Working principle and error analysis of DC tachogenerator

First, the working principle of DC tachogenerator: At no load, the output voltage of the DC tachogenerator is the armature induced electromotive force. Obviously the output voltage is proportional to the speed. In addition, the DC tachogenerator output is a pulsating voltage, and its alternating component has a significant influence on the speed feedback control system and the high-precision solving device.

Second, error analysis: DC output speed generator output voltage and speed must be strictly proportional to the relationship is difficult to achieve in practice, the actual output characteristics of the solid line in the figure, the main reasons for this nonlinear error are the following Three aspects: (1) Armature reaction DC speed measurement The armature current will generate an armature reaction when the generator is loaded. The demagnetization of the armature reaction reduces the air gap flux Φ0 and reduces the output voltage. From the output characteristics, the slope will decrease, and the larger the armature current, the more significant the demagnetization effect of the armature reaction, the more obvious the slope of the output characteristic is, and the output characteristic line becomes a curve.

(2) Influence of temperature If the DC tachogenerator is used for a long time, its excitation winding will generate heat, and its resistance value will increase with the increase of temperature, so the excitation current will decrease, causing the air gap flux to decrease and output. The voltage is reduced and the characteristic slope is reduced. The higher the temperature rises, the more pronounced the slope decreases, causing the characteristic to bend downward. A manganese-copper or a constant-constant copper oxide with a large resistance value and a small temperature coefficient can be connected in series in the excitation circuit to reduce the resistance change caused by the temperature change, thereby reducing the linearity error caused by the temperature.

(3) Contact resistance If the total resistance of the armature circuit includes the contact resistance of the brush and the commutator, the influence of the output voltage on the contact resistance voltage drop always varies with the load current. When the input speed is low, the contact resistance Larger, the output voltage that is not so large at this time becomes smaller, and the linearity error is large; when the current is large, the contact resistance is small and the value is basically stable, the linear error is relatively small. Much more.