18 Questions
What is one of the factors that practically differ the output characteristics of a tachogenerator from the linear law?
Effect of armature reaction
How does the demagnetization effect of armature reaction change with an increase in speed?
Voltage and current both increase
In what direction does the output voltage change compared to speed increase in a tachogenerator?
Increases slower than speed
What action can help reduce the demagnetizing effect of armature reaction in the load loop?
Increase resistance RL
What factor creates a defined inaccuracy in tachogenerator output characteristics compared to the ideal linear characteristic?
Nonlinear brush contact resistance
In which range do tachogenerators become insensitive to small changes in speed?
Range of small speeds
What is used to reduce the inaccuracy caused by the voltage drop under a pair of brushes in the tachogenerator?
Metal-graphite brushes
What is connected in series with the excitation winding to stabilize its resistance?
Thermistor
Why is a supplementary resistor with resistance Radd >> RL connected in parallel with the excitation winding?
To stabilize the resistance of the excitation winding loop
In a tachogenerator with a strong saturated magnetic field, how does a small change in excitation current affect the magnetic flux?
Practically does not influence the magnetic flux
What happens to the excitation current and output voltage of a tachogenerator when the excitation winding heats up?
Excitation current increases, output voltage decreases
How does a strong saturated magnetic system in a machine reduce the effect of demagnetization due to armature reaction?
Reduces demagnetization due to armature reaction
What happens to the size and weight of a tachogenerator when the saturation of its magnetic system increases?
Increases
In an unsaturated machine, what kind of relationship exists between the output voltage and the excitation current?
Proportional
What is used in a tachogenerator to compensate for the influence of temperature variation on the excitation winding?
Thermo sensitive shunt
Why does the magnetic conductance of the shunt reduce when the excitation winding heats up?
Excitation flux reduces
What leads to a divergence from the linear output characteristics of a tachogenerator from the idealized?
Inclined output curve under nominal speed
In DC tachogenerators, what form of error can arise due to inaccurately installing the brushes on the geometrical neutral line?
Symmetrical to the output voltage error
Study Notes
Tachogenerator Characteristics
- A rise in saturation of the tachogenerator's magnetic system increases the size and weight of the machine.
- Unsaturated machines have a proportional relation between output voltage and excitation current, which is necessary in some synchronized systems.
Compensating for Temperature Influence
- A thermo-sensitive shunt is used to compensate for temperature influence on the magnetic material of the tachogenerator.
- The shunt is made from an alloy that changes its magnetic conductance with heat variation.
Output Voltage and Errors
- The output voltage of the tachogenerator changes insignificantly due to the reduction in leakage flux.
- The per unit error in the output voltage is calculated as ∆v = ((Vid – Vd) / Vid )× 100%.
- In DC tachogenerators, the technical ability to accurately install the brushes on the geometrical neutral line gives rise to a symmetrical error.
Tachogenerator Speed and Accuracy
- The most common tachogenerator has a speed of n = 1500 …30000 rev/min.
- The output characteristics of the tachogenerator differ from the linear law due to armature reaction, nonlinear resistance, and temperature rise.
Armature Reaction and Demagnetization
- The demagnetization effect of armature reaction appears as the speed increases, causing the output voltage to increase slower than the speed.
- Increasing the resistance RL and reducing the speed can reduce the demagnetizing effect.
Brush Contact Resistance and Inaccuracy
- The nonlinear character of the brush contact resistance creates inaccuracy in the output characteristic.
- Using metal-graphite brushes or brushes with silver or gold soldered on can reduce the inaccuracy created by the voltage drop.
Excitation Winding and Temperature
- The heating of the tachogenerator's excitation winding leads to an increase in its resistance, reducing the excitation current and demagnetizing the output voltage.
- Connecting a thermistor or a supplementary resistor in series with the excitation winding can stabilize the resistance of the excitation winding loop.
Strong Saturated Magnetic Field
- A strong saturated magnetic field reduces the effect of excitation current on the magnetic flux.
- This reduces the demagnetization effect due to armature reaction, particularly for load currents.
Test your knowledge on the impact of tachogenerator magnetic system saturation and excitation winding on machine size and weight. Explore the relationship between output voltage and excitation current in unsaturated machines, as well as the use of thermo sensitive shunts to compensate for temperature influences.
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