Variable frequency power source operating system

The output frequency of a variable frequency power source for driving a motor which drives a rotary body such as a fan in a variable-speed control mode is fixed, when abnormality in a control signal supplied to the variable frequency power source from outside to determine the output frequency of the variable frequency power source is detected, at the output frequency of the variable frequency power source immediately before the detection of abnormality in the control signal, and thereby the abnormal variation of the revolving rate of the motor resulting from the abnormality in the control signal is avoided.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a system for driving the rotary component 
of a fan, a pump or the like by supplying power from a variable frequency 
power source (referred to as "VF power source" hereinafter) and more 
particularly to a VF power source operating system for operating a VF 
power source under an abnormal condition in which VF power source control 
signals can not be sent to the VF power source due to disconnection 
accident or short-circuit accident in the signal line. 
2. Description of the Prior Art 
A conventional VF power source operating system of this kind is shown in 
FIG. 1, in which indicated at 1 is a commercial power line, at 2 is a 
switch, at 3 is a VF power source for controlling the speed of a motor 4, 
at 6 is the rotary component of a fan, a pump or the like, at 5 is a 
mechanical coupler for coupling the motor 4 and the rotary component 6, at 
7 is a controller which gives a frequency control signal to the VF power 
source 3, at 8 is a control command given to the controller 7 and at 9 is 
a signal line interconnecting the controller 7 and the VF power source 3. 
In FIGS. 2 and 3, t.sub.1 and t.sub.3 are a time point when an increase 
control command is given and a time point when a decrease control command 
is given respectively, t.sub.2 and t.sub.4 are time points when the 
frequency of the output of the VF power source reaches the increased and 
decreased target frequencies, respectively, t.sub.5 is a time point when 
an accident occurred in the signal line 9 and t.sub.6 is a time point when 
the frequency of the output of the VF power source 3 reaches the frequency 
corresponding to the absence of the signal on line 9. 
The functions of this conventional VF power source operating system will be 
described hereinafter. In order to give a concrete description, the rotary 
component 6 is regarded as a fan for supplying its output (air) to the 
boiler, not shown, of a power plant. 
Referring to FIG. 1, the VF power source 3 receives power through the 
switch 3 from commercial power line 1 and provides an output for driving 
the motor 4. The revolving rate n of the motor 4 is represented by the 
following expression: 
EQU n=(120.times.F)/P (1) 
where F is the frequency of the output power of the power source and P is 
the number of poles of the motor. 
Accordingly, the revolving rate n is proportional to the frequency F of the 
output power of the power source, that is, the revolving rate n of the 
motor 4 varies according to the variation of the output frequency F of the 
VF power source 3. 
The motor 4 is coupled with the fan 6 with a coupler 5 and the fan 6 
supplies air to the boiler at an output air supply rate Q which is 
approximately proportional to the revolving rate n of the motor 4. 
In a power plant, the air demand of the boiler changes as the power demand 
of the associated power grid changes or the fuel supply mode of the boiler 
changes. In such a case, a control command 8 to change the air supply rate 
is given to the controller 7 and a control signal for deciding the air 
supply rate Q is given through the signal line 9 to the VF power source 3, 
and then the VF power source 3 provides an output of a frequency F 
corresponding to the required air supply rate Q. 
FIG. 2 is a time chart for explaining the normal actions of the 
conventional VF power source system shown in FIG. 1. In FIG. 2, when the 
control command 8 requests the increase of the air supply rate Q at a time 
point t.sub.1 and the decrease of the increase air supply rate to the 
original air supply rate at a time point t.sub.3, the output of the 
controller 7 is given through the signal line 9 to the VF power source 3. 
Ordinarily, the VF power source 3 is designed to respond to the request 
for the change of the output at a fixed increase rate or a fixed decrease 
rate, therefore the change of the output frequency F of the VF power 
source 3 is delayed slightly from the time points t.sub.1 and t.sub.3 of 
request for the change and the output frequency changing action of the VF 
power source 3 is completed at the time points t.sub.2 and t.sub.4. That 
is, if it is desired to change the output air supply rate Q of the fan 6, 
the revolving rate n of the motor 4 is changed by changing the output 
frequency F of the VF power source 3. 
FIG. 3 is a time chart showing the changes of controlled factors in case 
the disconnection of the signal line 9 including the loosening or the 
falling off of the connecting terminal, by way of example, has occurred. 
In this case, naturally, there is no change in the control command 8 and 
the control command 8 is requesting a fixed air supply rate Q. When the 
disconnection of the signal line 9 occurred at a time point t.sub.5, the 
input signal of the VF power source 3 becomes zero. Consequently, as 
explained in connection with FIG. 2, the output frequency F of the VF 
power source decreases with a slight delay to a lower limit value causing 
the decrease of the revolving rate n of the motor 4, and thereby the 
output air supply rate Q is decreased. 
Since the operation of a motor by means of a conventional VF power source 
has been performed in the manner as described hereinbefore, the output 
frequency of the VF power source 3 varies unnecessarily in case an 
accident occurs in the control signal to be given to the VF power source 
3, which entails unstable operation of the motor 4 and the rotary 
component 6 and the resulting accident in the system. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a variable 
frequency power source operating system capable of operating the 
associated variable frequency power source so that the sudden and great 
change of the revolving rate of the motor driven by the output of the 
variable frequency power source is obviated in case an abnormality 
occurred in the control signal for deciding the output frequency of the 
variable frequency power source. 
In an embodiment of the present invention, when an abnormality in the 
control signal for deciding the output frequency of a variable frequency 
power source is detected, the output frequency of the variable frequency 
power source is fixed to the frequency of the output which had been given 
immediately before the abnormality was detected. In another embodiment of 
the present invention, upon the detection of an abnormality in the control 
signal, the motor is disconnected from the variable frequency power source 
and connected to a commercial AC power source. In either one of these 
embodiments, the variation of the revolving rate of the motor is 
controlled within a limited range. 
In changing over the connection of the motor to the variable frequency 
power source to the connection to a commercial AC power source, the 
variable frequency power source is controlled from a time immediately 
after the detection of the abnormality so that the frequency of the 
variable frequency power source changes gradually to the same frequency as 
that of the commercial AC power source, if the output frequency of the 
variable frequency power source is greatly different from that of the 
commercial AC power source, and the changeover of the power source from 
the variable frequency power source to the commercial AC power source is 
executed upon the coincidence of the output frequency of the variable 
frequency power source with that of the commercial AC power source. 
Other objects, features and advantages of the present invention will become 
apparent from the description of the preferred embodiments thereof taken 
in connection with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
VF power source operating systems according to the present invention will 
be described hereinafter in connection with the accompanying drawings. In 
FIG. 4, the components indicated at reference characters 1 to 9 are 
substantially the same as those shown in FIG. 1, except that the 
controller 7 has a control signal generator 7B and the VF power source 3 
has an abnormal signal detector 3B. 
FIG. 5 shows the circuit of the control signal generator 7B, which will be 
described in detail afterward. As shown in FIG. 6, basically, the control 
signal generator 7B generates a control signal in the form of a train of 
pulses of a pulse width W1 or a pulse width W2 when the level of the 
control signal 8 is F1 or F2 respectively and supplies the control signal 
to the abnormal signal detector 3B. In FIG. 5, indicated at T.sub.1, 
T.sub.2, T.sub.3 and T.sub.4 are transistors, at R.sub.1 to R.sub.7 are 
resistances, at C.sub.1 and C.sub.2 are capacitors, at ZD is a Zener 
diode, at +Es is the voltage of a DC power source and at VC.sub.1 is the 
terminal voltage of the capacitor C.sub.1. 
The VF power source operating system according to the present invention is 
applied advantageously for controlling the air supply rate of the blower 
unit of a boiler as shown in FIG. 7, for instance. In this blower unit, 
the rotary component 6 consists of a shaft 6a coupled with a motor 4 with 
a mechanical coupler 5 and a fan 6b attached to the shaft 6a. The fan 6b 
is disposed within an air duct 12 to generate an air current in a 
direction indicated by the arrow 13. An air flow rate regulating mechanism 
14 provided as a subsidiary means to regulate the flow rate of the air in 
the air duct 12 includes a plurality of vanes 14a disposed within the air 
duct and a driving device 14b which regulates the vane angle according to 
an input control signal 15. Indicated at 12a and 12b are the inlet and the 
outlet of the air duct respectively. 
The functions of the VF power source operating system of the present 
invention will be described hereunder. In FIG. 6, the relation between the 
pulse width W.sub.i of the pulse wave and the magnitude F.sub.i of the 
control signal 8 is represented by 
EQU W.sub.i =K.multidot.F.sub.i (2) 
where K is a constant and i is an arbitrary positive integer. 
Accordingly, in the case of FIG. 4, 
EQU W.sub.1 =K.multidot.F.sub.1 and W.sub.2 =K.multidot.F.sub.2 (3) 
The control signal generator for generating pulse signals of pulse widths 
W.sub.1 and W.sub.2 is included in the controller 7 shown in FIG. 4. The 
frequency of an output power of the VF power source 3 for driving the 
motor 4 is dependent only on the pulse width W.sub.i of the pulse signal 
as defined by Expression (2). 
Referring to FIG. 5, in the control signal generator 7B, voltage V.sub.c 1 
rises at a time constant C.sub.1 R.sub.1 when the transistor T.sub.1 
changes from ON state into OFF state. When the voltage V.sub.c 1 exceeds 
the input voltage E.sub.i, the transistor T.sub.2, and the transistor 
T.sub.3, change from OFF states into ON states. 
When the transistor T.sub.3 is in the ON state, the base current of the 
transistor T flows through the Zener diode ZD to render transistor T.sub.1 
conductive to discharge capacitance C.sub.1. Upon discharge of capacitance 
C.sub.1, the transistors T.sub.2 and T.sub.3 return to the OFF state. The 
capacitor C.sub.2 is provided to secure a time period necessary to allow 
the voltage V.sub.c 1 to drop to zero volts by discharging the capacitor 
C.sub.1 through the transistor T.sub.1. 
If the input voltage varies, as shown in FIG. 8, between a level E.sub.I 1 
(a low value) and a level E.sub.I 2 (a high value), the time for the 
voltage V.sub.c 1 to reach the input voltage E.sub.I varies in proportion 
to the variation in E.sub.I. Therefore, the output signal E.sub.0 having 
pulses of pulse widths proportional to the input voltage E.sub.I and of 
input-frequency relation as shown in FIG. 8 is produced. 
If the signal line 9 is disconnected, the pulse magnitude H of the pulse 
signal being transmitted through the signal line 9 decreases to a reduced 
magnitude (one of reduced magnitudes including zero) at the input of the 
VF power source 3 even if any particular change does not occur in the 
control signal 8. Accordingly, the abnormal signal detector 3B for 
detecting the deviation of the pulse magnitude H of the pulse signal being 
supplied through the signal line 9 from the normal magnitude is provided 
to decide if the signal line 9 is in an abnormal condition, such as 
disconnection, and to prevent the change of the output signal of the VF 
power source 3 so that the existing operating condition of the motor is 
maintained. 
Although the invention has been described as applied to controlling the 
revolving rate of the fan of a power plant, the present invention is 
applicable for the same effects also to controlling a rotary component 
other than a fan, such as a pump, or as a VF power source operating system 
of a system other than a power plant. 
In the description given hereinbefore, the functions of the embodiment of 
the present invention have been described with reference to an abnormal 
condition where the signal line 9 was disconnected; however, the abnormal 
condition is not limited thereto and may be an accident such as 
short-circuit other than disconnection of the line 9. 
In FIG. 6, the minimum pulse magnitude and the maximum pulse magnitude of 
the pulse signal are zero and H respectively by way of example; however, 
the reference pulse magnitude is not necessarily limited to zero. 
Furthermore, in the description given above, the frequency control input 
applied to the VF power source 3 is a pulse signal of variable pulse 
width; however, a numerical signal produced through digital coding may be 
used instead of the pulse signal. 
FIG. 9 shows another embodiment of the present invention. This embodiment 
is different from the embodiment shown in FIG. 4 in respect of the 
insertion of switch 10 between a VF power source 3 and a motor 4 and the 
provision of an AC power supply line having a switch 11 for connecting the 
motor 4 to a commercial power supply. This embodiment changes over the 
power source of the motor 4 from the VF power source 3 to the commercial 
power supply when the abnormal signal detector 3B detects an abnormality 
in the control signal. This power source changeover action is performed 
preferably in cooperation with an air supply rate control mechanism 14 
shown in FIG. 7. 
When an abnormality in the control signal is detected while the system is 
operating with the vanes 14a of the air supply rate control mechanism 14 
fixed at the full open position for the purpose of energy conservation, 
the abnormal signal detector 3B cancels the fixation of the vanes 14a and 
changes the control mode of the air supply rate control mechanism 14 into 
an automatic control mode so that the degree of opening of the vanes 14a 
is regulated through a driving rod 14b according to an input control 
signal 15. Simultaneously, the abnormal signal detector 3B raises the 
output of the VF power source 3 near to the rated value thereof and 
increases the output frequency F to make the revolving rate of the motor 4 
approach the rated revolving rate. The output of the VF power source 3 is 
raised slowly and gradually. As the revolving rate of the motor 4 is 
raised, the resulting increase in the air supply rate is restricted within 
a safe range by reducing the degree of opening of the vanes 14a 
accordingly. After the output of the VF power source 3 has thus been 
increased near to the rated output, the switch 2 of FIG. 9 is opened and 
the switch 11 of FIG. 9 is closed to disconnect the motor 4 from the VF 
power source 3 and to connect the motor to the commercial power supply 1. 
This control procedure performed by this embodiment prevents the sudden 
change of the frequency of the input power supplied to the motor 4 over a 
wide range which occurs when the power source of the motor 4 is changed 
over simply from the VF power source 3 to the commercial power supply 1 
immediately after the abnormality in the signal line 9 has been detected, 
and thereby the variation of the air supply rate in changing the power 
source is limited within a small extent. 
Thus the power source of the motor 4 is changed over safely from the VF 
power source 3 to the commercial power supply 1 to continue the operation 
of the system even if any abnormality occurs in the signal line 9. 
Although the second embodiment of the present invention has been described 
as applied to controlling the revolving rate of the fan of a power plant, 
the present invention is applicable for the same effects also to 
controlling a rotary component other than a fan, such as a pump, or to a 
VF power source operating system of a system other than a power plant. 
The second embodiment of the present invention has been described in terms 
of functions in case of the disconnection of the signal line 9 by way of 
example, however, the abnormal condition is not limited thereto and may be 
an accident such as short-circuit. 
In FIG. 6, the minimum pulse magnitude and the maximum pulse magnitude of 
the pulse signal are zero and H respectively by way of example; however, 
the reference pulse magnitude is not necessarily limited to zero. 
Furthermore, in the action of the second embodiment, the power source of 
the motor is changed over from the VF power source 3 to the commercial 
power supply 1 after the output frequency of the VF power source has been 
raised near to the rated output frequency thereof; however, it is obvious 
that the process of changing the power source of the motor is not limited 
thereto and may be another process in which the power source is changed 
over immediately after the detection of an abnormal signal by the abnormal 
signal detector. 
Still further, in the description of this embodiment given above, the 
frequency control input applied to the VF power source 3 is a pulse signal 
of variable pulse width; however, a numerical signal produced through 
digital coding may be used instead of the pulse signal.