Control apparatus for inverter for driving AC motor

A control apparatus for controlling a PWM inverter, the PWM inverter receiving AC power and generating an AC output having a controlled frequency to be supplied to an AC motor, comprises a circuit for generating, on the basis of a given running command, a PWM signal controlling the inverter with a given sampling period, and a circuit for controlling the PWM signal generating circuit, whereby the AC motor generates an electromagnetic tone preselected so as to correspond to desired information when the AC motor is driven by output of the inverter controlled by the PWM signal. The above described desired information may be information representing the running state of the motor or selected music.

BACKGROUND OF THE INVENTION 
The present invention relates to a control apparatus of an inverter for 
driving an AC motor, and in particular to a control apparatus capable of 
effectively using electromagnetic vibration tones, i.e., noises generated 
from an AC motor such as an induction motor or a synchronous motor driven 
by a PWM inverter. 
As conventional countermeasures to noises generated from an AC motor driven 
by a PWM inverter, there are known a scheme in which the sampling 
frequency of the PWM inverter is raised up to the nonaudible frequency 
region and another scheme in which noises are changed to be white noises 
by changing the sampling frequency of the PWM inverter so that it may not 
concentrate to an identical frequency. The former scheme is described in 
1986 National Convention Record, IEE of Japan, Nos. 535 and 536, pp. 625 
to 626, for example. The latter scheme is described in 1988 National 
Convention Record, IEE of Japan, No. 523, pp. 619 to 620, for example. 
Upon occurrence of overcurrent, overspeed or abnormal state in a motor, 
this abnormal state is generally detected and displayed on a display 
device or informed of by means of an alarm device. 
In the former scheme among the above described conventional countermeasures 
to noises, attention is not paid to occurrence of high frequency noises 
caused by raising the sampling frequency, resulting in a problem of radio 
wave trouble. The latter scheme has a problem that noises offensive to the 
car still remain because the total pressure level of noises is not changed 
even if noises are made to be white. 
When an abnormal state of a motor is to be informed of, it is necessary to 
provide a display device or an alarm device in case of the prior art, 
resulting in a problem of an increase of the apparatus by that amount. 
SUMMARY OF THE INVENTION 
An object of the present invention is to make electromagnetic vibration 
tones generated by an AC motor nonoffensive to the ear. 
Another object of the present invention is to inform of an abnormal state 
or the like of an AC motor by using electromagnetic vibration tones 
generated from the AC motor. 
The above described objects of the present invention are achieved by, in a 
system in which the output of an AC power supply is supplied to an AC 
motor via a PWM inverter, changing the frequency of current ripple 
contained in the motor current according to an information tone to be 
generated as the electromagnetic vibration tone of the AC motor. 
As a result of changing the ripple frequency of a current contained in the 
motor current according to the desired information tone, the 
electromagnetic tone generated from the motor serves as an information 
tone, and it becomes nonoffensive to the ear and can be used as an alarm 
tone or the like.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred embodiments of the present invention will now be described by 
referring to drawings. 
FIG. 1 is a circuit configuration diagram of a control apparatus of an 
inverter for driving an AC motor according to an embodiment of the present 
invention. 
In FIG. 1, numeral 1 denotes a commercial three-phase AC power source, and 
numeral 2 denotes an inverter apparatus comprising an inverter section 2a 
and a converter section 2b. This inverter apparatus 2 is supplied with the 
output of the commercial three-phase AC power source 1 and outputs 
three-phase AC having voltages V.sub.u, V.sub.v and V.sub.w and a 
frequency f.sub.1 to drive a three-phase induction motor 3. Numeral 4 
denotes a tachometer generator, 5 a current detector, 6 a base drive 
circuit, and 7 a control circuit. This control circuit 7 comprises a PWM 
signal generating section 8, a command value generating section 9, and a 
sampling period control section 10 which in turn comprises a data table 11 
and an abnormal state detecting section 12. 
When a motor speed command .omega.* is inputted to the control circuit 7 as 
a running command, the command value generating section 9 calculates and 
outputs a voltage command value V* and an inverter frequency command value 
f.sub.I * on the basis of a detected current value i.sub.f and a detected 
speed value .omega..sub.f detected respectively by the current detector 5 
and the tachometer generator 4. This command value generating section 9 is 
provided to calculate a torque, which should be applied to the motor in 
order to correct the deviation of the motor speed .omega..sub.f from the 
command value .omega.*, on the basis of the above described deviation and 
the detected current value i.sub.f and then calculate the voltage command 
V* and the frequency command value f.sub.I * corresponding to the torque. 
As this command value generating section 9, a circuit as shown in 1988 
National Convention Record, IEE of Japan -Industry Application Society- 
Report No. 74, for example, is used. Upon receiving these command values, 
the PWM signal generating section 8 generates a PWM signal. This PWM 
signal is supplied to respective switching devices included in the 
inverter section 2a via the base drive circuit 6. 
FIG. 2 is a circuit configuration diagram of the inverter section 2a. FIGS. 
3A to 3D are diagrams for illustrating the operation of the inverter 
section 2a. The method for generating the PWM signal will now be described 
by referring to FIGS. 2 and 3A to 3D. 
As switching states of switching devices S.sub.1 to S.sub.6 of the inverter 
section 2a, there are eight combinations, i.e., (0, 0, 0), (1, 0, 0), (1, 
1, 0), (0, 1, 0), (0, 1, 1), (0, 0, 1), (1, 0, 1) and (1, 1, 1). Each set 
represents successively states of switching devices of phases U, V and W. 
In each set, "1" represents a state under which a switching device of 
positive side of one phase is conducting and a switching device of 
negative side of the phase is nonconducting, whereas "0" represents a 
state under which a switching device of positive side of one phase is 
nonconducting and a switching device of the phase is conducting. 
Voltage vectors V.sub.0 to V.sub.7 corresponding to these states become as 
shown in FIG. 3A. In case the output voltage is sinusoidal, a vector .phi. 
of magnetic flux generated by a stationary primary winding of the 
three-phase induction motor 3 so as to make interlinkage with a rotary 
secondary winding or a rotary cage conductor draws a circle rotating with 
an angular velocity .omega. as shown in FIG. 3B. In an angle 
.DELTA..theta. over which the interlinkage magnetic flux vector .phi. 
advances during a sampling period T.sub.s, the voltage vectors V.sub.0 to 
V.sub.7 are selected so that the magnetic flux vector may change along a 
circle drawn by the interlinkage magnetic flux vector .phi.. In case the 
voltage vectors are selected as V.sub.7 -V.sub.2 -V.sub.3 -V.sub.0 
-V.sub.2 -V.sub.3 -V.sub.7 as shown in FIG. 3C, for example, relations 
between sampling pulses and voltage vectors become as shown in FIG. 3D. 
Therefore, a PWM signal corresponding to a switching state associated with 
each voltage vector is outputted from the PWM signal generating section 8. 
FIGS. 4(a) to 4(d) are waveform diagrams showing the sampling pulse, 
voltage vector, output voltage and output current. 
In case voltage vectors as shown in FIG. 4(b) are selected in one period of 
the inverter operation, output phase voltages V.sub.U, V.sub.V, and 
V.sub.W, and output line voltage V.sub.UV become as shown in FIGS. 4(c) 
and 4(d). Therefore, the U-phase current i.sub.U of the motor 3 becomes as 
shown in FIG. 4(d), and the period of current ripple is substantially in 
proportion to the sampling period T.sub.s. This current ripple causes 
electromagnetic vibration of the stator core of the motor 3, resulting in 
noises. In this way, noises having a frequency nearly equivalent to the 
frequency of the current ripple, i.e., the sampling frequency (1/T.sub.s) 
are generated. 
On the other hand, the period T.sub.s of the sampling pulse is defined by 
the sampling period control section 10. The abnormal state detecting 
section 12 always monitors the detected current value i.sub.f and the 
detected speed value .omega..sub.f by comparing them with their limit 
values respectively set. Under a normal running state with respective 
detected values less than limit values, the abnormal state detecting 
section 10 outputs the constant sampling period T.sub.s. 
It is now assumed that a trouble occurs at time as shown in FIG. 5 and the 
detected current value i.sub.f or the detected speed value .omega..sub.f 
exceeds a current limit value i.sub.LIM or a speed limit value 
.omega..sub.LIM at time t.sub.1 as shown in FIG. 5(a). When overcurrent in 
the output current or overspeed of the motor is thus detected, the 
succeeding sampling period T.sub.s is changed. For example, a sampling 
period T.sub.s0 under the normal running state is changed successively to 
sampling periods T.sub.s1, T.sub.s2, T.sub.s3 and so on under the abnormal 
running state as shown in FIG. 5(b). It is now assumed that durations of 
these sampling periods are L.sub.1, L.sub.2, L.sub.3 and so on. As a 
result, the frequency of the electromagnetic vibration tone generated from 
the motor changes from nearly 1/T.sub.s0 successively to nearly 
1/T.sub.s1, 1/T.sub.s2, 1/T.sub.s3 and so on. Therefore, it is possible to 
inform of the abnormal running state by such a change in electromagnetic 
vibration tone. 
In order to change the sampling period from that of the normal running 
state to that of the abnormal running state, the sampling period control 
section 10 is provided. As shown in FIG. 6, the sampling periods T.sub.s1, 
T.sub.s2, T.sub.s3, - - -, T.sub.sn under the abnormal running state are 
so stored into the data table 11 as to be associated with their durations 
L.sub.1, L.sub.2, L.sub.3, - - -, L.sub.n. When the abnormal running state 
is detected, these data are called by the abnormal state detecting section 
12. The sampling period T.sub.s0 under the normal running state is changed 
successively to sampling periods T.sub.s1, T.sub.s2 and so on under the 
abnormal running state and outputted. Durations L.sub.1, L.sub.2, - - -, 
L.sub.n have such length that the human ear can distinguish the tone 
change, and are preferably 100 ms to several seconds. 
The processing heretofore described is performed by using a microcomputer. 
The flow of this processing is shown in FIG. 7. 
When an abnormal running state is detected, data T.sub.sn and L.sub.n are 
first loaded from the data table 11, and the sampling period is changed 
from T.sub.s0 to T.sub.sn. The duration L.sub.n is set into a counter, 
which is a register (not illustrated) provided in the PWM signal 
generating section 8. The sampling period remains at T.sub.sn until 
L.sub.n is successively decreased to reach zero. That is to say, the 
frequency of the electromagnetic vibration tone of the motor is maintained 
at nearly 1/T.sub.sn during L.sub.n. When the count reaches zero, the next 
data is loaded. An electromagnetic tone having a frequency corresponding 
to its sampling period is maintained for its duration. In the same way, 
the frequency of the electromagnetic vibration tone is hereafter changed 
according to the data stored in the data table. 
When an abnormal running state is detected in this embodiment, an alarm 
informing of this fact can be conveyed as a tone change by using an 
electromagnetic vibration tone generated from the motor. Therefore, it is 
not necessary to specially provide a display device or an alarm device for 
informing of the abnormal running state unlike the prior art. As a result, 
the apparatus can be provided at a lower cost. 
FIG. 8 shows another embodiment of the present invention. FIG. 8 is a 
configuration diagram of a control circuit capable of producing a musical 
sound at the normal running state of the motor. 
The sampling period control section 13 included in the control circuit 7 
comprises the data table 11 and a musical scale generating section 13. If 
data of the sampling period T.sub.s stored in the data table 11 is so set 
as to be limited to one period of the frequency of each musical interval 
of the musical scale included in music, it is possible to make the 
electromagnetic vibration tone of the motor nearly equivalent to the 
frequency of each musical interval of the musical scale. 
FIG. 9 shows relationship between musical interval and frequency. For 
example, reference tone (la) has a frequency of 440 Hz, and (la) higher 
than that by one octave has a frequency of 880 Hz. By dividing the 
interval between them into twelve sections in accordance with the 
frequency ratios shown in FIG. 9, frequencies of respective musical 
intervals are defined. By setting the reciprocal of each frequency as data 
of the sampling period T.sub.s and changing the duration L.sub.n of each 
musical interval in proportion to the length of the note, therefore, it is 
possible to play music by using electromagentic vibration tones of the 
motor. 
By using the present embodiment, it becomes possible to change 
electromagnetic vibration tones of the motor, which have heretofore been 
noises offensive to the ear, into music. 
Assuming now that the voltage is constant in the present embodiment, the 
power of the electromagnetic vibration tone of the motor becomes large at 
lower frequencies and small at higher frequencies. On the other hand, it 
is known that power spectrum distribution of comfortable tone with respect 
to frequency has large power in a low frequency region and has small power 
in a high frequency region as shown in FIG. 10. Therefore, it is known 
that the power spectrum distribution of FIG. 10 is also attained by making 
the electromagnetic vibration tones music as in the present embodiment, 
resulting in comfortable feeling. 
FIGS. 11 and 12 show other embodiments of the present invention. In the 
examples shown in FIGS. 11 and 12, the ripple frequency of the motor 
current is directly detected, and the sampling period is so changed as to 
attain the desired ripple frequency. The command value T.sub.s * of the 
sampling period is supplied from the abnormal state detecting section 12 
shown in FIG. 11 or the musical scale generating section 13 to a comparing 
and calculating section 701. On the basis of the command value T.sub.s * 
of the sampling period thus supplied and a ripple frequency f.sub.r 
supplied from a ripple frequency detecting section 702, the comparing and 
calculating section 701 performs the following proportion calculation and 
integration to derive the sampling period T.sub.s. 
##EQU1## 
K.sub.P and K.sub.I are proportion constant and integration constant, 
respectively. In the present embodiment, the precision in setting the 
ripple frequency of the motor current becomes high. It is thus possible to 
set the frequency of the electromagnetic tone generated from the motor at 
a predetermined value with higher precision so much. 
FIGS. 13 and 14 show other embodiments of the present invention. The 
command value 
##EQU2## 
of the current ripple frequency outputted from the sampling period control 
section 10 is inputted to a function generating section 705, which in turn 
outputs a permissible change width .DELTA.i* of current ripple. A 
comparing and calculating section 703 applies comparison and calculation 
to an instantaneous current command value i* outputted from an 
instantaneous current command generating section 704, the detected motor 
current value i.sub.f, and the .DELTA.i* to derive the PWM signal. 
Assuming now that the permissible change width of current ripple with 
respect to the instantaneous current command value i* is changed from 
.DELTA.i.sub.i * to .DELTA.i.sub.2 * at time t=t.sub.0 as shown in FIG. 
15, the frequency of current ripple is changed from f.sub.r1 to f.sub.r2. 
If .DELTA.i.sub.1 *&lt;.DELTA.i.sub.2 *, the relation f.sub.r1 &gt;f.sub.r2 
generally holds true. By controlling .DELTA.i* according to this relation, 
the ripple frequency f.sub.r of the motor current i.sub.f can be made 
equal to the desired value f.sub.r *. 
FIG. 16 shows another embodiment of the present invention. FIG. 16 is a 
circuit configuration diagram showing the case where the present invention 
is applied to a multi-inverter apparatus. 
An inverter apparatus 20 comprises a plurality of unit inverters 20a to 20n 
run so as to have running phases shifted by (180.degree./3n) respectively. 
Their output voltages are summed up by a multiple transformer 30. The 
resultant voltage is supplied to a three-phase induction motor 3 to drive 
the motor 3. Although not especially illustrated, each unit inverter is 
associated with a circuit equivalent to the control circuit shown in FIG. 
1. Sampling periods and their durations as shown in FIG. 17 stored 
beforehand into data tables 40a to 40n respectively associated with the 
unit inverters 20a to 20n. By doing so, the electromagnetic vibration tone 
generated from the motor 3 becomes a compound of frequencies nearly 
equivalent to sampling frequencies of the unit inverters. 
In the present embodiment, the information tone generated as the 
electromagnetic vibration tone when an abnormal running state has been 
detected can be made more complex than that of the above described 
embodiments. If a combination of frequencies to be composed is made to 
agree with frequency components of voice, for example, the information 
tone can be changed to a voice message such as a message "overspeed has 
occurred in the motor" to inform of the abnormal running state more 
clearly. 
By combining this embodiment with the embodiment shown in FIG. 8, it 
becomes possible to generate a chord as the electromagnetic vibration 
tone, resulting in more comfortable music. In case a plurality of motors 
are run in parallel, a similar effect can be obtained by using 
configuration as shown in FIG. 18. 
If the present invention is applied to house hold appliances using a PWM 
inverter drive motor system, it is possible to use effectively the 
electromagnetic vibration tones as an alarm, a message or BGM (background 
music), resulting in favorable feeling of use without noises. 
Further, if the present embodiment is applied to a plant system using a PWM 
inverter drive motor system, it becomes possible to use effectively the 
electromagnetic vibration tones especially as an alarm when abnormality 
occurs in the system.