Synchronous motor control device, compressor drive system, and air conditioner

A compressor drive system for controlling a permanent magnet synchronous motor used as a drive source of a compressor,

• the compressor drive system converting DC power supplied from a DC power supply into three-phase AC power, the compressor drive system comprising:
• an inverter main circuit to convert the DC power into the three-phase AC power by using a plurality of switching elements and supply the three-phase AC power to the permanent magnet synchronous motor;
• a current detector to detect a direct current flowing through a DC bus between the DC power supply and the inverter main circuit;
• a voltage detector to detect a DC voltage between a positive side of the DC bus and a negative side of the DC bus;
• and
• an inverter controller to generate a pulse width modulation signal from the direct current and the DC voltage for controlling each of the plurality of switching elements, and output the pulse width modulation signal to the inverter main circuit, wherein
• the inverter controller includes:
• a phase current reproducer to reproduce from the direct current, phase currents flowing to the permanent magnet synchronous motor;
• a current coordinate transformer to transform the reproduced phase currents into current on a control coordinate axis of a rotating coordinate system;
• a current controller to calculate a voltage command value of the permanent magnet synchronous motor in such a manner that the current on the control coordinate axis equals a specific value;
• and
• a limiter to limit a value of the voltage command value, wherein
• the limiter has a first limiter value (Vγδ_LIMIT 1) set to be capable of protecting the permanent magnet synchronous motor against demagnetization and protecting the inverter main circuit against overcurrent, and a second limiter value (Vγδ_LIMIT 2) set to be larger than the first limiter value and limit the voltage command value in steady state control,
• the first limiter value is set to a value smaller than or equal to Ra×OC_LEVEL×√(3/2), where Ra denotes a phase resistance value of the permanent magnet synchronous motor and OC_LEVEL denotes an overcurrent protection level capable of protecting the permanent magnet synchronous motor against demagnetization and protecting the inverter main circuit against overcurrent, and
• the limiter keeps the first limiter value when the permanent magnet synchronous motor starts up, and switches from the first limiter value to the second limiter value when the current value on the control coordinate axis reaches the specific value.