The present invention relates to a control system for controlling an inverter for providing AC output from DC input and, more particularly, to such a control system which is responsive to the temperature of the system sensed between the output of the inverter and the load rather than to a feedback voltage.
My prior U.S. Pat. No. 4,527,226 discloses a pulse width modulated (PWM) inverter, having a DC input, which may be employed, for example, on aircraft and the like to provide a waveform to a filter having an AC output which supplies power to loads. Load inverters typically include a pair of switches connected between the positive and negative terminals of the DC power supply and the AC load. The inverter switches, which may be transistors or the like, are controlled to provide a pulse width modulated waveform to the filter which, in response to this waveform, provides an AC output for supply to the load.
Known PWM inverters typically include a control system which operates the inverter switches to provide a single pattern pulse width modulated waveform to the filter. However, with any single pattern of pulse width modulated waveforms, distortion associated with power factor loads, i.e., reactive loads, may be produced in the output. Inverters employing known control systems to provide a single pattern pulse width modulated waveform typically do not take into account power factor loads which may introduce undesirable harmonics which are not easily filtered in the output of the inverter. In order to eliminate these harmonics from the inverter output waveform, the attenuation requirement of the output filter must typically be very large resulting in a heavy filter which is undesirable for many applications. Further, inverters employing such control systems typically do not compensate for voltage drops occurring between the DC power supply of the inverter and the point of regulation at which the AC output is applied to the load so that the peak amplitude of the AC output is not constant.
My prior U.S. Pat. No. 4,527,226 discloses an inverter control system which provides selection of an angle set defining an inverter output waveform in response to the power factor of the load and the normalized DC bus voltage so as to minimize the harmonic content of the inverter output waveform and to compensate for voltage drops occurring between the DC power supply and the load.
The inverter control system disclosed there includes an angle set look-up table for storing a number of angle sets, a plurality of which define pulse width modulated waveforms and at least one of which defines a quasi-square wave. The control system selects an angle set defining a pulse width modulated waveform in response to the power factor angle and a normalized DC bus voltage having a value which is greater than or equal to one. The angle set defining the quasi-square wave is selected by the control system in response to the normalized DC bus voltage having a value which is less than one indicating transient conditions.
The control system is also responsive to filter output current to detect fault conditions. In response to a detected fault condition, the control system selects at least one angle set defining a waveform to reduce the voltage at the load to zero while allowing the filter output current to increase to a given percentage of the rated current so as to enable actuation of circuit breakers such as those on aircraft.
These prior inverter control systems use the voltage supplied to the load as a feedback. The system compares the feedback voltage to a reference voltage and will control the inverter accordingly. The range of control for the voltage being supplied to the load will depend upon the gain of the inverter control system. This gain has a limiting effect on the ability of the control loop to provide control for a wide range of voltage fluctuations in the voltage being supplied to the load. For example, if the error between the feedback voltage and the reference voltage becomes too large, the gain required to bring the system back to reference may cause the control loop to go unstable. On the other hand, if the system can avoid using feedback voltage, the stability problems occasioned by the gain of the control loop can be minimized or avoided.