Operating point

The operating point is a specific point within the operation characteristic of a technical device. This point will be engaged because of the properties of the system and the outside influences and parameters. In electronic engineering establishing an operating point is called biasing.

The operating point of a system is the intersection point of the torque-speed curve of drive and machine. Both devices are linked with a shaft so the speed is always identical. The drive creates the torque which rotates both devices. The machine creates the counter-torque, e.g. by being a moved device which needs permanent energy or a wheel turning against the static friction of the track.

At the operating point the driving torque and the counter-torque are balanced, hence the speed does not change anymore.

A change in speed out of this stable operating point is only possible with a new control intervention. This can be changing the load of the machine or the power of the drive which both changes the torque because it is a change in the characteristic curves. The drive-machine system then runs to a new operating point with a different speed and a different balance of torques.

Should the drive torque be higher than the counter torque at any time then the system does not have an operating point. The result will be that the speed increases up to the idle speed or even until destruction. Should the counter torque be higher at any times the speed will decrease until the system stops.

Also in case of an unstable operating point the law of the balance of the torques is always valid. But when the operating point is unstable then the characteristics of drive and machine are nearly parallel. In such a case a small change in torque will result in a big change of speed. In practice no device has a characteristics which is so thin that the intersection point can be clearly expected. Because of parallel characteristics, inner and outer friction as well as mechanical imperfections the unstable operating point is rather a band of possible operating states instead of a point. Running at an unstable operating point is therefore undesirable.

The middle point on the curve in the third picture on the right is an unstable point, too. However the above-mentioned assumptions are not valid here. Torque and speed are the same but in case the speed will be increased only little then the torque of the drive will be much higher than the counter-torque of the machine. The same but vice versa applies when reducing the speed. For this reason this operating point does not have a stabilizing effect on the speed. The speed will run away to the left or the right side of the point and the drive will run stable there.

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Wikipedia