JPS6310673B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a variable speed motor drive system for an induction motor using an inverter, and more particularly to an inverter control system that automatically adjusts the slip frequency to the maximum torque-current efficiency.

Generally, the torque-slip frequency relationship of an induction motor is shown in FIG. 1, and the torque T has a certain peak value under a slip frequency F _S with the primary current I as a parameter. That is, the primary current I
= I ₁ , the slip frequency F _S = F _S1 and the torque T
has a maximum value T ₁ , and when the primary current I=I ₂ >I ₁ , the slip frequency F _S =F _S2 >F _S1 takes a maximum value T ₂ . Therefore, it is desirable to use the slip frequency F _S at which the maximum torque is generated depending on the value of the primary current I. To achieve this purpose, conventionally the slip frequency according to the primary current I is
Pre-program F _S with a pattern generator,
A method was used to generate an appropriate slip frequency F _S depending on the current used. However, according to this method, the characteristics of the electric motor to be used must be known in advance and the pattern generator must be programmed to suit the characteristics, making it inconvenient to apply to a wide range of electric motors. . Furthermore, in such applications, it is not possible to select the above-mentioned characteristics to be practically appropriate due to changes in machine constants due to inverter frequency, temperature characteristics of the motor, etc. It was unavoidable to use it.

The present invention aims to improve the above-mentioned drawbacks by constantly checking the characteristics of the motor during operation and automatically adjusting the slip frequency to the optimum slip frequency.

Fig. 2 shows an embodiment of the present invention, in which 1 is a torque setting device, 2 is a torque-to-current converter that converts the torque setting amount into a current command value, and 3 is a DC current converter that rectifies and filters the motor primary current. 4 is a subtracter that subtracts the output of the current detection circuit 3 from the current command; 5 is a current control amplifier that amplifies the output of the subtracter 4; and 6 is a torque detection circuit. The torque detection circuit 6 may be of any type as long as it detects the torque of the electric motor; it may be one that converts the detected value obtained using a mechanical detection element such as a strain gauge into torque-voltage; It can be anything you are looking for.

7 is a differentiator that differentiates the output of the torque detection circuit 6 over time; 9 is a divider that divides the output of the differentiator 7 by the output of the differentiator 8; 10 is an integrator that integrates the output of the divider 9 with time; 11 is an electric motor 12 is an adder that adds the output of the integrator 10 and the output of the speed detection circuit 11; and 13 is an inverter control circuit. This inverter control circuit 13 uses the output of the current control amplifier 5 as a voltage command.
The output of the adder 12 is inputted as a frequency command, and a gate pulse is supplied to the thyristor of the inverter so that the output voltage and output frequency correspond to the frequency command.

15 is an inverter that converts the power from the power supply device 14 into variable frequency/variable voltage by the inverter control circuit 13;
When is AC, a converter for AC-DC conversion is included. 16 is a current transformer that insulates and converts the motor primary current to generate a signal to the current detection circuit 3; 17 is an induction motor operated at variable speed by the inverter 15; and 18 is a gear coupled to the output shaft of the induction motor 17. The device 19 is a pulse pickup that generates pulses according to the rotation of the gear device 18 and outputs them to the speed detection circuit 11. The gear device 18 and the pulse pickup 19 are not limited to this example, and may be any device that can detect the rotational speed of the induction motor 17. Further, the inverter 15 and the inverter control circuit 13 exemplified here only need to control the frequency and voltage independently, and may be a known PWM inverter, current source inverter, or the like.

The operation of the circuit configuration thus constructed will now be described.

The output of torque setting device 1 is transferred to torque-current converter 2.
is converted into a current command, which is subtracted and amplified with the output of the current detection circuit 3 to control the inverter output voltage. By the action of this current control loop, the motor primary current is controlled to a predetermined constant value according to the torque setting.

On the other hand, the output of the integrator 10 is added to the output of the speed detection circuit 11 as a slip frequency command to control the inverter output frequency. These slip frequency F _S , inverter frequency F _INV , motor frequency F _M
There is a relationship between F _S + F _M = F _INV , and if the speed detection circuit 11 output is F _M ,
By making the frequency commands output from the adder 12 correspond to F _INV , the induction motor 17 operates at a slip frequency equal to the slip frequency command F _S .

Further, the output of the torque detection circuit 6 is differentiated by a differentiator 7, divided by a slip frequency command also differentiated by a differentiator 8 by a divider 9, and added to an integrator 10.

Now, on the characteristic curve shown in FIG. 1, the primary current I=
Consider I ₂ and slip frequency F _S =F _S1 .

If the output F _S of the integrator 10 is increasing, the differentiator 8
The output is positive, and the output of the differentiator 7 is in the region where the torque T increases as the slip frequency increases, so it is positive, and the output of the divider 9, that is, the input of the integrator 10, is positive, and F _S continues to increase. . Assuming that F _S decreases, the output of the differentiator 8 is negative, and the output of the differentiator 7 also becomes negative due to the decrease in torque T, which also causes the output of the divider 9 to become negative.
The output becomes positive and F _S starts to increase. In this way, F _S increases, and if it increases to around F _S = F _S3 , then the torque T will decrease as F _S increases, and the output of the divider 9 will always be becomes negative and F _S decreases. Thus F _S generates the maximum value T ₂ of torque T under primary current I ₂
Converges to F _S2 . Further, near F _S2 , the slope of the torque T with respect to F _S becomes steeper as the distance from F _S2 increases, and the output of the divider 9 also becomes larger, so that it quickly converges.

The circuit of this embodiment that operates in this way is an example of an automatic adjustment circuit using differentiators 7 and 8 and a divider 9, but this can be used as a sample-hold circuit, for example, to calculate the current torque and a certain period of time ago. It is also possible to change the design by using a method of comparing the torques of and sequentially modifying them to obtain the larger F _S. Furthermore, it is clear that any method of adjusting the slip frequency so that the torque value is maximized under a predetermined primary current value will have exactly the same effect.

As described above, according to the present invention, it is possible to provide a device that operates at a slip frequency at which the measured torque is the highest under a constant current regardless of the machine constants, and the practical effects are extremely large.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the torque-slip frequency characteristics of an induction motor, and FIG. 2 is a circuit diagram showing an embodiment of the present invention. 1...Torque setting device, 6...Torque detection circuit,
7, 8...Differentiator, 9...Divider, 10...Integrator, 13...Inverter control circuit, 15...Inverter, 17...Induction motor, T...Torque, F _S
...Slip frequency, I ₁ , I ₂ , I ₃ ...Primary current.

Claims (1)

[Claims] 1. In a variable speed motor drive device comprising an inverter that outputs variable frequency/variable voltage alternating current power and an induction motor connected to the inverter output, the current is controlled to a predetermined primary current value according to a current command. a first differentiator that is equipped with a control circuit and that differentiates the detected torque signal; a divider that takes the output of the first differentiator as one input; an integrator that integrates the output of the divider; a second differentiator for differentiating the integrator output, the output of the second differentiator is given to the divider as the other input, and the divider converts the output of the first differentiator into a second differentiator. An inverter control device characterized in that the integrator output is divided by a differentiator output, and is configured to obtain a slip frequency signal of the integrator output adjusted to obtain the maximum torque.