Offset Correction Circuit of PWM Inverter

This invention relates to a pulse width modulated (PWM) drive and, more particularly, to an offset correction circuit for current control.

As an example, a prior art PWM inverter having a current control circuit has a block configuration shown in FIG. 2. The DC power is taken via a rectifier 2 from an AC power source 1 and fed to an inverter main circuit 3, which supplies AC power with a PWM waveform having adjusted fundamental frequency and voltage to an induction motor 4 as a load. The control device of inverter main circuit 3 comprises a speed control amplifier 6 which is responsive to a difference signal on a line 6c from a summing junction 6d which compares a speed setting signal (Ns) on a line 6a and a detected signal on a line 6b from a speed detector 5 coupled to motor 4. The amplifier 6 performs a PI operation and provides an amplified signal on a line 6e to a sinusoidal wave generating circuit 7 which generates a sinusoidal signal on a line 7a for adjusting the frequency and voltage according to the speed instruction of amplifier 6. A current control amplifier 9 which amplifies (i.e., performs PI operation) an error signal on a line 9a from a summing junction 9b indicative of a difference between the magnitude of the current instruction signal on line 7a and a detected output current signal on a line 8a from a current detection circuit 8 responsive to the current provided by inverter main circuit 3 to the motor 4 by means of a sensor 8b. A PWM circuit 10 is responsive to an amplified difference signal on a line 9b from the amplifier 9 and provides PWM gating signals on a plurality of lines indicated generally by a line 10a in order to effect an appropriate output waveform for driving the motor by performing ON/OFF control for a corresponding plurality of switch elements in various phases of inverter main circuit 3.

In the conventional PWM inverter having a current control system, when there exists an offset in the current detector and current instruction (sinusoidal wave) that form the current control system, the offset voltage makes the output current of inverter main circuit 3 deviate from the ideal AC waveform as a DC component is overlapped on it, thus causing torque ripple and other problems.

In the conventional scheme, in order to compensate for the offset, as the motor is stopped, the offset amounts in the current instruction and the current detection signal are detected, and compensation signals corresponding to these detected offset amounts are applied during operation.

However, in long-term continuous operation, the offset amount varies as the temperature drifts. In this case, correction becomes impossible. In particular, the temperature drift is large for the current detector, and it is difficult to correct the offset caused by the current detection system. As temperature variation is also caused by seasons, proper correction is impossible. This is a problem.

The object of the present invention is to teach how to correct the offset in a current control for a PWM inverter and to show how to provide an offset correction circuit which can reliably and easily correct the offset in the current control system.

According to the present invention, in a PWM inverter having a current control system, by integrating the DC component of the current control amplifier and providing the integrated signal to said amplifier in a direction to eliminate the DC component, the offset of the current control system is corrected.

In a PWM inverter drive control system having a current control amplifier which provides the control signal of a PWM circuit in response to a difference signal obtained by comparing the magnitudes of a sinusoidal current instruction signal and a sensed signal indicative of the inverter output current, in further accord with the present invention, a type of offset correction circuit is provided comprising an integrating amplifier which integrates the DC component of the output of the said current control amplifier and an inverting amplifier which inverts the output of the integrating amplifier for summation with said difference signal in a direction to eliminate the DC component. Thus, when a DC component is generated in the output of the current control amplifier, the DC component is detected by the integrating amplifier, and with the aid of the inverting amplifier, the detected value is inverted and added to the input of the current control amplifier to eliminate the said DC component and thereby perform the offset correction. Due to this correction, the offset amount can be kept at nil as the correction is performed corresponding to the variation in the offset amount, such as that due to the temperature drift during operation.

These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as illustrated in FIG. 1 of the accompanying drawing.

FIG. 1 is a schematic diagram illustrating an application example of a best mode embodiment according to the present invention; and

FIG. 2 is a block diagram of a prior art PWM inverter control system.

FIG. 1 is a circuit diagram of the main portion of a single phase as an application example of a best mode embodiment, according to the teachings of the present invention. The symbols used here are identical to those used in FIG. 2. From an output current detection circuit 8, which may be responsive to a current transformer 8a or the like, the signal level of a detected current signal on a line 8c is adjusted to obtain a low-output-impedance sinusoidal detected signal on a line 8d. Current control amplifier 9 may consist of an operational amplifier 9.sub.1, a serial feedback circuit made of capacitor 9.sub.2, and resistors 9.sub.4 and 9.sub.5 of an input adder which serves the same function as the summing junction 9b shown in FIG. 2, except that the input adding circuit of FIG. 1 also has a resistor 9.sub.7 for performing input addition of an offset correction signal to be described later. Amplifier 9 performs PI operation for the error between the sinusoidal current instruction of sinusoidal wave generating circuit 7 and the sinusoidal detected signal of current detection circuit 8. In addition, current control amplifier 9 has an analog switch 9.sub.6 which is parallel to the feedback circuit of operational amplifier 9.sub.1 and is used to perform the ON/OFF control by a start/stop signal on a line 9.sub.8 of motor 4. In the PWM circuit 10, an output signal on a line 9.sub.9 from current control amplifier 9 and an output signal on a line 10.sub.3 from a triangular wave generator 10.sub.1 are taken as the inputs to a comparator 10.sub.2, and a PWM waveform with a width proportional to the output level and polarity of amplifier 9 is obtained.

In the illustrated case, in order to correct the offset, an integrating amplifier 11 an inverting amplifier 12 are arranged as shown. Integrating amplifier 11 is responsive to and integrates an output signal on the line 9.sub.9 of current control amplifier 9 by using operational amplifier 11.sub.1, feedback capacitor 11.sub.2 and input resistor 11.sub.3. Also, it has an analog switch 11.sub.4 in parallel to capacitor 11.sub.2. This analog switch is responsive to a signal on a line 11.sub.5 which turns the switch 11.sub.4 off (open circuited or non-conducting) when motor 4 is in operation or when the rotating speed is higher than a selected level (integration is allowed in this case); on the other hand, it turns on when motor 4 is stopped or the rotating speed is lower than the selected level (reset). Inverting amplifier 12 is responsive to an integrated signal on a line 11.sub.6 from integrator 11 and has an operational amplifier 12.sub.1 and resistors 12.sub.2, 12.sub.3. It inverts and amplifies the output signal on the line 11.sub.6 from integrating amplifier 11 and provides, via the resistor 9.sub.7, an offset correcting signal on a line 12.sub.4 to the summing junction in current control amplifier 9.

The integrating time constant T of integrating amplifier 11, which is the product of the capacitance C of capacitor 11.sub.2 and the resistance R of resistor 11.sub.3, is sufficiently larger than period 1/f of the inverter's output frequency f.sub.o to effectively integrate only the DC component thereof. In addition, resistance 9.sub.7 of current control amplifier 9 is much larger than the resistances of resistors 9.sub.4 and 9.sub.5.

In this configuration, the output of current control amplifier 9 has an ideal AC waveform. When there exists a DC component due to temperature drift of the current detector, etc., and when the DC component varies, this offset component is integrated and amplified by integrating amplifier 11 and it then has its polarity adjusted by inverting amplifier 12 so that a DC bias with a polarity and level which cancels the offset component is fed back to current control amplifier 9 and offset correction is thus realized. As this correction is performed corresponding to the offset component, the offset can be kept at nil at all times.

As mentioned, in the illustrated case, the integrating time constant T of integrating amplifier 11 has a time constant much longer than the period of the sinusoidal signal so that only the DC component is corrected. Also, analog switch 11.sub.4 may be turned off during operation or when the rotating speed of the motor is higher than a preset level, so that the offset correction is enabled. It is then turned on when the motor is stopped or in low speed operation so that the integrated output is reset. Also, as current control amplifier 9 is controlled OFF/ON in correspondence to the start/stop state of analog switch 9.sub.6, reset is performed while stopped. Control of these analog switches 11.sub.4, 9.sub.6 enables offset correction to be performed only during operation or when the rotating speed is higher than a selected or preset level. Hence, it is now possible to prevent the adverse influence of offset on the current control system.

Although the invention has been shown and described with respect to a best mode embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions, and additions in the form and detail thereof may be made therein without departing from the spirit and scope of the invention.