For a couple of decades, appliance makers have been trying to justify the added cost of brushless DC (BLDC) motor replacements for brush and AC motors. In general, only high cost/high-reliability applications have been able to justify the change based on reliability and performance factors such as speed control, torque ripple, jitter, and EMI.

Conventional PCB.

Quadrant Systems Inc. (QSI) has taken a systems development approach to improving BLDC motor technology that has resulted in improved BLDC motors of all sizes from a few watts to hundreds of watts. The two following examples illustrate improvements in size, cost, and performance.

Commutation

Enlarge this picture Fig. 1. Rotor magnetization and magnetic profile.

Existing commutation techniques utilize three Hall elements, three Hall ICs, or sensorless commutation. Sensorless systems eliminate the three required Halls sensors, saving a component cost of 10 cents to 45 cents, the mechanical placement of sensors, and a bundle of wires. Sensorless commutation systems require a much more complex chip, more discrete parts, and cannot generally start with a heavy load. However, sensorless technology is standard in HDD and other disk drive systems where space is at a premium, starting load is minimal, and speed/jitter control is critical.

In order to commutate a conventional 3-phase motor, three sensors are used. This requires three Hall sensors embedded in the motor with five to 12 wires and decoding logic. The Quadrant Systems, SingleSense® (1) method facilitates 3-phase commutation with no discrete Hall sensor.  One sensor is combined with the control chip, so there is not a sensor component. In the SingleSense motor, the rotor is modified to contain magnetic signals, which allow one Hall sensor to control 3-phase commutation, utilizing simple logic. (6)

Enlarge this picture Fig. 2. Zener clamps and BUD.

A schematic drawing of one embodiment of the rotor magnet is shown in Fig. 1, where the A, B, and C black lines represent the stator poles. The radial magnetization provides the main rotor magnet poles. The small, axial magnetization implants and the radial magnet edge magnetization pattern provides the commutation signal.

The magnetic profile created by both axial and radial flux of the rotor is also shown in Fig. 1.  The on-chip Hall sensor detects the North, and South magnetic field, which with simple logic turns on the drivers for phases A, B and C.

Drive technology

Enlarge this picture Fig. 3. BUD commutation sequence.

The motor’s drive technology is called BUD™ (2), which stands for bi-phase unipolar drive. This drive allows the commutation of a multi-phase BLDC motor with three power devices instead of six, yielding a significant cost saving.

It is a motor phase switching technology that provides two driven phases, as in a typical bipolar drive motor. However, it accomplishes this with a 3-step unipolar drive sequence instead of the 6-step sequence used for a standard bipolar drive.

BUD technology eliminates the need for separate Zener protection and extends the ability to use low cost unipolar drive from motors of a few Watts to approximately 100 Watts without a cost penalty to the control circuit. (See Fig. 2).

Enlarge this picture Fig. 4. Conventional radial motor vs. vertical motor. Outward facing stator shown above.

As one power FET turns off, the stored energy in the motor coil must be discharged. Without a clamp, such as a power Zener or BUD winding, the power FET will be subject to hundreds of Volts and be destroyed. The Zener will avalanche and dissipate the energy. As currents/motors become larger, the Zener cost becomes prohibitive, making bipolar drive a lower cost solution, requiring 3 additional power FETS and a more complex controller. In the BUD system, the clamping is accomplished through the mutual inductance of the bifilar windings, so the stored energy is transferred to the complementary phase (e.g. B " B’).

This approach also provides more balanced motor drive, as compared to standard unipolar drive, because 2/3 of the motor phases are being powered as in bipolar drive. This kind of drive saves controller cost, but uses two times the wire, 10 percent more stator material, and has higher torque ripple as compared to a  bipolar drive. The cost penalty on the motor side is considerably less than the cost savings on the controller side. Fig. 3 shows the first two of the three commutation sequences.

Magnetics

Enlarge this picture Fig. 6. 1-Watt, 3-phase drive system PCB and block diagram.

To better understand the vertical architecture motor, which has been named the Powder Keg® (3), one must first understand the basic motor architecture of conventional brushless permanent magnet motors (BLDC).

The stator poles of conventional (horizontal) BLDC motors face inward toward the motor shaft or outward away from the shaft. This form of architecture is necessary because the stator is made of a stacked series of steel laminations. Because laminations are magnetically two dimensional, (in terms of efficient magnetic flux transfer), sound engineering requires that the stator pole laminations lie in a plane perpendicular to the motor axis, pointing directly at the rotor magnet.

The Powder Keg motor magnetic circuit is three-dimensional, whereas laminated steel is uni-dimensional. The three-dimensional approach takes full advantage of the isotropic magnetic properties of SMC (Soft Magnetic Composite) material such as Hoganas’ Somaloy™ 700 (5).

The SMC vertical stator structure is isotropic; its flux passes in all directions. The vertical structure uses bobbin-wound coils, provides multiple rotor magnet configuration options, and has an interior available for load functions such as gear heads or electronics.

Fig. 4 illustrates a vertical SMC stator compared to a laminated stator. The windings of the vertical structure are wound on bobbins and placed on the stator poles. On a laminated stator, the winding is usually placed directly on the insulated iron poles, which is a much slower and complex operation with lower yields.

Appliance motors

Fig. 7A. Powder Keg motor (black) replaced radial motor (white) in refrigerator evaporator fan assembly.

A motor system that combines SingleSense, BUD and Powder Keg technologies provides a lower cost and a smaller size than conventional BLDC alternatives. Despite the simplified three-step drive, this system delivers equivalent performance in most applications.

One example is a 1-Watt, cooling-fan motor (4), shown in Fig. 5. As seen on the circuit board, using SingleSense eliminates five components used in the conventional fan’s board.

The drive system for this fan, shown in Fig. 6, uses an IC from USM-III, called Pecos. The 3-phase SingleSense type controller doubled the efficiency of the fan, compared to the traditional single-phase controller, without increasing the price.

Another example is an evaporator fan motor for a refrigerator. Most motors in such applications have typically been a shaded-pole design to achieve the lowest cost. However, the efficiency of shaded-pole is generally accepted as poor. With the growing interest in improving energy efficiency, appliance makers have been gradually moving to 2-phase and 3-phase BLDC technology at 12 VDC.

In this particular refrigerator application, the reduced motor diameter dimension permits improved airflow. (See Fig. 7.) The Powder Keg motor has a diameter of 42 mm. The conventional radial motor has a face area of 61 mm x 75 mm, which restricts the airflow. (See Fig. 7A.) By contrast, the Powder Keg® motor does not interfere with airflow. (See Fig. 7B.) 

Fig. 7B. Prototype of new evaporator fan motor assembly using a Powder Keg motor that does not restrict air flow.

When all the aspects are weighed together, it has been shown that a motor system combining Powder Keg, BUD, and SingleSense technologies can deliver a lower cost brushless motor solution than the other available alternatives, while at the same time, capable of delivering equivalent performance in many applications. In addition, a SingleSense integrated control technology and Powder Keg motor system can even be price competitive with brush type DC motor systems in certain applications. The motor system cost savings for a Powder Keg motor with BUD/SingleSense control can be as high as 60 percent when compared to a conventional 3-phase, bipolar-driven, laminated motor.

At higher power (greater 100 W power output) the Powder Keg motor can be used with conventional 3-phase bipolar control technology and still result in a system cost reduction. Also, the resulting motor system is typically smaller and lighter. QSI has built Powder Keg motors covering the power output range of 2 W to more than 500 W. The technology is currently being licensed to motor and semiconductor manufacturers and others.

For more information, email: Brad Marshall at bradusm3@aol.com, or Christian C. Petersen at cpetersen@quadrantsi.com

References

1. SingleSense® is a registered trademark name of Petersen Technology Corp. U.S. patent # 6,891,343, 7,026,773 assigned to Petersen Technology Corp.
2. BUD™  a trademarked name of Petersen Technology Corp. U.S. patent #’s 7,166,948, 7,202,620 assigned Petersen Technology Corp.
3. Powder Keg® Patent is a registered trademark name of Petersen Technology Corp. U.S. patent #’s  6,617,747, 6,707,224,  and others assigned to Petersen Technology Corp.
4. “Motor Technology Upgrades Cooling Fans,” Marshall, Petersen, Hodohara, Power Electronics Magazine, August 2007.
5. SMC material such as Hoganas’ Somaloy™ 700
6. “Powder Keg Motors with BUD and Single Sense Control in Automotive Applications,” Christian Petersen, 11-12 June 2007, UK Magnetics Seminar, Helsingborg, Sweden.
7. AKM data sheet.
8. Cost study by major systems supplier, private communication.
9. “Novel System Design for Drive, Commutation and Magnetics,” Marshall, Petersen, Power Electronics, Conference Dallas TX October 2007.