Characteristics of Brushless DC Micromotors with a Bridge Commutator for Star and Delta Winding Connections
Abstract Generated abstract
The paper compares star and delta phase connections in three-phase brushless DC micromotors using a bridge semiconductor commutator. Treating the motor as a synchronous machine supplied by an inverter tied to rotor speed, the authors analyze voltage harmonics and the effect of rotor magnetic-field shape, then test an 8 W model under equivalent supply-voltage conditions and loads. The analysis and measurements indicate that star and delta connections are practically equivalent when the third harmonic of the rotor-magnet field is absent or negligible, while a measurable third harmonic produces circulating currents in the delta connection and slightly reduces its energy performance.
Full Text
CHARACTERISTICS OF BRUSHLESS DC MICROMOTORS WITH A BRIDGE COMMUTATOR FOR STAR AND DELTA WINDING CONNECTIONS
E. T. Dikii, G. A. Parkhomenko
(Presented by the scientific seminar of the Departments of Electrical Machines and General Electrical Engineering)
The rapid development of semiconductor technology and automation now makes it possible to replace, in DC micromotors, the brush-and-commutator assembly with a contactless semiconductor commutator. This replacement makes it possible not only to preserve all the advantages of DC micromotors in comparison with AC micromotors, but also to give them the principal advantage of AC motors—contactlessness, which is one of the main factors in determining the reliability and service life of micromotors. The increase in the weight and dimensions of brushless micromotors in comparison with commutator DC machines is temporary in nature. The rapid development of microelectronics—advances in micromodule technology and film electronics, the creation of solid-state circuits and, finally, functional solid-state circuits—will in the near future make it possible to produce commutators whose dimensions will approach those of brush-and-commutator apparatus.
At present, various versions of brushless DC micromotors are being developed and studied, differing in the circuit design of the semiconductor commutator, in the structural design of the rotor-position sensor, and in the micromotor itself.
A brushless micromotor is considered as a structural combination of a synchronous motor with an inverter, whose output-voltage frequency is determined by the rotational speed of the motor rotor. This method is the most convenient for analyzing the operation of a three-phase DC micromotor with a bridge commutator when the motor windings are connected in star and in delta, as shown in Fig. 1.
In the present article a comparison is made of the characteristics of three-phase brushless DC micromotors with the windings connected in delta and in star. This is due to the fact that recently, in the development of brushless micromotors of the indicated type, there have been disagreements on this question.
Fig. 1.
The brushless three-phase DC micromotor under consideration has three windings placed in the slots of the stator, the axes of which are displaced in space by 120 electrical degrees. The windings are commutated by a semiconductor commutator according to signals from a transformer-type rotor-position sensor [1]. The rotor magnet is made of the alloy YuNDK-35T5 in the form of a four-pole star. The shapes of the phase voltage supplied to the motor windings for the case of delta and star connection are shown in Fig. 2 [2].
Fig. 2
Expansion of the voltage curves into a Fourier harmonic series shows that they do not contain higher harmonics that are multiples of three [3]. When the phases of a micromotor are connected in delta, the harmonics that are multiples of three coincide in phase and form a closed system of currents, which create additional losses. Thus, if it is assumed that the magnetic field in the air gap of the machine, produced by the rotor magnet, has a sinusoidal form, then the delta and star phase-connection circuits are equivalent for the motor under consideration. That is, if the same phase voltage of the first harmonic is applied to the motor phases connected in delta or in star, then the mechanical and energy characteristics of the motor should be identical. It follows from what has been said that the mechanical and energy characteristics of a brushless micromotor should not change when the winding circuit is reconnected from delta to star, and vice versa, if at the same time the motor supply voltage is increased or decreased, respectively, by a factor of $\sqrt{3}$. In practice, however, the magnetic field in the air gap of the machine has not a sinusoidal, but a flattened form.
To obtain a sinusoidal field form, it is necessary to make a nonuniform air gap under the poles of the rotor magnet, which is associated with technological difficulties in machining the magnets.
Expansion of the magnetic-field curve into a Fourier harmonic series and experimental studies show that it contains higher harmonics that are multiples of three. The higher harmonics of the magnetic field that are multiples of three induce corresponding emfs in the motor windings, which, when connected in delta, form a closed system of currents. However, in the machines under consideration the level of harmonics that are multiples of three is not large and, in comparison with the first harmonic, amounts to several percent. Of these, only the third harmonic of the magnetic field has a noticeable influence. Taking into account the above, in the micromotors under consideration the star winding connection has certain energy advantages over the delta connection.
For a practical check of the characteristics of a brushless DC micromotor, a motor model with a power of 8 W was made, and characteristics were taken with the phase connections in delta and star at different voltages and identical loads.
The pole-coverage coefficient of the rotor magnet in the manufactured specimen is equal to 0.65. The level of the third harmonic of the magnetic field in the air gap of the machine, relative to the fundamental, was 5.2%.
The test results of the model specimen are summarized in Table 1. The table presents the results of tests of the micromotor characteristics with winding data for voltages of 27 V and 15.6 V, respectively, for star and delta connection. The table gives the test results for the same motor, but with different winding data, at voltages of 27 and 46.5 V with the same slot-fill factor of the stator by winding copper.
The test results of the model specimen confirm the correctness of the arguments set forth above and make it possible to draw the following conclusions:
Table 1
Characteristics of brushless dc micromotors with bridge connection of the motor windings in star and delta
| Quantity, units | Winding connection $\lambda$ | 0 | 40 | 80 | 120 | 160 | 200 | 240 | 280 | 320 | Winding connection $\Delta$ | 0 | 40 | 80 | 120 | 160 | 200 | 240 | 280 | 320 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| $U = \mathrm{V}$ | 27 | $27/\sqrt{3}=15.6$ | ||||||||||||||||||
| $M$, g·cm | 0 | 40 | 80 | 120 | 160 | 200 | 240 | 280 | 320 | 0 | 40 | 80 | 120 | 160 | 200 | 240 | 280 | 320 | ||
| $I$, mA | 100 | 150 | 200 | 260 | 320 | 365 | 420 | 480 | 532 | 176 | 262 | 360 | 453 | 560 | 642 | 736 | 835 | 935 | ||
| $n$, rpm | 3900 | 3750 | 3650 | 3500 | 3400 | 3260 | 3140 | 3000 | 2880 | 3900 | 3750 | 3620 | 3450 | 3350 | 3220 | 3080 | 2920 | 2770 | ||
| $\eta$, % | — | 38 | 55.6 | 61.8 | 64.6 | 68 | 68.6 | 67 | 65.6 | — | 37.8 | 53 | 60.2 | 63 | 66 | 66.2 | 64.5 | 62.4 |
The same motor, but with modified data
| Quantity, units | Winding connection $\lambda$ | 0 | 40 | 80 | 120 | 160 | 200 | 240 | 280 | 320 | Winding connection $\Delta$ | 0 | 40 | 80 | 120 | 160 | 200 | 240 | 280 | 320 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| $U = \mathrm{V}$ | $27\sqrt{3}=46.5$ | 27 | ||||||||||||||||||
| $M$, g·cm | 0 | 40 | 80 | 120 | 160 | 200 | 240 | 280 | 320 | 0 | 40 | 80 | 120 | 160 | 200 | 240 | 280 | 320 | ||
| $I$, mA | 60 | 86 | 120 | 152 | 186 | 225 | 254 | 285 | 320 | 120 | 170 | 230 | 285 | 340 | 395 | 455 | 515 | 575 | ||
| $n$, rpm | 3850 | 3700 | 3550 | 3450 | 3350 | 3250 | 3140 | 3050 | 2940 | 3900 | 3800 | 3700 | 3600 | 3420 | 3300 | 3200 | 3100 | 3000 | ||
| $\eta$, % | — | 38 | 52.3 | 60 | 63.5 | 65.6 | 65.7 | 65 | 64.7 | — | 34 | 49 | 57.8 | 61.2 | 63.6 | 64 | 64 | 63.6 |
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In brushless three-phase DC micromotors with a bridge commutator, the winding phase-connection schemes in star and delta are practically equivalent in the absence of the third harmonic of the rotor-magnet field curve.
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The presence of the third harmonic in the magnetic-field curve in the air gap of the machine somewhat worsens the energy performance of the micromotor when the windings are connected in delta.
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V. V. Tsokanov, O. A. Kossov. Brushless DC electric drive. Elektrichestvo, 1963, No. 1.
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G. A. Parkhomenko. Currents in a brushless DC micromotor. Collection of papers of the Institute of Electrodynamics, Academy of Sciences of the Ukrainian SSR, Naukova Dumka, 1966.
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I. E. Ovchinnikov, N. I. Lebedev. Contactless DC motors for automatic devices. Nauka, 1966.