In general, the development and changes in the structure of DC motors mainly lie in the frame. Although many new patents have emerged regarding commutators, only the so-called "tight-coil type" is currently widely used. Therefore, our discussion will only focus on the frame. Typically, in DC motors with a salient-pole stator, the stator yoke and frame are integrated, and thus, they are made of cast steel or steel plate with good magnetic permeability. With the expansion of the application range of DC motors, the increase in capacity, the adoption of rectified power supplies, and advancements in design technology, DC motor frames have undergone corresponding changes and developments in terms of structure and materials.
1. Diversification of shapes
As discussed in previous posts, besides box-type and split-frame motors, DC motors were among the first to adopt polygonal frames. These were primarily used for railway traction and marine motors to save installation space. It is well known that for DC motors with salient-pole stators, the stator space in a cylindrical frame cannot be well utilized. Therefore, this polygonal frame design was also developed for general DC motors, making motors of the same capacity lighter and smaller. As for DC motors designed for specific machinery, the two are often structurally integrated, thus further diversifying the motor's structural types.
2. Material stamping
For large rolling mill motors with frequently fluctuating loads, especially those with cyclical impact loads, the frame is no longer made of a single piece of cast steel or thick steel plate to suppress eddy currents generated within the frame and meet commutation performance requirements. Instead, it is constructed from thin steel plates or stacked silicon steel sheets. For DC motors powered by rectified power supplies, even smaller capacity motors use laminations to reduce eddy current losses caused by the AC component of the power supply within the frame and its adverse effects on commutation performance. This is very suitable for mass-produced DC motors. For thin steel plate laminations, the surface is not painted. For some laminations made of silicon steel sheets, the surface can be painted, similar to the stator core of AC motors.
3. Stator Hidden Polarization
For a long time, DC motor stators have used a salient-pole structure. Due to the inefficient use of internal space and the need to accommodate brushes and commutators, the capacity of a DC motor with a coaxial height is much smaller than that of an induction motor. The aforementioned development towards lamination in the frame has promoted the salient-pole structure of DC motor stators. In this structure, the commutating pole windings are placed in the larger slots, while the excitation windings and compensation windings are placed in the smaller slots. Typically, the compensation windings are placed near the slot opening; ventilation holes can also be punched on the back of the smaller slots. The rotor (armature) is the same as that of the salient-pole structure.
4. Advantages of using a salient pole structure
The salient-pole structure firstly reduces the height of the main poles. For motors with the same center height, a larger armature outer diameter can be used, thereby increasing the motor capacity. Secondly, the excitation windings are distributed in each slot, increasing the heat dissipation area, thus increasing the current density of the excitation windings, reducing the amount of copper used in the windings, and making the heat generation more uniform. Thirdly, leakage flux between the main poles and between the main poles and the commutating poles is reduced. Fourthly, the symmetry between the magnetic circuits of each pole can be well guaranteed through die design and stamping processes. Fifthly, in some cases, it is possible to use the same laminations and frame as induction motors. In this case, the capacity of the DC motor can reach more than 2/3 of that of an induction motor with the same laminations.
Disclaimer: This article is a reprint. If it involves copyright issues, please contact us promptly for deletion (QQ: 2737591964). We apologize for any inconvenience.
