1. Introduction
Electrical and electronic equipment design in the 21st century is invariably focused on energy efficiency. Most home appliances have already been replaced by energy efficient upgrades: inverter-based designs have replaced conventional solutions for air conditioners and refrigerators, LEDs have supplanted fluorescent lights, and LED TVs have substituted CRT-based televisions, to name a few examples [
1]. Among all household appliances, electric fans are one of the most widespread in both urban and rural environments [
2]. Ceiling fans are extensively used in summer seasons all around the world, constituting approximately 6–9% of the total energy used in the residential sector. Improvements in ceiling fan design offer enormous potential for energy savings. The most significant improvement is the use of a brushless DC (BLDC) motor in place of an induction motor, which can reduce power consumption by 50% [
3,
4]. Although the technology of permanent magnet BLDC motors for fans is more complex, the rising energy cost has tipped the return on investment in its favor [
5,
6]. Two types of PM motors are currently being well received for ceiling fan applications, namely the BLDC motor and the permanent magnet synchronous motor (PMSM). Both these machines are very similar, except that the BLDC has a trapezoidal back EMF, while the PMSM has a sinusoidal back EMF. BLDC motors are commonly used in designs that require cost-effective solutions, due to their low cost of manufacturing [
7].
A comparison of induction motors with BLDC and PMSM motors is given in
Table 1. The usage of BLDC motors is substantially increasing in ceiling fans as well as other household appliances. These appliances are commonly powered by single-phase utility electricity supply, which is rated country-wise as 220 V (50 Hz) or 110 V (60 Hz). DC-DC converters are used to convert this single-phase AC to the required DC Voltage levels that are fed to a voltage source inverter (VSI). A combination of this DC-DC converter and the VSI forms a BLDC motor drive that controls the motor speed in the target appliance. The speed and torque are the output characteristics of the fan motor. Both these parameters are controlled by the sensorless field oriented control (FOC) algorithm. Even with tight regulation through properly designed control loops, torque ripple is created in BLDC motors due to changes in the stator current ripples caused by inductance and resistance in the motor windings [
8,
9]. This ripple can cause extremely undesirable effects such as motor vibration, acoustic noise, and other physical faults that may develop over time.
The torque of a BLDC motor is given in Equation (
1):
where
,
, and
represent the EMF of the motor windings.
,
, and
represent the stator currents, and
depicts the angular speed. As the angular speed will be constant in steady state operation, the magnitude of EMF across each of the windings will also remain constant. Therefore, the parameter that is responsible for the torque ripple is the stator current in each phase.
There are two main sources of stator current ripples. The first source is the commutation of the BLDC motor through the VSI that causes a ripple that has a frequency corresponding to the fundamental frequency at which the VSI is being switched. There are several mitigation strategies for this kind of ripple. First, the magnetic design of the stator and rotor can be improved by altering the machine design [
10,
11,
12]. Other techniques include the design of a torque controller [
13,
14], a current shaping model [
15,
16], input voltage controller [
17], feedforward current controller [
18], and direct torque control [
19,
20] methods. The second source of ripple is the DC power supply of the VSI. If the DC source supplying the VSI contains current and voltage ripples, they will be reflected in the stator currents. Since motor drives are commonly supplied by DC-DC converters, the output ripples of the converter will have a frequency that corresponds to the switching frequency of the MOSFET in the DC-DC converter. In order to mitigate the stator current ripple, the output current ripple of the converter must be minimized. This is why converters that exhibit a continuous output current are suitable for BLDC motor drive applications.
In addition to the requirement of continuous output current, power supplies for BLDC motor drives must comply with IEC standards to maintain input power quality at an accepted level [
21]. Presently, different topologies along with power factor correction (PFC) methods are being used in power supply designs to comply with these standards. In the open literature, several contributions have been reported regarding PFC single-phase AC-DC switch mode power supplies that exhibit near-unity PF and low THD [
22]. These supplies are general purpose and may be used in a variety of applications, such as any analog or digital circuitry, motor drives, and battery chargers. However, very few efforts have been reported in the existing literature that uncover the need for better power quality, specifically for BLDC motor drives. The designs proposed in [
23,
24,
25] and [
26] are not target-specific, i.e., they may be used in any BLDC motor which is supplied by a single-phase AC power source.
Table 2 shows a comparison between different BLDC motor drive designs.
However, every application comes with some specific requirements that must be considered in the design process. Therefore, application-specific design is required in order to develop the motor drive that fulfills these specific requirements. There have been a few efforts to use different converters for the specific application of BLDC ceiling fan drives. In [
27], a non-isolated buck–boost converter-based motor drive for a BLDC ceiling fan has been presented. This design is also non-isolated, hence only suitable for ceiling fans operating at high voltage. In [
28,
29], the use of Zeta and SEPIC (single ended primary inductance converter) converters, respectively, has been proposed for a ceiling fan motor drive. These designs are isolated and use DCM with the voltage follower technique to achieve near-unity PF and low THD. Furthermore, they vary the DC link voltage of the VSI, which is also the output of the DC-DC converter in order to vary the motor speed. This speed control method decreases the switching losses in the VSI by operating it at a lower switching frequency but increases the switch and body diode conduction losses in the high and low side switches of the VSI [
30].
Table 3 shows the advantages and disadvantages of different BLDC motor driver designs for ceiling fans.
Due to the above limitations, FOC has been preferred as the method of speed control for sensorless and hall-sensor-based BLDC motor drives, especially since more powerful and inexpensive microcontrollers are now readily available to handle the comprehensive computations required by FOC [
31]. In [
32,
33], a design of a BLDC ceiling fan drive that uses FOC for speed control has been proposed. It can be inferred from the recent literature [
32,
33] that the designs of commercial appliances are increasingly using FOC for the speed control of motors. Another important aspect of the fan controller from the perspective of user experience is the number of pre-defined speed levels that can be set by the user. The literature indicates that the possibility of controlling the fan speed generally enhances thermal comfort for users [
34]. Traditional ceiling fan motor controllers limit the control of speed to a few pre-defined levels, which may limit the comfort of the user. The stepless speed control can give more control to the user and hence more comfort.
Keeping in view the application-specific design requirements for BLDC ceiling fans, this work aims to design and test the performance of an efficient isolated Cuk converter supply with integrated magnetics for the motor drive of the BLDC fan. The significant contributions of this work are as follows:
- 1.
Power supply design with negligible ripples: Presenting the design methodology for the development of an isolated PFC Cuk converter with integrated magnetics for low-power applications. This enables a design that is compact, efficient, and provides stable power supply with nearly zero high frequency ripples.
- 2.
BLDC fan operation with low torque ripple: Using the Cuk converter to incorporate continuous input and output currents in the AC-DC supply design. This minimizes the current ripples and ensures less torque ripple in the BLDC fan motor.
- 3.
IEC standards compliance: Deploying the isolated Cuk converter in continuous conduction mode (CCM) with the current multiplier technique. This enables power factor correction (PFC) and a total harmonic distortion (THD) reduction along with an additional feature of over-current protection.
- 4.
Failure modes analysis: A comprehensive analysis with common fault conditions of the ceiling fan and their impact on the proposed power supply is presented.
The rest of this paper is organized as follows: the preliminary concepts used in this work are discussed in
Section 2. The AC-DC PFC power supply topology, along with the design methodology, is discussed in
Section 3. The system performance validation that includes the modeling and simulation approach is described in
Section 4. The results and performance comparison with similar topologies are discussed in
Section 5. The concluding remarks are presented in
Section 6.
Appendix A includes the list of acronyms used throughout the paper, the notations along with their units, and the parameters of the ceiling fan BLDC motor in
Table A1,
Table A2 and
Table A3 respectively.
2. Background
Many topologies are deployed to improve power quality in AC-DC converters. They are primarily used according to the required load characteristics and power level.
Figure 1 shows the classification of isolated power supplies according to their input to output voltage characteristics, namely the buck, buck–boost, and boost supplies. The buck and boost power supplies are preferred in applications where the output power requirement exceeds 500 W [
22].
The single-phase buck–boost AC-DC converters are generally preferred and suited for low-power applications less than 500 W [
22]. As represented in
Figure 1, these converters are further classified as Flyback, Cuk, SEPIC, and Zeta isolated AC-DC converters. These designs have some degree of similarity as all of them offer buck–boost input to output characteristics by using a single active switching device (MOSFET) and a high frequency transformer for isolation. The transformer enables the use of multiple regulated outputs in these supplies. They may be operated in discontinuous or continuous conduction modes to improve the power quality. These converters can also be implemented with integrated magnetics to further decrease the component count, size, weight, and cost. Power quality improvement through input current shaping is extensively used in these power supplies [
22].
A brief overview of the Flyback, SEPIC, Zeta, and Cuk power supply topologies is presented here. The isolated Flyback converter’s high frequency transformer provides isolation, electrical safety, lower cost, and simple control. This power supply is a popular choice in low-power applications for designers due to the lower component count and simple control [
22]. The isolated Cuk converter exhibits brilliant power quality at the input AC as well as output DC side. In this design, energy is transferred through capacitors; therefore, the input and output currents are continuous. This design offers very low switching current ripple, wide range of input and output voltage, small size, natural protection against inrush current, and high overall conversion efficiency [
22]. The isolated SEPIC-based power supply is another design that exhibits brilliant power quality at the input AC and output DC side. Low component count, small size, and fast dynamic response make this design a popular choice for applications where a high degree of efficiency, reliability, and power quality are required [
22]. The isolated Zeta converter-based power supply is relatively new and therefore interests designers. It uses a high side switch and provides protections such as inrush current, short circuit, and overload. It is also preferred for high-power applications such as telecom power supplies and some motor drives [
22].
The isolated Flyback converter has been used extensively in commercial power supply applications for decades; one limitation of this topology is that the magnetic energy is temporarily stored in the coupled circuit core. Thus, for a specific magnetic material, the maximum energy transfer is restricted by core volume. The core volume is utilized more effectively if the magnetic energy transfer is through instantaneous transformer action rather than transfer with intermediate magnetic energy storage. The other three converters address this limitation by transferring electrical energy through magnetic transformer action [
35]. The results of SEPIC and Zeta converters have been reported previously in BLDC ceiling fan drives, but the isolated Cuk converter is not reported to have been tested in such an application. The SEPIC and Zeta converters exhibit continuous current on the input and output sides, respectively, whereas the Cuk converter exhibits continuous current on the input and output sides. As a result, the ripple in input and output currents is lower than the other converters. This low ripple, integrated magnetics, and low size and cost are characteristics that make the Cuk converter an interesting choice in this particular application.
5. Results and Discussion
In the light of the simulations obtained through the PLECS software presented in the previous section, the effect of the switching frequency of the VSI (10 kHz) can be observed in
Figure 16 in the stator currents
and motor torque
ripple. The harmonics of this switching ripple can also be visualized in the figure. The result shows that the DC-DC converter switching frequency (50 kHz) ripple does not cause any significant motor torque ripple. In a conventional design, however, this switching frequency ripple would have affected the stator currents
, and the same would have also been reflected in the motor torque
. Therefore, it can be concluded that the proposed power supply design is suitable for BLDC fan motors, as there are negligible motor torque ripples.
In view of the failure modes analysis presented in the above section, the power supply tolerates over-current faults by limiting the input current
of the converter, as seen in
Figure 17. The value of the MOSFET duty cycle
D is updated at the switching frequency
= 50 kHz according to the current and voltage feedback. Therefore, in the case of an over-current event, the response time of the loop will be one switching interval of the DC-DC converter. In this case,
= 20
s. As a result of this current limiting protection feature, the DC link voltage
also decreases sharply. Moreover, as shown in
Figure 18, the converter also tolerates inverter failure without causing any damage. Therefore, it can be concluded that the two common faults discussed in this paper do not negatively affect the system, making it reliable.
The input power quality of the converter is in line with the requirements specified in
Table 4. The PF is found to be 0.99 whereas the THD is measured as 4.72% as per the simulation at rated load. The power quality indices of the other topologies (buck–boost, SEPIC, Zeta) implemented with their own PFC techniques that have been reported in the literature in [
27,
28,
29] have also been compared with the simulation results of the proposed design.
Figure 19 represents the comparison of the four designs. The data of the proposed design have been obtained through simulation, while the data of the other three designs have been taken from the previously reported work in the literature [
27,
28,
29]. Since every fan motor is designed to operate for different specifications, it is inappropriate to compare the power quality parameters as a function of the fan speed. Therefore, the PF and THD have been plotted as a function of the real power supplied by the AC-DC supply. This can be used to evaluate their performance and compare these designs. It can be inferred from
Figure 19 that all four converters exhibit similar performance with minor differences. All these designs comply with existing standards, exhibiting near-unity PF and low THD in all operating conditions. Therefore, it can be concluded that the proposed CCM Cuk converter provides the abovementioned advantages with no compromise on input power quality.
Table 7 below describes the indicative cost of the major components required in each of the four converters. The table shows the quantity and description of each of the components required. Since quantification of the cost is a subjective topic having dependencies on several design choices, the cost of each component has been discussed qualitatively. Intuitively, it may seem that the Cuk converter is an expensive option due to overall higher component count, but the integrated magnetics and highly efficient transformer design are cost effective since they deliver the same power with smaller sized magnetics.
By inspection of the component count and type, it can be seen that the Flyback converter is the cheapest option, despite its inefficient and bigger transformer, which explains its commercial success. The proposed Cuk converter with integrated magnetics ranks second, while the SEPIC and Zeta converters follow behind. Such trade-offs between cost and performance always exist in most designs. The proposed converter offers an optimized choice between cost and performance.