JPH04289797A - Voltage control method for pwm inverter - Google Patents

Abstract

PURPOSE:To suppress overexcitation of motor during an interval after start of deceleration before stoppage of motor while enhancing the torque at the start of motor. CONSTITUTION:A comparator 3 compares an output frequency command value fed from a PWM inverter 11 through a setter 1 with an actual output frequency value fed from the PWM inverter 11 through a soft start/stop circuit 2 and makes a decision whether a motor 12 is accelerating or decelerating. Any one of voltage weakening rate pattern generators 6A, 6B is then selected based on the comparison result thus suppressing overexcitation of motor during an interval after start of deceleration before stoppage of motor.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a voltage control method for a so-called PWM (Pulse Width Modulation) controlled inverter.

0002

2. Description of the Related Art Conventionally, a so-called voltage weakening control method has been known in which a motor is controlled by changing the output voltage depending on the magnitude of the output current using the output current of an inverter device as a parameter.

0003

[Problems to be Solved by the Invention] However, in this method, control is performed to strengthen the voltage in the low frequency region for loads with large mechanical loss that require high starting torque when starting the inverter. However, since the voltage is strengthened during deceleration, the motor may become over-excited, and the excitation current may flow up to the inverter's current limit value, leading to accidents such as burnout. There is a risk. Therefore, an object of the present invention is to suppress the overexcitation phenomenon of the motor during deceleration or stop while achieving high torque during starting.

0004

[Means for Solving the Problems] In order to solve the above problems, the present invention compares the output frequency of a PWM inverter that controls an electric motor with its frequency command value, and based on the comparison result, the electric motor operates at an accelerated speed. The present invention is characterized in that it determines whether the motor is operating at a slow speed or during deceleration operation, selects a predetermined one from a plurality of output voltage weakening rate patterns according to the result of the judgment, and controls the motor based on the selected pattern.

[0005]

[Operation] By determining the operating mode of the motor (accelerating or decelerating) and selecting a desired one of multiple pre-prepared voltage weakening patterns according to the determination result, To suppress the overexcitation phenomenon of the motor from deceleration to stop while achieving high torque.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a block diagram showing an embodiment of the present invention. In the figure, 1 is a frequency setter, 2 is a soft start/stop circuit, 3 is a comparator, 4 is a voltage command generator, 5 is a multiplier, 6A and 6B are pattern generators, and 7 is a PW.
M (pulse width modulation) circuit, 8 is a carrier generator, 9 is a base drive circuit (BD), 10 is a current detector, 11 is an inverter main circuit, 12 is a motor (induction motor), 13
is a current transformer, and SW is a switch. The soft start/stop circuit 2 includes, for example, a function generator or the like, and outputs a signal for soft starting or soft stopping the motor 12 in accordance with the frequency command value from the frequency setter 1. This output is used as the inverter output frequency (actual value), and is compared with the frequency command value from the frequency setter 1 by the comparator 3. Thereby, the operation mode is determined. In other words, when the frequency command value < inverter output frequency, or when there is an inverter output frequency without a driving command (forward or reverse driving command), it is determined that the motor is decelerating, and when the frequency command value ≧ the inverter output frequency, it is determined that the motor is accelerating. (operating normally).

Since the comparator 3 outputs "1" or "0" depending on the comparison result, for example, if it is set to output "0" during acceleration and "1" during deceleration, this output will The on/off operation of the switch SW is selected, and one of the pattern generators 6A and 6B is selected depending on the comparison result. Therefore, by setting the pattern generator 6A to a voltage weakening pattern suitable for acceleration and the pattern generator 6B to a voltage weakening pattern suitable for deceleration, optimal operation can be achieved. This pattern generator 6
The output from A or 6B is multiplied by the output from voltage command generator 4 in multiplier 5 to determine the inverter output voltage, but the output voltage from pattern generator 6A or 6B is multiplied by the output from voltage command generator 4 in multiplier 5. It is determined according to the inverter output current from the converter 10. Note that although two types of pattern generators are used here, it is also possible to further subdivide the pattern generators for acceleration and deceleration to provide two or more types.

The inverter output voltage command from the multiplier 5 is given to the PWM circuit 7, where it is compared with the carrier signal from the carrier generator 8 to generate a PWM signal, and based on this PWM signal, the base drive circuit 9 is activated. The switching elements of the inverter main circuit 11 are driven through the inverter main circuit 11, and the electric motor 12 is operated. It should be noted that although it is assumed that software processing is performed by a microcomputer or the like within the dashed line in the figure, it is of course possible to implement the processing using hardware.

[0009]

[Effects of the Invention] According to the present invention, by appropriately selecting a voltage weakening pattern, it is possible to suppress overexcitation of the motor during deceleration or stop while achieving high torque during starting. is obtained.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the invention.

[Explanation of symbols]

1 Frequency setter 2 Soft start stop circuit 3 Comparator 4 Voltage command generator 5 Multiplier 7 PWM (pulse width modulation) circuit 8 Carrier generator 9 Base drive circuit (BD) 6A pattern generator 6B Pattern generator 10 Current detector 11 Inverter main circuit 12 Motor (induction motor) 13 Current transformer SW switch

Claims (1)

[Claims] 1. The output frequency of a PWM inverter that controls an electric motor is compared with its frequency command value, and based on the comparison result, it is determined whether the electric motor is in acceleration operation or deceleration operation. A voltage control method for a PWM inverter, characterized in that a predetermined one is selected from output voltage weakening rate patterns, and a motor is controlled based on the selected pattern.