Handbook of Small Electric Motors MAZ Part 12 doc

CHAPTER 5 ELECTRONICALLY COMMUTATED MOTORS Chapter Contributors Duane C. Hanselman Dan Jones Douglas W. Jones William H. Yeadon 5.1 5.1 BRUSHLESS DIRECT-CURRENT (BLDC) MOTORS* This chapter covers performance calculations and characteristics of the most com- mon electronically commutated motors. The first motor to be covered is the brush- less direct-current (BLDC) motor in several configurations. Next, several configurations of the stepper motor are discussed. This is followed by the switched-reluctance motor, which is really a specialized configuration of the variable-reluctance stepper motor. Brushless direct-current motors (BLDCs) are so named because they have a straight-line speed-torque curve like their mechanically commutated counterparts, permanent-magnet direct-current (PMDC) motors. In PMDC motors, the magnets are stationary and the current-carrying coils rotate. Current direction is changed through the mechanical commutation process. In BLDC motors (Fig. 5.1), the magnets rotate and the current-carrying coils are stationary. Current direction is switched by transistors. The timing of the switching sequence is established by some type of rotor-position sensor.A typical rotor assem- bly for an inner-rotor configuration with sensors and commutation magnet is shown in Fig. 5.2. The three white devices mounted on the printed-circuit board are Hall- effect switches.They are positioned next to the larger-diameter magnet wheel, which causes them to switch high and low as the wheel changes from north to south as it * Section contributed by William H.Yeadon,Yeadon Engineering Services, PC, except as noted. rotates. The angular position of the Hall devices are adjusted to provide the opti- mum firing angle for the application. Figure 5.3 shows via magnetic field–viewing film the relative position of the motor magnet transition zone with respect to the Hall device. The stator assembly for this is shown in Fig. 5.4. This motor has a low- energy-product solid-plastic ring magnet with magnetic poles superimposed on it. Another type of motor has a rotor with solid magnet arcs, as shown in Fig. 5.5. 5.2 CHAPTER FIVE FIGURE 5.1 BLDC motor. FIGURE 5.2 BLDC rotor assembly. ELECTRONICALLY COMMUTATED MOTORS 5.3 FIGURE 5.3 BLDC rotor showing magnetic pole transitions. FIGURE 5.4 BLDC stator assembly. 5.1.1 Basic Configuration of BLDC Motors Outer-Rotor Motors. These motors are generally used where relatively high rotor inertia is beneficial to system performance. Common applications are computer disk drives and cooling fans. Construction is shown in Figs. 5.6 and 5.7. The rotor assembly in Fig. 5.6 consists of a flexible magnet with four poles mag- netized on it. It is enclosed by a magnetically soft steel cup or housing.A shaft which 5.4 CHAPTER FIVE FIGURE 5.5 BLDC rotor assembly with core segment magnets. FIGURE 5.6 BLDC outer rotor assembly. allows the rotor to turn with respect to the stator is attached to the center of the steel housing. The stator assembly (Figs. 5.8 and 5.9) consists of a lamination stack with coils of wire wrapped around the pole pieces. This is supported by a mounting base which also contains a bearing support and a control circuit. This motor’s poles are alter- nately magnetized N-S-N-S. As the rotor turns, the currents are turned off in one winding set and turned on in the other set by the Hall switch. This results in an S-N-S-N magnetization in which the S poles are induced at the interpoles and the poles where no current exists in the windings. This keeps the rotor turning. A close observation of this structure shows ELECTRONICALLY COMMUTATED MOTORS 5.5 FIGURE 5.7 Outer-rotor BLDC motor rotor and stator assembly. FIGURE 5.8 Outer-rotor BLDC stator diagram. that it is in fact a single-phase motor and, as such, will have rotor positions where there is zero torque. This could result in failure to start. This is overcome by placing interpoles between the wound poles, as shown in Fig. 5.10. The magnet provides some position bias, which always results in rotation when the pole is energized. Some other methods used to start these single-phase motors are shown in Figs. 5.11 and 5.12. In the case of Fig. 5.11, the reluctance torque produced by the magnet 5.6 CHAPTER FIVE FIGURE 5.9 Outer-rotor BLDC wound stator assembly. FIGURE 5.10 Outer-rotor motor diagram. causes the rotor to line up over the pole. When the coils are energized, the field resulting from the energized coil appears to move across the face of the pole in the direction of the wider section of the pole that is closest to the gap. This starts rota- tion, which may then be maintained by alternately reversing the winding currents. The structure shown in Fig. 5.12 is another method of providing an unbalanced mag- netic circuit which causes the rotor to have a preferred resting position in the unen- ergized state that is different from that of the energized state. All of the motors just discussed are single-phase motors.Although this is proba- bly the case for most outer-rotor motors, it is quite possible to build a multiphase motor in this configuration, as shown in Fig. 5.13. ELECTRONICALLY COMMUTATED MOTORS 5.7 FIGURE 5.11 Stator with reluctance notches. FIGURE 5.12 Stator with reluctance holes. Inner-Rotor Motors. If one simply inverts the outer-rotor motor diagrammed in Fig. 5.8, the device shown in Fig. 5.14 is generated.This is also a single-phase device, and it would operate in the same fashion as the outer-rotor motor. Some observations need to be made about these motors. The outer-rotor motor has much more magnetic material than the inner-rotor device, which means it is capable of more flux when the identical materials are used. It would be necessary to use a higher-energy-product magnet to get the same performance from an inner- rotor motor. The inertia of the inner-rotor motor is lower because of its smaller rotor diame- ter.Therefore, it accelerates more rapidly than the outer-rotor motor. 5.8 CHAPTER FIVE FIGURE 5.13 Multiphase outer-rotor motor. FIGURE 5.14 Single-phase inner-rotor motor. Most inner-rotor motors have multiple phases in an effort to reduce the starting problems associated with single-phase motors.The stators may have salient poles or distributed windings. Figure 5.15 illustrates a three-phase four-pole salient-pole machine. Here the rotor is a magnetically soft steel core with magnet arc segments bonded to it.As the phases are switched in sequence A-B-C, the rotor moves to line up with the subsequent phase. When the rotor has moved 180° electrical, it is neces- sary to reverse the current in the windings, starting with phase A, in order to prop- erly polarize the stator poles with respect to the magnet poles. ELECTRONICALLY COMMUTATED MOTORS 5.9 FIGURE 5.15 Multiphase inner-rotor motor. In order to properly switch the coil currents at the right time and in the right order, the control electronics must know the rotor position. This is usually accom- plished by means of Hall-effect devices, encoders, or counter-emf sensing. Salient-pole machines have inherently high torque perturbation characteristics because of the abrupt permeance changes as the rotor moves from pole to pole.This can be reduced by distributing the windings over several stator teeth per pole. This is illustrated in Fig. 5.16. Special Configurations—Slotless Brushless Inner-Rotor Motors. One variation of the distributed-winding multiphase brushless motor is the slotless brushless motor.As shown in Fig. 5.17, the winding distribution is similar to that of the stator shown in Fig. 5.16. However, the stator section has no teeth. The advantage of this type of motor is that there are no variations in permeance as the rotor moves.Thus, there are no torque perturbations or cogging. There is, however, a price to be paid. The lack of teeth increases the effective air gap, thereby lowering the available flux. Another variation is the axial airgap motor.This motor is illustrated in Fig. 5.18. It has a stator with triangular coils (Fig. 5.19) and a disc rotor which is alternately magnetized NS-NS-NS-NS through its thickness. The stator coils (Fig. 5.20) are mounted to a nonmagnetic substrate, while the rotor disc is mounted to a magneti- cally soft steel.The advantage of this motor is that, like the other slotless motor, it is low cogging, but it also has relatively high inertia. 5.10 CHAPTER FIVE FIGURE 5.16 Inner rotor distributed-wound stator. FIGURE 5.17 Inner rotor slotless stator. Summary. Brushless DC motors can be put into three basic categories according to their structure: ● Outer-rotor motors ● Inner-rotor motors ● Special-configuration motors Table 5.1 presents a relative comparison of the inner- and outer-rotor types.This assumes the same physical size motor operating at the same speed and torque. [...]... combinations of teeth and poles that will work for motors having two, three, four and more phases Common numbers of teeth for two-phase motors are 8, 12, 16, 24, and 48 Common three-phase numbers are 6, 9, 12, 15, 24, 36, and 48 There are obviously other numbers of teeth that will work for these motors These are just some of the common ones As the number of teeth selected increases, the number of slots... assumption that all parts of the rotor rotate around the center of the shaft For the outer-rotor motor in Fig 5.28, calculate the inertia of each part separately, then add the parts together for the total rotor inertia First, calculate the inertia of the shaft: ΂ ΃L Ds Jshaft = 0.0184 ᎏ 2 FIGURE 5.28 Outer-rotor inertia 4 shell 5.21 ELECTRONICALLY COMMUTATED MOTORS Then calculate the inertia of the disk:... configuration This pattern is popular for resolver windings and some low-cog BLDC motors The following equation yields the pitch: number of slots 15 ᎏᎏᎏᎏᎏ = ᎏ = 11⁄4 12 number of phases × number of poles (5.4) The winding throw is shown in the following equation: 15 number of slots ᎏᎏᎏᎏᎏ = ᎏ = 33⁄4 4 number of phases × number of poles (5.5) Equation (5.5) identifies the winding pitch or throw as either... electrical = 24° × 2 = 48° electrical (5.7) A 48° electrical pitch does not equal the desired 60° pitch, so there must be torque loss Figure 5.41 illustrates the winding pattern for a 12- slot 2-pole fractional-pitch winding A full winding pitch would possess a value of 6 with a throw of 1 to 7 This winding pattern has a winding pitch of 5⁄6 (fractional) or a throw of 1 to 6 This group of fractional-pitch... pitch of the 15-slot stator 360° SP = ᎏᎏ number of slots 360 SP = ᎏ = 24° mechanical 15 (5.6) Now, since this design is a four-pole BLDC design, there are two full electrical cycles for one full mechanical cycle The next equation defines the relationship between electrical and mechanical degrees for any four-pole design Degrees electrical = degrees mechanical × number of pole pairs ∴ Degrees electrical... constant-pitch 4-pole 12- slot stator, 3-phase hookup, 30° mechanical, 60° electrical FIGURE 5.36 Constant-pitch 4-pole lap winding pattern with adjacent coil layout ELECTRONICALLY COMMUTATED MOTORS 5.29 throws The following equation describes the method used to determine the two variable winding pitches: number of stator slots VSP1 = ᎏᎏᎏ + 1 number of poles number of stator slots VSP2 = ᎏᎏᎏ − 1 number of poles... practical If one uses a 12- slot stator winding, there are two full-pitch integral lap windings available, one for two poles and the other for four poles P × Ph = nS where (5.1) P = number of poles Ph = number of phases S = number of stator slots n = integer number 1, 2, 3, n If P = 2, Ph = 3, and S = 12, then n = 2, an integer Figure 5.30 shows the basic winding-slot pattern for a 12- slot 2-pole 3-phase... and 30 degrees An important rule to remember when establishing part dimensions is that the width of any part should be 1.5 times the thickness of the material Failure to follow this rule results in part distortion and die wear The material should be selected based on cost, induction, and core loss requirements ELECTRONICALLY COMMUTATED MOTORS FIGURE 5.25 FIGURE 5.26 5.19 Inner-rotor tooth shapes Inner-rotor... (90)(2) The density ratio is divided by 2 because of the flux split Att = Ttw ⋅ Lstk 115 ∴Ttw ∝ ᎏ ≈ 2 60 Ary = Ywr ⋅ Ls 115 ∴Ywr ∝ ᎏ ≈ 0.75 × total tooth width 75(2) In the case of the outer-rotor motor, the length of the shell Ls is usually longer then the stator stack, so Ywr is more in the range of 30 to 40 percent of total tooth width To estimate the width of the teeth, assume that the area available... Previously defined is the number of phases, which is three here Next in importance is the number of poles The use of two poles is waning, and the use of six or eight poles is increasing Four-pole BLDC motors are among the most popular used today Twoand four-pole BLDC motor designs are used here, but the rules for two and four poles can be extended to higher pole counts The number of stator slots (and teeth) . num- bers of teeth for two-phase motors are 8, 12, 16, 24, and 48. Common three-phase numbers are 6, 9, 12, 15, 24, 36, and 48. There are obviously other numbers of teeth that will work for these motors. . the assumption that all parts of the rotor rotate around the center of the shaft. For the outer-rotor motor in Fig. 5.28, calculate the inertia of each part separately, then add the parts together for. remember when establishing part dimensions is that the width of any part should be 1.5 times the thickness of the material. Failure to follow this rule results in part distortion and die wear.