Working point
The operating point , also known as the operating point or operating state , is a specific point in the characteristic map or on the characteristic curve of a technical device that is assumed due to the system properties and external influences and parameters.
Drives
The operating point of a drive is the intersection of the torque / speed characteristics of the drive machine and the driven machine. Both machines are coupled via a shaft , so the speed is always identical. A torque is applied by the drive machine, which sets both machines in rotation. The work machine applies a counter-torque by z. B. moves a medium or rotates a vehicle wheel against the static friction of the tire on the road.
- If the drive torque is greater than the counter torque, the speed of the drive increases.
- If the counter torque is greater than the drive torque, the speed of the drive is reduced.
At the working point, the drive torque and the counter torque are in equilibrium, so that the speed no longer changes.
- A change in speed immediately generates a change in torque at the operating point, which counteracts the change in speed.
A speed change from this point is only possible through a renewed control intervention, a change in the drive torque or the counter torque by changing the characteristic of the drive or work machine. The characteristics change z. B. by increasing the drive power or the load. The system then runs towards a new operating point with a different speed and a different torque equilibrium.
If a drive does not have an operating point because the drive torque is higher than the counter-torque at any speed, the drive will run through , i.e. increase the speed steadily to idle speed or to destruction. The same applies to a counter-torque that is always higher, then only the opposite direction of rotation results. Often, however, it is also the case that the counter-torque at low speed is lower than that of the drive machine, but then increases very steeply with the speed. In this case, the drive speed is very low; in the borderline case, the drive then stands still due to internal friction.
Stable and unstable working points
There are stable and unstable working points. In both cases the above speed and torque equilibrium applies. In the unstable operating point, however, the characteristics of the drive machine and the driven machine run almost parallel, so that a small change in torque can cause a large change in speed. In practice, no machine has an infinitely thin characteristic curve, so that the unstable working “point” is actually a wide, long band of possible operating states between which the drive constantly changes due to internal and external friction and torque ripples. Operation at the unstable operating point is therefore very undesirable.
The middle state on the third drawing on the right is also unstable. This state is often called an unstable operating point , but the above assumptions do not apply to the operating point here. Here speed and torque are the same, but at this point the torque of the drive machine is immediately higher than the counter torque of the driven machine when the speed is increased slightly, and vice versa when the speed is reduced. This does not lead to the effect of the operating point stabilizing the speed, on the contrary. The speed can "run away" on both sides, the drive will settle either to the left or to the right operating point.
Desired and unwanted working points
In the lower right case, the asynchronous machine drives a conveyor belt. This type of machine has an almost constant counter-torque over the entire speed range. An unfavorable choice of drive machine (type and size) results in three possible operating points, each with the same torque. Of course, the operating point with the highest speed is desirable, because this is where the highest mechanical power (proportional to torque times speed) results, while at the other operating points the majority of the electrical power (proportional only to the torque) is converted into heat in the drive machine. In addition to the poor energy balance, this can also cause the prime mover to overheat.
In the case shown on the right, there is also the fact that the drive does not only achieve the more favorable operating state with the same torque and higher speed (and thus higher mechanical power) at the right operating point due to the "torque valley" between the undesired operating point and the unstable point, which is technically necessary for this drive machine can reach. In the case of drives with constant counter-torque, a speed-dependent clutch can be used to prevent “getting stuck in front of the mountain”. The constant counter-torque is only applied when the drive machine has reached a speed beyond the unstable point without load and the drive is therefore safely approaching the right, desired operating point. Alternatively, a drive machine with a torque that is almost constant over the speed can be used. In the past , this was done with a shunted DC machine , nowadays asynchronous machines with a current displacement rotor or asynchronous machines are used together with a frequency converter .
electronics
The operating point of a circuit is the idle state in the absence of a signal. It is described by a specific point on the characteristic curve . From this point on, the current or voltage changes when a useful signal is applied. In order to achieve the most undistorted, symmetrical signal transmission possible, the operating point is normally placed in the middle of the characteristic curve, i.e. H. between maximum and minimum voltage or current. This operating state is also called A operation .
If asymmetrical modulation is required, the operating point is moved to the edge of the characteristic ( B operation, C operation ). A separate transistor must then be provided for each half-wave of the signal (positive / negative wave) ; both transistors are arranged in the form of a push-pull output stage . This technology is used in power amplifiers because it allows a low quiescent current to flow through the transistors, while the high quiescent current required in A operation heats the transistor more strongly.
The picture shows a transistor amplifier and its behavior with different settings of the operating point. The signal level can be changed with the potentiometer P 1 , the operating point with R and P 2 . If it is in the middle between maximum (operating voltage) and minimum voltage (ground), the signal can be controlled symmetrically around the operating point. Shifting the operating point upwards leads to the signal peaks colliding with the operating voltage, and shifting downwards against the ground potential. This will distort the signal. Distortion also occurs when the amplifier is overdriven (due to an input signal that is too large) . In this case, the range between maximum and minimum voltage is no longer sufficient for the signal. It is also said that the amplifier starts clipping . This mainly creates odd harmonics, which are important when calculating the degree of harmonic distortion . The even-numbered harmonics are of subordinate importance, since they are perceived as far less disruptive. In order to reduce the ratio of odd-numbered harmonics to even-numbered harmonics, there are so-called soft-clipping circuits.
Frequently used working points
Described on the basis of tube circuits, analogous guidelines apply to transistor circuits.
The A operation
In A operation, the operating point is slightly above the center of the grid voltage-anode current characteristic curve (limited by the abscissa and ordinate in the second quadrant ). A mode is used in almost all pre-stages and output stages (control current I max / quiescent current I r = 1). The dynamic range is limited by the grid voltage-anode current characteristic.
The B operation
In B operation, the grid bias is at the point on the characteristic curve at which the anode quiescent current begins to flow significantly (control current I max / quiescent current I r ≥ 10). An amplifier stage in B mode does not differ significantly from anode rectification because it also only amplifies the positive half-oscillations of an approximately sinusoidal and constant component-free signal curve. However, it is common practice to also amplify the other half oscillations with the help of a second stage in push-pull . The combination of both components results in the complete signal. The dynamic range of the signal in this operating mode can be twice as large as the dynamic range of the tube in A mode. The output is (theoretically) four times the value compared to the A operation.
The AB operation
The characteristic curve of a tube has a curvature (less steepness ) at the anode current start point , which leads to the signal being distorted in the vicinity of the zero crossing. This transfer distortion of the push-pull amplifier that occurs during B operation can be reduced by choosing an operating point with a slightly larger anode current. The maximum power is a little lower (control current I max / quiescent current I r ≈ 5).
The C operation
In C mode, there is no anode current without a triggering signal (control current I max / quiescent current I r ≥ 100). The non-linear distortions are made harmless by filtering the transmitter output stages. The flywheel effect of the filters leads to a regeneration of the cut off parts of the signal. Cutting off part of the half- oscillations is therefore only important in envelope demodulation with a very high degree of modulation.
The audio company
In the historical audion and the grid rectification used for electron tubes , the operating point shifts depending on the signal. With larger signals, the effective slope of the tube is reduced. This special property was of great importance for the good adjustability of the feedback .
See also
literature
- Ulrich Tietze, Christoph Schenk: Semiconductor circuit technology . 12th edition. Springer, Berlin 2002, ISBN 978-3-540-42849-7 .