Permanent magnet direct current (PMDC) motors nowadays successfully replace analogical motors with electromagnetic excitation. PMDC have a higher output-input ratio and less mass on unit of power. In the process of planning and exploitation of PMDC motors it is necessary to prevent the danger of demagnetization effect of permanent magnets. The demagnetizing action of armature reaction can result in demagnetization effect of permanent magnets and to the loss of capacity to the PMDC. These factor ground research actuality. The aim of the article is a rough estimate of firmness to demagnetization effect of PMDC motor on the basis of the simplified linear model. Firmness to demagnetization effect of PMDC motor is examined on the example of widespread bipolar construction with the magnets which magnetized in radial direction. Position-finding of operating point is considered on a diagram to the magnet of PMDC motor with the use of graphic method. For this realization it is necessary to know parameters to the magnet and magnetic core of PMDC motor and also to define the reaction of anchor Fa. It is considered three components of magnetomotive force (MMF) of armature. It is necessary to take into account the most unfavorable case of reaction of armature of PMDC motor, when this reaction is accepted by equivalent it to the cross component. For the estimation of firmness to demagnetization effect of PMDC motor the equivalent circuit of magnetic core is used. On this stage of planning to the PMDC motor the yet not expected leakage fluxes of the magnet and saturation characteristic of motor. An equivalent circuit contains MMF of the magnet Fm and reactions of armature Fa and magnetic resistance. An equivalent circuit is described by the system of linear equations that is made after the Kirchhoff's circuit laws. The decision of the system is found in relation to unknown magnetic flux of permanent magnet Фm. Demagnetization curve of the magnet B (H) is presented as equation of line. The final formula is got for the maximally possible value of reaction of armature Famax, that will not result in the loss of magnetic properties a permanent magnet. This formula it is expedient to use on the initial stage of planning of PMDC motor.
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