TECHNICAL FIELD

The present invention relates to a rotating machine drive system that drives a rotating machine.

BACKGROUND ART

The efficiency of a rotating machine that is driven at variable speed by an inverter device is normally represented by an efficiency curve obtained by changing the rotation speed under a constant load condition, and the efficiency reaches a peak in one of rotation zones of the required rotation range. To realize device energy saving, it is important to improve the efficiency curve over a wide rotation range, and reduce power loss of the rotating machine.

However, it is known that the efficiency of a conventional rotating machine drops in a low-speed rotation zone. One of the factors of this is that a conventional rotating machine is designed to have a small inductance so as to expand the rotation range. In a conventional rotating machine having a small inductance, a harmonic component corresponding to the switching frequency of the inverter device is superimposed on a substantially sinusoidal current waveform supplied to the rotating machine. Because of this, the harmonic component of iron loss generated in the core of the rotating machine, AC copper loss generated by the skin effect and the proximity effect of the stator winding, and the like become larger, which leads to a decrease in the efficiency of the rotating machine.

Meanwhile, it is known that an increase in the inductance of the rotating machine at the stage of designing can reduce the current value and also reduce the harmonic component. As a result, the efficiency in the low-speed rotation zone can be improved. However, there are problems that the efficiency in the high-speed rotation zone becomes lower, and the rotor cannot be driven in the high-speed rotation zone and loses synchronism.

To solve such a problem, there is a technique for switching the connection of the stator winding between a low-speed rotation zone and a high-speed rotation zone, as disclosed in PTL 1. In a case where connections are switched while the rotating machine is being driven, arcs are generated at the switching contact points, and the life of the contact points is shortened. PTL 1 discloses a technique for switching connections after reducing torque commands and putting the inverter main circuit into a non-energized state so as to avoid contact arcs.

CITATION LIST

Patent Literature

PTL 1: JP 5-3694 A

SUMMARY OF INVENTION

Technical Problem

The cause of generation of a contact arc is that, at the moment when the distance between contact points decreases to a very short distance during a contact opening/closing operation, the electric field strength of the portion increases, resulting in dielectric breakdown and an excessive flow of current. For this reason, even if the contact portions are in a non-energized state, generation of contact arcs cannot be avoided in a configuration in which a potential difference is generated between contact points, and a circuit that can be energized in a contact closed state is formed. In such a configuration it is difficult to prolong the life of the contact points.

This problem is also caused in a case where winding connections are switched during a transition between the low-speed rotation zone and the high-speed rotation zone. Even if winding connections are switched while the inverter main circuit is in a non-energized state, a potential difference is invariably generated between the line internal voltage Vac induced by the field magnetic flux of the rotating machine and the DC voltage Vdc supplied to the inverter device. Because of this, a potential difference is also generated between the contact points, and a circuit that can be energized in a contact closed state is formed. Therefore, there are problems that contact arcs are repeatedly generated every time winding connections are switched in accordance with acceleration/deceleration of the rotating machine, and the shortening of the life of the contact points leads to an increase in maintenance costs. Furthermore, as the risk of defects such as melting of the contact points becomes higher, the reliability of the drive system becomes lower.

Note that a method by which switching contact points are formed with semiconductor switching elements has been disclosed as a means to solve the above problems. By this method, however, the switching elements are always energized while the rotating machine is driven. Therefore, conduction loss is caused. That is, because new power loss is to be caused at the switching contact points, it cannot be said that the above method is effective in improving the energy efficiency of the entire drive system.

An object of the present invention is to change the rotation zone of a rotating machine without generation of contact arcs.

Solution to Problem

To achieve the above object, the present invention provides a rotating machine drive system that includes: 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 the 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. In a case where the rotation zone of the rotating machine is to be changed, the control device stops the 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 the high-speed rotation zone to the low-speed rotation zone, on condition that a line internal voltage induced by a field magnetic flux of the rotating machine is lower than the DC voltage of the DC power supply.

Advantageous Effects of Invention

According to the present invention, the rotation zone of a rotating machine can be changed without contact arc generation. As a result, in a rotating machine that is operated at a variable speed by an inverter device, efficiency can be increased in a rotation range including both a low-speed rotation zone and a high-speed rotation zone.

The objects, structures, and effects other than the above will become more apparent from the embodiments described below.

