Effect of Rotor Designs on Thermal and Power Characteristics of Brushless DC Outer Rotor Flat Motors

The performance of a brushless DC (BLDC) motor depends on several factors, including overall topology (inner or outer rotor), winding configurations, number of pole pairs, slot configuration, and magnet composition. In outer rotor BLDC motors, the rotor construction plays a significant role in motor performance. This paper presents an analytical study of the performance characteristics of Portescap's 45ECF BLDC flat motor with three different rotor designs (closed, open slot, and integrated fan) operated at various speeds. The analysis also includes the power curves of the 45ECF motor for each rotor type.

INTRODUCTION

The performance optimization of the BLDC motor design has drawn the attention of various industry researchers over the past few decades. The BLDC motor is widely used in medical, automotive, aerospace, and major industrial automation applications to leverage its high efficiency, power density, torque-to-size ratio, and ease of control. A permanent magnet DC motor with an electronically controlled commutation system, the BLDC motor replaces the conventional brushed DC commutator and brush arrangement to make the motor more reliable and capable of operation at higher speeds. The three basic configurations of the permanent magnet BLDC motor prevalent in the industry today are the inner rotor, outer rotor, and axial gap disc design, each of which includes many different winding and pole configurations. The term 'radial' or 'axial' refers to the direction in which the magnetic field is imposed by permanent magnets. The motor is defined as a radial flux motor when the flux is imposed in the radial direction, while the axial flux motor is imposed in the axial direction.

OUTER ROTOR MOTOR

The structure of an outer rotor BLDC PM motor is shown in Figure 1. In the outer rotor configuration, the stator and rotor are swapped - the stator or windings of the motor are at the center of the motor, while the rotor and magnet assembly are placed to the outside. The stator consists of a multiphase winding on a laminated core, while the rotor consists of permanent magnets (segments or molded ring) fixed to the inner surface of a steel cup-like component. The cup is attached to the motor core shaft and rotates freely on bearings. Through the sequential excitation of the stator windings, continuous motion of the rotor is achieved. The three different configurations of rotors (closed, open slot, and integrated fan) are shown in Figure 3.

POWER LOSS IN MOTORS

Motors dissipate energy losses as heat and in other forms such as vibrations and internal core losses. While all of these losses may reduce the efficiency of a motor, excessive heat losses can be a major contributor to premature motor failure. Copper and iron losses significantly contribute heat to the motor, while lesser contributors include friction and windage losses. The heat dissipation rate depends on the motor geometry, environment, material, and motor speed. Energy losses determine the overall efficiency and thermal behavior of the motor. The major performance losses for an open rotor BLDC motor follow.

•	Copper losses are losses dissipated in the form of heat from the resistance of the stator copper windings and are the predominant contributor to excessive motor heat. The equation used to calculate copper losses is as follows: Pcu=Rp* I2 (Watt) Where Pcu = copper loss in Watts Rp = Phase to Phase resistance in ohm I = DC-current input in amps.
•	Iron core losses are the losses generated in the iron core due to harmonic flux or alternating magnetic fields and consist of hysteresis and eddy current losses.
•	Mechanical losses consist of friction and windage losses. Friction losses are due to bearing friction while windage losses are due to the drag generated by air on the outer rotor.

METHODOLOGY

The complexities of accurate measurement collection, coupled with the wide expansion of BLDC motor applications, have predicted BLDC motor losses as one of the most sought-after research topics in academia and industry circles. The ultimate goal is to compare the thermal performance of all three variants and to find the effect of rotor geometry on torque performance. The assumption is that copper losses in the stator are the main contributor to motor heat. All other losses contribute to the thermal resistance of the motor, which is mathematically determined by motor construction and speed variables (Equivalent thermal resistance (Rth)eq). If speed is constant and the load increases, the iron and mechanical losses will tend to be constant with the temperature rise primarily due to copper losses only. With equivalent thermal resistance, the maximum torque can be calculated at any speed based on empirical data at a specific load point. After recording the temperature and equivalent resistance at various data points, an accurate power graph can be created (speed vs torque).

The thermal resistance of a material is the reciprocal of thermal conductance and is described as the temperature difference required in a material to transfer 1 watt of power. The expression for thermal resistance at equilibrium condition is:

The maximum torque of motors of each type at various speeds are calculated using the equivalent thermal resistance (Rtheq ). The formula for maximum torque is:

THERMAL TESTING

To compare motor performance characteristics by construction type, BLDC motors with closed rotor type, open slot rotor type, and integrated fan rotor type are tested at various torque and speed points with the following instruments:

1.	Portescap's 45ECF slotted flat motor with thermal sensors
2.	Power supply
3.	BLDC driver
4.	Dynamometer with closed loop feedback system
5.	RMS multimeter

Torque is applied to the motor with a dynamometer. The temperature rise of the coil is monitored and recorded with a thermal sensor mounted on the motor coil and data from the RMS multimeter. The trend of temperature rise over time is recorded until the motor reaches its steady state temperature (thermally stable). Motors are tested at speeds ranging from 0 to 8,000 RPM with increments of 2,000 RPM. Resistance is measured when the coil temperature reaches a steady state. Power loss and thermal resistance of the motor at each speed are calculated based on the measured resistance, current, and stable coil temperature.

The thermal resistance of the open slot type rotor motor and the integrated fan type rotor motor decreases rapidly with an increase in speed as shown above, while the thermal resistance of the closed type rotor decreases slightly (up to 3,000 RPM) and then increases drastically (5,000 RPM onwards).

Replacing the closed rotor with the open slot rotor increases the maximum torque from 54 mNm to 80.5mNm at 8,000 RPM, which is approximately a 47% increase in torque capacity. The open slot rotor with the integrated fan further improved maximum torque capacity to 113 mNm, which is 40% greater than the open rotor design.

The maximum torque vs speed is plotted for motors with a closed type rotor, open slot type rotor, and integrated fan type rotor. The motor with an integrated fan type rotor resulted in the highest power rating.

THERMAL TESTING SUMMARY

Due to the fan circulation, the integrated fan rotor design motor had the highest heat dissipation property. With an increase in speed, the heat dissipation of this motor will improve. The motor with an integrated fan type rotor can draw more input current and provide higher torque. The motor with the open slot type rotor had a medium power rating with a moderate heat dissipation property. The motor with the closed rotor had the least heat dissipation property. With an increase in speed, the iron and core losses of the closed rotor design also increased and, above 5,000 RPM, increased rapidly with a significant decrease in the power capability of the motor.

CONCLUSION

Choosing the appropriate setup for a brushless DC outer rotor flat motor presents a notable challenge, necessitating a deep comprehension of the particular performance attributes that the application values. Companies specializing in micro- motion technology, such as Portescap, excel in guiding design engineers through the selection process to pinpoint the most suitable motor for achieving the desired outcome.

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