Operating point

The operating point and operating point or condition referred to is a certain point in the characteristic diagram or of the characteristic curve of a technical device, which is taken up because of the system characteristics and the external influences acting on, and parameters.

• 2.1 Commonly used operating points 2.1.1 The A- operation
• 2.1.2 The B operation
• 2.1.3 The AB operation
• 2.1.4 The C operation

Drives

The operating point of an actuator is the intersection of the torque / speed characteristic of the prime mover and the driven machine. Both machines are coupled by a shaft, so the speed is always the same. Of the engine, a torque is applied, the ability of the two machines in rotation. The machine takes in a counter-torque by, for example a medium moves or rotates a vehicle against the static friction of the tire on the ground.

• Is greater than the driving torque of the counter torque, the speed of the drive increases.
• If the counter-torque greater than the drive torque, reduces the speed of the drive.

At the working point torque and counter torque in equilibrium, so that the speed does not change.

• A speed change generated at the operating point immediately torque change, which counteracts the speed-change.

A speed change from this point out is possible only by a new control intervention, a change in the drive or the counter-torque by changing the characteristic of driving or driven machine. The characteristics change, for example, to by increasing the drive power or the load. The system then runs back to a new operating point with a different speed and other moment equilibrium.

If a drive is not working, because the drive torque at each speed is higher than the resistive torque, so will go through, so increase the speed steadily until the idle speed or destroy the drive. The same applies to any time a higher counter-torque, then there is merely the reverse direction of rotation. But it is often the case that the counter torque is at a low speed less than that of the engine, but then rises very steeply with speed. In this case, a very small speed of the drive, in the limiting case the drive is because of the internal friction silent then.

Stable and unstable operating points

There are stable and unstable operating points. In both cases, the above speed and torque balance applies. However, in the unstable operating characteristics of the engine and working machine to run nearly in parallel, so that a small variation of the torque can cause a large change in rotational speed. In practice, no machine has an infinitely thin curve, so that it is a " point " is actually unstable when working around a wide, long ribbon of possible operating states between which the drive constantly changes due to internal and external friction and Momentenwelligkeiten. An operating at unstable operating point is very undesirable.

The median state on the third drawing on the right is also unstable. Often these states are also unstable operating point is called, but the above assumptions for the operating point does not apply here. Here the speed and torque are indeed the same, but the torque of the engine in case of slight increase in speed is immediately higher than the resistive torque of the machine at this point, vice versa, when the speed reducing. This leads not to the stabilizing effect, the rotational speed of the operating point, on the contrary. The speed can " run away " to both sides, the drive will settle either on the left or on the right operating point.

Desirable and undesirable operating points

In the lower right case, the asynchronous machine drives a conveyor belt. This kind of machine has a nearly constant torque over the entire speed range. In an unfavorable choice of the prime mover (type and size) there are three possible operating points, each with the same torque result. Desirable is, of course, the operating point with the highest speed, as there results in the maximum mechanical power ( proportional to torque times speed), while the majority of the electrical power ( proportional only to the torque ) into the engine to heat the other operating points. Besides the poor energy balance, the prime mover can also become overheated.

In the case shown on the right that the drive the more favorable operating condition with the same torque and higher speed (and thus higher mechanical power) just adds the right working through the technical reasons for this driving machine " Drehmomentental " between the undesired operating point and the unstable point not alone can achieve. For drives with constant load torque can be a "stuck in front of the mountain" using a speed-dependent coupling prevent. The constant counter moment will be created only if the engine has reached a speed beyond the unstable point of no load and the drive therefore safe running towards the right, the desired operating point. Alternatively, a driving machine with over the speed almost constant torque can be used. This was achieved earlier with a shunt DC machine, it is now being asynchronous with current displacement runner or asynchronous along with a frequency converter.

Electronics

The operating point of a circuit is the resting state in the absence of a signal. It is described by a specific point on the characteristic curve. From this point to modify current or voltage, when a useful signal is applied. In order to achieve the highest possible undistorted, balanced signal transmission, usually lays one the operating point in the center of the curve, that is, between the maximum and minimum voltage and current requirements. This mode is also called A- operation.

If an asymmetric modulation is desired, one moves the operating point to the edge of the characteristic curve (B- mode, C- mode). One has then for each half cycle of the signal ( positive / negative wave) provide a separate transistor; Both transistors are arranged in a push-pull output stage. Of this technique, use is made in power amplifiers, because it is a low quiescent current through the transistors flows while the A operation necessary, high bias current heats the transistor stronger.

The picture shows a transistor amplifier and its behavior at different settings of the operating point are shown. By the potentiometer P1, the signal level can be changed by R and the operating point P2. It lies in the middle between the maximum (operation voltage) and a minimum voltage (ground ), then the signal can be controlled symmetrically around the operating point. Shift of the operating point upwards means that the signal peaks abut the operating voltage at shift down to the ground potential. The signal is distorted as a result. For distortion occurs also when the amplifier is overdriven ( by a too large input signal ). In this case, the range between maximum and minimum voltage is no longer sufficient for the signal. It also says that the amplifier starts to clip. These arise primarily odd harmonics, which tip the balance in the calculation of Klirrgrades. The even harmonics are of minor importance, as these are widely perceived less annoying. To reduce the ratio of the odd harmonics to even harmonics, there are so -called soft clipping circuits.

Commonly used operating points

Described in terms of tube circuits for transistor circuits are analog guidelines.

The A- operation

The A mode, the operating point lies slightly ( defined by the abscissa and ordinate in the second quadrant ) over the center of the grid voltage - anode current characteristic. The O operation is in almost all precursors, as well as power amplifiers used ( dowry current Imax / quiescent current Ir = 1). The dynamic range is limited by the grid voltage - anode current characteristic.

B- mode

In the B - mode, the bias is at the point of the characteristic curve at which the anode bias current begins to flow appreciably ( dowry current Imax / quiescent current Ir ≥ 10). An amplifier stage in the B operation does not differ materially from the anode rectification because they too only amplifies the positive half cycles of an approximately sinusoidal and equal share free waveform. However, it is customary to amplify the other vibrations by means of a second stage in push-pull as well. By combining the two components results in the complete signal. The dynamic range of the signal may be twice as great in this mode, as the dynamic range of the tube in class A mode. As performance results compared to the A- operation of the (theoretically) four times the value.

The AB operation

Has the characteristic of a tube on the anode current application point, a curve ( lower slope) which means that the signal is distorted in the vicinity of the zero crossing. This occurring during operation B of the push-pull amplifier crossover distortion can be reduced by an operating point is selected at a slightly higher anode current. Maximum power is something less ( dowry current Imax / quiescent current Ir ≈ 5).

The C operation

When C operation flows without which activates signal no anode current ( dowry current Imax / quiescent current Ir ≥ 100). The non-linear distortions are rendered harmless by filtering at the transmitter output stages. The flywheel effect of the filter leads to a regeneration of the severed parts of the signal. The performance of a part of the semi-oscillations in the envelope demodulation is therefore only at very high modulation depth of meaning.

The audio mode

In the past Audion and the lattice rectification thereby used in electron tubes, the operating point moves signal-dependent. For larger signals, the effective transconductance of the tube decreases. This particular property was very important for the good adjustability of the feedback.