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A bipolar electric motor is an electric motor with only two (hence ''bi-'') poles to its stationary field. They are an example of the simple brushed DC motor, with a commutator. This field may be generated by either a permanent magnet or a field coil. The 'bipolar' term refers to the stationary field of the motor, not the rotor.〔 The rotors often have more than two poles, three for a simple motor and potentially more for a high-power motor. A two-pole rotor has the disadvantage that it is not self-starting in all positions and so requires to be flicked to start. == Early motors == The first DC electrical motors, from the Gramme motor of the 1870s onwards, used bipolar fields. These early machines used crudely designed field pole pieces with long magnetic circuits, wide pole gaps and narrow pole pieces that gave only a limited flux through the armature. These fields were usually horseshoe-shaped, with either permanent horseshoe magnets or else either one or two field coils at some distance from the poles. Early insulated wire was insulated, if at all,〔The first electromagnets were wound with bare copper wire, the only sort then available, and insulated with strips of cloth laid on the windings as they were wound.〕 with wrappings of cotton thread. These coils could only handle a low temperature rise before overheating and burning out with a short circuit. The coils were thus long and shallow, sometimes of only a single layer of wire, which required a long core simply to contain their size. Single small coils could be mounted horizontally, but the most common arrangement used two tall coils side by side. To improve the efficiency of the magnetic circuit, it was realised that multiple magnetic paths could be provided through the same armature. The two coils were now separated and placed at the sides of the motor, with their iron core as a sideways figure-8 circuit and the armature in a central pole gap. Flux from both coils passed through this gap. This gave a magnetic circuit that was shorter overall and thus had fewer magnetic losses. The more compact coil windings were made possible by the use of shellac for impregnating the windings and improving the reliability of their insulation. Later designs, from around 1900, became more compact with shorter, more efficient magnetic circuits. The field coils now moved into short, squat ''internal'' coils around the pole pieces themselves.〔 The remainder of the magnetic circuit was a double-sided circular path around the casing of the motor. Whilst primarily designed to be more efficient, this also gave a far more compact layout in terms of space. This circular layout also represented the end of the bipolar motor as an industrial power source. It was possible to place a second set of field coils and pole pieces within the same size of casing, giving a four-pole arrangement. Because of the more efficient provision of field flux around the entire circumference of the armature, this give a motor of almost twice the power, for the same armature current.〔 Armature current, and the associated commutator and brushgear, represented one of the most expensive parts of the motor to manufacture. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「bipolar electric motor」の詳細全文を読む スポンサード リンク
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