This looks like a standard for driving two motors, but upon closer examination, we see that M1 is controlling a motor pump for lubricating oil. The main drive motor, controlled by motor starter M2 is controlled by a pressure switch.
By pressing the start button, motor M1 will start to pump lube onto the bearings of the main motor. Once the pressure meets the minimum threshold and closes the pressure switch contacts, motor M2 will be allowed to start. This arrangement ensures the motor M2 will not run if there is insufficient lubrication pressure.
The circuit will continue to operate as a standard three-wire circuit providing until either the stop button is pressed, or an occurs. If an overload occurs in motor M1, then M2 will continue to run until the pressure switch drops open. If an overload occurs in M2, it will stop while motor M1 will continue to keep lubrication pressure up.
In motor control terminology, a three-wire circuit utilizes a magnetic motor starter with a holding contact, along with momentary contact pushbuttons. A three-wire circuit provides low-voltage-protection.
A device that controls the flow of electrical power to a motor. It is designed to safely start and stop a motor, and provide overload protection.
A contact that under normal conditions does not have continuity through it. When the contact changes its state it permits the flow of current by closing its contacts. Can be associated with pushbuttons, pilot devices or magnetic contactors.
Circuits with low-voltage protection will not automatically turn back on when voltage is restored following a power outage. Examples include the microwave or power tools.
A moderate and gradual rise in the value of current over a relatively long period of time that is caused by excessive amounts of current drawn by a motor due to too much load being put on the motor.