Motor Drive Electronic Commutation Patterns

The understanding of BLDC motor commutation in BLDC motors is as important as brush placement in brush dc motors. The Hall device is the single component that made brushless dc motors possible with their product announcement in the mid-1970s.

They are still necessary in positioning applications, but recently many suppliers have utilized the PM motor's Ke signals to commutate or switch to the appropriate windings at the appropriate time.

The Hall device is an electronic switch that can be used to turn on and turn off the appropriate stator windings sequentially. Positioning of the Hall device is an important condition, with higher rotor pole counts being more sensitive to its location based on the multiple effect of electrical to mechanical degrees. For example, the eight-pole rotor is 4 times more sensitive than a two-pole rotor to positioning the Hall device for optimum commutation.

ILL. 91 Generated voltage signals from a BLDC motor: voltage out-put versus position of two adjacent stator phases. Rotor velocity  1000 rpm.

ILL. 92 Location of Hall sensors at the beginning of the C  B  commutation cycle.

ILL. 93 Delta configuration: ( a) excitation pattern, and ( b) commutation pattern.

TABLE 18 Commutation Sequence-Three-Phase Four-Pole Stator-rotor mechanical degrees | Drive windings | Positive | Negative

0-30 E C B 30-60 D A B 60-90 F A C 90-120 E B C 120-150 D B A 150-180 F C A 180-210 E C B 210-240 D A B 240-270 F A C 270-300 E B C 300-330 D B A 330-360 F C A One straightforward method of locating the Hall sensors is to measure the generated voltage from the PM brushless motor and align the switching pattern of the Hall device for that winding by using an oscilloscope. Ill. 91 shows the generated voltage signals from two of three phases of a BLDC motor. The angular positions are measured, and the Hall devices are placed at specific positions shown in Ill. 92.

Just substitute a four-pole magnet pattern in place of the photo interrupter in Ill. 92, and the positions for the coils are shown for a 12-slot stator with elements of the 12 coils located every 3  mechanical. A delta winding is the line configuration.

Ill. 92 displays the sensor signals X1, Y1, and Z1 for two turn-on events and two turn-off events per 36  mechanical or 72  electrical. The excitation pattern for the six power devices is shown in Ill. 9 3a. The commutation pattern is shown in Ills. 10.9 3b and Table 10.18, which define the commutation sequence for a three-phase four-pole delta line configuration. Ill. 94 displays the torque versus position waveform for the delta line configuration for the 12 commutation points per full mechanical rotation.

Ills..95 and 96 and Table 19 illustrate equivalent patterns for a full-wave Y line configuration. Many other patterns are possible, particularly with Hall devices that turn on and off for only N or S rotor magnet poles. Using a separate magnet for commutation will also accomplish the same task. Most BLDC motor designs use the rotor magnets' leakage flux to energize the electronic switch.

Although relatively expensive, the Hall device does possess temperature limitations (above 12  C). Many servomotor suppliers use resolvers for position signals; their ability to generate absolute position signals allow the resolver and its companion R and D circuit to commutate the motor as well.

ILL. 94 Torque versus position waveform for the delta line con-figuration: (top) winding F  TAC, (middle) winding D  TAB, and (bottom) winding E  TCB.