Patent GB-676757-A (GB)
Title: Improvements in a compressor plant
Inventor: ROSENSCHOLD JOHN RUTGER MUNCK
IPC: F04B49/06
Filed: 1950-09-07
Abstract: 676,757. Control of motors. AKTIEBOLAGET ATLAS DIESEL. Sept. 7, 1950. No. 22045/50. Class 38 (iii). [Also in Group XXVIII] A compressor plant comprises a compressor, electric motor driving means for producing compression work at different speeds, means operable independently of the voltage applied to said driving means for changing the speed of the compressor, a mechanism for unloading the compressor when the speed is being increased and a device operated in response to a condition resulting from the compression work to actuate said speed changing means. An electric motor M is coupled to a compressor K supplying a reservoir B, a relay operated unloading valve S being operative to throttle the inlet to the compressor or to hold open the inlet valve. The motor M is controlled by a main switch O1 and a pole changing switch 02. Switches T1, T2 are operable in response to pressure existing in the reservoir B. Initially when a switch 03 is closed, the relay of the valve S is energized through the contacts 1, 2 of a relay R1, and a valve 10 connects a pipe 9 from the reservoir B to a pipe 12 leading to the unloading device on the compressor and at the same time closes the normally open communication between the pipe 12 and an opening 11 to atmosphere. At the same time the switch O1 is actuated through the winding of the relay R1, and the switch T1. After a time delay the relay R1 opens the contacts 1, 2 but the relay of valve S remains energized through relay R2 and switch T2. The relay R2 after a time delay closes contacts 3, 4, which operates the switch 02 to decrease the number of poles in the motor M which speeds up to the maximum speed. Subsequently the relay R3 energized through contacts 3, 4 closes contacts 5, 6 to short circuit the relay of valve S which closes the pipe 9 and vents the pipe 12 to atmosphere putting the pump K on load. At a given pressure the switch T2 trips and the switch 02 operates to reduce the motor speed. At a still higher pressure the switch T1 trips and operates switch O1 to shut down the compressor, relay R1 is thereby de-energized and contacts 1, 2 close to operate the valve S to unload the compressor. A drop in pressure closes switch T1 to start the motor at low speed and a still further drop closes switch T2 to increase the speed. At each increase of speed the valve S operates to unload the compressor. Alternatively the switches T1, T2 may be mechanically, pneumatically or hydraulically coupled to the switches O1, 02 and instead of being pressure operated may be responsive to a factor which is a function of the pressure e.g. temperature of the air, power output of the motor, torque or the temperature in the refrigerating space if the compressor is used for that purpose. The motor M may be connectable for more than two speeds. In a modification the single motor is replaced by two motors having the same or different speeds driving the compressor through different reduction gears, the motor which is energized carrying the idle motor or alternatively the motors may be clutched in or out. In a further modification a single speed motor drives the compressor through a gear box and the switch 02 is replaced by a device for changing the gear ratio. Two motors of different speeds may be directly coupled to the compressor and while one is energized the other is carried along idly. Current limiting devices may be included in the motor circuits. The pressure responsive device may be arranged to control the pressure oil and cooling water circuits. Specification 676,758, is referred to.