A power generating system for adjusting coupling an induction motor, as a generator, to an A.C. power line wherein the motor and power line are connected through a triac is described. The triac is regulated to normally turn on at a relatively late point in each half cycle of its operation, whereby at less than operating speed, and thus when the induction motor functions as a motor rather than as a generator, power consumption from the line is substantially reduced

Nola, F. J.

English

NASA Technical Reports Server

United States Patent 1191 [I11 4,388,585 
Nola [45] Jun. 14, 1983 
[54] ELECTRICAL POWER GENERATING 
[75] Inventor: Frank J. Nola, Huntsville, Ala. 
[73] Assignee: The United States of America as 
SYSTEM 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[21] Appl. No.: 243,683 
[22] Filed: Mar. 16, 1981 
[51] Int. Cl.3 ............................................... H02P 9/00 
[52] U.S. CI. ........................................ 322/47; 322/29; 
322/35; 322/95 
[58] Field of Search ....................... 322/29, 32, 47, 89, 
322/90, 95, 2 R, 35; 290/44, 55 
~561 References Cited 
U.S. PATENT DOCUMENTS 
3,946,242 3/1976 Wilkerson ........................... 322/2 R 
3,982,170 9/1976 Gritter et al. ..................... 322/32 X 
20 
4,242,628 12/1980 Mohan et al. .................... 322/47 X 
OTHER PUBLICATIONS 
“Energy Saving in AC Generators”, Nola; NASA 
Tech. Briefs; vol. 5, No. 2; pp. 134, 135; Summer, 1980. 
Primary Examiner-R. J. Hickey 
Attorney, Agent, or Firm-L. D. Wofford, Jr.; J. R. 
Manning; J. H. Beumer 
1571 ABSTRACT 
A power generating system for adjustably coupling an 
induction motor, as a generator, to an A.C. power line 
wherein the motor and power line are connected 
through a triac. The triac is regulated to normally turn 
“on” at a relatively late point in each half cycle of its 
operation, whereby at less than operating speed, and 
thus when the induction motor functions as a motor 
rather than as a generator, power consumption from the 
line is substantially reduced. 
1 Claim, 2 Drawing Figures 
n 
U 
22 
14 
https://ntrs.nasa.gov/search.jsp?R=19830020048 2020-03-22T00:45:58+00:00Z
U.S. Patent Jun. 14, 1983 4,388,585 
AC 
LINE 
FIRING 
ANGLE - 
CONTROL 
A 
FIG. 2 
- I4 
LOW WIND 
8 VELOCITY 
4,3 n n,5 85 
1 
ELECTRICAL POWER GENERATING SYSTEM 
ORIGIN O F  THE INVENTION 
The invention'descriged herein was made by an em- 
ployee of the U.S. Government and may be manufac- 
tured and used by or for the Government of the United 
States for governmental purposes without the payment 
of any royalties thereon or therefor. 
TECHNICAL FIELD 
This invention relates to electrical power generation, 
and particularly to an auxiliary generating system for 
coupling an induction motor type generator to an A.C. 
power line for adding power to that line. 
10 
15 
BACKGROUND ART 
In the past few years, there has been a considerable 
effort to develop new sources of electrical power. In- 
cluded has been the development of systems primarily 2o 
designed for providing power to a single enterprise, for 
example, a household, with any excess generated power 
being fed back to a power line of a public utility provid- 
ing a primary source of power for that household. Fre- 
quently, the auxiliary or local power generating unit is 25 
in the form of a windmill, and there are times when little 
or insufficient power is available from it alone. Thus, as 
a matter of convenience, in order to preserve a continu- 
ous interconnection of power to on-site electrical de- 
vices to be powered, the windmill and public utility 30 
power lines are connected together. 
Windmill generators have typically been of the direct 
current type, and thus in order to achieve compatibility 
with public power lines, which are of alternating cur- 
rent powder, the output of such a generator must be 35 
converted to alternating current power. This is accom- 
plished by switching means operating synchronously 
with the frequency, typically 60 cycles, of the power 
line. In addition to effecting frequency compatibility, 
there must be both voltage amplitude and phase com- 40 
patibility between the generated output and the power 
line voltage. All in all, such a coupling system is neces- 
sarily complex and costly. 
As an alternate to the direct current generator, induc- 
tion motor/generator units are sometimes used with 45 
windmill generating systems. While the induction mo- 
todgenerator has not seen great use as a generator in 
the past, it is perhaps the most widely used type of 
motor, and thus is widely available and at a reasonable 
cost. 50 
The power input to an induction motor is given by 
the product of the applied voltage, the current, and the 
cosine of the phase angle between the voltage and cur- 
rent (E I Cosine a). In a heavily loaded motor, the cur- 
rent will tend to be in phase with the voltage. When 55 
unloaded, the current will typically lag the voltage 70" 
to 80". If an external force tends to drive the shaft 
higher than synchronous speed, the phase lag will con- 
tinue to increase. When the force is sufficient to cause 
the phase lag to be 90", the power input to the motor is 60 
zero since cosine 90" =O. At this point, the mechanical 
energy applied to the shaft is exactly equal to the mag- 
netizing losses, and there is no net energy being re- 
turned to the A.C. buss. As the driving force continues 
to increase, the phase angle becomes greater than 90". 65 
The cosine of angles greater than 90" is negative, indi- 
cating negative power flow. The motor is now generat- 
ing and returning energy to the A.C. buss. Further 
increase in driving force causes the phase lag to ap- 
proach 180" as the full generatiing capacity of the ma- 
chine is reached. 
Significantly, the induction generator requires no 
synchronization or voltage regulation circuitry to cou- 
ple its output to a power line. It inherently functions as 
a generator when it is driven above its synchronization 
speed, a speed equal to the frequency of the power line 
divided by the number of pairs of poles that it contains, 
typically the speed being 1,800 rpm in the case of a 
4-pole device. It, like a direct current generator, is typi- 
cally connected to a power line when its speed is suffi- 
cient for the production of power which, in the case of 
the induction motodgenerator, is at sync speed. Be- 
yond this speed, and in the range of approximately five 
percent of the sync speed, this type device provides 
increasing power output to a power line, this increase 
occurring as the phase lag of current with respect to 
voltage increases above 90", an angle which persists at 
the sync speed. 
Despite the obvious advantages of the induction mo- 
tor/generator over a D.C. generator as described, the 
former has one significant disadvantage. It must draw 
field excitation power from the power line that is con- 
nected to it. This excitation current is drawn during a 
portion of each half cycle of the A.C. line voltage when 
current and voltage are of the same polarity, which, in 
the case of a lightly driven generator, is only slightly 
less than one-half of each half cycle. Thus, in such case, 
it can only function as a generator during the remaining 
slightly more than one-half of each half cycle, and thus 
its net output as a generator is essentially slight. At 
higher speeds, the ratio of power drawn to power deliv- 
ered improves. 
It is the object of this invention to effectively reduce 
the portion of each half cycle where current is drawn 
by the generator, and thus substantially improve its 
efficiency, particularly at low velocity drive levels 
which, in the case of windmill operation, may persist for 
a substantial portion of the time of operation. 
Disclosure of the Invention 
In accordance with this invention, an induction mo- 
todgenerator is mechanically driven and electrically 
coupled to an A.C. power line through a thyristor or 
thyristors operated to retard the time of interconnection 
to a relatively late point in each half cycle of the A.C. 
line voltage, whereby the energizing power supplied by 
the line to the motodgenerator may be decreased, and 
thereby the net power delivered to the line may be 
increased. Typically, a triac would be employed as the 
thyristor. Alternately, two SCR devices connected in 
antiparallel may be employed instead. 
Brief Description of the Drawings 
FIG. 1 is a schematic illustration of an embodiment of 
FIG. 2 is a graphical presentation of characteristics of 
the invention. 
operation of the invention. 
Best Mode For Carrying Out the Invention 
Referring to the drawings, an A.C. induction motor/- 
generator 10 is mechanically driven through shaft 12 by 
a propeller 14 of a windmill 16. In this illustration, a 
115-volt (or other voltage) A.C. power line 18 is cou- 
pled to terminals 20 and 22 and thence to the circuit of 
this invention. Thus, terminal 20 is connected to one 
3 
4,3 88.585 
4 
terminal 24 of generator 10, and a second power termi- 
nal 22 is connected through triac 28 to terminal 26 of 
generator 10, connection being via conventional triac 
power terminals MF1 and MT2. The firing angle for 
triac 28 is set by a conventional firing angle control 30, 
which is connected to power line 18 and supplied a 
trigger voltage to the gate terminal 32 of triac 28. 
As a feature of this invention, triac 28 is controlled to 
be turned “on” at a selected point, a relatively late 
point, in each half cycle of the A.C. voltage cycle as 
supplied by line 18. Typically, this voltage is at approxi- 
mately 150” into each half cycle. From this point, and 
until the 180” point when the line voltage passes 
through zero, the power line actually supplies an ener- 
gizing field current to generator 10 to enable immedi- 
ately following operation as a generator during a por- 
tion of the next half cycle. The turn “on” point is set no 
later than needed to effect generator operation for the 
particular generator used. Thus, power drain by the 
generator is minimized. 
Most significant in the present invention is the appli- 
cant’s utilization of the characteristic of the triac to turn 
“off” only after current goes through zero following a 
turn “on” of the triac as described. Significantly, this 
zero crossing by the current occurs when the generator 
ceases to deliver power to the line after which point the 
generator would consume power. Thus, there is 
achieved an automatic optimum control of the period of 
coupling between the line and generator. 
The operation described will be better appreciated by 
reference to the waveforms shown in FIG. 2 illustrative 
of the employment of induction motodgenerator 10 
with and without the present invention. Waveforms A, 
which will be first examined, are illustrated of windmill 
operation for a relatively low wind velocity and for the 
conventional case where triac 28 is omitted and genera- 
tor 10 and power line 18 are directly connected. Curve 
el is representative of line voltage of A.C. power line 
18. Curve il is illustrative of current flow between an 
A.C. power line and an induction motodgenerator. 
Waveform B of FIG. 2 illustrates, comparatively, the 
electrical operation of the system constructed in accor- 
dance with the present invention as illustrated by the 
circuit of FIG. 1, and wherein operation is for a like 
(low) wind condition to that depicted by waveforms A. 
In the applicant’s system, triac 28 is keyed “on” at ap- 
proximately the 150” point ti in each half wave voltage 
cycle of power line 18 as described above. Voltage 
curve e2 is illustrative of the voltage present on genera- 
tor 10. Current curve i2 is illustrative of current flow 
concurrent with voltage, both terminating at point t2 
during each half cycle. 
Significantly, it is to be noted with respect to wave- 
forms A and B that when current and voltage are of a 
like polarity, power is being extracted from the line by 
a generator to energize its field; and when current and 
voltage are of an unlike polarity, the generator func- 
tions to furnish power back to the line. 
To examine first the operation of the conventional 
case, and with reference to waveforms A, it is to be 
noted that power is supplied by a power line to an in- 
duction motodgenerator from time point t3 to time 
point t4, and power supplied by the generator to the 
power line from time point t4 to time point t5. By exami- 
nation of the curves during the two intervals; it is to be 
appreciated that there would occur only a slightly 
greater magnitude of power flow (current times volt- 
age) from the generator to the line than from the line to 
5 the generator. Thus, there is little net power supplied to 
the line. 
In contrast, by the employment of the applicant’s 
system, with the same wind velocity, the power sup- 
plied the line by the generator exceeds the power con- 
10 sumed by a greater margin. This is illustrated in wave- 
form B by comparing the combination of voltage and 
current curves e2 and i2, respectively, for the power 
consumption interval from time point t 1 to time point to 
and the power generation interval from time point to to 
Waveform C illustrates by voltage curve e3 and cur- 
rent waveform i3 operation of the system shown in FIG. 
1 for a relatively high wind velocity condition. Here, as 
will be noted, the energization period for the windmill 
20 is the same as shown in waveforms B, from time point 
ti to time point to, but the power generation portion 
from to to the end of the current half cycle t2 is signifi- 
cantly expanded. 
The applicant has not attempted to detail require- 
25 ments for driving an induction motodgenerator at a 
range just above its synchronization speed, as such ar- 
rangements may vary substantially. It is to be under- 
stood, however, that depending upon the optimum op- 
erating speed of the drive element, e.g., propeller 14, 
30 there would be appropriate gearing between the drive 
element and the shaft of the induction motodgenerator. 
The present invention has its principal effect in im- 
proving efficiency when the generator is driven, in the 
lower portion of its speed range, as in the case of wind- 
35 mill operation depicted in waveforms B. It enables a 
lower threshold of windmill speed for power genera- 
tion, and in its lower velocity range of operation, it 
enables a greater electrical output. Considering now 
that in many installations environmental winds are less 
40 than maximum most of the time, it is to be appreciated 
that the present invention provides a valuable contribu- 
tion to the art. 
15 time point t2. 
I claim: 
1. An electrical generating system comprising: 
an induction motodgenerator device having a power 
rotary power means for coupling a rotating move- 
an alternating current power supply line having a pair 
triac means, including at least one triac, having a 
control input and a pair of power terminals; 
said power terminals of said device and power line 
being interconnected through said power terminals 
control means connected between one of said power 
terminals and said control input of said triac for 
delaying the turn “on” point of said triac with 
respect to the period of each half cycle of alternat- 
ing current power to normally limit the power 
transfer from said power line to said device; 
whereby a higher net transfer of power from said 
device to said power line may be obtained. 
45 
shaft and a pair of power terminals; 
ment to said power shaft; 
50 of power terminals; 
55 of said triac, and 
60 
* * * * *  
65 


Electrical power generating system

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