High/low DC voltage motor vehicle electrical system

An automotive electrical system with a generator has outputs for loads requiring a high and low voltage, e.g., a heater and a battery. A field-controlled a.c. generator with three-phase output windings has a low voltage main terminal and an auxiliary terminal for high voltage loads. A step-down transformer and a first rectifier connect the windings and the main terminal while a second rectifieer directly connects the windings and the auxiliary terminal. A selector switch having a open and closed positions is connected between the main terminal and the auxiliary terminal. A voltage regulator for controlling current through the generator field is connected to the main terminal. When the selector switch is open the generator provides a dual voltage output i.e., a high voltage level at the auxiliary terminal and a low voltage level at the main terminal; in the closed position of the switch the regulator is connected to the auxiliary terminal and reduces the generator output thereof to said low voltage level while the output of the first rectifier is reduced to an ineffective level, due to the step-down effect of the transformer. The selector switch thus controls the generator output between a dual voltage output (switch open) and a low voltage output (switch closed).

This invention relates to an electrical system for an automotive vehicle 
which provides not only the usual low-magnitude DC voltage for energizing 
the ordinary low DC voltage loads of the vehicle, but which also provides 
a high-magnitude DC voltage capable of energizing a high DC voltage 
auxiliary load. 
The common motor vehicle electrical system includes a storage battery, a 
three-phase AC generator, a three-phase full-wave rectifier, and a voltage 
regulator. The battery provides standby power at a DC voltage of 
predetermined low magnitude (e.g., a nominal fourteen volts) between a 
main power terminal and system ground. The engine driven generator 
typically includes output windings across which a three-phase AC voltage 
is produced at an amplitude determined by the amount of current fed 
through a field winding. The rectifier acts to convert the three-phase AC 
voltage to a DC voltage between the main power terminal and system ground 
for charging the battery and for supplying the other low-magnitude DC 
voltage vehicle loads. The voltage regulator is responsive to the DC 
voltage appearing between the main power terminal and system ground to 
control the amount of current fed through the field winding of the 
generator so as to cause the three-phase AC voltage put out by the output 
windings of the generator to have an amplitude correct to establish and 
maintain the DC voltage at the predetermined low magnitude. 
In a motor vehicle electrical system of the above common type, it is 
sometimes necessary or desirable to provide electrical power to an 
auxiliary load requiring energization from a DC voltage of substantially 
greater magnitude (e.g., 50 to 75 volts) than the low-magnitude DC voltage 
ordinarily provided in such system. A high-power windshield heater element 
is one example of such a high DC voltage auxiliary load. The present 
invention provides a high-magnitude DC voltage power supply applicable to 
a motor vehicle electrical system of the above-described type and capable 
of advantageously energizing a high DC voltage load such as a high-power 
window glass heater. 
According to the invention, the foregoing common low-magnitude DC voltage 
automotive electrical system is modified by the addition of three 
elements: a three-phase AC voltage step-down transformer connected between 
the output of the engine driven three-phase generator and the input of the 
three-phase full-wave rectifier, a second three-phase full-wave rectifier 
connected between the output of the three-phase generator and an auxiliary 
power terminal, and a selector switch connected between the auxiliary 
power terminal and the main power terminal. In operation, when the switch 
is closed, the first rectifier is disabled and the three-phase AC voltage 
put out by the generator (as controlled by the voltage regulator) has a 
first amplitude correct when rectified by the second rectifier to 
establish and maintain the DC voltage between the main power terminal and 
system ground at the predetermined low magnitude. Conversely, when the 
switch is opened, the first rectifier is enabled and the three-phase AC 
voltage put out by the generator (as controlled by the voltage regulator) 
has a second higher amplitude correct when stepped down by the transformer 
and rectified by the first rectifier to establish and maintain the DC 
voltage between the main power terminal and system ground at the 
predetermined low magnitude. Additionally, with the switch opened, the 
second higher amplitude of the three-phase AC voltage put out by the 
generator is also correct when rectified by the second rectifier to 
establish and maintain a DC voltage between the auxiliary power terminal 
and system ground which is higher than the predetermined low-magnitude DC 
voltage by a voltage step-up ratio which is the converse of the voltage 
step-down ratio of the transformer. The latter high-magnitude DC voltage 
may be applied to energize a high DC voltage accessory load such as a 
high-power windshield heater. 
In another aspect of the invention, where the voltage regulator is of the 
type in which the DC voltage for supplying the field winding of the 
generator is derived from a separate rectifier, three further elements are 
provided: a second switch operable in unison with the selector switch, and 
third and fourth rectifiers. The third rectifier is connected from the 
output of the three-phase generator through the second switch to a field 
winding supply terminal for rectifying the three-phase AC voltage put out 
by the generator to provide a DC voltage of approximately the 
predetermined low magnitude for energizing the field winding when the 
second switch is closed. The fourth rectifier is connected between the 
output of the three-phase transformer and the field winding supply 
terminal for rectifying the stepped-down three-phase AC voltage put out by 
the transformer to provide a DC voltage of approximately the predetermined 
low magnitude for energizing the field winding when the second switch is 
opened. In this manner, the field winding of the generator is always 
energized by a DC voltage of approximately the predetermined low magnitude 
regardless of the state of the selector switch and the amplitude of the 
three-phase AC voltage put out by the generator.

Referring to FIG. 1 of the drawing, a motor vehicle electrical system 
includes a storage battery 10 connected between a main power terminal 12 
and system ground 14 for providing standby power at a DC voltage of 
predetermined low magnitude, e.g., a nominal 14 volts. Also connected 
between the main power terminal 12 and system ground 14, as represented by 
block 16, are various low-magnitude DC voltage loads of the type normally 
found in a motor vehicle. It will be understood that each of the loads 16 
may include appropriate switches and like devices for controlling the 
application of the low-magnitude DC voltage to energize such load. 
A three-phase AC generator 20 includes output windings 22a, 22b and 22c and 
a field winding 24. Preferably, the generator 20 is of the type where the 
phase windings 22a, 22b and 22c are stationary and the field winding 24 is 
mechanically driven in rotation by the vehicle engine 26 through an 
approximate drive linkage 28. In operation, a three-phase AC voltage is 
developed across the output windings 22a, 22b and 22c having an amplitude 
determined by the amount of current fed through the field winding 24 and 
having a frequency determined by the rotating speed of the field winding 
24. 
The generator output windings 22a, 22b and 22c are arranged in a wye or 
star configuration in which each is connected between a common neutral 
node 30 and a different associated one of a set of generator output 
terminals 32a, 32b and 32c. As will be appreciated by those skilled in the 
art, the line-to-line phase voltage components of the three-phase AC 
voltage produced by the generator 20 appear between the respective output 
terminals 32a, 32b and 32c. Alternatively, the generator output windings 
22a, 22b and 22c could be connected in a delta configuration if desired. 
A three-phase voltage step-down transformer 34 includes a set of primary 
windings 36a, 36b and 36c, and a corresponding set of secondary windings 
38a, 38b and 38c. The transformer primary windings 36a, 36b and 36c are 
each connected between different associated pairs of the generator output 
terminals 32a, 32b and 32c. The transformer secondary windings 38a, 38b 
and 38c are each connected between different associated pairs of 
transformer output terminals 40a, 40b and 40c. In operation, the 
transformer 34 is effective to step down the amplitude of the three-phase 
AC voltage put out by the generator 20 in accordance with a predetermined 
voltage step-down ratio, e.g., 4 to 1. Alternatively, the transformer 34 
could be an autotransformer. 
A first three-phase full-wave bridge rectifier 42 includes positively poled 
diodes 44a, 44b and 44c each connected between a different associated one 
of the transformer output terminals 40a, 40b and 40c, respectively, and 
the main power terminal 12. The first rectifier 42 further includes 
negatively poled diodes 46a, 46b and 46c each connected between a 
different associated one of the transformer output terminals 40a, 40b and 
40c, respectively, and system ground 14. In operation, the first rectifier 
42 is effective to rectify the stepped-down three-phase AC voltage put out 
by the transformer 34 to provide a first full-wave rectified DC voltage 
between the main power terminal 12 and system ground 14, provided that the 
rectifier 42 is appropriately forward biased or enabled. 
A second three-phase full-wave bridge rectifier 48 includes positively 
poled diodes 50a, 50b and 50c each connected between a different 
associated one of the generator output terminals 32a, 32b and 32c, 
respectively, and an auxiliary power terminal 52. The second rectifier 48 
further includes negatively poled diodes 54a, 54b and 54c each connected 
between a different associated one of the generator output terminals 32a, 
32b and 32c, respectively, and system ground 14. In operation, the second 
rectifier 48 is effective to rectify the three-phase AC voltage put out by 
the generator 20 to provide a second full-wave rectified DC voltage 
between the auxiliary power terminal 52 and system ground 14. 
A voltage regulator 56 is responsive to the DC voltage appearing between 
the main power terminal 12 and system ground 14 to control the amount of 
current fed through the generator field winding 24 to cause the amplitude 
of the three-phase AC voltage developed across the generator output 
windings 22a, 22b and 22c to be correct to establish and maintain such DC 
voltage at the predetermined low magnitude, e.g., a nominal 14 volts. For 
this purpose, the voltage regulator 56 may, for example, be of the type 
shown in U.S. Pat. No. 3,098,964 or in U.S. patent application No. 
775,172. 
A selector switch 58, operable between opened and closed states, is 
connected between the main power terminal 12 and the auxiliary power 
terminal 52. When the switch 58 is closed, the main and auxiliary power 
terminals 12 and 52 are connected together. In this condition, the 
full-wave rectified DC voltage provided by the second rectifier 48 is 
applied through the switch 58 between the main power terminal 12 and 
system ground 14. The voltage regulator 56 is responsive to this DC 
voltage to control the amount of current fed through the generator field 
winding 24 such that the three-phase AC voltage developed across the 
generator output windings 22a, 22b and 22c is at a first amplitude correct 
when rectified by the second rectifier 48 to establish and maintain the DC 
voltage between the main power terminal 12 and system ground 14 at the 
predetermined low magnitude, e.g., a nominal 14 volts. Due to the voltage 
step-down action of the transformer 34, the positively poled diodes 44a, 
44b and 44c of the first rectifier 42 are reverse biased, and so, the 
first rectifier 42 is disabled. 
When the switch 58 is opened, the main and auxiliary power terminals 12 and 
52 are disconnected from each other. In this condition, the positively 
poled diodes 44a, 44b and 44c of the first rectifier 42 are forward biased 
and the first rectifier 42 is enabled to provide a full-wave rectified DC 
voltage between the main power terminal 12 and system ground 14. The 
voltage regulator 56 is responsive to this DC voltage to increase the 
amount of current fed through the generator field winding 24 such that the 
three-phase AC voltage developed across the generator output windings 22a, 
22b and 22c is at a second greater amplitude correct when stepped-down by 
the transformer 34 and rectified by the first rectifier 42 to establish 
and maintain the DC voltage between the main power terminal 12 and system 
ground 14 at the predetermined low magnitude. 
Further, when the switch 58 is opened, the full-wave rectified DC voltage 
provided by the second rectifier 48 is applied between the auxiliary power 
terminal 52 and system ground 14. With the three-phase AC voltage put out 
by the generator 20 at the second greater amplitude, the DC voltage 
appearing between the auxiliary power terminal 52 and system ground 14 is 
at a predetermined high magnitude which is greater than the predetermined 
low magnitude of the DC voltage appearing between the main power terminal 
12 and system ground 14. Specifically, the high-magnitude DC voltage is 
greater than the low-magnitude DC voltage by a voltage step-up ratio which 
is the converse of the voltage step-down ratio of the transformer 34. For 
example, if the voltage step-down ratio of the transformer is 4:1, then 
the high-magnitude DC voltage will be approximately four times greater 
than the low-magnitude DC voltage, i.e., for a low-magnitude DC voltage of 
14 volts, the high-magnitude DC voltage would be approximately 56 volts. 
The latter high-magnitude DC voltage may be utilized to energize a high DC 
voltage accessory load 60 such as a high-power windshield heater. 
FIG. 2 illustrates a further embodiment of the invention applicable where 
the voltage regulator 56 is of the type in which the DC voltage for 
supplying the field winding 24 of the generator 20 is derived from a 
separate rectifier. Examples of this type of voltage regulator are 
disclosed in U.S. Pat. Nos. 3,469,168 and 3,539,864. Like numerals are 
used to denote like elements in FIGS. 1 and 2. 
In addition to the elements shown in FIG. 1, FIG. 2 includes a second 
switch 62, and third and fourth three-phase half-wave rectifiers 64 and 
66, respectively. The second switch 62 is operated in unison with the 
selector switch 58. The third rectifier 64 includes positively poled 
diodes 68a, 68b and 68c each connected from a different associated one of 
the generator output terminals 32a, 32b and 32c, respectively, through 
the second switch 62 to a field winding supply terminal 72 associated with 
the voltage regulator 56. The fourth rectifier 66 includes positively 
poled diodes 70a, 70b and 70c each connected from a different associated 
one of the transformer output terminals 40a, 40b and 40c, respectively, to 
the field winding supply terminal 72. 
In operation, when the switches 58 and 62 are closed, the third rectifier 
64 is effective to rectify the three-phase AC voltage produced at the 
generator output terminals 32a, 32b and 32c to provide a DC voltage of 
approximately the predetermined low magnitude for energizing the field 
winding 24 (with a current determined by the voltage regulator 56). 
Further, with the switch 62 closed, the fourth rectifier 66 is reverse 
biased or disabled. Alternatively, when the switches 58 and 62 are opened, 
the third rectifier 64 is disabled and the fourth rectifier 66 is enabled. 
In this condition, the fourth rectifier 66 is effective to rectify the 
stepped-down three-phase AC voltage produced at the transformer output 
terminals 40a, 40b and 40c to provide a DC voltage of approximately the 
predetermined low magnitude for energizing the field winding 24 (with a 
current determined by the voltage regulator 56). In this manner, the field 
winding 24 is always energized by a DC voltage of approximately the 
predetermined low magnitude regardless of the state of the selector switch 
58 and the amplitude of the three-phase AC voltage put out by the 
generator 20. 
It is to be noted that the foregoing embodiments of the invention are 
disclosed for purposes of illustration only and are not intended to limit 
the invention in any way. As will be appreciated by those skilled in the 
art, various alterations and modifications to the illustrated embodiments 
may be made without departing from the spirit and scope of the invention.