JPH04248384A - Control method for direct current braking of inverter - Google Patents

Abstract

PURPOSE:To increase the switching frequency by reducing the switching losses of inverter switching elements at the time of DC braking of an induction motor. CONSTITUTION:As for the switching of two sets of switching elements selected from each upper and lower arm of different two phases in the bridge construction of the main circuit of a voltage-type inverter which performs DC braking control of its load, an induction motor, and connected in series in pairs, one of the above-mentioned both elements is brought into a continuous conducting state for a period T2, and the other is turned on and off with a required conducting period and with a pulse period T1 (T1<<T2) needed for DC braking voltage required. And this state is reversed and repeated in turn for the above- mentioned both elements, and the above-mentioned DC braking voltage with a required pulse frequency 1/T1 is obtained.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC braking control method for a load induction motor of a voltage source inverter.

0002

2. Description of the Related Art As a conventional direct current braking control method for this type of inverter, there is known a method as shown in FIG. 3, which shows operating waveforms during control operation. FIG. 2 is a basic circuit diagram of the main circuit of a voltage source inverter that uses an induction motor as a load.

First, in FIG. 2, 1 is a rectifier circuit that rectifies input from an AC power source, C is a smoothing capacitor, and 3 is an induction motor. Reference numeral 2 designates a bridge-configured inverter having switching elements such as power transistors shown in the drawing in which commutating diodes (not shown) are connected in antiparallel to each arm of the upper and lower arms of each phase. The voltage is converted into an alternating current voltage of a predetermined voltage and a predetermined frequency, and power is supplied to the electric motor 3 to drive it at variable speed. When DC braking of the electric motor 3 is performed, for example, switching transistors TR1 and TR2 shown in the drawing are connected to the inverter bridge. The switching elements of the upper and lower arms of two different phases are paired to perform a switching operation, and a pulsed DC voltage whose average value is a predetermined value is supplied to the electric motor 3 to brake it.

Next, FIG. 3 is an operational waveform diagram of each part of the circuit when the conventional DC braking control method is applied to the inverter circuit shown in FIG. and lower arm transistor TR2 are both conductive (O
N) and cutoff (OFF) operation are repeated, and the DC braking voltage V to the motor 3 is determined as a voltage pulse train whose pulse time width is the ON timing overlap period of both transistors TR1 and TR2. The pulse time width is controlled by a control circuit (not shown) so that the voltage V becomes a predetermined value.

[0005]

[Problems to be Solved by the Invention] Generally, in order to reduce the noise of an induction motor that is in a DC braking state by an inverter, or to reduce the ripple of the current flowing through it, the pulse frequency of the DC voltage forming the pulse train supplied by the inverter should be as high as possible. desirable. On the other hand, the switching loss of each switching element of the inverter main circuit that outputs the DC voltage increases as the pulse frequency increases, and the upper limit of the pulse frequency is restricted from the standpoint of the switching loss.

Therefore, in the conventional DC braking control method as described above, if the pulse frequency f=1/T1 of the DC voltage V is increased as shown in FIG. 3, the switching loss in the switching transistors TR1 and TR2 will increase. As a result, it may be difficult to increase the pulse frequency f to its desired value.

In view of the above, it is an object of the present invention to provide a DC braking control method for an inverter that significantly reduces switching loss compared to the conventional method and enables the pulse frequency to be further increased. It is.

[0008]

[Means for Solving the Problems] In order to achieve the above object, the inverter DC braking control method of the present invention changes the output of a voltage source inverter that drives an induction motor at variable speed from a predetermined AC output to a predetermined DC output. The DC braking control method for the inverter brakes the electric motor by braking the electric motor, the upper arm of a certain phase in the main circuit bridge configuration of the inverter operating intermittently in pairs to obtain the predetermined DC output. Regarding the operation of the switching element and the other one-phase lower arm switching element, the period T is the same as the predetermined DC output given as a pulse train having a predetermined time width.
1 and a period T2 which is sufficiently larger than the period T1, and if a continuous conduction state is established for the period T2, the intermittent operation is performed at the period T1 during the subsequent period T2. Repeat at 2T2,
Further, when one of the two elements is in the continuous conducting state, the other element is in the discontinuous state, so that the operating states of the two elements are staggered by the period T2. The period T1 is set to correspond to an operating frequency as high as possible in consideration of noise suppression during operation of the motor and the allowable value of switching loss of the switching element, and the period T2 is set to correspond to an operating frequency that is sufficiently large compared to the period T1. The thermal time constants of both switching elements are set to be sufficiently smaller than the thermal time constants of both switching elements.

[0009]

[Operation] The DC braking voltage V for the induction motor of the inverter, which forms a voltage pulse train having a predetermined period T1 and pulse time width ΔT and is controlled so that the average DC voltage thereof becomes a predetermined value, is determined by the main circuit bridge configuration of the inverter. The TR shown in Figure 2 from the upper and lower arms in different two phases inside
This can be obtained by controlling two sets of switching elements selected and connected in series, such as TR1 and TR2, intermittently at the period T1, with the overlapping period of conduction of both elements set to the time width ΔT.

In the present invention, instead of controlling the two sets of switching elements to be turned on and off together at the period T1 as in the conventional method, and to make the conduction overlapping period of the two elements equal to the time width ΔT, One of the switching elements of the pair is brought into a continuous conduction state during a period T2 (where T2 >> T1), and the other element is brought into and out during a period T1 and a conduction period ΔT, and such a state is alternately maintained for both of the above-mentioned elements. In either method, the DC voltage V can be obtained as a voltage pulse train with a period of T1 and a pulse duration ΔT.

However, if we look at the number of times of switching of both the elements during the period 2T2, if we take the conventional method as N1 and the present invention as N2, then both the periods T1
If T2 = T1, then N2 = N1, but if T2≫T1, then N2≈N1/2. Therefore, the switching loss in both of the above-mentioned elements, which changes proportionally to the number of times of switching, is also reduced by approximately half in the case of the present invention compared to that in the conventional method under the condition T2≫T1. In the present invention, the period T2 is selected to be as large as possible compared to the period T1 within a sufficiently smaller range than the thermal time constants of both elements, and the period T1, which is the pulse period of the DC voltage V, is is the same as that in the conventional method, while significantly reducing the switching loss in both elements.Conversely, if the switching loss is the same, the period T1 is significantly shortened, and therefore 1
It is possible to significantly increase the pulse frequency of the DC voltage V defined by /T1.

0012

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the operational waveform chart during control operation shown in FIG. The main circuit configuration of a voltage source inverter whose load is an induction motor controlled according to FIG. 1 is the same as that shown in FIG. 2 as in the conventional method. As shown in FIG. 1, the transistors TR1 and TR2 perform the same switching operation with a time difference of a period T2, and each has a period of 2
It operates repeatedly in T2, and in period T2, the period T1
If conduction (ON) and cutoff (OFF) operations are performed, continuous conduction (ON) is performed during the following period T2. Therefore, the DC voltage V is between the two elements TR1 and TR.
It is obtained as a voltage pulse train having a voltage pulse width of a conduction state overlap period ΔT with 2 and repeating at a period T1, and the pulse width ΔT is controlled so that the average voltage of the voltage pulse train becomes a predetermined value.

Next, both the elements T during the period 2T2
If we look at the number of switching operations of R1 and TR2 as a pair of conduction (ON) and cutoff (OFF) operations, in the case of the example shown in FIG.
In the case of following the conventional method shown in , the number of times is 6. Therefore, the switching loss of the two elements according to the present invention is reduced to approximately 4/6 of that of the conventional method, and conversely, if the loss during the conventional method is allowed, the pulse frequency of the DC voltage V (1/T1) is allowed to increase by about 6/4 times that of the conventional method.

[0014]

According to the present invention, when a voltage-source inverter performs DC braking control of its load induction motor, the voltage-source inverter selects from each of the upper and lower arms of different two phases in the main circuit bridge configuration of the inverter and connects them in series in pairs. Two sets of connected switching elements are connected, and one of the two elements is connected for a period T2.
is in a continuous conduction state, and the other is brought into and out with a required conduction period at the pulse period T1 (however, T1 <<T2) of the required DC braking voltage, and such a state is alternately inverted and repeated for both of the elements. By doing so, it becomes possible to significantly reduce the switching loss of the two elements when obtaining the DC braking voltage having the required pulse frequency 1/T1, and therefore, for the same allowable loss, the DC braking voltage This makes it possible to significantly increase the pulse frequency.

[Brief explanation of the drawing]

[Fig. 1] Operation waveform diagram of each part of the inverter main circuit during DC braking control, showing an embodiment of the present invention.

[Figure 2] Basic circuit diagram of the main circuit of a voltage source inverter with an induction motor as a load

[Figure 3] Operation waveform diagram of each part of the inverter main circuit during DC braking control showing an example of the conventional method

[Explanation of symbols]

1 Rectifier circuit 2 Inverter 3 Induction motor C Smoothing capacitor

Claims (2)

[Claims] 1. A DC braking control method for an inverter, which brakes the motor by changing the output of a voltage type inverter that drives an induction motor at variable speed from a predetermined AC output to a predetermined DC output, the method comprising: The operation of the upper arm switching element of one phase and the lower arm switching element of another phase in the main circuit bridge configuration of the inverter, which operate intermittently in pairs to obtain an output, has a predetermined time width. A period T1 that is the same as the predetermined DC output given as a pulse train and a period T2 that is sufficiently larger than the period T1 are defined, and if a continuous conduction state is established for the period T2, the period T2 will be the same as that of the predetermined DC output given as a pulse train.
1, and such operation is performed intermittently at a cycle of 2T.
2, and when one of the two elements is in the continuous conduction state, the other element is in the discontinuous state, so that the mutual operating states of the two elements are set with a time difference of the period T2. A direct current braking control method for an inverter, characterized in that: 2. In the DC braking control method for an inverter according to claim 1, the period T1 corresponds to an operating frequency as high as possible in consideration of noise suppression during operation of the electric motor and an allowable value of switching loss of the switching element. and the period T2 is set as the period T1.
A direct current braking control method for an inverter, characterized in that the DC braking control method is set to be sufficiently larger than the thermal time constants of the switching elements and sufficiently smaller than the thermal time constants of both the switching elements.