JPS63171188A - Arithmetic circuit for magnetic flux command value - Google Patents

Abstract

PURPOSE:To effectively use memory, by measuring the cycle of the speed pulse, the reciprocal of a speed, and by using the cycle as the address of a storage element storing the data of a magnetic flux command value. CONSTITUTION:By a counter 2, a time interval between given speed pulse and following speed pulse is measured by means of counting clock from a clock generating circuit 1. Accordingly, the output of the counter 2 is to be in inverse proportion to the cycle of the speed pulse namely a speed. The output of the counter 2 is latched by a latch circuit 3 when the output overflows the counter 2 or when the speed pulse is generated. To a processing circuit 5, the input of a magnetic flux command value stored in a ROM 4 is provided by reading out the output signal of the latch circuit 3 as address signal. As a result, the capacity of memory is determined only by the effective resolution of data, and the memory can be effectively used.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a variable speed drive device that performs magnetic flux weakening control on an electric motor, and particularly to a magnetic flux command value calculation circuit thereof.

[Conventional technology]

2. Description of the Related Art Power conversion devices using power semiconductor elements are widely used as variable speed drive devices for electric motors.

This is because, for example, if a self-extinguishing element is used, the magnitude, frequency, and phase of the voltage and current supplied to the motor can be arbitrarily controlled. However, since the input power supply voltage is limited by the withstand voltage of the element, the maximum value of the induced voltage of the motor is also determined by this.

On the other hand, the induced voltage e of the motor can be expressed by the following formula.

e=k・φ・n・・・・・・・・・・・・
(1) Here, n is the rotational speed of the motor, φ is the magnetic flux, and 1(
is a proportionality constant determined by the electric motor. That is, if the magnetic flux φ is constant, as the speed n increases, the induced voltage e also increases in proportion. Therefore, if the maximum value of the induced voltage of the motor is not determined by the rated voltage of the converter, the maximum rotation speed of the motor is limited by the rated voltage of the converter when the magnetic flux is constant. However, in variable speed drive applications, there are many applications that require a wide range of speed control at high speed 1, rather than requiring any torque at low speed at high speed. Therefore, when speed control is performed up to the high speed range, the magnetic flux φ is kept constant until the induced voltage e reaches the rated value, and when the speed n is higher than that, the magnetic flux φ is decreased in inverse proportion to the increase in speed n. There is a method of controlling the speed by keeping the induced voltage e at a constant value. This is called flux weakening control, and the speed in this region is called the base speed. Here, at the base velocity (
1) By adding the subscript “B” to the various quantities in the equation, the magnetic flux φ in the flux weakening region can be expressed by the following equation. Also, when these relationships are illustrated, it becomes as shown in Figure 3. .

B φ−φB・− (however, n≧nB) ・・・・・・
12i The magnetic flux φ in this equation is determined by the magnetic flux command value when performing magnetic flux weakening control, and various methods have been proposed in which the calculation of equation 121 is performed to obtain this value. In particular, with the recent development of microprocessors, there is a tendency to avoid the effects of drift and offset by digitally performing the control and calculations necessary for variable speed drives. It needs to be done digitally. A digital speed detector is also required for this purpose.

On the other hand, in the field of AC variable speed drives, the speed detector must also be non-contact, so the pulse generator (PLG) is commonly used as a speed detector that satisfies both requirements.
) to digitally process the signal from this PLG,
A method of calculating speed is commonly used.

FIG. 4 is a block diagram showing a conventional example of a calculation circuit that calculates a magnetic flux command value using such a pulse generator. This is because the counter 12 counts speed pulses proportional to the speed from a pulse generator, etc., while the timing circuit 1
1, a launch signal is sent to the latch circuit 13 at a constant cycle,
Further, a reset signal is applied to each of the counters 12. As a result, the output of the launch circuit 13 is proportional to the average speed of the motor within one cycle, and therefore, formula 12) is calculated to output the magnetic flux command value. On the other hand, since the relationship between speed and magnetic flux is non-linear as shown in Figure 3, complex software is required to actually calculate it. 2) The relationship of the formula is memorized in advance to obtain the magnetic flux command value. In other words, in this method, the magnetic flux command value is further corrected based on the relationship "inversely proportional to speed" as shown in Figure 2, or when the speed of an AC machine becomes low, the voltage drop due to leakage inductance cannot be ignored. There are cases where the magnetic flux command value is further weakened due to the relationship "inversely proportional to speed", so calculating the magnetic flux command value from the speed is often more complicated, and furthermore, it is easy to change the data. It is widely used.

[Problem that the invention seeks to solve]

Next, problems when using a memory element will be explained using an example in which the relationship between speed and magnetic flux is simply inversely proportional.

In general, the input of a memory element is called an address, and the output is called data.The resolution of each element can be determined arbitrarily, but the effective resolution is determined by the relationship between input and output.

That is, when data at one address is compared with data at the next address, if there is a change in the data, it is effective to further increase the resolution of the address, but if there is no change, increasing the resolution is meaningless. Therefore, in general, when the address changes by 1 in the vicinity where the amount of change in the output is maximum, the data also changes by approximately the resolution of the data.
Determine the input resolution (the data resolution determined in this way is called the effective resolution). For this reason, the capacitance of the storage element has the disadvantage that, although the effective resolution of the output is constant, it increases in proportion to the speed control range in which flux weakening control is performed.

This point will be explained in detail using specific numerical values.

Now, the base velocity nB shown in Fig. 3 is nB=1500rp.
m, resolution of magnetic flux command value e8 bit (minimum change amount is 1
/2' = 1/256). The magnetic flux command value in the flux weakening control region is given by the above equation 121, so two points in this region are defined as φ1 and 9, respectively. If φ2, then φ
1-φB・(-) φ2-φ8・(-) ・・・・・・・
・・・・・・(31n2, and here, if the difference between φ and φ is the minimum change in magnetic flux, then 1/256・φB−φ, −φ2 ・・・・・・・・・
...+41. By substituting equation (3) into equation (4) and setting n2 = n1 + Δn, the following is obtained. Substituting n, -150Orpm when the change in magnetic flux is maximum, into equation (5), Δn = 5.8
8 rpm. From this, the speed resolution is 5 rp
When the speed control range is 300 rpm, the storage element capacity is required to be 600 bytes (300o15), and up to twice the speed of 6000 rpm, 1200 bytes are required, which is proportional to the speed control range. Also, 300
Δn near 0 rpm is about 23.6 rpm, and unless the address changes by 5, the output does not change, and the capacity of the storage element is wasted. Furthermore, 6000 rp.
In the vicinity of m, the address does not change by about 19, which is 95 in Δn.

Therefore, when using the magnetic flux command value, which is a nonlinear function of speed, by storing the data in a storage element in advance, the capacity of the storage element is determined only by the effective resolution of this data, and the memory The purpose is to make effective use of.

[Means for solving problems]

A counter for counting the period of a pulse signal having a frequency proportional to the motor speed using a clock signal having a period shorter than this period, and a memory for storing a magnetic flux command value having a predetermined flux weakening characteristic are provided.

[Effect]

This invention was made based on the fact that the magnetic flux command value is basically inversely proportional to the speed in the magnetic flux weakening region.
Measure the period of the speed pulse with a frequency proportional to the speed, that is, the reciprocal of the speed, and then manually (address) the entire memory element.
By doing so, the capacity of the storage element can be determined only by the effective resolution of the magnetic flux command value, and the memory can be used effectively.

〔Example〕

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

The difference between this and FIG. 4 is that in FIG. 4, the number of speed pulses within a certain period of time is counted, so the counter 12
While the output of the counter 2 in FIG. 1 is proportional to the average speed, the counter 2 in FIG. It's in being. Therefore, the output of the counter 2 is inversely proportional to the period of the speed pulse, that is, the speed. This can be expressed as a formula as follows.

φ=φB・(T/TB) In this method, the period of the speed pulse becomes longer at low speeds,
Counter 2 overflows, so it is not suitable for normal speed detection, but in order to calculate the magnetic flux command value as shown in Figure 3, -〇 - Speeds below the base speed NB are constant. Since it is sufficient to output it as a value, it can be said that it is suitable for speed detection for calculating a magnetic flux command value.

Here, FIG. 1 will be specifically explained using a numerical example.

Now, the number of speed pulses P is 500 per rotation.
(pulse/rev), resolution of speed and magnetic flux is 8 pits, base speed NB is 1500 (rpm), clock frequency fct from clock generation circuit 1, xTh3
．． When the frequency is 5 MHz, the signal S output by the counter 2 and the launch circuit 3 at the base speed is as follows. Therefore, the capacity of the memory element is required to be 280 bytes or more, but in this case we will set it to 300 for now.In this example, the resolution of the magnetic flux command value φ is 8 pintos (1/256), and as shown in equation (6). Since the period is proportional to the magnetic flux, the period resolution must also be 8 pits or more, but at the above clock frequency it is 1/280, so n'6 is satisfied.

An example of data written to the memory element in the above case is
Shown in Figure 2. Here, the horizontal axis represents the address of the storage element, which represents the period, and the vertical axis represents the data, which corresponds to the magnetic flux command value. The number of rotations corresponding to the period is shown in the figure, but even if the speed control range expands to a high speed range, the usage area of the storage element will only expand to the smaller address, so the capacity of the storage element will not change, and this will increase the magnetic flux. This shows that it is determined only by the resolution of the command value.

〔Effect of the invention〕

According to this invention, the magnetic flux command value calculation circuit does not measure the speed, but measures the period of the speed pulse, which is the reciprocal of the speed, and stores it in the memory element that stores the data of the magnetic flux command value. Since it is used as an address, the capacity of the memory element is weakened and the magnetic flux is controlled regardless of the speed control range.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing an embodiment of this invention, and FIG.
Figure 3 is a graph showing the relationship between speed, induced voltage, and magnetic flux command value when performing magnetic flux weakening control; Figure 4 is a conventional magnetic flux command value calculation circuit. It is a block diagram which shows an example. Code explanation 1...Clock generation circuit, 2.12...Counter,
3.13... Launch circuit, 4.14... ROM (memory element), 5.15... Arithmetic processing circuit, 6... OR gate, 11... Timing circuit,

Claims (1)

[Claims] In a motor drive device that performs at least magnetic flux weakening control using a pulse signal with a frequency proportional to motor speed, the period of the pulse signal is counted by a predetermined clock signal having a higher frequency than the pulse signal. A magnetic flux command value calculation comprising: a counter; and a memory for storing a magnetic flux command value having a predetermined flux weakening characteristic, and the magnetic flux command value is obtained by retrieving the contents of the memory according to the output of the counter. circuit.