Working Principle and Error Analysis of DC Tachogenerator

DC tachogenerator is a speed measuring element, which converts the speed signal into DC voltage signal for output. In this paper, Starlight power generation equipment will briefly describe the working principle of DC tachogenerator and the reasons for error analysis.

1、 Working principle of DC tachogenerator: when the load is no load, the output voltage of DC tachogenerator is the armature induced electromotive force. Obviously, the output voltage is proportional to the speed. In addition, the output of the DC tachogenerator is a pulsating voltage, and its alternating component has obvious influence on the speed feedback control system and high-precision solution device.

2、 Error analysis: It is difficult to strictly maintain the proportional relationship between the output voltage and speed of the DC tachogenerator in practice. Its actual output characteristics are solid lines in the figure. The reasons for this nonlinear error are mainly the following three aspects:

(1) Armature reaction When the DC tachogenerator is loaded, the armature current will produce armature reaction, and the demagnetization effect of the armature reaction will make the air gap flux Φ 0 decreases to reduce the output voltage. From the perspective of output characteristics, the slope will decrease, and the larger the armature current is, the more significant the demagnetization effect of armature reaction will be, the more obvious the slope of output characteristics will decrease, and the output characteristics will become a curve.

(2) Influence of temperature If the DC tachogenerator is used for a long time, its excitation winding will heat up, and its winding resistance will increase with the increase of temperature, so the excitation current will decrease, resulting in the decrease of air gap flux, output voltage and characteristic slope. The higher the temperature rises, the more obvious the slope decreases, making the characteristics bend downward. A manganin or constantan resistor with large resistance and small temperature coefficient can be connected in series in the excitation circuit to reduce the resistance change caused by temperature change, thus reducing the linear error caused by temperature.

(3) Contact resistance If the total resistance of the armature circuit includes the contact resistance of the brush and the commutator, the output voltage is always affected by the voltage drop of the contact resistance and changes with the load current. When the input speed is low, the contact resistance is large, making the output voltage, which is already small, smaller, resulting in large linear error; When the current is large, the contact resistance is small and basically tends to be stable, the linear error is relatively small.

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