resistance of the phase winding is 0.0351 Ω , and the leakage inductance of the phase end is 0.205 mH.

B. ELECTROMAGNETIC FIELD CALCULATION AND ANALYSIS
The FFT of no-load back EMF with the skewed slots, the torque curve under rated load and 3 times weak magnetic field, the FFT of current and terminal voltage under 3 times weak magnetic field, and the stability curve of the motor after 60min operation (ambient temperature 40 ℃ ) calculated by using the transient temperature field are shown in Figs. [8][9][10][11]. As shown in Fig.8, the no-load induction back of EMF after the slots are skewed has a good sinusoidal degree. And the RMS of no-load phase voltage is 150.6 V.     As shown in Fig.9 to Fig. 11, the average output torque is 14.25 kN.m (power is 111.9 kW) at the rated speed of 75 r/ min, the output torque of the motor is 4.812 kN.m (power is 113.3 kW) at a constant power area of 225 r/ min, the RMS of motor current is 250 A(353.61/sqrt(2)), and the phase terminal voltage is 200.71 V( ( 283.8/sqrt(2) ) ) (line terminal voltage is 347.62 V), which meets the output voltage capacity of the frequency converter. It shows that the motor can achieve 3 times constant power flux weakening.
As shown in Fig.12, the highest temperature is 94 ℃ after the motor runs for 60 minutes, and the temperature rises about 54 K. The highest temperature appears at the end of the stator winding, and the rotor magnetic steel temperature is about 52 ℃. Natural cooling is safe and VOLUME XX, 2017 10 reliable for short-time S2 (30 minutes) and 180 ℃ UH class permanent magnetic.

C. CO-SIMULATION ANALYSIS BASED ON CONSTANT TENSION
The linear velocity V2 on the outside of the reel must also remain constant to maintain constant tension. Then, when V2 remains constant, the speed of the motor decreases with the increase of the coil diameter, which is as follows where 2 V is the linear velocity, i is the ratio of the drum, and D is the diameter of drum. the torque of drum can be given below.
Therefore,the product of torque and speed is obtained as here, 2

K= 2
Vi  , when 2 V remains constant, K is constant, and F is constant tension.
The MotorSolve+Simulink co-simulation based on vector analysis is shown in Fig.13.Only part of the simulation curves of determining solutions is given because of the limitation of space. The co-simulation torque curve, current curve, and speed response curve under constant tension are shown in Fig. 13-15, respectively.   (1) The torque curve and the speed curve have a certain overshoot, and the speed regulation performance is good, and the load steady-state error is relatively small, 14000 N.m load about 0.02 seconds to restore the given speed. Under constant tension control, the speed response under the given speed curve has good tracking performance. Error and overshoot are small.
(2) The method of MotorSolve and Simulink cosimulation can optimize motor parameters from the perspective of system matching, which is more in line with reality.

Ⅴ.DEMONSTRATION APPLICATION AND TEST
A low-speed and high-efficiency PMSM and a driving frequency converter were manufactured to verify the analysis results and the correctness of the method. The demonstration application and test were carried out on a 45 T EWMW in a deep-sea test platform, as shown in Fig.16.Then, it was certified by the China Classification Society.The pre-interference and post-processing waveforms of rotating transformer signals are shown in Fig.17, respectively. It is obvious that the transition plate of non-magnetic material reduces the interference of the brake to the position signal, and the effect is obvious. The measured line voltage curve of the motor under rated load is shown in Fig.18, and the aver RMS of line voltage is 288.5 V. TABLE III shows that the part of the simulation agrees well with those measured values.   The main conclusions by the simulation analysis and test are listed as follows: (1) The PMSM simulation value is in good agreement with the actual test value. The motor can meet three times the field weakening speed regulation and 2.3 times the short-time overload capacity. The measured value of motor efficiency is 94.3% [27].The gear efficiency is about 96%, the inverter efficiency is 98.3%, and the motor efficiency is 94.3%,therefore, the efficiency of the system can be easily obtained as 0.96  0.983  0.943=88.9%.
The error between the calculated temperature rise (50 K) and the PMSM experimental value (45.5 K) is about 8.5 K. The main reason is that the actual motor base and casing are connected to the WMW and have good thermal conductivity. Therefore, the actual short-time operation of the motor for 30 minutes adopts natural cooling, which fully meets the requirements, and the operation is safe and reliable.
(2) The accuracy of the position signal of PMSM has a great influence on the constant tension control of the EWMW and the follow-up and dynamic response of the large impact load. In this paper, it is proposed that after the magnetic isolation measures of non-conductive materials and the large gap of the reluctance resolver are adopted, the electromagnetic field of the electromagnetic brake has little interference to the signal of the position sensor, and the waveform is perfect.
(3) The permanent magnetization drive system of EWMW is a complex and large-scale system, involving the motor and its drive, upper computer communication, control, mechanical transmission, etc., which needs to be further studied.

Ⅵ.CONCLUSION
The design and application of low-speed and highefficiency PMSM and drive systems are difficult due to the special load characteristics and requirements of EWMW. Taking a set of 110kW 75r / min PMSM and its drive system as an example, a parameter matching design method of low-speed and high-efficiency PMSM based on finite element, field circuit coupling, and co-simulation is proposed in this paper. At the same time, it also describes some design matters of structural matching. Finally, through the development of the prototype and the demonstration application and test on the 45t electric anchor winch, the test results show that the performance indexes fully meet the design requirements and verify the correctness and effectiveness of the matching design method. It has certain reference and guiding significance for the design and application of PMSM and drive system of WMW.

ITEM
The results of FEA