A rotating machine drive system comprising: a rotating machine that includes a plurality of windings; a phase current detecting circuit that detects a phase current to be supplied to the rotating machine; an inverter device that includes: an inverter circuit that converts DC power from a DC power supply into AC power; and a control device that controls power conversion being performed by the inverter circuit on a basis of the phase current detected by the phase current detecting circuit, the inverter device operating the rotating machine at a variable speed; and a winding switching device that switches connections of the plurality of windings in accordance with a command from the control device, wherein, when a rotation zone of the rotating machine is to be changed, the control device stops a current supply from the inverter circuit to the rotating machine, and switches the rotation zone of the rotating machine from a low-speed rotation zone to a high-speed rotation zone, or from a high-speed rotation zone to a low-speed rotation zone, on condition that a line internal voltage induced by a field magnetic flux of the rotating machine is lower than a DC voltage of the DC power supply, and wherein the control device controls a rotary frequency of the rotating machine, estimates the line internal voltage on a basis of the phase current detected by the phase current detecting circuit, and issues a command for switching connections of the plurality of windings to the winding switching device at the rotary frequency at which the estimated line internal voltage is lower than the DC voltage, characterized in that when the line internal voltage is higher than the DC voltage either before or after the connection switching before the current supply from the inverter circuit to the rotating machine is stopped, the control device reduces the field magnetic flux of the rotating machine by reducing an excitation current command value to perform demagnetization control to make the line internal voltage lower than the DC voltage both before and after the connection switching. a rotating machine that includes a plurality of windings; a phase current detecting circuit that detects a phase current to be supplied to the rotating machine; an inverter device that includes: an inverter circuit that converts DC power from a DC power supply into AC power; and a control device that controls power conversion being performed by the inverter circuit on a basis of the phase current detected by the phase current detecting circuit, the inverter device operating the rotating machine at a variable speed; and an inverter circuit that converts DC power from a DC power supply into AC power; and a control device that controls power conversion being performed by the inverter circuit on a basis of the phase current detected by the phase current detecting circuit, the inverter device operating the rotating machine at a variable speed; and a winding switching device that switches connections of the plurality of windings in accordance with a command from the control device, wherein, when a rotation zone of the rotating machine is to be changed, the control device stops a current supply from the inverter circuit to the rotating machine, and switches the rotation zone of the rotating machine from a low-speed rotation zone to a high-speed rotation zone, or from a high-speed rotation zone to a low-speed rotation zone, on condition that a line internal voltage induced by a field magnetic flux of the rotating machine is lower than a DC voltage of the DC power supply, and wherein the control device controls a rotary frequency of the rotating machine, estimates the line internal voltage on a basis of the phase current detected by the phase current detecting circuit, and issues a command for switching connections of the plurality of windings to the winding switching device at the rotary frequency at which the estimated line internal voltage is lower than the DC voltage, characterized in that when the line internal voltage is higher than the DC voltage either before or after the connection switching before the current supply from the inverter circuit to the rotating machine is stopped, the control device reduces the field magnetic flux of the rotating machine by reducing an excitation current command value to perform demagnetization control to make the line internal voltage lower than the DC voltage both before and after the connection switching.