40 while permittinq radial movement of the rotor 21 and counterweights 30 and 40 as a unit in operation as the magnetic field moves about the stator 20 through the selective activation of some of the windings as described above rotor piston 21 tends to follow the magnetic field and coacts with the base 20 1 of stator 20 in the manner of the coaction of the piston and cylinder of a rolling piston compressor the rotor piston 21 thus rotates about the coaxial axes of shafts 21 1 and 2 while the rotor piston 21 and counterweights 30 and 40 rotate as a unit about coaxial axes of shafts 24 1 and 26 1 since the weight portions 30 3 and 40 3 are on the opposite side of shafts 24 1 and 26 1 from the center of gravity of rotor piston 21 the unit can be dynamically balanced with the correct selection or design of the counterweights 30 and 40 using standard moment of inertia equations to balance the rotor piston 21 with the counterweights 30 and 40 since the gas loads change with the compression process there will be unbalance at some time since the counterweights do not accommodate these changes however the initial selection of the counterweight can chose some stage of the compression stroke at which balance is established if a liquid slug for example was in the trapped volume of the compressor its incompressibility would create an excess pressure because slots 30 2 and 40 2 are provided rotor piston 21 can move away from the.