How to control surge problem in multistage centrifugal air compressor?

In the anti-surge control of multi-stage centrifugal air compressors with constant speed (electric motor), the following devices are used:

1.- Valve of butterfly of entrance (IBV) or

2.- Input guide palettes (IGV)

These devices act automatically through the control system to control the discharge pressure, opening or closing the passage of air in the suction to change the pressure in the eye of the impeller of the first stage. Generally, at the start-up of the compressor these devices are closed and then open until the design operating point or motor overload control is reached. The percentage of opening of the IBV or the angle of the IGV is defined in the control strategy, modifying the head and efficiency curves of the compressor to move the operation point away from the surge point.