Apparatus for controlling an input power to an induction motor

An input power control apparatus for use with an AC induction motor adjusts a firing angle to allow the motor to run at near a rated speed of the motor and then to find a minimum power point at some voltage level in a variation of the applied voltage.

FIELD OF THE INVENTION 
The present invention relates to an AC induction motor; and, more 
particularly, to an apparatus for controlling an input power to an AC 
induction motor for energy saving. 
DESCRIPTION OF THE PRIOR ART 
An AC induction motor including three-phase or single-phase type is 
certainly one of the most commonly used in industrial and commercial 
applications. For an induction motor that runs at a constant speed, an 
efficiency of the induction motor can be improved by controlling an input 
voltage applied to the induction motor. It is known that under a lightly 
loaded condition, a reduction in the voltage applied leads to an energy 
saving in operating the motor, by achieving high efficiency for the given 
load condition. One of the most popular candidates for the energy saving 
is a thyristor ac voltage controller employing a power factor control 
technique. Such a technique is disclosed in U.S. Pat. No. 4,052,648, 
issued to F. J. Nola, wherein a line voltage and a current applied to an 
induction motor are sampled and a power input to the induction motor is 
decreased in proportion to a detected phase displacement between the 
current and the voltage applied to thereby provide less power to the 
induction motor. However, the constant power factor angle control system 
suggested by Nola is known not so effective in power saving under varying 
operating condition, e.g., changing load condition in an induction motor. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide an 
apparatus for controlling an input power to an AC induction motor for 
energy saving. 
In accordance with the present invention, there is provided an apparatus 
for use with an AC induction motor having a switching device to apply an 
input power to the motor for controlling the input power, said apparatus 
comprising: means for detecting a phase voltage for each phase of the 
motor; means for deriving a phase signal from each of the phase voltages 
to determine a firing angle; means for detecting an input current for each 
phase of the motor; means for detecting a switch voltage across the 
switching device; means for sampling the detected switch voltage at a 
predetermined sampling rate; means for calculating an input power value 
drawn to the motor by averaging the summation of the products of the phase 
voltages and the currents whenever the detected switch voltage is sampled; 
means, having a plurality of memory locations, for storing the calculated 
input power values, wherein each of the calculated input power value is 
stored in a memory location indicated by the level of the sampled switch 
voltage corresponding thereto; means for comparing a first power sum with 
a second power sum, wherein the first power sum is the summation of the 
calculated input power values stored in a plurality of memory locations 
located at a lower level with respect to the level of the sampled switch 
voltage and the second power sum is the summation of the calculated input 
powers stored in the plurality of memory locations located at a higher 
level with respect to the level of the sampled switch voltage; and means 
for increasing the firing angle by a first predetermined amount at a time 
if the first power sum is greater than the second power sum and for 
decreasing the firing angle by the first predetermined amount at a time if 
the first power sum is less than the second power sum.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, there is shown a schematic diagram of an input power 
control apparatus 100 for use with a three-phase AC induction motor 10 in 
accordance with the present invention. A switching device 30 is placed in 
series with the induction motor 10 and includes three pairs of identical 
thyristors, each pair of thyristors being connected back-to-back in series 
with each phase of the induction motor 10, respectively. The respective 
thyristor pairs in the switching device 30 are conducted with thyristor 
gate control signals, Sa, Sb and Sc, from the control apparatus 100 so 
that each of input voltages, Va, Vb, Vc, on supply lines 22, 24, 26 is 
applied to its corresponding phases of the induction motor 10 through each 
of output terminals 23, 25, 27 of the switching device 30, which will be 
fully discussed hereinbelow with reference to FIG. 2. 
In FIG. 2, the input power control apparatus 100 is shown to include a 
phase voltage detector 200, a phase detector 300, a phase current detector 
500, a switch voltage detector 600, a driver 700 and a controller 800. 
The voltage detector 200 detects voltages for three phases of the motor 10 
by using line voltages on the lines 24, 26. FIG. 5A shows only one of the 
measured phase voltages, which will be applied to the controller 800 and 
the phase detector 300. 
The current detector 500 measures an input current for each phase of the 
motor 10. FIG. 5B shows a measured current for one phase. The measured 
current for each phase is also provided to the controller 800 for 
measuring input power value drawn to the induction motor 10 in connection 
with the measured voltages. 
On the other hand, the phase detector 300 derives from each of the phase 
voltages a phase signal and provided the same to the controller 800. FIG. 
5C represents only one of the detected phase signals wherein each of 
positive pulses represents a cycle of condcution of a thyristor in a 
forward direction in each thyristor pair and a zero-crossing point at a 
rising edge in each positive pulses represents a trigger point at which 
the thyristor is conducted. A distance delayed from the trigger point is 
referred to as a firing angle. The delay of the zero-crossing point leads 
to an increase of the firing angle, which results a decrease of a voltage 
applied to the motor. 
The switch voltage detector 600 measures an appropriate ac switch voltage 
across the switching device 30, wherein the measured switch voltage, as 
shown in FIG. 5D, is also provided to the controller 800. The driver 700, 
in response to a firing angle control signal for each phase, Ga, Gb, Gc, 
provided from the controller 800; generates the thyristor gate control 
signal, Sa, Sb Sc, to trigger each thyristor in each pair, as shown in 
FIG, 5E and 5F for a pair of thyristors. 
In the controller 800, the switch voltage is sampled at a predetermined 
sampling rate and input power value for each phase is calculated at the 
sampling rate by averaging the summation of the products of the voltages 
and the currents. Each of the calculated power values is then stored in a 
look-up table 900 (shown in FIG. 4) incorporated in the controller 800. 
The controller 800 adjusts the firing angle based on the sampled switch 
Voltage and the calculated power value, resulting in a variation of a 
voltage applied to the motor, which, in turn, changes the speed of the 
motor. 
In accordance with the invention, by adjusting the firing angle, the motor 
speed is adjusted near to a rated speed of the induction motor that gives 
a maximum efficiency of the motor. Thereafter, the controller finds a 
voltage level giving a minimum power by further varying the applied 
voltage. However, in case where the load of the motor is abruptly changed 
while a motor speed is kept constant, the applied voltage is accordingly 
changed, which, in turn, results in a loss of motor speed. Accordingly, in 
order to maintain the high efficiency of the motor, it is necessary to 
quickly adjust the motor speed to near the rated speed. 
In order to run the motor at near the rated speed, as shown in FIG. 3 which 
illustrates a plot of switch voltage versus load factor, the present 
invention employs a predetermined minimum switch voltage and a 
predetermined maximum switch voltage which have values below and above by 
some levels with respect to a switch voltage to be generated at the rated 
speed for a given load condition. As can be seen from FIG. 3, when a load 
approaches zero value, each of the predetermined minimum and maximum 
switch voltages converges to a point "A" of a peak switch voltage. The 
predetermined minimum switch voltage is set to be zero when a load factor 
is 60% and the maximum switch voltage is set to be zero when the load 
factor is 90%, which defines a minimum switch voltage line 52 by 
connecting the point A with 60% load point and a maximum switch voltage 
line 54 by connecting the point A with 90% load point. Accordingly, under 
a given load condition, the Y value on the Line 52 corresponding to the 
given load point gives the minimum switch voltage and the Y value on the 
line 54 corresponding to the given load point gives the maximum switch 
voltage for the given load. In this connection, the minimum and maximum 
switch voltage lines Y1 and Y2 can be defined as follows: 
EQU Y1=-(M/N1)X+M; and Y2=-(M/N2)X+M 
wherein M represents the peak switch voltage; and N1 and N2 represent a 
minimum load factor, i.e., 60% of the motor and a maximum load factor, 
i.e., 90%, of the motor, respectively. Accordingly, the predetermined 
minimum and the maximum switch voltages for a given load condition can be 
obtained from the equation. 
If necessary, a memory can be employed to store the minimum and the maximum 
switch voltages for a set of preselected load points. 
In accordance with the invention, the minimum and the maximum switch 
voltages are further limited to below 80% of the peak switch voltage, as 
indicated by a line 56, in order to avoid an extreme alternation of the 
motor speed occurred near the peak switch voltage. 
For a given load condition, if the sampled switch voltage is less than the 
minimum switch voltage, the controller 800 increases the firing angle by a 
first predetermined amount at a time, thereby decreasing the applied 
voltage to the motor or lowering the motor speed, until a switch voltage 
to be sampled becomes greater than the minimum switch voltage; however, if 
the sampled switch voltage is greater than the maximum switch voltage, the 
controller 800 decrease the firing angle by the first predetermined amount 
at a time, thereby increasing the applied voltage to the motor or raising 
the motor speed, until a switch voltage to be sampled becomes less than 
the maximum switch voltage. 
Once the motor is controlled to run at near the rated speed by measuring 
the sampled switch voltage, the controller 800 performs a minimum power 
process as is described hereinbelow. 
FIG. 4 shows an exemplary diagram of the look-up table 900 explaining the 
minimum power process in accordance with the invention. 
As shown in FIG. 4, the look-up table 900 has a plurality of memory 
locations for storing the calculated power values, with a one-to-one 
correspondence to the level of the switch voltage shown in FIG. 3, wherein 
a calculated input power value previously stored in a memory location 
indicated by the level of the sampled switch voltage, corresponding 
thereto is updated with a new one when the switch voltage is sampled. 
Then, whenever the calculated power value is stored in its corresponding 
memory location, the controller 800 compares a first power sum with a 
second power sum, wherein the first power sum is the summation of the 
input power values stored in a plurality of memory locations, e.g., at 
least three memory locations 31, 32, 33 located at a lower level with 
respect to the level of the sampled switch voltage, Vsw, and the second 
power sum is the summation of the input power values stored in a plurality 
of memory locations, e.g., at least three memory locations 35, 36, 37 
located at a higher level with respect to the level of the sampled switch 
voltage, Vsw. If the first sum is greater than the second sum, then the 
controller 800 increasing the firing angle by a second predetermined 
amount at a time, to thereby decrease the applied voltage to the motor 
until the first sum equals to the second sum; otherwise, the controller 
800 decreasing the firing angle by the second predetermined amount at a 
time, to thereby increase the applied voltage until the second sum becomes 
to equal to the first sum. Accordingly, by repeating the above operation, 
a minimum power point will be found at a certain position for a given load 
in FIG. 3. 
In accordance with the invention, the first amount of the increment of the 
firing angle is greater than the second amount of the increment. 
Accordingly, the rated speed control responds much more quickly than the 
minimum power control in adapting to a rapid change of load condition. 
Such controller 800 may be implemented in a 80196 single chip 
microcontroller, which is available from Intel in USA. 
FIG. 6 presents a flow diagram explaining the control operation executed by 
the controller of the present invention. 
The control process begins with a step 62, wherein the controller 800 
samples a switch voltage across the switching device at the sampling rate. 
And, in a step 64, an input power value is calculated when the switch 
voltage is sampled. 
In a step 66, the calculated power is then stored in a memory location in 
the look-up table indicated by the level of the sampled switch voltage. 
Thereafter, in steps 68 and 70, the sampled switch voltage is compared 
with a minimum switch voltage and a maximum switch voltage to check 
whether the sampled switch voltage falls within therebetween. If it is 
determined that the sampled switch voltage is less than the minimum switch 
voltage, the control process passes to a step 72 to increase the firing 
angle by the first amount at a time until a switch voltage to be sampled 
becomes greater than the minimum switch voltage. On the contrary, if it is 
determined that the sampled switch voltage is greater than the maximum 
reference switch voltage, the control process flows to a step 74 to 
decrease the firing angle by the first amount at a time until a switch 
voltage to be sampled becomes less than the minimum switch voltage. 
Once the sampled switch voltage falls within the limit range, the 
controller 800, as in step 76, compares a first sum and a second sum in 
the look-up table. 
If it is determined that the first sum is greater than the second sum, the 
control process proceeds to a step 78 to increase the firing angle by the 
second predetermined amount at a time until the first sum equals to the 
second sum. Otherwise, the control process advances to a step 80 to 
decrease the firing angle by the second predetermined amount at a time 
until the second sum equals to the first sum. 
While the present invention has been shown and described with respect to 
the preferred embodiments, it will be apparent to those skilled in the art 
that many changes and modifications may be made without departing from the 
spirit and scope of the invention as defined in the appended claims.