Load control for wind-driven electric generators

A generator load curve is precisely matched to a wind-driven motor characteristic by means of a rotor speed-responsive tachometer effecting stepwise control of field current in the generator. Several variations of the tachometer circuit are described. Field current is controlled by an amplifier, and voltage regulation is effected by an override circuit disabling the amplifier.

BRIEF SUMMARY OF THE INVENTION 
In generating electric power by means of wind-driven generators, it is 
difficult to attain optimum efficiency because of a combination of 
factors. First, wind speed always varies unpredictably. Secondly, typical 
generator load curves are imcompatible with the characteristic curves of 
typical wind-driven rotors. As a result, a simple system in which a 
wind-driven rotor is arranged to drive a generator is, at best, optimally 
efficient only at one particular wind velocity. 
It can be shown, for example, that, using a given alternator having a 
constant field excitation in combination with a multiple-blade wind-driven 
rotor, if the rotor diameter is relatively large, the generating 
efficiency of the system approaches a maximum as wind speed increases from 
zero to a given velocity, and efficiency then drops off with further 
increases in speed. On the other hand, if the size of the rotor blades, or 
the number of blades, is decreased, a condition is reached where the 
torque produced by the wind-driven rotor is insufficient to overcome the 
load presented by the generator. When this occurs, the generator cannot 
increase in speed as wind speed increases. Consequently, generator speed 
is necessarily low, and the system cannot take advantage of the increased 
power available in high winds. In either case, the rotor-generator system 
is inefficient because it does not take full advantage of available wind 
power at all times. 
Some recognition has been given in the past to the foregoing problems, as 
demonstrated, for example, by the following U.S. Pat. Nos.: 2,339,749, 
issued Jan. 25, 1944 to J. R. Albers; 2,178,679, issued Nov. 7, 1939 to E. 
M. Claytor; 1,366,844, issued Jan. 25, 1921 to J. A. Snee, Jr.; 2,148,804, 
issued Feb. 28, 1939 to E. M. Claytor; 1,142,538, issued June 8, 1915 to 
J. A. Snee, Jr. et al.; 2,470,797, issued May 24, 1949 to P. H. Thomas; 
and 2,360,792, issued Oct. 17, 1944 to P. C. Putnam. The foregoing patents 
propose various sytems for improving the performance of wind-driven 
generators by correlating the power requirements of the generators to the 
power output characteristics of wind-driven rotors. 
Claytor U.S. Pat. No. 2,179,679 is of particular interest because it 
describes a wind-driven generator control system which switches shunt 
resistors in and out of the field circuit in accordance with wind speed, 
in order to produce a modified generator loading curve which approximates 
the locus of the peaks of the propeller curves. 
Albers U.S. Pat. No. 2,339,749 is also of interest in that it describes a 
system in which field current is controlled through a resistance varied by 
means of governor-operated cam so that, as rotor speed increases, 
resistance is gradually increased, then decreased. The system of Albers is 
at least theoretically capable of producing a better match between the 
generator load curve and the rotor curve for the reason that field current 
is controlled in accordance with rotor speed rather than directly by wind 
speed as is the case in Claytor. Where control is attempted in accordance 
with wind speed, matching of the curves is imperfect because wind speed 
and rotor speed correspond to each other only under steady-state 
conditions. The theoretically perfect matching achieved by the mechanical 
system of Albers has not been achieved heretofore in systems with 
non-mechanical speed sensing. 
The present invention constitutes an improvement over the systems described 
by Claytor, Albers and the others in several important respects. One 
object of the invention is to achieve improved performance in terms of the 
efficiency of operation by providing more precise matching of the rotor 
and generator curves than is achieved by Claytor. Another object is to 
provide precise matching of the rotor and generator curves over a wide 
range of rotational speeds. Another object of the invention is to achieve 
such precise matching at a low cost by the use of an all-electronic system 
rather than a mechanical one. Another object of the invention is to 
provide a novel matching system incorporating voltage regulation. Still 
another object of the invention is to provide a matching system having 
novel and improved sensing means for determining rotor speed. 
The principal building blocks of the invention are the generator speed 
sensor and the generator field current control, which is responsive to a 
signal produced by the speed sensor. 
The speed sensor establishes at least four contiguous speed ranges which 
together consitute a wide range of rotational speeds. For each of the 
speed ranges into which the wide range is divided, the speed sensor 
establishes a predetermined amplitude for its output signal. The speed 
sensor is capable of different signal amplitudes for adjacent speed 
ranges. However, because the ultimate purpose of the apparatus is to match 
the load curve of the generator to the available wind power, in some cases 
it is necessary to produce a field current which does not vary 
monotonically with generator speed. The sensing means is therefore 
designed so that, with a proper choice of components, any desired signal 
amplitude can be produced for any given one of the generator's speed 
ranges. Thus, while the signal amplitudes for adjacent speed ranges are 
different, it is entirely possible for two separated speed ranges to be 
assigned identical signal amplitudes. The foregoing objective is 
preferably achieved by utilizing a source of constant voltage together 
with a resistive dropping network having a plurality of selectable 
resistors. Selected resistors are connected into the dropping network in 
accordance with generator speed. 
The preferred way of selecting resistors for connection into the dropping 
network is to generate a pulse train the repetition rate of which is 
proportional to generator speed, and to count the number of pulses 
occurring in a predetermined time interval. Counting is carried out 
repetitively for successive time intervals, and selective switching of 
resistors is controlled by a register which holds a count, established in 
a first time interval, temporarily until a new count is established in a 
subsequent time interval. 
Another way of selecting resistors is to utilize an array of analog voltage 
comparators together with a tachometer circuit which establishes a varying 
d.c. voltage which increases with increasing generator speed. The 
comparators effect connection of appropriate resistances into the dropping 
network in dependence on generator speed. 
The pulse train which is counted or the varying d.c. voltage which is 
delivered to the analog voltage comparators is derived in either of two 
ways. 
In one alternative, an amplifier having a high input impedance is connected 
to one of the phase windings in the generator stator. When the generator 
armature rotates, an a.c. signal is produced in the stator. This a.c. 
signal is present even when the field is not energized (which is 
preferably the case at very low speeds), as a result of residual 
magnetization in the armature core. The a.c. signal is amplified by the 
high-impedance amplifier, and the amplifier output is either used to 
produce a pulse train for counting, or to produce a varying d.c. signal 
for delivery to an array of analog comparators. 
In another alternative the magnetic pick-up is positioned adjacent the fins 
of the generator's cooling fan. The fins of the cooling fan are unevenly 
spaced in order to avoid a "siren" effect, but this produces a non-uniform 
pulse train at the output of the magnetic pick-up. A prescaling counter 
operates on the output of the magnetic pick-up to produce a uniform train 
of pulses for counting. The prescaling counter may also be used in 
conjunction with the array of analog voltage comparators. 
The control means, the other principal building block of the invention 
comprises an amplification circuit connected to receive the signal 
produced by the sensing means. The output of the amplification circuit is 
connected to deliver current to the field winding. By utilizing an 
amplification circuit to control field current together with a plurality 
of selectable resistors in a voltage dropping network, it is a very simple 
matter to utilize any number of selectable resistors to determine the 
number of steps deemed necessary to achieve the required degree of 
precision in matching generator loading to available wind power. It is 
also a simple matter to make empirically derived adjustments in resistor 
values by first using variable resistors ("potentiometers"), making 
adjustments in their resistances and then choosing fixed resistors with 
corresponding values. In summary the control means in accordance with the 
invention makes it very easy to achieve very precise matching. 
Other objects of the invention and further details and advantages will be 
apparent from the following detailed description when read in conjunction 
with the drawings.

The circuit of FIG. 1 can be used in conjunction with any one of the 
circuits of FIGS. 2, 3 and 4. Interconnections are made by connecting 
correspondingly numbered interconnection terminals (blocks 1 and 2). When 
the circuit of FIG. 3 is used, only interconnection terminals 2 are 
connected. 
Detailed Description 
FIG. 1 shows a three-bladed wind-driven rotor supported atop a tower 10. 
The rotor is preferably of the type described in the copending application 
of Kevin E. Moran, Ser. No. 676,535, Filed Apr. 12, 1976. While, as shown 
in the Moran application, the generator is supported atop the tower with 
the rotor and driven by the rotor through a belt, the generator in FIG. 1 
is illustrated schematically by rotor 12 together with a set of three 
armature windings 14, 16 and 18, interconnected in a Y cofiguration. The 
generator rotor 12 is driven by wind-driven rotor 8 through a transmission 
20, which may be a belt, or gearing, or some other form of mechanical 
power transmission. The generator is typically a three-phase, seven pole 
device, resembling a conventional automotive alternator. The armature 
windings are connected through an array 22 of diodes to a storage battery 
24 for charging the same. As shown, each of the armature windings is 
connected through a first diode to the positive terminal of batter 24 and 
through a second diode to the negative terminal of the battery, the diodes 
being connected in appropriate directions to effect proper charging of the 
battery and to prevent discharge. Thus, for example, winding 18 is 
connected through diode 26 to the positive battery terminal and through 
diode 28 to the negative battery terminal. In addition to its being 
connected through diodes to battery 24, winding 16 is also connected to 
interconnection terminal 1. 
Field current is delivered to rotor 12 through a pair of brushes, a first 
brush being connected through line 30 to ground, and the second brush 
being connected through line 32 to a field current control circuit 
generally designated 34. The field current control circuit comprises an 
operational amplifier 36, which can be for example, one of the four 
independent dual-input amplifiers in the LM3900 quad amplifier available 
from Nation Semiconductor Incorporated, 2900 Semiconductor Drive, Santa 
Clara, Calif. The "+" input of amplifier 36 is connected through resistor 
38 to interconnection terminal 2 which is connected to receive, from a 
speed-sensing circuit, a signal, the amplitude of which varies as a 
function of generator speed. The "-" input of ampifier 36 is connected to 
receive a sample of the voltage across the field winding, there being a 
connection from the field winding, through line 32 and through voltage 
dropping resistors 40 and 42 to ground, and from the junction of resistors 
40 and 42, through resistors 44 and 46, to the "-" input of amplifier 36. 
Diode 48, connected between line 32 and ground, protects amplifier 36 from 
back e.m.f. induced in the field winding. Capacitor 50, also connected 
between line 32 and ground, filters out the ripple voltage which would 
otherwise appear across the field winding as a result of operation of the 
alternator, so that the ripple voltage does not interfere with precise 
control of field current. 
The output of amplifier 36 is connected through resistor 52 to the base of 
NPN transistor 54. The collector of transistor 54 is connected through the 
resistor 56 to a positive voltage supply terminal 55, labeled V.sub.B. 
(Terminals labeled V.sub.B are connected to the positive terminal of 
storage batter 24). The collector of transistor 54 is also connected 
through resistor 58 to the base of PNP transistor 60. The collector of 
transistor 60 is connected to the base of NPN transistor 62. The emitter 
of transistor 60 and the collector of transistor 62 are connected together 
to terminal 55. The emitter of transistor 62 is connected to line 32, and 
a protective diode 64 is connected between the emitter and collector of 
transistor 62. 
When the voltage of the signal at interconnection terminal 2 is zero, the 
output of amplifier 36 is low, and transistor 54 is cut off so that the 
battery voltage at terminal 55 is applied to the base of transistor 60. 
When the base of transistor 60 is thus held positive, there is no current 
at the base of transistor 62, and transistor 62 is thus cut off, 
preventing current from entering the field winding of the generator. When 
a positive signal appears at interconnection terminal 2, however, the 
output of amplifier 36 goes positive, and causes transistor 54 to conduct. 
The base of transistor 60 is brought to ground potential, and transistor 
62 conducts, applying current to the generator field winding. 
The description thus far does not take into account the negative feedback 
path provided by resistors 40, 42, 44 and 46. This negative feedback path, 
in cooperation with the gain of the amplification circuitry causes the 
voltage across the field to follow the voltage of the signal at 
interconnection terminal 2. The voltage ratio of this circuit is 
determined primarily by the attenuation in the feedback path since 
resistors 38 and 46 are equal and the amplifier has a high gain. The 
values of dropping resistors 40 and 42 in the feedback path are preferably 
chosen so that the voltage at the "-" terminal of amplifier 36 is 
one-third the voltage across the field winding of the generator. Because 
of this ratio in the feedback path, under equilibrium conditions the 
voltage across the field winding is approximately three times the voltage 
at the terminal 2. The amplification circuit allows for precise control of 
generator field current from a very low power signal applied to terminal 
2. This amplification circuit, because of its feedback loop, also provides 
for a controlled field current which is substantially independent of 
variations in battery voltage. 
The field current control circuit of FIG. 1 also includes a voltage 
regulator circuit for setting the maximum voltage at the output of the 
alternator. The main purpose of the voltage regulator is to prevent 
overcharging of the storage battery. 
The voltage regulator circuit is constructed as follows. A pair of 
resistors 66 and 68 are connected in series between ground and terminal 
70, which is connected to the positive side of the storage battery, and 
accordingly labeled V.sub.B. Preferably, the resistance of register 66 is 
approximately 3 time the resistance of resistor 68, so that the voltage 
appearing across resistor 68 is about one-fourth of the battery voltage. 
This voltage, appearing at the junction between resistors 66 and 68 is 
applied to the "-" input of amplifier 72 through resistor 74. Amplifier 72 
is another operational amplifier, similar to amplifier 36. Signals applied 
to its "+" and "-" inputs have opposite effects on its output. A smoothing 
capacitor 76, e.g. 10 microfarads, is connected across resistor 68 to 
prevent any switching of the voltage regulator which might be caused by a 
ripple voltage at the alternator output. Terminal 78 is connected to the 
positive side of a separate five volt supply. A voltage dropping network 
is provided by the series combination of variable resistor 80 and fixed 
resistor 82, connected between terminal 78 and ground. The wiper of 
resistor 80 is connected through resistor 84 to the "+" input terminal of 
amplifier 72. The values of resistor 80 and 82 are preferably so chosen 
that the voltage, measured between the wiper of resistor 80 and ground can 
be varied from a minimum of 3.5 volts to a maximum of 5 volts. The output 
of amplifier 72 is connected through resistor 86 to the base of NPN 
transistor 88. The emitter of transistor 88 is connected to ground, and 
the collector is connected through resistor 90 and diode 92 to junction 94 
between resistors 44 and 46 in the feedback path of amplifier 36. Resistor 
96 is connected between positive terminal 70 and the junction between 
resistor 90 and diode 92. In normal operation, transistor 88 is 
conductive, and the voltage at the anode of diode 92 is lower than the 
lowest non-zero control voltage applied to terminal 2. However, the 
connection through resistor 96 through positive terminal 70 provides for a 
high positive voltage at the anode of diode 92 when transistor 88 is cut 
off. 
The operation of the voltage regulator circuit involves a comparison of the 
alternator output voltage (at terminal 70) with a reference voltage 
established by the setting of variable resistor 80. The comparison is 
accomplished by amplifier 72. Assuming, for the sake of illustration, that 
resistors 66 and 68 are chosen to produce a voltage drop to one -fourth 
the voltage at terminal 70, when the voltage at terminal 70 is below a 
value which is four times the voltage at the wiper of resistor 80, the 
output of amplifier 72 is high, and transistor 88 is conductive. Under 
these conditions, the voltage at the anode of diode 92 is low (typically 
one volt). Assuming that the voltage across the generator field is zero, 
the small voltage at the anode of diode 92 produces a small current at the 
"-" input of amplifier 36, forcing the output of amplifier 36 to a low 
condition. The voltage at interconnection terminal 2 will be either zero 
or very slightly positive under these circumstances. 
The speed sensing circuits (the details of which are yet to be described) 
produce discontinuous stepwise variations in the voltage amplitude at 
interconnection terminal 2, and the lowest non-zero value is typically 
about 2 volts. With a non-zero voltage at terminal 2, the output of 
amplifier 36 goes high, and current is applied to the generator field. The 
voltage across the field produces a positive voltage at junction 94 which 
reverse-biases diode 92, so that the voltage at the anode of diode 92 has 
no effect on the field current. 
If the voltage at the output of the alternator exceeds the value which is 
preset by the adjustment of resistor 80 (i.e. the voltage at terminal 70 
is more than four times the voltage at the wiper of resistor 80), then the 
output of amplifier 72 switches to a low condition, transistor 88 is cut 
off, and the battery voltage at terminal 70 is applied to the anode of 
diode 92. This overrides the normal operation of the field current 
control, by applying a high positive voltage to the "-" input of amplifier 
36. When this occurs, no field current is applied, regardless of the 
signal level at interconnection terminal 2. 
A resistor 98 is connected between the output of amplifier 72, and its "+" 
input. Resistor 98 provides a small amount of positive feedback to prevent 
the output of amplifier 72 from switching rapidly between its high and its 
low position when the voltage at terminal 70 hovers about the critical 
value. The effect of resistor 98 is to increase the difference between the 
amplitudes of the "-" and "+" inputs as the generator output voltage rises 
above the reference level. Resistor 84 assists resistor 98 in 
accomplishing this objective, in that it permits the "+" input of 
amplifier 72 to be modified more easily by current in resistor 98. 
Resistors 84 and 74 are preferably equal so that the regulation range is 
determined only by resistors 66, 68, 80 and 82. 
With resistors 80 and 82 chosen to provide for a variation in the voltage 
at the wiper of resistor 80 from 3.5 to 5 volts, and with resistors 66 and 
68 chosen to produce a voltage drop ratio of four, the voltage regulator 
can be set to limit the generator output voltage to any chosen value from 
14 to 20 volts. The range of the voltage regulator can be easily modified 
by changing the values of these resistances. The range of 14 to 20 volts 
is desirable in wind-driven power plant systems, where a 12 volt storage 
battery is located at a distance from the generator. The limits of 14 to 
20 volts allow for losses in the cable connecting the generator to the 
storage battery. 
The general function of the circuit in FIG. 2 is to sense the rotational 
speed of the generator and to provide a signal for delivery to the control 
circuit of FIG. 1, which signal has an amplitude varying as a function of 
generator speed. In the circuit of FIG. 2, generator speed is sensed by 
counting pulses derived from the generator armature. To this end, a 
connection is made, through interconnection terminal 1, between winding 16 
of the generator armature (FIG. 1) and the base of an NPN transistor 100. 
Transistor 100 is interconnected with another NPN transistor 102 in a 
Darlington configuration. The collectors of both transistors are connected 
through terminal 104 to the positive side of the storage battery, and the 
emitter of transistor 102 is connected through resistor 106 to ground. An 
output is taken from the emitter of transistor 102, so that the Darlington 
configuration acts as a buffer, having a high input impedance and a gain 
of unity. 
The signal applied by the generator armature to the base of transistor 100 
is an alternating voltage superimposed on a positive d.c. level, the 
latter resulting from reverse leakage in the rectifiers which connect the 
armature to the storage battery. The frequency of this a.c. coltage varies 
with rotational speed of the generator, and the a.c. voltage is present 
even when the field is not energized, as a result of residual magetization 
in the core of the field winding. 
The emitter of transistor 102 is connected through resistor 108 and 
capacitor 110 to ground. The resistor and capacitor together constitute a 
low-pass filter for eliminating radio-frequency interference. The output 
of the filter is taken from the ungrounded side of capacitor 110. Diode 
112 limits the negative excursion of the filter output to -0.07 volts, and 
prevents high negative voltages which occur when the field is energized, 
from causing damage to amplification circuitry receiving the output of the 
low-pass filter. The output of the filter is coupled through capacitor 114 
and resistor 116 to the "-" input of operational amplifier 118, which is 
similar to amplifiers 36 and 72. The "+" input of amplifier 118 is 
grounded, and a resistor 120 is connected between the output of the 
amplifier and its "-" input. Another resistor 122 is connected between the 
output and ground. The function of resistor 120 is to provide negative 
feedback, controlling the gain of the amplification circuitry. Thus, the 
voltage gain from the emitter of transistor 102 to the output of amplifier 
118 is determined by the values of resistors 120, 116 and 108. Typically, 
resistor 120 has a resistance value about 200 times the sum of the values 
of resistors 116 and 108, so that the voltage gain is approximately 200. 
This results in saturation of amplifier 118 at voltage peaks, producing 
rectangular pulses at output line 124. Resistor 122 is provided in order 
to allow the output of amplifier 118 to reach a "zero" logic level. 
The circuitry of FIG. 2 thus far described acts as a combined amplifier and 
pulse-shaper, and produces, at line 124, a series of pulses which are 
suitable for actuating the logic circuitry which will not be described. 
The logic circuitry repetitively establishes a predetermined time 
interval, repetitively counts the number of pulses in line 124 which occur 
in the predetermined time interval, and establishes, at interconnection 
terminal 2, a signal amplitude which depends upon the number of pulses 
counted in a given counting cycle. 
Line 124, which carries the series of pulses, is connected to the trigger 
input T of a monostable multivibrator 126. The multivibrator is a National 
Semiconductor LM555 timer, with appropriate external connections for 
monostable operation, as shown. Thus, a timing capacitor 128 is connected 
between a "threshold" terminal and ground. Resistors 130 and 132, the 
latter being adjustable, are connected between the positive 5 volt supply 
at terminal 134, and the ungrounded side of capacitor 28 for charging the 
capacitor to establish the timing interval. Output terminal U is connected 
tO deliver a positive-going timing pulse to line 136. The leading edge of 
the positive-going timing pulse occurs when the trigger input at T 
switches from a high to a low condition. 
Line 124 is also connected to one of the inputs of a two-input NAND gate 
138, the other input of which is connected to line 136 to recieve the 
output of timing multivibrator 126. The output of NAND gate 138 is 
connected through line 140 to one of the inputs of a second two-input NAND 
gate 142. The output of NAND gate 142 is connected through line 144 to 
input terminal I of counter 146. Counter 146 is a four-bit binary counter 
having outputs A, B, C and D, providing divisions by 2, 4, 8 and 16 
respectively. Counter 146 has a pair of reset terminals R, both of which 
are connected to an internal gate. An input terminal E is connected to the 
input of the second flip-flop in the counter, and an external connection 
must be made through line 148 between terminals E and A in order to 
connect the output of the first flip-flop in the counter to the input of 
the second flip-flop. 
The outputs of the counter at terminals B and D are connected to the inputs 
of two-input NAND gate 150, the output of which is connected through line 
152 to the remaining input of NAND gate 142. The purpose of gate 150 is to 
prevent the counter from counting beyond a count of fifteen in which event 
its output would correspond to a count of zero and cause faulty control. 
Since the maximum usable count in the system is a count of ten, it is 
possible to utilize terminals B and D to limit the count, since they both 
go "high" at a count of 10. when this occurs, the output of gate 150 
disables gate 142, and pevents further pulses from being counted. 
Line 136, which is connected to output terminal U of multivibrator 126, is 
connected through capacitor 154 to the reset terminals R of counter 146. 
Resistor 156 is connected between reset terminals R and ground to keep the 
reset terminals normally at ground potential to allow counting to take 
place. Diode 158, connected across resistor 156 prevents the reset 
terminals from going negative at the end of a timing pulse and causing 
damage to the counter. 
The binary-coded output from terminals A, B, C and D of counter 146 is 
connected through lines 158, 160, 162 and 164, to the inputs of a four-bit 
latch 166. The latch is a National Semiconductor DM7495 four-bit 
parallel-in parallel-out shift register. The shift register has a pair of 
clock inputs, one of which, J, is connected to the output of multivibrator 
126 through line 136. (The other clock input is disabled.) The shift 
register is designed so that data transfer takes place on the negative 
transition of the clock pulse. When the clock input at J switches from a 
high to a low condition, the latch retains the binary-coded information 
from counter 146 until the J input again switches from a high to a low 
condition. The output of latch 166 is delivered to lines 168, 170, 172 and 
174 to a binary-to-decimal decoder 176. Decoder 176 is a National 
Semiconductor DM74145 BCD-to-decimal decoder/driver, which translates a 
four-bit binary input to a decimal output. As shown, there are ten 
outputs, three of which are connected to line 178 directly through lines 
180, 182 and 184. The remaining outputs of decoder 176 are connected to 
line 178 through resistors 186-198. These resistors have resistance values 
selected in accordance with the desired functional relaionship between 
generator speed and signal amplitude at interconnection terminal 2. 
Decoder 176 functions in such a way that all of the outputs are 
open-circuited except for the single output corresponding to the decimal 
equivalent of the binary input, the latter being connected to ground. The 
outputs of the decoder may be thought of as numbered from "zero" to 
"nine," with line 184 being connected to the "zero" output, line 182 being 
connected to the "one" output, and so on. The decoder grounds line 184 
when the output of the latch corresponds to a binary "zero". It grounds 
line 182 when the output of the latch corresponds to a binary "one," and 
so on. At a count of nine, the decoder output connected to resistor 198 is 
grounded. When a count of ten or more is present at the inputs, all of the 
outputs are open-circuited. 
Line 178 is connected to a 5 volt supply terminal 200 through variable 
resistor 202 and fixed resistor 204. Line 178 carries a variable voltage 
depending on the condition of decoder 176. A connection is made from line 
178, through resistor 206, to interconnection terminal 2. A capacitor 208 
is connected between terminal 2 and ground. The combination of resistor 
206 and capacitor 208 acts as an averaging circuit to prevent the decoder 
from causing mechanical stresses in the rotor-generator system by imposing 
instantaneous changes in loading. 
In the operation of the circuit of FIG. 2, as the generator rotor rotates, 
the alternating current signal at interconnection terminal 10 is 
translated into a train of positive-going pulses at line 124. The 
repetition rate of these pulses is directly proportional to generator 
speed. 
Initially the output of monostable multivibrator 136 is in a low condition. 
As a consequence, the output of gate 138, at line 140, is high. At the end 
of a first pulse appearing at line 124, the logical condition of line 124 
switches from "high" to "low". This triggers multivibrator 126 through its 
trigger terminal T, and the output at terminal U switches to a high 
condition for an interval of time predetermined by the adjustment of 
resistor 132. The interval typically is 0.0225 seconds. 
The output of multivibrator 126 at terminal U does three things. First, 
when the output of multivibrator 126 is in a high condition, it enables 
gate 138, permitting delivery of pulses to the counting circuitry. 
Secondly, the leading edge of the output pulse at terminal U, when applied 
to capacitor 154, produces a short, positive-going reset pulse which is 
applied to reset terminals R of counter 146 to reset the count of the 
counter to zero. Finally the transition of the multivibrator output from a 
high to a low condition causes latch 166 to retain the count in the 
counter until the next time a similar transition takes place. This mode of 
operation, provides for updating of the information retained in the latch 
before it becomes obsolete as a result of changing rotor speed. 
Thus, a first pulse in line 124 triggers monostable multivibrator 126, and 
counter 146 is reset. Since the output of multivibrator 136 enables gate 
138, subsequent pulses in line 124 produce corresponding pulses in line 
140. Assuming that line 150 is in a high condition, gate 142 is also 
enabled, and corresponding pulses are delivered to the counter input 
terminal I. Since there are two logic reversals by reason of the two NAND 
gates, the pulses at the counter input are positive-going and correspond 
to the pulses in line 124. 
The counter stops counting pulses when the output of multivibrator 126 goes 
"low" disabling gate 138. At the same time, the transition from "high" to 
"low" at terminal J of latch 166 causes a data transfer to take place 
within the latch, and output lines 168-174, which had been selectively 
energized in accordance with the last preceding count, are energized in 
accordance with the present count in the counter, and remain so energized 
until the end of the next output pulse from monostable multivibrator 126. 
Accordingly updating of the information retained in the latch takes place 
instantaneously. This is important for effective control of the signal at 
interconnection terminal 2. 
The binary-to-decimal decoder 176 translates the output of the latch to 
decimal form. For counts from zero to two, line 178 is grounded, and the 
voltage at interconnection terminal 2 is zero. For counts between three 
and nine, the voltage at terminal 2 is determined by the values of 
resistors 186-198. At a count of ten, line 178 is open-circuited, and a 
maximum voltage is applied to terminal 2. 
FIGS. 5 and 6 illustrate the operation of the combined speed sensing 
circuitry and control circuitry shown in FIGS. 1 and 2. In FIG. 5, curve 
210 is a typical characteristic generator load curve under saturated field 
conditions. For consistency with the other curves, curve 210 depicts 
mechanical input power to the generator rather than electrical output 
power. The mechanical input power corresponds to electrical output power 
divided by generator efficiency. The mechanical input power depicted by 
curve 210 corresponds to the maximum possible electrical output power from 
the generator for any given generator speed. 
Curve 212 is a typical curve depicting available wind power for a given 
mechanical system. This curve can be interpreted as meaning simply that, 
in a given mechanical system, for a given amount of available wind power 
corresponding to the ordinate of a point on the curve, the generator speed 
will not exceed the value of the abscissa of that point. 
It is characteristic of most generators that the curve corresponding to 
curve 210 is concave downwardly. Similarly, it is characteristic of most 
wind-driven rotors, that the curve corresponding to curve 212 is concave 
upwardly. In a system in which these curves retain their original shapes, 
curve 210 can be moved to the right by using a smaller generator, or curve 
212 can be moved to the left by using a larger rotor. However, if the 
relationship between the curves is thus modified so that they do not cross 
each other, then the system exhibits its maximum efficiency only at the 
speed where the curves most closely approach each other, and is rather 
inefficient elsewhere. 
On the other hand, if the curves cross each other as shown, there exists a 
range of generator speeds for which the mechanical load imposed by the 
generator on the wind-driven rotor exceeds the available wind power acting 
on the rotor. The speed of the generator cannot exceed the speed at the 
low end of this range, since there is not enough wind power to cause the 
generator speed to increase. 
In order to alleviate this problem, the circuitry of FIGS. 1 and 2 modifies 
the generator characteristic so that, instead of following curve 210, it 
follows curve 214, which is shown by a broken line. The object of the 
circuitry of FIGS. 1 and 2 is to cause curve 214 to match curve 212 
throughout the range in which generating loading exceeds available wind 
power. Above the upper end of this range, the circuit causes full field 
excitation to be applied, and curve 214 follows curve 210. 
FIG. 6 shows, by curve 216, how field current in the generator (current in 
line 32) varies with generating speed in a typical system. At the low end 
of the speed range, field current is held at zero. This is desirable, 
since, at low speeds, a generator will not produce a usable output, yet 
the field winding draws current. With current in the field cut off, drain 
on the storage battery is reduced. At a particular speed near the low end 
of the range, field current suddenly increases from zero to a relatively 
high value. As generator speed is further increased, field current 
decreases in a stepwise manner, and then increases until it reaches a 
point at which it levels off at a maximum value. The speed at which field 
current begins to decrease with increasing generator speed corresponds 
closely to the low end of the speed range in FIG. 5 in which curve 210 is 
above curve 212. The point at which field current levels off at its 
maximum value corresponds closely to the upper end of this range. 
The variations in generator field current depicted in FIG. 6 result in a 
modified generator characteristic curve 214, which, as shown, closely 
follows curve 212 in the range in which curve 212 is below curve 210, and 
closely follows curve 210 above this range. 
The speed sensing circuitry of FIG. 2 is calibrated in the following 
manner. First, the abscissa is found for the point P in FIG. 5 at which 
wind power begins to exceed the mechanical load posed by the generator. At 
this point, full field voltage should be applied. Consequently, the timing 
of monostable multivibrator 126 (FIG. 2) is adjusted so that the duration 
of its output pulse is equal to the time required for ten pulses to enter 
the counter at a generating speed corresponding to point P. Thus, for 
example, if point P corresponds to a speed 3800 r.p.m., and the generator 
is a 7 pole generator producing 7 cycles per revolution, the pulse rate in 
line 124 (FIG. 2) will be 443 pulses per second. At this rate, the time 
duration required for ten pulses to be counted is 0.0225 seconds. 
Monostable multivibrator 126 is adjusted accordingly by adjustment of 
resistor 132. 
Next, a determination is made of the "cut-in" speed, that is the minimum 
speed at which field current is applied. In general, the cut-in speed will 
be chosen to be near the lowest speed at which the generator will produce 
an output. Once the cut-in speed is determined, appropriate outputs of 
decoder 176 (FIG. 2) are connected directly to line 178. For example, if 
it is determined that the generator cannot provide an output until its 
speed exceeds 1100 r.p.m., the first three outputs of decoder 176 are 
connected directly to line 178 through lines 180, 182 and 184. This means 
that the cut-in speed will correspond to a decimal count of three, which 
is equivalent to 1140 r.p.m. Consequently, below 1140 r.p.m., the voltage 
applied to interconnection terminal 2 is zero, and the field current is 
accordingly zero. This reduces drain on the battery. At 1140 r.p.m., 
however, that is at a count of three pulses, decoder 176 connects resistor 
186 to ground, and the voltage at interconnection terminal 2 is determined 
by the voltage dropping network comprising resistors 202, 204 and 186. 
The values of resistor 186-198 are then carefully chosen to produce a 
stepwise variation in field current which will result in as close as 
possible a match between curve 214 and wind power curve 212. This can be 
accomplished by initially using variable resistors for resistors 186-198, 
adjusting their values for a precise match of the curves, and thereafter 
substituting fixed resistors. 
The basic shapes of curves 210 and 212, corresponding respectively to the 
generator and wind-driven rotor units, are similar for similar units, and, 
accordingly, resistors 186-198 do not have to be changed from one unit to 
the next. However, such differences as do occur between units can be 
compensated for by adjustment of the timing of monostable multivibrator 
126 through adjustment of resistor 132, and by adjustment of resistor 202, 
which determines the voltage at interconnection terminal 2. Differences in 
the power output capability of wind-driven rotors can be accommodated by 
adjustment of resistor 132. Differences in the generator outputs can be 
accommodated by adjustment of resistor 202. Once the values of resistors 
186-198 are appropriately chosen, precise matching of the wind-driven 
rotor and the generator is dependent only on these two controls. Together, 
these two controls provide considerable flexibility in the adjustment of 
the position and shape of curve 216 which in turn determines position and 
shape of the power response curve 214. 
From the foregoing, it will be seen that the circuit of FIGS. 1 and 2 
possesses a number of advantages: in particular, the ability to achieve 
optimum efficiency in operation by obtaining a precise matching of the 
wind power and generator load curves. 
Turning now to FIG. 3, an alternative speed sensing circuit is shown, 
utilizing a magnetic pick-up 218 located closely adjacent the blades of 
the cooling fan 220 of the generator. Magnetic pick-up 218 is one of a 
number of commonly available permanent magnet proximity transducers, such 
as model VR-375-1250 ST available from Transducer Systems, Inc., 710 
Davisville Road, Willow Grove, Penna. The nominal output for this 
particular unit is a two volt peak across a 10,000 ohm load. 
The blades of the generator cooling fan are not symmetrically spaced from 
each other. The purpose of the non-symmetrical spacing is to prevent the 
production of sirenlike noise when the cooling fan rotates. Unfortunately, 
the non-symmetrical spacing of the blades causes the series of pulses 
generated by a pick-up device cooperating with the blades to be uneven. If 
an attempt were made to use such a series of uneven pulses to drive the 
circuit of FIG. 2, erratic results would be obtained, preventing a good 
match between the wind power curve and the modified generator load curve. 
The circuit of FIG. 3 is similar to the circuit of FIG. 2, in that it 
repetitively establishes a predetermined time interval, and repetitively 
counts the number of pulses occurring in said interval, establishing a 
signal amplitude dependent on the count. However, in order to avoid the 
effect of the unevenness in the pulse train generated by the magnetic 
pick-up, an additional counter is provided, together with associated logic 
circuitry, and the time interval established by the monostable 
multivibrator is lengthened. The additional counter produces an even train 
of pulses, the repetition rate of the pulses being proportional to 
rotational speed of the generator. 
The amplification and pulse-shaping circuitry receiving the output of the 
pick-up through line 222 is similar to the corresponding circuitry in FIG. 
2. It comprises a Darlington circuit 224, filtering means 226, a clamping 
diode 228, and an operational amplifier 230, delivering an output to line 
232. 
The additional counter which produces an even pulse train comprises counter 
234, which is a four-bit binary counter similar to counter 146 of FIG. 2. 
Line 232 is connected to input terminal I of counter 234. The 
divide-by-sixteen output at terminal D and the divide-by-four output at 
terminal B are both connected to the inputs of a two-input NAND gate 236. 
The output of gate 236 is connected to one of the inputs of a second 
two-input NAND gate 238, the output of which is connected to both reset 
terminals R of counter 234. 
The pulses in line 232 are delivered to the trigger input T of monostable 
multivibrator 240, which corresponds to multivibrator 126 of FIG. 2 except 
that it produces a longer output pulse at its output terminal U. The 
output at terminal U is connected through line 242 to the remaining input 
of two-input NAND gate 238. The output at terminal U is also coupled to 
reset terminals R of counter 244, which corresponds to counter 146 of FIG. 
2. Also, the output of multivibrator 240 is connected through line 246 to 
the clock terminal J of latch 248 to effect data transfer in the latch at 
the trailing edge of the output pulse from multivibrator output terminal 
U. 
Counter 244 is driven in accordance with the pulses delivered by the D 
output of counter 234. The D output of counter 234 is connected through 
line 250 to an input of two-input NAND gate 252, the output of which is 
connected to input terminal I of counter 234. The other input of gate 252 
is connected to the output of a two-input NAND gate 254, which derives its 
inputs from output terminals B and D of counter 234. The purpose of gate 
254 is similar to that of gate 150 in FIG. 2, namely to stop counter 244 
at a count of ten. 
The four outputs at terminals A, B, C and D of counter 244 are connected to 
the inputs of latch 248, and the outputs of latch 248 are delivered to 
decoder 256, which is connected to an interconnection terminal 2 in the 
same manner as decoder 176 of FIG. 2. The circuit of FIG. 3 is used in 
conjunction with the control circuitry of FIG. 1, there being a connection 
between the two circuits at interconnection terminal 2. Since generator 
speed is sensed by a different means, interconnection terminal 1 in FIG. 1 
is not used. 
In general, the operation of the circuit of FIG. 3 is similar to that of 
the circuit of FIG. 2, the principal difference being that counter 244 
operates on the output of counter 234 rather than directly on the output 
of the amplification and pulse-shaping circuitry. At the end of a first 
pulse in line 232, the output of multivibrator 240 at terminal U goes 
"high". At this time the count in counter 234 is zero, and the ouput of 
gate 236 is also "high". With two "highs" at its input, the output of gate 
238 is "low", and counter 234 is enabled. Subsequent pulses are counted 
until a count of ten is reached, whereupon output terminals D and B of 
counter 234 are both in a logical "high" condition. At this point, the 
output of gate 236 switches to a low condition, producing a "high" at the 
output of gate 238, regardless of the state of the output of multivibrator 
240. The "high" at the output of gate 238 resets counter 234. The counter 
continues to count starting again at a count of zero. 
At a count of eight, the output of counter 234 at terminal D goes "high". 
This output returns to a low condition upon resetting of this counter. As 
a consequence, there is produced in line 250 a short pulse which is 
inverted by gate 252, and counted by counter 244. It should be noted that 
cooling fan 220 has eleven fins. The pulse produced by the first of these 
fins triggers monostable multivibrator 240, but is not counted by counter 
234. The remaining ten pulses in a single rotation of the generator are 
counted by counter 234. As a result, for every eleven pulses in line 232, 
one pulse is produced in line 250 for counting by counter 234. The pulses 
at the output of counter 234 are evenly spaced so long as the generator is 
operating at a constant speed. Consequently, the uneven spacing of the 
blades on fan 220 does not interfere with the proper operation of the 
counter, latch and decoder. More specifically, since only one pulse is 
produced at line 250 for each complete rotation of the generator, it does 
not matter what position the generator shaft is in when counting by 
counter 244 begins. 
The time interval for the high condition at output U of multivibrator 240 
should be seven times that of the corresponding time interval in the 
circuit of FIG. 2. Resistor 258 and capacitor 260, which determine the 
"on" time for the multivibrator are chosen accordingly. 
FIG. 4 shows another alternative circuit which can be interconnected with 
the circuit of FIG. 1 through interconnection terminals 1 and 2 to provide 
a complete system. 
In the circuit of FIG. 4, pulses generated in the generator armature are 
delivered from interconnection terminal 1 to an amplification and 
pulse-shaping circuit similar to that shown in FIGS. 2 and 3. The 
amplification and pulse-shaping circuit comprises a Darlington amplifier 
262, filtering means 264, a clamping diode 266, and an operational 
amplifier 268. It is of particular importance that the pulses produced by 
the amplification and pulse-shaping circuit be rectangular in shape, since 
they are delivered to a tachometer circuit 270, which will not operate 
properly unless the pulses at its input are rectangular. Accordingly, the 
gain of the amplification and pulse-shaping circuitry is set so that even 
the smallest pulses appearing at interconnection terminal 1 cause 
saturation in amplifier 268. 
The output of amplifier 268 is coupled through resistor 272 and capacitor 
274 to tachometer circuit 270. The tachometer is preferably a so-called 
"frequency doubling" tachometer, although other well-known tachometer 
circuits can be used. Frequency doubling is desirable as it reduces ripple 
in the d.c. output of the tachometer and simplifies filtering at the 
tachometer output. The tachometer circuit comprises an operation 
amplifier, which is desirably one unit in an LM3900quad amplifier. 
(Amplifier 268 can be another unit in the same quad.) Capacitor 274 is 
connected to the "+" input, and capacitor 276 is connected between the 
output and the "-" input. Also connected between the output and "-" input 
is the series combination of variable resistor 278 and fixed resistor 280. 
Diode 282 is connected between the "-" and "+" inputs of amplifier 270, 
the anode of the diode being connected to the "-" input. 
Tachometer 270 produces a d.c. output in line 284, the voltage level of 
which is proportional to the pulse rate at its input. The tachometer is 
calibrated by adjustment of resistor 278 so that a given pulse rate at the 
input can be made to correspond to a desired d.c. level at the output. 
An array of 4 voltage comparators 286, 288, 290 and 292 is provided. Each 
of these comparators is preferably one unit of a National Semiconductor 
LM339 quad comparator. A string of resistors is connected between a 
positive supply terminal 294 and ground. These resistors 296, 298, 300, 
302 and 304 constitute a voltage dividing network, providing predetermined 
reference voltage levels at juntions 306, 308, 310 and 312. Jucntion 306, 
which is at the lowest potential of the four junctions is connected 
through resistor 314 to the "+" input terminal of comparator 286. Junction 
308, which is at the next highest potential is connected to the "+" input 
of comparator 288 through resistor 316. Junction 310 is connected to the 
"+" input of comparator 290 through resistor 318. Junction 312 is 
connected to the "+" input of comparator 292 through resistor 320. 
The tachometer output, in line 284, is connected to the "-" input terminals 
of the comparators respectively through resistors 322, 324, 326 and 328. 
Each "-" input is connected to ground through a capacitor, capacitors 
being provided at 330, 332, 334 and 336. These capacitors, together with 
resistors 322-328 provide filtering of the tachometer output. The 
resistors 322-328 also provide isolation between the comparators. 
The outputs of comparators 286-292 are taken from the open collectors of 
internal transistors, and the internal circuitry of each comparator is 
such that the output transistor is conductive when the voltage level at 
the "-" input exceeds the voltage level at the "+" input, thereby 
grounding the output. Otherwise, the output transistor is cut off so that 
the comparator output is an open circuit. 
The output of comparator 286 is connected through resistors 338 and 340 to 
a positive supply terminal 342. Junction 344, between resistors 338 and 
340, is connected through resistor 346 to interconnection terminal 2. A 
capacitor 348 is connected between terminal 2 and ground, and together 
with resistor 346, forms an averaging circuit for the output signal at 
terminal 2. It should be noted at this point that resistors 340 and 338 
act as a voltage dropping pair, and that, when the only comparator with 
its output grounded is comparator 286, the voltage at interconnection 
terminal 2 is dependent upon the value of resistor 338. 
The output of comparator 288 is connected through resistors 350 to a 
positive supply terminal 352. The output of comparator 288 is also 
connected through resistor 354 to the base of NPN transistor 356, the 
emitter of which is grounded, and the collector of which is connected to 
terminal 2. The output of comparator 288 is also connected, through 
resistor 358 to the base of NPN transistor 360. The emitter of transistor 
360 is grounded, and its collector is connected through resistor 362 to 
the "-" input of comparator 288. 
The output of comparator 290 is connected through resistor 364 to junction 
344. Resistor 364 corresponds to resistor 338 at the output of comparator 
286, and has a similar function. The output of comparator 290 is also 
connected to the base of PNP transistor 366. The collector of this 
transistor is grounded, and its emitter is connected through line 368 to 
the "-" input of comparator 286. 
The output of comparator 292 is connected through diode 370 to the "-" 
input of comparator 290, and through diode 372 to the "-" input of 
comparator 286. 
In each of comparators 286, 290 and 292 a resistor is connected between the 
output and the "+" input. These resistors, 374,376 and 378 are similar in 
function to resistor 98 of FIG. 1; that is they provide positive feedback 
to prevent oscillation. 
The circuit of FIG. 4 senses generator speed and applies a control signal 
to interconnection terminal 2, the amplitude of the control signal varying 
in a stepwise manner in accordance with generator speed. In the case of 
FIG. 4, the amplitude of the signal at terminal 2 has four possible 
values, depending on the voltage level at line 284. 
When the generator is not turning, the outputs of all four comparators are 
open-circuited. With the output of comparator 288 open-circuited, the base 
of transistor 356 is positive by reason of its connection to the positive 
supply terminal 352. Accordingly, transistor 356 is conductive, and ground 
interconnection terminal 2, causing its voltage to be very nearly zero. It 
is preferred that the collector of transistor 356 be connected directly to 
terminal 2, rather than to junction 344 for two reasons. First, the 
connection directly to terminal 2 reduces the quiescent current drawn by 
the circuit when no power is being generated, while still allowing 
resistors 340, 338 and 364 to have relatively low values. It is important 
that these resistors have relatively low values to prevent the resistors 
374 and 376 from affecting the signal level at terminal 2. Secondly, by 
connecting the collector of transistor 356 directly to terminal 2, the 
collector-emitter saturation voltage is kept at a minimum, making the 
voltage at terminal 2 as small as possible during quiescent conditions. 
Before describing the complete operation of the circuit, it is important to 
note the function of transistor 360. Transistor 360, together with 
resistors 362 and 324 allows for precise turn-off, turn-off hysteresis 
when the circuit switches between the condition when the voltage at 
terminal 2 is zero and the condition in which the voltage at terminal 2 is 
positive. When transistor 360 is conductive, which is the case when the 
output of comparator 288 is open-circuited, the voltage at the "-" input 
of comparator 288 is determined by the voltage in line 284 from the 
tachometer, and the values of resistors 324 and 362, these resistors 
acting to produce a voltage drop at the "-" input of comparator 288. When 
this voltage exceeds the voltage at junction 308, the output of comparator 
288 is grounded, and transistors 356 and 360 are both cut off. Since 
transistor 360 is cut off, the voltage at the "-" input of comparator 288 
is the same as the voltage in line 284. This being the case, the output of 
comparator 288 cannot become open-circuited again until the tachometer 
output voltage in line 284 falls below the voltage level at juntion 308. 
Thus, the output of comparator 288 switches from an open condition to a 
grounded condition at a higher tachometer output level than the level at 
which it switches from a grounded to an open condition. This hysteresis 
allows for the fact that the blades of the wind-driven rotor will slow 
down when field excitation is applied. It eliminates a continual switching 
of the field excitation during low wind velocity conditions. 
As previously stated, when the generator is not turning, the output of 
tachometer 270 in line 284 is approximately zero, and the outputs of all 
of the comparators 286, 288, 290 and 292 are open-circuited. Transistor 
356, however, is conductive, and consequently the voltage level at 
terminal 2 is approximately equal to zero. The values of the resistors 
296-304 in the dropping network are chosen so that the voltages at 
junctions 306, 308, 310 and 312 correspond to the voltage levels at the 
output of tachometer 270 for four predetermined generator speeds. 
When the generator begins to speed up, and the voltage in line 284 exceeds 
the voltage at junction 306, the output of comparator 286 is grounded by 
the switching of the output transistor in the comparator. Nothing happens 
at terminal 2, however, until the voltage level at the output of the 
tachometer reaches the voltage level at the "-" input of comparator 288. 
When this occurs, the output of comparator 288 becomes grounded, and 
transistors 356 and 360 are both cut off. When transistor 356 becomes cut 
off, the voltage level at terminal 2 is established by the voltage 
dropping circuit comprising resistors 340 and 338. This condition prevails 
even if the generator speed drops, unless it drops to a level such that 
the voltage in line 284 falls below the voltage level at junction 308, 
whereupon the output of comparator 288 becomes open-circuited, and 
transistor 356 conducts, causing the voltage at terminal 2 to drop to 
zero. 
Assuming that, instead of dropping, the generator speed increases beyond 
the level required to cause the output of comparator 288 to switch, the 
voltage in line 284 eventually reaches the level of the voltage at 
junction 310. At this point, the output of comparator 290 is connected to 
ground. This causes transistor 366 to conduct, ground the "-" input of 
comparator 286. The output of comparator 286 opens, and resistor 338 is no 
longer instrumental in determining the voltage level at terminal 2. 
Rather, the voltage level at terminal 2 is now determined by the dropping 
network comprising resistors 340 and 364. It will be noted tha the 
presence of isolating resistor 322 is important because it allows 
transistor 36 to ground the "-" input of comparator 286 without seriously 
affecting the tachometer output. 
As generator speed further increases, the voltage in line 284 reaches the 
voltage level at junction 312. The output of comparator 292 becomes 
grounded when this occurs, and the "-" inputs of comparators 286 and 290 
are simultaneously grounded by the output of comparator 292 through diodes 
372 and 370 respectively. (Isolating resistors 322 and 326 are important, 
since they allow this to occur). With the outputs of comparators 286 and 
290 both open-circuited, resistors 338 and 364 are both out of service, 
and the voltage at interconnection terminal 2 is approximately equal to 
the supply voltage at terminal 342. 
Summarizing the foregoing, the circuit of FIG. 4 is capable of applying 
four different voltage levels to interconnection terminal 2, the first 
voltage being zero, the second voltage being determined by resistors 340 
and 338, the third voltage being determined by resistors 340 and 364, and 
the fourth being the full supply voltage at terminal 342. In this circuit, 
it should be noted that resistors 338 and 364, which determine the two 
intermediate voltage levels at terminal 2, are independent of each other 
in that, at any one of these two intermediate levels, the voltage at 
terminal 2 is determined by only one of these resistors. The independence 
of these two voltage levels makes it easy to adjust the output function of 
the circuit, since the adjustment of the voltage level for one step will 
not affect the voltage level for another step. 
The circuit of FIG. 4 can be easily modified to provide more than four 
voltage levels at the output terminal by the installation of additional 
comparators corresponding to comparator 290, together with appropriate 
logic (corresponding to transistor 366 and diodes 370 and 372) to insure 
that the voltage levels for individual steps are independently determined. 
While three specific embodiments of the invention have been described, it 
will be apparent that many modifications can be made to produce other 
specific embodiments. For example, the tachometer and comparator array of 
FIG. 4 can be used in conjunction with the magnetic pick-up of FIG. 3. 
Preferably, such a combination would take advantage of the prescaling 
counter 234. Any one of the circuits of FIGS. 2, 3 and 4 can be modified 
to provide a greater number (or a lesser number) of field current steps. 
It will also be apparent that, in any of the circuits described herein, 
various alternatives to the components specifically disclosed can be used, 
and that numerous other modifications can be made without departing from 
the scope of the invention as defined by the following claims.