JPH03265486A - Inverter control system - Google Patents

Abstract

PURPOSE:To prevent fluctuation of ripple current even upon modification of the connection of winding by varying the PWN carrier frequency of an inverter according to the primary impedance of motor winding at the time of connection of winding. CONSTITUTION:Since signals (b), (c) are applied, respectively, on minus and plus inputs of a comparator 29, output thereof goes High thus turning switches 25A, 27A ON and turning switches 25B, 27B OFF. Under this state, output of an integrator 21 is obtained by integrating the input signal (a) to the integrator 21. When the output increases and exceeds over the comparison voltage of the comparator 29, output thereof goes Low thus turning the switches 25A, 27A OFF and turning the switches 25B, 27B ON. Consequently, the output Vout drops below the comparison voltage -Es of the comparator 29. Since the oscillation frequency is determined by a reference voltage, an input resistance and an integration capacitor, the oscillation frequency can be modified by regulating any one of them.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an inverter drive using a winding switching control method.

[Prior Art] Conventionally, in order to expand the constant output control range of spindle drives of machine tools, the motor windings are sometimes connected to be changed and driven by an inverter (for example, Japanese Patent Laid-Open No. 60-21.999
Publication No. 3).

Specifically, in FIG. 5, 1 is an inverter that controls the motor 7, 7 is the motor, and 71 to 73 are three-phase windings of the motor 7. 91 and 92 are switches for changing the connection of the windings of the electric motor 7.

When the switch 91 is closed and the switch 92 is opened, the motor windings 71 to 73 become star-connected (hereinafter referred to as Y-connection).
When the switch 91 is opened and the switch 92 is closed, the motor windings 71 to 73 become delta connections (hereinafter referred to as Δ connections).

If the impedance of one phase in Δ connection is Z, the impedance of one phase in Y connection is 3Z.

The magnitude of ripple current generated when an induction motor is driven by a voltage-type PWM inverter is determined by the PWM voltage waveform and its impedance. .

[Problems to be Solved by the Invention] However, in the prior art, since the PWM carrier frequency of the PWM voltage waveform was constant, there was a problem in that the magnitude of the ripple current described above varied depending on the winding connection.

In particular, in an inverter with a current minor loop, the current loop gain is limited by the ripple current value, so changing the winding connection is an important issue that affects current response.

An object of the present invention is to prevent the magnitude of ripple current from changing even if the winding connections are changed.

[Means for Solving the Problems] In order to solve the above problems, the present invention provides a polyphase induction motor that can change the winding connection, a switch for changing the connection, and an inverter that has a winding switching control function. In a variable speed drive system, the inverter's PW varies depending on the primary impedance of the motor windings when each winding is connected.
The multiphase induction motor is controlled by changing the M carrier frequency.

[Function] The PWM carrier frequency is changed in accordance with the winding connection so that the ripple current remains constant even if the winding connection is changed, and since the switching loss also changes as the PWM carrier frequency changes, overload is avoided. The load protection level can be adjusted to prevent damage to the power elements of the inverter.

[Example code] The present invention will be described with reference to embodiments shown in the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention, where an inverter l is a converter section 2 using a rectifying element such as a diode.
, a main circuit section consisting of an inverter section 3 using self-extinguishing elements such as transistors, a smoothing capacitor 4, and a control circuit section consisting of a current detector 5 and a control circuit 6. 7 is an induction motor whose winding connection can be changed, 8 is an encoder for speed detection attached to the induction motor 7, and 9 is a fifth
This is a winding switching device for changing winding connections, which includes the switches 91 and 92 explained in the figure. Further, the control circuit 6 includes a speed control section 11. which controls the speed of the induction motor 7. A current command generator 12 for vector control, etc., a constant memory 13 that supplies control constants to the current command generator 12 according to each winding connection, and a voltage command by inputting the current command and the detection signal from the current detector 5. A current control section 14 that outputs
, carrier signal generator for PWM control] 5. PWM control unit 16 that generates a signal to drive the transistor of the inverter unit 3 from the voltage command and the PWM carrier signal;
The overload protection unit 17 performs overload protection operation based on the detection signal from the current detector 5, and the winding selection command generated inside the inverter based on an external command or motor speed.
It is composed of a constant memory 13, a carrier signal generator 15, an overload protection section 17, and a winding selection command generator 18 that provides a winding connection signal to the winding switching device 9.

It is assumed that when the winding selection command is Y connection, the PWM carrier frequency and overload protection level are set as standard.

Next, when the winding selection command is changed to Δ connection, the winding impedance decreases to 1/3, so if the PWM carrier frequency is not changed, the ripple current will triple. However, in order to keep the current control gain constant regardless of the winding connection, it is necessary to keep the ripple current constant.

Since the ripple current is determined by the PWM voltage waveform and the motor winding impedance, in order to achieve constant ripple current control, the carrier frequency only has to be made high. The PWM carrier frequency of the carrier signal generator 15 input to the control unit 16 may be changed.

For example, a triangular wave oscillator is shown in FIG. 2 as a representative example of the carrier frequency generator 15. 21 is an integrator, 22A, 22
B is the integrating capacitor of the integrator 2, 23A123B is a switch for switching the integrating capacitors 22A and 22B based on the signal from the winding selection command generator 18, 24 is the input resistance of the integrator 2, and 26A126B is the input to the integrator 2 A reference voltage source 25A125B is a switch 28A that switches the polarity of a reference voltage source 26A126B input to the integrator 21.
, 28B is a reference voltage source that determines the peak value of the triangular wave signal;
27A and 127B are switches for changing the polarity, and 29 is a comparator that compares the output of the integrator 21 with the reference voltage sources 28A and 28B and outputs a logic signal.

First, switch 23A is turned off and switch 23B is turned on. Now, if the time is tl, as shown in Fig. 3, the signal ■ is added to the single power of the comparator 29, and the signal ■ is added to the -manual power, so the output becomes H level, and the switch is activated by the signal ■. 25A, 27A are on, switches 25B, 2
7B is turned off.

In this state, the output of the integrator 21 is ■. , 1. is integrator 2
Since it is obtained by integrating the input signal 1, the reference voltage source 2
The voltage of 6A is -E, and the voltage of reference voltage source 28A is R.
6. If the voltage of the reference voltage source 28B is -E, then Vout =: -' f:0-2Jnee-dt-
E, -= (1)C22BR24 It is expressed as follows. C22B represents the capacitance of the integrating capacitor 22B, and R24 represents the resistance value of the input resistor 24.

When this output Vljul rises and exceeds the comparison voltage E of the comparator 1 notor 29 at time t2, the comparator output is inverted and becomes the L level, and the signal ■ turns off the switches 25A and 27A, and turns off the switches 25B and 27B.
turns on.

Therefore, the reference voltage E from the reference voltage source 26B is applied to the integrator 21, as shown by the signal ■, and its output voltage V. u+ becomes at time t and smell, and the output ■. , decreases until it falls below 1E, which is the comparison voltage of the comparator 29, at time t4.

The triangular wave oscillator automatically repeats this operation and outputs a triangular wave signal. The oscillation frequency of this oscillator is expressed by formula (1), (
2) As shown, it is determined by the reference voltage E1, R8, input resistance R, and integrating capacitor C22B. therefore,
To change the oscillation frequency, you can operate any one of these.

Generally, a method of changing the capacitor capacity is often used, and in this case, the capacitor capacity was changed using the winding switching signal 18. If integrating capacitor C22B is selected for Y connection, integrating capacitor C22B is selected for Δ connection.
22A is selected. Naturally, the carrier frequency is raised by the Δ connection (so the capacitor capacity is C22
A < C22B.

On the other hand, if the same current is applied during Δ connection as when Y connection is applied,
Since the PWM carrier frequency is high, the switching loss of the transistor in the inverter section 3 increases.

Therefore, in order to prevent thermal breakdown of the transistor, the loss generated during energization must be suppressed to the same level lj in the Δ connection and in the Y connection. The overload protection section 17 switches and uses an overload protection operation level, which is a current/time product shown in FIG. 4, in accordance with a signal from a winding connection command generator 18.

That is, when connecting )', the conventional overload protection level is applied, and when connecting Δ, the overload protection level is lowered than when connecting Y.

As described above, by adding means for changing the PWM carrier frequency and means for changing the overload protection level based on a winding switching command to a conventional winding switching type inverter drive, the winding connection can be improved. Constant current control responsiveness can be obtained regardless of the current control.

Although this embodiment describes a vector control inverter, current responsiveness can be improved by using a similar technique for a V/f controlled winding switching type inverter.

[Effects of the Invention] As described above, according to the present invention, ripple current can be kept constant regardless of the winding connection, current control responsiveness can be kept constant, and winding switching with stable characteristics can be achieved. This has the advantage that it is possible to provide an inverter of this type.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a block diagram showing an embodiment of the present invention.
WM carrier command generation circuit, FIG. 3 is an explanatory diagram showing overload protection characteristics, FIG. 4 is a characteristic diagram showing overload protection operation levels, and FIG. 5 is a configuration diagram of a conventional winding switching type inverter. l... Inverter, 2... Converter section, 3... Inverter section, 5... Current detector, 6... Control circuit,
7...Induction motor, 8...Encoder, 9...Winding switching device Figure ot1 Electric 2 Electric! t4

Claims (1)

[Claims] 1. In a variable speed drive system consisting of a polyphase induction motor whose winding connection can be changed, a switch for changing the connection, and an inverter having a winding switching control function, each winding An inverter control method characterized in that the multiphase induction motor is controlled by changing the PWM carrier frequency of the inverter according to the primary impedance of the motor winding at the time of connection. 2. The inverter control system according to claim 1, which has a function of adjusting the overload protection operation level of the inverter in accordance with changes in the PWM carrier frequency of the inverter.