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Slotless Motor Design & Analysis

Subject: Design of XXXXX Motor with Improved Km.

Elinco are pleased to provide  design and analysis for the improvement of the XXXXX motor. We have created a MotorSolve model of the xxxxxxxx. This is the XXXXX that we received from Elinco and tested here. We are providing three options for improvement of the Km of this motor.

The designs include the MotorSolve files for your use. The data includes the active materials with dimensions and winding information for the designs. We will recommend suitable vendors to provide the stator assemblies and the magnet materials used in the improved designs.


“Design initial motor: Model the existing motor to calibrate the simulations against test data, prepare a new design and select a winding scheme for higher Km performance. Deliverables for this phase would be all data required to prepare production docs of active magnetic components including materials, winding details PM rotor construction and simulated performance results using MotorSolve.”


The design goal is to increase the KM of the XXXXX. We assume that we are designing to make improvements to the existing motor supplied. This motor exhibits a KM of 3.16 oz-in/√(Ω’s). In phase one of this project we expected an improvement of 30% to 50%.


The following information references TABLE B. All mechanical dimensions are in millimeters. The values for Km have been normalized to reflect the improvement with respect to the sample motor provided by Elinco. All values reflect performance at 20″C. Conversations with Juergen Matzen indicate the customer believes that high temperature will not be an issue due to the low duty cycle and the ambient temperature. The following steps characterize the methodology used to realize the improvements indicated in TABLE B. Keep in mind that all of the models attempt to keep the KT similar to the sample motor.

1. Characterize existing motor by testing and disassembling a motor provided
1.1. Measure actual performance (resistance, inductance, back EMF)
1.2. Disassemble and measure active components (lamination, magnet, and winding)
1.3. Examine the winding layout

2. Create model in MotorSolve that duplicates the performance of the sample motor

2.1. Stator

2.1.1. Lamination (ID, OD, thickness, and length of stack)
2.1.2. Winding configuration (number of turns provided by Elinco)
2.1.3. Winding convention used in the models is shown by the colored line graduating to broad colored line indicating start and finish of coil respectively in the cross section (see Fig. 3)
2.1.4. End turn length
2.1.5. Wire diameter
2.1.6. Slot fill

2.2. Rotor

2.2.1. Magnet (ID, OD, and length)
2.2.2. Material properties (validated by duplicating the back EMF of                               sample motor)

3. Use validated MotorSolve model for design iterations to improve KM

3.1. Design 1 – Improvement by changing winding configuration only

3.1.1. New winding configuration
3.1.2. Same lamination as sample
3.1.3. Same magnet as sample

3.2. Design 2 – Improvement by changing winding configuration and active materials

3.2.1. Use new winding configuration from above
3.2.2. Reduced lamination ID for reduction in magnetic air gap
3.2.3. Improved magnet material (same dimensions as sample motor)

3.3. Design 3 – Improvement by changing winding configuration,active                                          materials and number of poles

3.3.1. Use new winding configuration for four pole motor to present                               maximum possible KM improvement potential in the frame size

3.3.2. Improved magnet material, four poles, larger magnet ID & OD

Two XXXXX motors were provided for evaluation and characterization (Fig. 1). One motor was disassembled (housing was actually cut in half), which allowed measurement of various mechanical features of the motor. We measured critical dimensions to facilitate constructing a model using MotorSolve. The dimensions are shown in TABLE B. One of the motor was used to measure performance (TABLE B) and determine the coil layout (Fig. 2) after disassembly.

TABLE B shows the correlation between the actual motor and the MotorSolve model. The comparison of the measured back EMF wave shape and the model wave shape is evident in Fig. 6 & 7.


A model was created using MotorSolve to simulate the xxxxxxxxxxx motor. All materials in the model reflect the dimensions and the test results obtained from the sample motor. The bare magnet wire diameter used for the model was .315 mm. This yielded the best correlation with the sample motor for resistance and slot fill factor. The KM, which is the item for improvement is within 1.7% of the actual motor. This is well within the tolerance that one might find in normal production. The material used for the magnet has the properties in the range of a typical N35SH grade Neodymium-Iron-Boron material (Br=1.19T). The B/H ratio, as determined from the sample motor is .59 (based on magnet and lamination dimensions).

Figure 7 Back EMF for MotorSolve model of xxxxxxxxxxx


This design is a change in winding configuration only (see Fig. 8). All dimensions and materials are the same as the MotorSolve model of the sample motor (with the exception of the magnet wire diameter). The winding configuration changes are in the form of; the number of coils per phase, the coil location, and the fact that more of the existing winding area is used. The same number of total conductors is used; however, the magnet wire diameter has increased due to the additional area available for copper. It should be noted that the winding factor has decreased from 100% to 96.6%. The winding factor is a figure of merit for the coil placement with respect to the rotor magnet. This said 96.6% is still quite good.

With this change, an improvement of 31% is predicted for the value of KM. This is of course due to the increased copper area facilitated by the improved conductor distribution, and therefore a lower resistance. The magnetic circuit remains unchanged, which means that the amount of saturation in the lamination stack is equivalent to the existing sample motor, i.e. the B/H ratio is unchanged from the model of xxxxxxxxxx.


Here we have taken a different approach. The magnet will stay the same in dimensions; however, the material properties have changed. The Br increases from 1.19 Tesla, as in previous designs to 1.33 Tesla. This is comparable to an N45SH Neodymium-Iron-Boron material (Br=1.33T).
Along with the change in magnet material, the inside diameter of the lamination was reduced (see Fig. 9). This smaller diameter creates a smaller air gap and therefore improves the magnetic circuit (B/H ratio=.76). This improvement allows for a reduction in the total conductors, which in turn allows for a larger diameter magnet wire than used in the sample motor. This, of course lowers the total resistance of the motor for a predicted improvement in KM of 39%. Although the air gap flux density is higher, the lamination is thicker therefore saturation is a little less than the sample motor. This means core losses will be about the same as the sample motor.


This design changes the magnetic circuit by configuring the motor as a four-pole motor (see Fig.10). The lamination will retain the same dimensions and material as the sample motor. The magnet material’s Br is higher than that of the model or that of design one. It is comparable to N38SH (Br=1.24T). There will be four arc segments (#90″ mechanical) to facilitate complete saturation of the magnet during magnetization. The magnet dimensions change to an OD of 14 mm and an ID of 8 mm (3 mm thick). Of course, the winding configurations will change to accommodate the four-pole configuration. The winding factor is 100%. The increased magnet thickness reduces the air gap (3 mm) to improve the B/H ratio to 1. Due to the four-pole configuration, the lamination has about the same flux density as than the other designs. A four-pole motor will have twice the magnetic frequency as the two-pole machine. Therefore, the Iron losses will be higher than that the two-pole machine. The graph (Fig. 11) shows the theoretical total iron loss (in Watts, M19, .014”) for two poles vs. four poles at speeds up to 30,000 RPM. Based on the information received about the motion profile of this application, this should not be an issue. That is to say that if the motor spends very little time at the elevated (>10krpm) speeds, the iron losses should not adversely affect the overall performance.

The core loss chart (Fig. 11) is based on the data listed in the United States Steel catalog for 29 gauge, M19 fully processed lamination steel with a flux density of 1.2 Tesla. It is the total loss, which includes hysteresis and eddy current losses. The lamination stack size used for the comparison is; 20 mm ID, 24.6 mm OD, and a stack length of 64.5 mm.


We have provided four MotorSolve models for XXXX motor. The models have all of the active material information required to build the three improved motor designs. Also included in the model information is the winding layout for each design. The first model is a representation of the sample motor provided. This model was used to generate designs 1, 2, and 3. The calculated performance of the model matches the measured performance of the motor (xxxx) within 1.7%. The three designs provided improve the value of Km, which is the goal of the project. Note that the B/H ratio of the sample motor, as well as that of design 1 is .59; consequently, the operating point of the magnet (Fig. 12). This is due to the air gap being much larger than that of the magnet length. The implication is that magnet may be suspectible to demagnetization at elevated temperature combined with high current. However, no such problem has been reported with respect to the existing motor. Keep in mind that this is a representation based on N35SH material. We do not know what the sample motor material is. Based on N35SH material used to model the sample motor, the aluminium demagnetization current (11 amps peak, TABLE A) is met at a rotor temperature of  60ºC. The models that have improved B/H ratio are less susceptible to this phenomenon.

Results for Design 1: 

In design 1 the KM is improved by approximately 30% over the sample motor provided. This improvement is accomplished by changing the winding configuration to make better use of the available winding area. All of the dimensions of the existing motor were maintained, with exception of the magnet wire diameter. The magnet material modeled in the sample motor is used in design 1 as well. The improvement in the KM is due to the reduction in the winding resistance while maintaining the value of KT.

Results for Design 2:

Design 2 predicts an improvement of 39% over the sample motor by changing the inside diameter of the lamination while maintaining the magnet dimensions. A higher energy magnet is used in this design. This reduction in the air gap as well as a higher flux density allows for a reduction of the number of turns used for the coils (keeping the KT the same). The lower number of turns makes room for a larger wire diameter. The reduced turn count and larger wire diameter allow for a lower resistance. This design also has the advantage of an improvement in the B/H ratio to .76.

Results for Design 3:

Design 3 completely changes the magnetic circuit to a four pole device. The predicted improvement in KM over the sample is 53%. The lamination has the same dimensions as the sample motor. The winding configuration is changed to a four-pole design. The magnet material is changed from that of the sample motor to a slightly higher energy material. The magnet thickness is increased to three millimeters. The air gap is also three millimeters. This improves the B/H ratio to 1 which is the highest of the three designs. The magnets are arranged in the form of four arc segments. This will allow for better magnetization; however it will make the fabrication more involved than that of the sample motor and the other designs. The other designs use the cylinder magnet. In addition, the magnets will require a form of safety containment in the form of a thin tube of non-magnetic material placed around the outer diameter of the arc segments. This can take the form of a thin (.3mm to .5mm) stainless steel sleeve that is pressed onto the rotor assembly (magnet arcs and shaft). The rotor can also be wrapped with carbon filament or fiberglass roving. This option requires additional equipment and is not the recommended method. This design will produce a rotor with increased inertia. This will of course affect the acceleration and deceleration of the rotor by either reducing the values or requiring higher current to achieve the same values.

To assemble the stator with maximum magnet wire, fill for all of the designs presented requires layer winding the coils on appropriate tooling using bondable wire. The bondable coating will be activated during the winding process using a jet of hot air directed at the point of the winding where the magnet wire meets the previous conductor. Once the coil is wound it can be removed from the tooling and placed in additional tooling used to form (compress) the coil shape to accommodate insertion in the stator lamination stack with appropriate insulation (see paragraph 1.2 through 1.8, ES373). After all of the coils are formed and inserted, tooling is required to ensure that the inside diameter of the coil is maintained to provide the working air gap while the coils are bonded in place.

Balancing of the rotor may be accomplished in the same manner as the sample motor; however we recommend that a material such as brass be used for the area of material removal. This should allow for easier balancing due to removal of less material. The sample material appeared to have rather large areas of material removed from the rings.

Designs 1 and 2 give a fair increase in KM with little impact on the method in place to fabricate the existing motor. The main difference is in the winding. Of course, the lamination for design 2 is a different dimension and the magnet material is different. Again, all of the models use M19, 29 gauge steel. Design 3 offers the largest gain in KM. Of course, this will require the most effort of the three designs. Further analysis of the rotor assembly and magnet containment will be required. With respect to the magnet material, it is becoming increasingly more difficult to obtain NdFeB magnet material with enhanced thermal properties (addition of Dysprosium). Once a design is settled, we will address the subject of assembly and tooling.

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