JPS6120234B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an operating system for an AC motor, and particularly relates to an operating system when an AC motor being driven by a commercial frequency power source is switched to being driven by a variable frequency power source.

AC motors have traditionally been used in many applications at a constant speed, and induction motors and synchronous motors have been used as motors for fans, blowers, etc. Recently, with the widespread use of stationary variable frequency power supplies, there is a tendency to conserve resources and energy by operating the motor at low speed when the rated output is not required for the above-mentioned applications. is becoming more prominent. Since this invention is particularly effective for induction motors among AC motors, the following description will be made for induction motors.
FIG. 1 shows an example of a circuit widely used for the above-mentioned applications, in which an inverter device is used as a variable frequency power source. In Fig. 1, the induction motor 13 is connected to the AC power supply 11 through the switch 121.
(hereinafter abbreviated as commercial operation) from the AC power supply 11 to the transformer 15 - rectifier 1
6 - DC reactor 17 - Inverter 18 - Transformer - 19 - Induction motor 1 through switch 122
There is an operation using a second route (hereinafter abbreviated as inverter operation) for driving the 3. The transformer 15 and the transformer 19 adjust the voltage so that an effective output can be obtained for the semiconductor elements used in the rectifier 16 and the inverter 18, and are not particularly important elements for this invention. . A current transformer 21 for detecting the output current 1 is provided between the inverter 18 and the transformer 19, and feeds back a current signal to the inverter control circuit 22. Figure 2 is the first
It is a figure for explaining the driving method of a figure. In FIG. 2, the induction motor 13 is started at time t ₁ and accelerated according to the second path in FIG. 1 until the speed reaches n ₁ . When the switch 122 is switched to the switch 121 at time t ₂ to enter commercial operation, the speed drops once during the switch, but after the switch is completed, it starts accelerating again, and at time t ₃ the speed returns to around the synchronous speed.
Enter constant speed operation at n ₂ . When the deceleration command is entered at time _t4 , the switch 121 opens and the switch 122 closes to form the second path, but if the inverter operation is not started, the deceleration will continue in a free run state.

In the free run state, decelerate until time t ₅ , and at time t ₅
If the inverter operation starts at the frequency corresponding to the set speed n ₁ when the
Thereafter, the deceleration can be continued and the stop can be completed at time _t6 . This time t ₅ can be detected as the time corresponding to the set speed n ₁ if the induction motor 13 is equipped with a means for detecting the speed of a rotary generator, a pulse oscillator, etc.; When not equipped, it is common to use a timer or other time factor to set the speed when the load is the heaviest, that is, assuming the shortest time from the start of deceleration to the set speed _n1 . However, in this case, when there are various fluctuations in the load and the load becomes the lightest compared to the state described above, when the inverter operation is started at time _t5 , the induction will be as shown in the curve in Figure 2b. The speed of the electric motor 13 is n' ₁ , and the pattern is that deceleration cannot be started until the speed is pulled to the set speed n ₁ at time t ₅₁ , and deceleration is completed at time t ₆₁ . Therefore, from the start of inverter operation at time t ₅ until time t ₅₁ , inverter operation is performed at the set speed n ₁ using another time element.
must continue to operate at the frequency corresponding to
Even if the inverter operation is started between time t ₅ and time t ₅₁ under normal load, there is a drawback that deceleration operation cannot be immediately started.

This invention was made in order to eliminate the above-mentioned drawbacks, and utilizes the characteristics of voltage and current peculiar to an induction motor, which are generated by starting inverter operation at a set frequency and passing through a synchronous speed with respect to that frequency. The present invention aims to provide an AC motor operation method that can reliably bring the AC motor into inverter operation by detecting the speed of the AC motor.

FIG. 3 shows a characteristic diagram when the operating method of the present invention is implemented. In Fig. 3, a is the synchronous speed n ₀ ,
The characteristic torque T M and current _{I M peculiar} to the induction motor are shown with respect to the operating speed n' ₀ corresponding to the load torque and the speed n _M of the induction motor. b indicates the output current I output from the inverter with respect to the speed. In FIG. 3, switch 121 is switched from commercial operation to switch 12.
2, and after a certain set time has elapsed, the induction motor 13 reaches the speed _nM . At this time, assume that inverter operation is started at a frequency corresponding to speed n ₀ (n ₀ <n _M ). The current control value L _L1 set for normal inverter operation is the current I _M
In the lower range, the current I _M1 continues to flow along the solid line, and when the speed n _M is n ₀ , it passes through the exciting current L ₀ and settles at a speed n' ₀ corresponding to the load torque.
By determining by the current transformer 21 and control circuit 22 in FIG. 1 that the output current I has passed through the excitation current _I0 on the way, it is determined that the induction motor 13 has been drawn into normal inverter operation. Therefore, deceleration operation can be started immediately. Further, the current limit value before normal inverter operation is at a level that allows the excitation current to be determined, and by lowering it to the level _i12 in FIG. 3b, there is no need to allow for wasted capacity.

FIG. 4 is an example of a circuit for carrying out the operation method of the present invention, and shows the control circuit 22 in FIG. 1 in detail. In the figure, speed standard 3
Reference numeral 11 is a speed reference, which is a speed reference at which the induction motor 13 should be operated after being brought into inverter operation. Both speed references are selected by switches 321 and 322 to provide signals to input limiting circuit 33. This output is divided into two paths: a path from the voltage control circuit 34 to the current control circuit 35 to the rectifier 16, and a path from the oscillator 36 to the frequency dividing circuit 37 to the inverter 18. The output current is fed back from the current transformer 21, and this signal is converted to the excitation current by the comparator circuit 38.
It compares with the signal of I ₀ setting 39 and turns switches 321 and 322 on and off. When the output current I of the comparator circuit 38 is greater than or equal to the excitation current _I0 , the switch 322 is turned on to maintain the speed reference at _n0 , and when the excitation current _I0 has passed, the switch 321 is turned on and pulled in. A speed reference n for later operation is given.

FIG. 5 shows another characteristic diagram in which the present invention is implemented. In FIG. 5, after starting the inverter operation, a certain constant current is forced to flow as the output current of the inverter. If this current value is, for example, the value of the excitation current I ₀ in Fig. 3b, then as the speed n _M decreases from this state, the terminal voltage V _M of the induction motor will gradually increase, and the synchronous When the speed n ₀ is reached, the terminal voltage V _M rises to the normal value of V/F. This voltage is transferred to the transformer 23 in FIG.
By making this determination by the control circuit 22, it can be determined that the induction motor 13 has been drawn into normal inverter operation.

FIG. 6 is a circuit example showing another embodiment of the present invention. A particular difference from FIG. 4 is that a comparison circuit 40 compares the feedback signal of the output voltage with the terminal voltage V _M setting. The output of this comparator circuit 40 is used to turn on and off switches 321 and ₃₂₂ as shown in _FIG . It is used to turn on and off switches 431 and 432 connected to the output signal (current reference) of circuit 34.

As explained above, according to the present invention, when the system is drawn into operation using a variable frequency power source, it is possible to determine whether the operation is performed using a variable frequency power source by detecting the amount of electricity specific to an AC motor. Therefore, it is possible to provide a driving method with the following advantages.

(1) After being drawn into operation by the variable frequency power supply, deceleration operation can be started immediately, saving unnecessary system time.

(2) There is no need for unnecessary current to flow before the variable frequency power source is used for operation.

(3) The commercial power source may be another CVCF (constant voltage constant frequency power source) or variable frequency power source, providing a high degree of system flexibility.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram for implementing the present invention, FIG. 2 is a diagram for explaining the operating method shown in FIG. 1, FIG. 3 is an operating characteristic diagram of an embodiment of the present invention, and FIG. FIG. 5 is a circuit diagram showing one embodiment of the invention, FIG. 5 is an operating characteristic diagram of another embodiment of the invention, and FIG. 6 is a circuit diagram showing another embodiment of the invention. 11... AC power supply, 121, 122... Switch, 13... Induction motor, 15, 19... Transformer, 16... Rectifier, 17... DC reactor,
18... Inverter, 21... Current transformer, 22...
Control circuit, 23...transformer.

Claims (1)

[Claims] 1. After switching the drive power source of the AC motor from a constant frequency power source to a variable frequency power source, the speed of the AC motor is determined by detecting an amount of electricity peculiar to the AC motor caused by passing through a synchronous speed. AC motor operating method.