JPH0382397A - Speed controller for induction motor - Google Patents

Abstract

PURPOSE:To balance a plurality of input voltages and to prevent an overvoltage due to unbalance of the input voltages by connecting a plurality of power converters in series with a DC power source, and controlling it based on the differential voltage of the input voltages. CONSTITUTION:A positive terminal of a DC power source 1 is connected in series with a first 3-phase inverter 6 and a second 3-phase inverter 7. 3-phase induction motors 8A, 8B are connected to the inverters 6, 7. The difference of the input voltages of the inverters 6, 7 is detected by a differential voltage detector 19, and input to a frequency controller composed of an absolute value calculator 20 and a subtracter 22. A frequency controller controls the frequencies of the AC powers of the inverters 6, 7. Thus, generation of an overvoltage is prevented to prevent the damage of the inverter.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a speed control device for an induction motor, and more specifically, the present invention relates to a speed control device for an induction motor. This paper proposes a speed control device for an induction motor that drives each motor separately.

[Conventional technology]

FIG. 2 is a circuit diagram of a variable speed drive device disclosed in, for example, Japanese Unexamined Patent Publication No. 63-23589. For example 750V
The positive terminal of the DC power source 1 of the overhead contact line is connected to the positive input terminal of the first three-phase inverter 6 via a series circuit of a power switch 2 and a filter reactor 3. The negative terminal is the negative input terminal of the second three-phase inverter 7 -
is connected to.

Respective positive and negative input terminals + of the three-phase inverter 6.7.

- filter capacitors of equal capacitance 4.
5 are connected separately, and their 3-phase inverter 6
and 7 are connected in series.

The output terminals U, V, W of the three-phase inverter 6 are connected to the first primary winding 81 of the three-phase induction motor 8 and the three-phase inverter 7.
The output terminals U□V, , W, are connected to the second primary winding 82 of the three-phase induction motor 8.

The first of these. The second primary windings 81, 82 are configured to generate a rotating field with a predetermined phase difference.

In addition, the filter reactor 3 and the filter capacitor 4
and 5 constitute an inverse type smoothing filter circuit.

Next, the operation of this variable drive device will be explained. Power switch 2
When the filter capacitor 4.5 is turned on, the filter capacitor 4.5 is charged by the DC voltage of the DC power source 1 at a voltage division ratio of 1:1.

The three-phase inverters 6 and 7 convert the divided DC voltage into a three-phase AC voltage and supply the first inverter to the three-phase induction motor 8. second 1
It is supplied to the next winding 81,82.

Here, the current flowing through the filter reactor 3 is ■.

The terminal voltage of the filter capacitor 4 is EC1+, the terminal voltage of the filter capacitor 5 is Ecz, the DC current flowing into the 3-phase inverter 6 is the DC current flowing into the 3-phase inverter 7! The output of the two- and three-phase inverter 6 is assumed to be Pl, and the output of the three-phase inverter 7 is assumed to be P2. Now, the characteristics of two three-phase inverters 6.7.

Or the first one of the three-phase induction motor 8. Second primary winding 81.8
The impedance of 2 is unbalanced and PI>P
In the case of , the following formula holds true in the initial state if the internal loss of the three-phase inverter 6.7 is ignored.

P+ =Ee+X I+ >Pt =EczX I
t ”(1)P+ +Pg = (Ec++Ecz
) X I...(2) Also, in the initial state, EcI-Ecz -(3)
Then, according to equations (1) and (2)! t>L...(4)
becomes.

Therefore, the terminal voltage Eel of the filter capacitor 4 decreases, and the terminal voltage ECt of the filter capacitor 5 increases.

Therefore, the terminal voltage Eel continues to decrease until I+=L=I, and the terminal voltage EC! continues to increase and PI-Ec
lX I > ECtX I ””Pi ”'(5
) and will be balanced.

Due to this phenomenon, an imbalance occurs in the DC voltage that is the input voltage of the three-phase inverters 6 and 7, and the outputs of the three-phase inverters 6 and 7 become unbalanced.

[Problem to be solved by the invention]

In conventional variable speed drive devices, if there is an imbalance in the characteristics of the components such as the control rectifying elements of each of the two three-phase inverters that drive the three-phase induction motor, or if the primary winding of the three-phase induction motor If there is an unbalance in impedance, there is a problem that the input terminals of the two three-phase inverters will be unbalanced, and the three-phase inverter may be damaged by the overvoltage caused by the unbalance.

In view of such problems, the present invention aims to balance the input terminals of a plurality of power converters that individually supply AC power to induction motors, and thereby prevents damage to the power converters due to overvoltage caused by unbalanced input voltages. An object of the present invention is to provide a speed control device for an electric motor.

[Means to solve the problem]

The speed control device for an induction motor according to the present invention includes a plurality of power converters that are connected in series to a single DC power source and separately supply converted AC power to the plurality of induction motors;
It includes a differential voltage detection circuit that detects a difference between the input terminals of each power converter, and a frequency control circuit that controls the frequency of AC power of the power converter based on the differential voltage detected by the differential voltage detection circuit. .

[Effect]

The plurality of power converters separately supply AC power to the plurality of induction motors. The differential voltage detection circuit detects the voltage difference between the input voltages of the respective power converters.

The frequency control circuit controls the frequency of the AC power of the power converter in relation to the differential voltage detected by the differential voltage detection circuit.

As a result, when the input voltages of each power converter become unbalanced, the frequency of the AC power of the power converter changes. And the input voltages are balanced.

〔Example〕

The present invention will be described in detail below with reference to drawings showing embodiments thereof.

FIG. 1 is a circuit diagram of a speed control device for an induction motor according to the present invention. For example, the positive terminal of the direct current power source l of the overhead contact line is connected to the positive input terminal of the first three-phase inverter 6 via a series circuit of the power switch 2 and the filter 3, and the negative terminal is connected to the second three-phase inverter 7. It is directly connected to the negative input terminal of -. The negative input terminal of the three-phase inverter 6 and the positive input terminal of the three-phase inverter 7 are connected, and the three-phase inverters 6 and 7, which are power converters, are connected in series. Positive and negative input terminals + of 3-phase inverter 6 (7).

- between the resistor 9 (11) and the voltage sensor 10 (12)
A series circuit with a filter capacitor 4 (5) is connected to the resistor 13 (15) and a chopper 14 (
16) is connected in series circuit with resistor 13 (15).
The flywheel diodes FDI are connected in parallel to the AC output terminals U1 (U2), V+ (Vz), and Wl of the three-phase inverter 6 (7).
2nd) 3-phase induction motor 8A (8B) primary winding 8A
w (8Bw). 3 phase induction motor 8
Frequency f+u(f□) related to the rotation speed of A (8B)
Pulse generator PG+ (PG
The output voltage of M) is input to the adder 17 (1B).

The voltage sensors 10 (12) are connected to three-phase inverters 6 (
7), and the output voltage of the voltage sensor 10 (12) is input to the positive terminal + (negative terminal -) of the differential voltage detection circuit 19.
Reference numeral 9 detects the difference between the input voltages of the three-phase inverters 6 and 7. The differential voltage ΔE, which is the output of the differential voltage detection circuit 19, is input to the absolute value calculation circuit 20. Differential voltage ΔE
If is positive, the coefficient K.

The output voltage multiplied by is input to the negative terminal of the subtracter 22. On the other hand, when the differential voltage ΔE is negative, the output voltage obtained by multiplying the coefficient - by 1 is input to the negative terminal - of the subtracter 23 . The output of the subtracter 22 (23) is then input to the three-phase inverter 7 (6) as an inverter frequency control signal. The absolute value calculation circuit 20 and the subtracters 22 and 23 constitute a frequency control circuit.

The output of the torque pattern setter 24 that gives the absolute amount of torque of the three-phase induction motors 8A and 8B is input to the torque-slip frequency setter 25, and the torque-slip frequency setter 25 sets the slip frequency according to the torque pattern. Determine f. The output voltage associated with the slip frequency f is applied to the adders 17 and 18. Adder 17 with inverter frequency f INVI + f INVI.
The 18 output voltages are individually applied to the respective positive terminals of the subtracters 22, 23.

Next, the operation of the speed control device for an induction motor configured as described above will be explained. When the power switch 2 is turned on, the filter capacitor 4.5 is 1:l due to the DC voltage of the DC power supply 1.
It is charged at a partial pressure ratio of . The DC voltages of the charged filter capacitors 4.5 are separately supplied to three-phase inverters 6.7, which convert the DC voltages into three-phase AC voltages. The respective three-phase AC voltages are then supplied to the primary windings 8Aw and 88w of the three-phase induction motors 8A and 8B.

And each terminal voltage EC of filter capacitors 4 and 5
, EC, the output voltage of the voltage sensor 10.12 is input to the differential voltage detection circuit 19, and the terminal voltage E Cl
The differential voltage ΔE of rEcz will be detected, but the impedances of the primary windings 8A, 8B of the 3-phase inverter 6.7 and the 3-phase induction motors 8A, 8B are balanced, so the differential voltage ΔE cannot be detected. If not, the output voltage of the absolute value calculation circuit 20 becomes 0, and the subtracter 2
The input to the negative input terminal - of 2.23 becomes 0.

On the other hand, the torque pattern setter 24 supplies an output voltage corresponding to a preset torque pattern to the torque frequency setter 18, and applies the output voltage to the adder 1.
7.Enter each in 18. Then adder 17.18
adds the output voltage related to the slip frequency f to the output voltage related to the rotational speed Fl+rxz of the three-phase induction motor, and inputs the added output voltage to the subtracters 22 and 23. The output voltage of each of the subtracters 22 and 23 is determined by the three-phase inverter 6.
．． 7 is input to the three-phase inverter 6.7 as a signal to control the inverter frequency. In other words, a rotating field slightly faster than the rotation speed of the three-phase induction motors 8^, 8B is applied, and the three-phase induction motors 8A, 8B are rotated by the torque pattern setter 24.
It rotates to output the torque set in . In this way, the characteristics of the three-phase inverter 6.7 and the three-phase induction motor 8
If the impedances of the primary windings 8Aw and 88w of A and 8B are balanced, the output voltage of the subtracter 22.23 is balanced and the inverter frequency of the 3-phase inverter 6.7 is controlled equally, The input voltages of the phase inverters 6 and 7 are balanced.

Now, for example, the primary winding 8As of a three-phase induction motor 8^, 8B
If the impedance of m8B, 1 is unbalanced, that is, the primary winding 8AM of the three-phase induction motor 8A, 8B
, 88W input impedance, respectively. ．． 2. Assume that there is a relationship Zl>Zf.

At this time, when the three-phase inverter 6.7 is started, the input current 1,, I, of the three-phase inverter 6.7 becomes I r =
K X E cI/Z t ・”(6)1 *
-K X Eeg/Zt (7), where K is a constant.

Therefore, if Zt>Zt, in the initial state,
If EcI-EcI, then 1 from equations (6) and (7)
, <I! ...(8), the terminal voltage Eel of the filter capacitor 4 increases, the terminal voltage Ect of the filter capacitor 5 decreases, and EcI>Ecg
becomes.

The outputs of the voltage sensors 10.12 that have detected these terminal voltages ECM and Ecll are input to the differential voltage detection circuit 19, and the differential voltage detection circuit 19 detects both terminal voltages Ec1. Outputs the differential voltage of EB +ΔE.

This difference voltage ΔE is multiplied by a coefficient by an absolute value calculation circuit 20 and input to a subtracter 22 . The subtractor 22 thereby subtracts the difference voltage 1ΔE from the output voltage associated with the inverter frequency fINvl. In other words, the subtracter 22 includes the output voltage related to the slip frequency f, the pulse generator PG,
f is the sum of the rotational speed f□ detected by the output voltage and the related output voltage.
Since the output voltage related to the inverter frequency fINVI consisting of s+fMl is manually input, the output voltage of the subtracter 22 is 1ΔE for r+, lv+-, and the low output voltage related to the frequency lower than the inverter frequency fIMVI is Human power is applied to the three-phase inverter 7 to increase the inverter frequency according to ΔE. As a result, the terminal voltage EC2 of the filter capacitor 5 increases, approaches the terminal voltage Ec1, and becomes balanced.

On the other hand, on the other hand, the terminal voltage EC of the filter capacitor 5! increases and the terminal voltage ECI of the filter capacitor 4 decreases so that EcI<Ect, the differential voltage output by the differential voltage detection circuit 19 becomes -ΔE. This differential voltage -ΔE is multiplied by -5 by the absolute value detection circuit 20 and input to the subtracter 23. As a result, the output voltage of the subtracter 23 becomes f+Hvz Kl ΔE through the same operation as the subtracter 22 described above, and the low output voltage associated with a frequency lower than the inverter frequency f IMVI is input to the three-phase inverter 6, and its The inverter frequency will be increased according to ΔE. As a result, the terminal voltage Eel of the filter capacitor 4 increases, approaches the terminal voltage Ect, and becomes balanced. In this way, the input voltages of the three-phase inverter 6.7 are always controlled to be balanced.

Therefore, even if the characteristics of the three-phase inverter 6.7 or the impedance characteristics of the primary windings 8A11 and 88w of the three-phase induction motors 8A and 8B are different, the input voltage of the three-phase inverter 6.7 will be unbalanced. No overvoltage is generated, and damage to the three-phase inverter 6.7 due to the overvoltage is prevented.

In this embodiment, a speed control device for a three-phase induction motor, which is the main motor of an electric car, has been described.
If the configuration is such that each three-phase inverter drives the three-phase induction motor separately, the same effect can be obtained not only in electric vehicles.

Also, in this example, the speed of the three-phase induction motor was controlled;
It is not limited to phase induction motors.

〔Effect of the invention〕

As detailed above, according to the present invention, when a plurality of three-phase inverters connected in series and connected to a single DC power source are used to drive three-phase induction motors corresponding to each one. In addition, the input voltages of each three-phase inverter can always be balanced. Therefore, overvoltage due to unbalanced input voltage does not occur, and 3.
It is possible to provide a speed control device for a three-phase induction motor that can prevent damage to a phase inverter.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of a speed control/shelf device for an induction motor according to the present invention, and FIG. 2 is a circuit diagram of a conventional variable drive device. 1...DC power supply 2...Power switch 4.
5... Filter capacitor 6.7... 3-phase inverter 8^, 8B... 3-phase induction motor 11.12.
...Voltage sensor 19...Differential voltage detection circuit 20
...Absolute value calculation circuit 22.23...Subtractor 24.
...Torque pattern setter 25...Torque-slip frequency setter PCI, PG2...Pulse generator Note that in the drawings, the same reference numerals indicate the same or equivalent parts.

Claims (1)

[Claims] (1) Induction motor speed control that uses a power converter that converts DC power to AC power, supplies the AC power to the induction motor, and controls the rotation speed of the induction motor by changing the frequency of the AC power. In the device, a plurality of power converters are connected in series to a single DC power supply and each supply AC power to a plurality of induction motors separately, and a voltage difference between the input voltages of each of the power converters is detected. What is claimed is: 1. A speed control device for an induction motor, comprising: a differential voltage detection circuit; and a frequency control circuit that controls the frequency of the alternating current power in relation to the output of the differential voltage detection circuit.