Circuit arrangement for starting detection of three-phase generator

In a circuit arrangement for measuring a starting of rotation of a generator having an exciting winding and three-phase windings, a voltage regulator regulates an output voltage of the generator by influencing an exciting current, the three-phase windings are connected to one another at a point, one of the phase windings is connected to ground via a first resistor, another of the phase windings is connected to ground via a second resistor, a voltage across the first resistor is evaluated as a rotational-speed-dependent alternating voltage, this voltage is compared with a threshold voltage, and the starting of the rotation of the generator is detected when the measured voltage exceeds the threshold voltage.

BACKGROUND OF THE INVENTION 
The present invention relates to a circuit arrangement for measuring the 
rotational speed of a generator. More particularly, it relates to a 
circuit arrangement for a starting detection of a three-phase generator. 
In motor vehicles, three-phase generators which excite themselves after the 
motor driving them has started up are usually used to generate the 
electrical energy. So that this self-excitation occurs reliably and 
safely, measures which improve the self-excitation are usually initiated 
directly after the motor vehicle engine starts. 
One customary method consists in using the charge monitoring lamp to 
pre-excite the generator. For this purpose, during starting, energy is fed 
to the exciter winding from the battery via the charge monitoring lamp. 
Another method consists in providing an additional lead between the 
battery and the exciter winding of the generator and conducting an 
additional exciter current via this connection during the starting 
process. 
The aforesaid measures are disclosed, for example, in the German 
Offenlegungsschrift 38 43 161 and the corresponding U.S. Pat. No. 
5,107,198. In the known solutions, additional means for detecting the 
start of the engine are required. 
However, in generator voltage controller systems, it is also known to use 
the rotational-speed-dependent alternating voltage in a phase winding of 
the three-phase generator in order to detect starting. In such a case, a 
voltage measurement is carried out in the voltage controller, during which 
voltage measurement the voltage is measured between the phase and earth 
and, when a threshold value is exceeded, it is detected that the engine is 
starting and thus the three-phase generator is beginning to rotate. 
This type of starting detection has the disadvantage that, if the switch-on 
threshold is too high, the cut-in speed of the generator is too high and, 
if the threshold is too low, the signal-to-noise ratio becomes too small, 
so that starting may be detected erroneously. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a circuit 
arrangement of the above mentioned general type, which avoids the 
disadvantages of the prior art. 
More particularly, it is an object of the present invention to provide a 
circuit arrangement of the above mentioned type, which has the advantage 
that either the threshold voltage can be increased when the cut-in speed 
is the same, as a result of which the signal-to-noise ratio is improved, 
or that the cut-in speed can be lowered. The threshold voltage here is the 
voltage which, when exceeded, permits the start of rotation of the 
generator to be detected. 
This advantage is achieved in that the voltage between one phase and earth 
is evaluated in a generator with star-connected phase windings, while a 
different phase winding is connected to earth via a further resistor. By 
means of this measure, a voltage divider is obtained which consists of the 
input resistor of the voltage meter and the additional resistor, this 
voltage divider being connected to the phase-to-phase voltage between the 
two phase windings. This phase-to-phase voltage is higher than the phase 
voltage by a factor .sqroot.3. 
Furthermore, it is advantageous that the threshold voltage which, when 
exceeded, permits the start of rotation of the generator to be detected, 
can be set easily by suitable selection or variation of the further 
resistor. 
The novel features which are considered as characteristic for the invention 
are set forth in particular in the appended claims. The invention itself, 
however, both as to its construction and its method of operation, together 
with additional objects and advantages thereof, will be best understood 
from the following description of specific embodiments when read in 
connection with the accompanying drawings. FIG. 1 shows a circuit 
arrangement according to the invention, and in FIG. 2 an equivalent 
circuit diagram is illustrated which permits the relationships to be more 
easily recognized.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a three-phase generator G with the phase windings U, W 
and V which are connected to one another at a common center point MP. The 
phase windings U, V and W are connected to the diodes or Zener diodes of 
the rectifier bridge GL via connections u, v, w. 
The rectifier diodes GL are connected via connections B+ to the positive 
pole of the battery and via B- to the negative pole of the battery B or 
earth. In each case a further diode or Zener diode D7, D8 can be connected 
between the center point MP between the phase windings and B+ or B-. The 
diodes of the rectifier bridge are designated by D1 to D6. 
The phase windings u, v, w have voltages Uu, Uv, Uw. The voltage Uu is 
present between the connection u of the winding u as shown in FIG. 1. The 
voltage U.sub.u is present between the connection u and the ground as 
shown in FIG. 1. 
In order to measure the rotational speed n.sub.G of the generator, the 
resistor R1 across which the voltage U1 drops is connected between the 
connection v of the phase winding V and earth. This resistor R1 is the 
internal resistor or the input resistor of a voltage detection device S. 
The rotational speed of the generator G can be determined from the voltage 
U1, which is a rotational-speed-dependent alternating voltage, in that for 
example the period of the alternating voltage is determined and the 
rotational speed is calculated from it in a known manner. By evaluating 
the level of the voltage U1 and comparing it with a threshold voltage US, 
a rotation of the generator can be detected when the voltage U1 exceeds 
the voltage US. 
An additional resistor R2 across which the voltage U2 drops is connected 
between the connection w and earth. This resistor substantially improves 
the voltage evaluation at R1, so that the detection of the rotation of the 
generator is also improved. 
The evaluations are usually also carried out in the voltage regulator R. 
In FIG. 2 an equivalent circuit diagram with the voltage sources Uv and Uw 
is illustrated, the said diagram indicating the voltage division in a 
circuit with the resistor R2. In this arrangement, the two voltage sources 
replace the phase windings V and W with the voltages Uv and Uw. 
If a generator phase which is different from that used for detection of the 
rotational speed in the regulator is connected to earth via a resistor R2, 
a voltage divider consisting of R2 and the input resistor R1 of the 
voltage-detection and the rotational-speed-detection circuit of the 
voltage regulator is connected to the phase-to-phase voltage Uvw of the 
generator in the arrangement selected in the exemplary embodiment. This 
voltage is higher than the phase voltage by the factor .sqroot.3. 
Since the voltage to be evaluated is greater than one of the phase voltages 
by the factor .sqroot.3, in comparison with rotational-speed measurement 
using the evaluation of a phase voltage, there is a greater voltage range 
which can be utilized to determine the start of rotation of the generator. 
Thus, the voltage limit value which, when exceeded, permits rotation of the 
generator to be detected can be increased in the voltage range which is 
made higher by .sqroot.3, as a result of which the signal-to-noise ratio 
is enlarged and the risk of rotation of the generator being erroneously 
detected is reduced. 
However, it is also possible to operate with a relatively low limit value; 
as a result of the voltage which is increased by .sqroot.3, in comparison 
with the evaluation with a phase voltage alone, rotation of the generator 
is detected earlier and the excitation can be initiated at an earlier 
point; the so-called cut-in speed of the generator is then lower in 
comparison with evaluation using one of the phase voltages without the 
additional resistor according to the invention. 
By varying the value of R2, any desired voltage to earth can be set at the 
fixed input resistor R1 of the voltage regulator. The threshold voltage at 
R1 can thus be reduced by means of R2, so that the rotation of the 
generator is detected by the voltage regulator at lower rotational speeds 
of the generator when the measured voltage exceeds the threshold voltage. 
After it has been detected that the generator is rotating, the start of the 
internal combustion engine is also detected and the voltage regular R can 
directly initiate measures which bring about an improvement in 
pre-excitation. 
Reference E identifies an exciting winding of the generator while reference 
I.sub.E identifies an exciter current which flows through the exciting 
winding and controls the voltage regulator R so that the initial voltage 
of the generator increases to a desired value. These generator components 
are known from the prior art and described for example in U.S. Pat. No. 
5,107,198. 
It will be understood that each of the elements described above, or two or 
more together, may also find a useful application in other types of 
constructions differing from the types described above. 
While the invention has been illustrated and described as embodied in a 
circuit arrangement for starting detection of three-phase generator, it is 
not intended to be limited to the details shown, since various 
modifications and structural changes may be made without departing in any 
way from the spirit of the present invention. 
Without further analysis, the foregoing will so fully reveal the gist of 
the present invention that others can, by applying current knowledge, 
readily adapt it for various applications without omitting features that, 
from the standpoint of prior art, fairly constitute essential 
characteristics of the generic or specific aspects of this invention. 
What is claimed as new and desired to be protected by Letters Patent is set 
forth in the appended claims.