engines. Practice has shown that engines with constant magnets in a quadrupole implementation are very reliable, and they have a back-up brush unit and winding armature. These engines have a high start-up momentum to nominal momentum ratio for the range of powers which are used and thus have a good overload capacity, which has a good effect on the reliability of the drives, but requires the use of protective couplings. Despite the presence of a brush collector, the engines have a good working capacity in vacuum. To further increase the lifetime of the engines and other elements in open space one can fully hermetize the hulls and use direct current engines, direct current collectorless engines, etc. When alternating current engines are used, the element which is most damaged by vacuum is eliminated, the brush collector. However, in the majority of spacecraft the main source of electrical energy is a direct current power supply system. Thus, their use is associated with a number of drawbacks, mainly: 1) the need for on-board DC-AC converters; 2) the large mass and size of the engines themselves; 3) lower efficiency (in small power engines); 4) worse overload capacity (lower ratio of the launch momentum . For example, in the drives of the docking mechanism and locks of the docking frame for the Apollo APDA, AC electrical engines were used. These engines were supplied by on-board static converters (based on semiconductor elements) of a 27 V direct current to a 115 V alternating current with a frequency of 400Hz. Each of these two converters had a mass of about 15 kg. Each drive had two engines connected kinematically through a differentia!; the power of one electrical engine was ~100W, with a mass of -2.5 kg. The small ratio made it possible to avoid the protective coupling, although it placed more stringent requirements on the deactivation of the electrical engines in the approach to the mechanical support. The casings of the drives were fully hermetized, although in this case it is clear that this was not necessary. Each drive, not considering the voltage converter, weighed about 10 kg. When disconnected the shaft of the engine was held immobile by a built-in friction brake coupling activated when voltage was sent to its winding. Figure 3.14 shows the schematic of control of the Apollo APDA, including a schematic for the commutation of electrical engines. The reliability of the scheme
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