Vane compressor with delivery pressure control

An air compressor of sliding vane eccentric rotor type includes a stator, which includes an inlet and an outlet and defines a substantially cylindrical bore, and a rotor eccentrically rotatably mounted in the bore. The rotor is connected to be rotated by a three-phase asynchronous electric motor (2) of pole amplitude modulated type which is switchable between low speed six pole operation and high speed four pole operation under the control of a controller (4). A pressure sensor communicates with the compressor outlet and is connected to the controller which is arranged to produce a first signal when the compressor discharge pressure falls below a first threshold value and a second signal when this pressure rises above a second threshold value. Each of the three electrical power supply lines (6) of the motor is associated with a respective impedance (10), which is connected in parallel with a shunt path including a respective switching means (8), which is switchable under the control of the controller, whereby when the switching means (8) is closed the impedance (10) is shunted and is effectively switched out of the associated power line (6) and when the switching means (8) is open the shunt path is interrupted and the impedance (10) is effectively switched into the power supply line. The controller is arranged so that when the compressor is operating at low speed and the first signal is produced the motor is switched to operate at high speed and when the compressor is operating and high speed and the second signal is produced the compressor is switched to operate at low speed. The controller is also arranged so that when the compressor is switched on or switched between high and low speeds the impedances (10) are switched into the power lines (6) for a predetermined period of time and are switched out of the power lines at all other times.