JPH05954B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field of the Invention The present invention relates to a group starting control device for an AC motor;
A small-capacity variable voltage/variable frequency converter is used to monitor the load current of each motor, and software starts up many small AC motors installed in process lines, etc. to prevent large starting currents from flowing to the motors. , reduce the heat loss during AC motor startup, reduce the rotational impact force on the machine, increase the allowable number of AC motor startups, and divide a large number of AC motors into groups to make variable voltage variable for one or a small number of AC motors. The frequency converter is intended to activate each group sequentially.

Many small AC motors of several kilowatts to several tens of kilowatts are used in process lines and the like as power sources for hydraulic pressure, blowers, and the like. These devices operate continuously while the process line is in operation, and the motor capacity is often determined by the load during rated operation.
Further, to start these motors, a full voltage starting method is adopted in which the voltage and frequency of the rated operating voltage of the motor is applied, that is, the motor is directly connected to a commercial power source. Figure 1 shows the circuit diagram of the full voltage starting method, and Figure 2 shows the torque and current characteristics.

When starting the AC motor 3, use the motor control panel 2.
The contactor 4 housed in the motor is closed, and the voltage V and frequency f supplied by the power source 1 are applied to the motor 3 to start it. The starting current Is at this time is 5 to 6 times the rated current I of the motor 3. Ts is the starting torque generated by the electric motor (cage type induction motor) 3, S is the slip, rpm
is the rotational speed, and W indicates the operating range.

Conventional technology and problems Small AC motors in process lines are normally started around the time the line starts operating, and remain in operation until the line ends, but in reality they are idling without being loaded. There is also quite a period of time.
Even in no-load operation, a predetermined amount of power is consumed, which is not preferable from the viewpoint of energy saving. To avoid this, it is best to start it only when necessary and stop it when it is not needed, but in this case, of course, you will have to start and stop it frequently. However, as mentioned above, the starting current is very large compared to the operating current, so increasing the starting frequency increases heat loss during starting, exceeding the heat tolerance, leading to burnout of the motor 3, and damage to connected equipment. There is even a risk of damage due to unnecessary mechanical force being applied to the product. Moreover, when a plurality of electric motors 3 are started at the same time, a large current flows, and the voltage V of the power source 1 drops, which adversely affects other devices connected to the same power source. Because of the above-mentioned problems, these AC motors 3 have been operated continuously, especially when the work is stopped for a short time.

OBJECTS OF THE INVENTION The present invention attempts to save energy by frequently starting and stopping an AC motor, and also to avoid adversely affecting the motor, power supply, and the like.

Configuration of the Invention The AC motor group starting control device of the present invention includes a variable voltage/variable frequency conversion device, a current detection device,
The conversion device has a control panel, and is capable of repeatedly outputting gradually increasing voltage and frequency, and reduces the rate of increase of the voltage and frequency based on the output of the current detection device, and starts a plurality of motor groups one by one. The current detection device detects the maximum overcurrent of each motor in the motor group and outputs it, and the control panel has a power supply and a power supply for each motor group. and between the converter and each motor group, the contactors are opened and closed to connect one motor group to the converter, and to adjust the output voltage of the converter. and when the frequency increases to a value corresponding to that of the power source, the electric motor group is switched and connected from the converter to the power source, which will be described in detail below with reference to embodiments. explain.

Embodiment of the invention The starting current Is and starting torque Ts of the AC motor 3 are as follows:
This can be roughly expressed by equations (1) and (2).

Is＝K ₁・Vs ……(1) Ts＝K ₂・Vs ² /f _s ……(2) Here, Vs is the applied voltage at startup, f _s is the frequency at startup, K ₁ , K ₂ is a constant determined from the resistance and leakage reactance of the AC motor 3. As is clear from equations (1) and (2), starting current Is and starting torque Ts can be controlled by voltage Vs and frequency f _s , and Vs and f _s can be controlled by variable voltage/
It can be easily controlled with a variable frequency converter.

FIG. 3 shows a circuit configuration of a startup control method when starting using the variable voltage/variable frequency converter 5. This circuit includes a power supply 1 to the AC motor 3, a motor control panel 2' that houses a contactor for selecting the power supply to the AC motor 3, and a voltage Vs to be supplied at startup,
A variable voltage/variable frequency converter 5 that outputs a frequency f _s , a current detector 6 that detects a starting current Is, and an overcurrent detector 7 that has an overcurrent setting device and detects an overcurrent Is' that is equal to or higher than a set value _I1 . It consists of The conversion device 5 includes a thyristor and outputs a voltage V and a frequency f that gradually increase over time, and is a known device. The time rate of change of voltage and frequency can be increased or decreased depending on the input signal. The motor control panel 2' is provided with contactors 4a and 4b that reverse opening and closing, and at startup, the contactor 4a is closed and the contactor 4b is open.

Therefore, at startup, the input voltage and frequency to the AC motor 3 become the output voltage and frequency of the variable voltage/variable frequency converter 5 supplied with power from the power source 1.
These voltage Vs and frequency f gradually rise from zero to slowly start the AC motor 3. In this circuit, the starting current Is is detected by the current detector 6 at the time of startup, and if the current signal exceeds the value _I1 set by the overcurrent detector, the slope of the rise in the voltage Vs and frequency _fs is detected. The starting current Is is controlled to be equal to or less than a constant value _I1 . FIG. 4 shows the relationship between voltage Vs, frequency _fs , and overcurrent Is in FIG. 3. In this example, the starting current Is reaches the set value _I1 at time t1, so the overcurrent detector 7 converts the current Is' = K (Is - _I1 ), where K is a proportionality constant, as a control signal. As a result, the conversion device 5 changes the time rate of change α1 of the voltage V and the frequency f to α2. The rate of change α1 is set in advance in the conversion device 5, and the rate of change α2 is the rate of change α1 changed by the signal Is′ from the detector 7. The amount of change is determined by the value of Is', and when this V, f control starts, the starting current Is is limited to a constant value.

When the voltage Vs and the frequency _fs reach the same level as the voltage V and the frequency f of the commercial power source 1 (time t2), the converter 5 generates a signal to activate the contactor housed in the motor control panel 2'. 4a is opened and 4b is closed, the power supply is switched from the A side to the B side route, and the normal operating state is entered.

A variable voltage/variable frequency converter 5 may be provided for each motor 3, or a variable voltage/variable frequency converter 5 may be provided for each motor 3, or a variable voltage/variable frequency converter 5 may be provided for each motor 3.
Providing the variable frequency converter 5 would result in a very large equipment cost. Therefore, in the present invention, a plurality of AC motors (all motors in a process line) 3 are divided into groups, and a variable voltage/variable frequency converter 5 having a capacity commensurate with the total capacity of the AC motors 3 in each group.
One or a small number of AC motors are provided, and AC motor groups (M, N, . . . groups) are sequentially started for each group.
FIG. 5 shows an embodiment of the present invention.

In Fig. 5, 3 is the aforementioned small AC motor, all the motors are divided into groups G1, G2, etc., each is provided with a motor control panel 21, 22, etc., and the commercial Power is supplied from the power source 1 or the variable voltage/variable frequency converter 5. l1 is a common bus line supplied from power supply 1, l2
is a starting bus line to which power is supplied from the converter 5, which is shared by all the electric motors and has a capacity that can start any group of electric motors at once. Each electric motor is provided with a current detector 6 that detects its starting current Is,
Further, each group is provided with overcurrent detectors 71, 72, . . . and maximum overcurrent detectors 81, 82, . The overcurrent detectors 71, 72, . . . have detection parts similar to the overcurrent detector 7 in FIG. Starting current Isi (i=1, 2, 3,...
...n. (same below) and compare it with the set value I _1i ,
Output I′si=Ki (Isi−I _1i ). Detector 81, 8
2, ... is a maximum value detection circuit consisting of a diode, etc., and the output of each detection section of detectors 71, 72, ...
Take out the largest one of Isi (i=1, 2,...n), and
c, 22c to the converting device 5. The control panels 21, 22, . . . input the output signals of the contactor 21b that connects the motor group to the operating bus l1, the contactor 21a that connects the starting bus 12, and the detectors 81, 82, . Contacts 21c, 22
c..., which are opened and closed by a sequencer and a start-up completion detector provided in the conversion device 5. Initially, 21a and 21c of the first group G1 are closed, and the first group G1 is closed.
When the first activation ends, 21b closes, the others open, and then the second group of 22a and 22c close, and so on, and so on, and so on.

FIG. 6 shows details of the main parts of FIG. 5. The variable voltage/variable frequency converter 5 includes a rectifying section 51 that includes a rectifying thyristor and a smoothing capacitor and outputs a variable DC voltage, a frequency converting section 52 that includes a thyristor and converts the DC to alternating current with a variable frequency, and these components. A voltage control unit 53 and a frequency control unit 54 that perform gate control of the thyristor, a command unit 55 that determines the rising slope angle of the voltage and frequency, and a starting unit that detects whether the starting voltage and frequency have reached the voltage and frequency of the commercial power supply. It consists of a completion detection section 56 and the like.

When the voltage and frequency of the starting bus _l2 become equal to those of the operating bus (the voltage and frequency match and the frequencies are synchronized), the detection unit 56 energizes the relay Ry, opens and closes the contactor on the control panel, and , and a little later closes contact Rya and opens contact Ryb. These relay contacts are connected to the maximum output voltage of the command section 55.
It is inserted into the output circuit of the frequency setter VR1, and at startup, the contact Ryb is closed and the contact Rya is open, so the output of the setter VR1 is input to the adder/subtractor A1. The next stage A2 is also an adder/subtracter, but if we ignore this for now, the output of the adder/subtracter A1 is applied to the servo motor SM, which is controlled by the control units 53 and 54.
Operate variable resistor VR2 that outputs the command value Vs to. The command value is negatively fed back to the adder/subtractor A1, and therefore the command value Vs is the value set by the setting device VR1 (this value corresponds to the voltage and frequency of the commercial power supply).
up to, at a rate of increase that follows the rotational speed of the servo motor SM. Although not shown, a saturation element such as an amplifier is inserted into the servo motor drive circuit, and its saturation value is made changeable so that the above rate of increase can be set arbitrarily.

The overcurrent detector 71 includes a converter (usually a resistor) 71a that converts the output current of the current detector 6 into voltage;
Setter 71b that outputs set value I ₁ j, addition/subtraction point 7
1c and a buffer amplifier 71d (usually composed of operational amplifiers). Although the overcurrent detector 71 is provided for the current detector 6 of each electric motor 3, the configuration is the same, and only the set value I ₁ j is different. The maximum overcurrent detector 81 includes a diode group 81a and a buffer amplifier 81b. The above-mentioned sequencer SQ may be provided in the conversion device 5 or in an appropriate location such as the control panel 21. FIG. 7 is a time chart for explaining the operation.

The operation will be described with reference to FIGS. 5 to 7. At startup, the group G1 is first activated by the variable voltage/variable frequency converter 5. At this time, the sequencer SQ receives the startup command S, turns on the power of each part, and closes the contactors 21a and 21c. In the command unit 55, the contact Ryb is initially closed and the feedback voltage from the variable resistor VR2 is 0, so a constant voltage determined by the saturation element is applied to the servo motor SM, and the servo motor SM rotates at a speed according to this voltage and receives the command. Value Vs
gradually increases from 0. As a result, the output voltage and frequency of the variable voltage/variable frequency converter 5 start to gradually increase, and the motor group G1 starts to start. The starting current Is of each AC motor 3 in the motor group G1 is detected by the current detector 6 of each motor, and the starting current Is of each AC motor 3 in the motor group G1 is detected, and the value set individually by each detection part of the overcurrent detector 7 is detected in the same way as explained in FIG. When an overcurrent I'si greater than _I1i flows, the maximum current detector 81 selects the maximum overcurrent I'si of each motor, and sends the signal to the adder/subtractor A2 of the command unit 55 of the converter 5. The starting current Isi is controlled to be equal to or less than the overcurrent setting value _I1i by changing the slope of the rise of the input voltage Vs and the frequency _fs at the time of starting. When the output voltage Vs and frequency _fs of the converter 5 become equal to those of the power supply 1, the relay Ry of the detection unit 56 operates,
Contactors 21a and 21c are opened and contactor 21b is closed. As a result, the motor group G1 is switched from the starting bus line l2 to the operating bus line l1, and enters the operating state.
Subsequently, after a delay of time T, contact Ryb is opened and Rya is closed, and the servo motor SM of the command unit 55 is reversed to return the sliding arm of the variable resistor VR2. When this return operation is completed, the sequencer SQ closes the contactors 22a and 22c of the next motor group G2 and converts the converter 5.
The motor group G2 is started with the output voltage and frequency gradually increasing. The same goes for the rest, and each motor group is slowly started one after another and brought into operation while preventing overcurrent from flowing.

Note that the overcurrent setting I _1i in the overcurrent detector 7 when performing group start-up control is set as a ratio to the rated current of each AC motor 3, since each motor in the group has a different capacity.

Furthermore, if the variable voltage/variable frequency converter 5 is used to stop these AC motors 3 by regenerative control, it is possible to perform shock-free (acceleration/deceleration) control in the same way as when starting them, and also to reduce rotational energy. can be recovered as electrical energy, increasing the energy saving effect.

As explained above in detail, according to the start control method of the present invention, it is possible to slowly start a group of motors using a small-capacity variable voltage/variable frequency device, and when unnecessary The operation can be stopped immediately and a high energy saving effect can be obtained.

[Brief explanation of drawings]

Figure 1 is a diagram explaining the conventional starting control method of a small AC motor, Figure 2 is a diagram showing the torque current-speed characteristics of a general AC motor, and Figure 3 is a diagram explaining the start-up control method of a conventional small AC motor. Figure 4 is a diagram explaining the starting method that controls the starting current. Figure 4 is a graph explaining the behavior of the AC motor current, starting voltage, and frequency when started using a variable voltage/variable frequency device. Figure 5 6 is a block diagram showing an embodiment of the present invention, and FIG. 7 is a time chart for explaining the operation. 1...Power supply, 2...Motor control panel when starting at full voltage, 2'...Motor control panel when using a variable voltage/variable frequency converter, 3...AC motor, 4
...Contactor at full voltage startup, 4a...Variable voltage,
Contactor at startup in variable frequency converter, 4b...
... A contactor that switches to the voltage and frequency on the power supply side when using a variable voltage/variable frequency converter, 5... Variable voltage/variable frequency converter, 6... Current detector, 7... Overcurrent detector, 8 ... Maximum current detector, G1, G2 ... Motor group.

Claims (1)

[Claims] 1. A variable voltage/variable frequency conversion device 5, current detection devices 6, 7, 8, and control panels 21, 22, . . . , the conversion device gradually increasing voltage and frequency. It has a capacity that can repeatedly output, reduce the increase rate of the voltage and frequency by the output of the current detection device, and start a plurality of motor groups G1, G2, . . . one group at a time, and the current detection device The device detects the maximum overcurrent of each motor in the motor group and outputs it, and the control panel is connected between the power supply and each motor group, and between the converter and each motor group. contactors 21a, 21b, 22a, 22b, . . . connected between the A group starting control device for AC motors, characterized in that when the frequency increases to a value corresponding to that of the power source, the motor group is switched and connected from the conversion device to the power source.