An apparatus comprising: an electric motor having a can for separating a process gas, flowing inside the can and external to an electric motor rotor, from a cooling oil contained in an electric motor stator compartment surrounding the electric motor rotor; a compressor having a rotor connected to the electric motor rotor wherein a part of the compressor's process gas is diverted through the can of the electric motor; and a pressure equalizer comprising: at least one bellows hermetically connected to a housing in order to create a first reservoir of the cooling oil and a second reservoir of the process gas inside the housing and to prevent the mixing of the process gas with the cooling oil inside the housing; at least one spring configured to provide compression on the cooling oil such that the cooling oil pressure in the first reservoir is greater than the process gas pressure in the second reservoir; a connection from the first reservoir to the electric motor stator compartment outside of the can of the electric motor; and a connection from the second reservoir to the process gas inside of the can of the electric motor; whereby the process gas can not come into contact with the electric motor stator, in particular electric motor laminations and electric motor windings; and wherein the electric motor stator cooling is achieved by cooling oil which flows by natural convection inside the stator compartment. an electric motor having a can for separating a process gas, flowing inside the can and external to an electric motor rotor, from a cooling oil contained in an electric motor stator compartment surrounding the electric motor rotor; a compressor having a rotor connected to the electric motor rotor wherein a part of the compressor's process gas is diverted through the can of the electric motor; and a pressure equalizer comprising: at least one bellows hermetically connected to a housing in order to create a first reservoir of the cooling oil and a second reservoir of the process gas inside the housing and to prevent the mixing of the process gas with the cooling oil inside the housing; at least one spring configured to provide compression on the cooling oil such that the cooling oil pressure in the first reservoir is greater than the process gas pressure in the second reservoir; a connection from the first reservoir to the electric motor stator compartment outside of the can of the electric motor; and a connection from the second reservoir to the process gas inside of the can of the electric motor; at least one bellows hermetically connected to a housing in order to create a first reservoir of the cooling oil and a second reservoir of the process gas inside the housing and to prevent the mixing of the process gas with the cooling oil inside the housing; at least one spring configured to provide compression on the cooling oil such that the cooling oil pressure in the first reservoir is greater than the process gas pressure in the second reservoir; a connection from the first reservoir to the electric motor stator compartment outside of the can of the electric motor; and a connection from the second reservoir to the process gas inside of the can of the electric motor; whereby the process gas can not come into contact with the electric motor stator, in particular electric motor laminations and electric motor windings; and wherein the electric motor stator cooling is achieved by cooling oil which flows by natural convection inside the stator compartment.