Output voltage control apparatus of a permanent magnet alternator

A permanent magnet alternator including a flux diversion member resiliently and magnetically coupled to the permanent magnet rotor for rotation therewith, an auxiliary (control) permanent magnet rotor rigidly coupled to the flux diversion member for rotation therewith, an auxiliary stator disposed about the auxiliary rotor, and control means for variably loading the auxiliary stator to adjust the rotary position of the flux diversion member relative to the main permanent magnet rotor. The flux diversion member comprises a plurality of magnetic flux conducting vanes which coact with the rotor magnets such that variation of its rotary position relative to the rotary position of the permanent magnet rotor varies the stator flux, and hence the alternator output voltage.

The present invention is directed to output voltage control of an 
alternator having a permanent magnet rotor, and more particularly to an 
adjustable flux diversion apparatus for carrying out the control. 
BACKGROUND OF THE INVENTION 
The output voltage of a permanent magnet alternator is proportional to the 
product of the rotor speed and the stator flux produced by the rotor 
magnets. When the alternator is driven by a variable speed source, such as 
the engine of a motor vehicle, the output voltage and power likewise vary. 
Instead of employing an external voltage regulator, various flux diversion 
arrangements have been proposed for regulating the alternator output 
voltage by varying the stator flux. See, for example, U.S. Patent No. 
2,610,993 to Stark, issued Sept. 16, 1952, and the European Patent 
Application No. 058,025, filed Jan. 29, 1982. 
SUMMARY OF THE PRESENT INVENTION 
The present invention is directed to an improved flux diversion apparatus 
for voltage control of an N-pole alternator having a permanent magnet 
rotor, including a flux diversion member resiliently and magnetically 
coupled to the permanent magnet rotor for rotation therewith, an auxiliary 
permanent magnet rotor rigidly coupled to the flux diversion member for 
rotation therewith, an auxiliary stator disposed about the auxiliary rotor 
and control means for variably loading the auxiliary stator to adjust the 
rotary position of the flux diversion member relative to the main 
permanent magnet rotor. The flux diversion member comprises N regions of 
soft magnetic material which coact with the rotor magnets such that 
variation of its rotary position relative to the rotary position of the 
permanent magnet rotor varies the stator flux and hence the alternator 
output voltage and power.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT 
FIG. 1 depicts a permanent magnet alternator 10 comprising a main machine 
12 and an auxiliary or control machine 14 disposed within a single housing 
member 16. The alternator drive shaft 18 is supported on the ball bearing 
assemblies 20, 22 mounted in the housing end plates 24, 26, the end plates 
being fastened to the housing member 16, as shown. In a motor vehicle 
application, a suitable pulley (not shown) is retained on shaft 18 via the 
key 28 and belt driven by the engine crankshaft. 
The main machine 12 comprises a three-phase six-pole laminated stator 
assembly 30 secured to the inner periphery of housing 16, a six-pole 
permanent magnet rotor 32 secured to the drive shaft 18, and a flux 
diversion member 34 disposed in the working air gap between the stator 30 
and rotor 32. The end leads 23 of the stator winding 25 are fed through a 
suitable opening in the end plate 24, as shown. The flux diversion member 
34 is supported at each end thereof by a nonmagnetic end ring 36, 38. The 
end rings 36, 38, in turn, are rotatably supported with respect to shaft 
18 on the sleeve bearings 40, 42. 
A pair of springs 44, 46 resiliently couple the shaft 18 to the flux 
diversion member 34 to produce a centering torque which tends to align the 
flux diversion member 34 with the rotor 32 as illustrated in FIGS. 1 and 
2a. Springs 44, 46 are preferably wrap springs; while normal spiral 
springs may alternately be employed, wrap springs have a nonlinear force 
characteristic which is used to advantage in the illustrated embodiment of 
this invention, as explained below. 
The auxiliary machine 14 comprises a three-phase two-pole laminated stator 
assembly 50 secured to the inner periphery of housing 16, and a two-pole 
permanent magnet rotor 52 secured to an axial extension of the nonmagnetic 
end ring 38. The end leads 54 of the stator windings 55 are fed through a 
suitable opening in the housing end plate 26 and connected to a load 
control unit 56, described below in reference to FIG. 4. 
As best seen in FIGS. 1, 2a and 2b, the flux diversion member 34 is defined 
by a cage of six spaced arcuate vanes 60 formed of soft magnetic material. 
The circumferential dimension of each vane 60 is substantially the same as 
that of a permanent magnet of the rotor 32. The vanes 60 may be attached 
to the support rings 36, 38 by a dovetail or similar locking fit 
In FIG. 2a, the flux diversion member vanes 60 are exactly aligned with the 
permanent magnets of rotor 32. Maximum stator flux is achieved in this 
orientation, as substantially all of the flux produced by the rotor 
magnets passes through the vanes 60 and into the stator 30. This is a 
magnetically stable orientation; over a limited range, any angular 
displacement of the flux diversion member 34 from the stable position 
results in a magnetic restoring (centering) torque due to the reluctance 
change between the rotor magnets and the vanes 60. 
The magnitude and direction of the magnetic restoring torque varies with 
the relative displacement between the magnets of rotor 32 and flux 
diversion member 34, as graphically depicted in FIG. 3a. As illustrated by 
the graph, the magnetic restoring force has a positive slope for relative 
displacements between 0 and 45 electrical degrees (15 mechanical degrees 
for the illustrated six-pole machine), and a negative slope for relative 
displacements between 45 and 90 electrical degrees (30 mechanical degrees 
for the illustrated six-pole machine). 
The resilient restoring force developed by the wrap springs 44, 46 augments 
the magnetic restoring force described above. It also varies with relative 
displacement of the rotor 32 and flux diversion member 34 but is 
unidirectional, as graphically depicted in FIG. 3b. The sum of the 
magnetic and resilient restoring forces, referred to herein as the overall 
restoring force, for relative displacements of 0-90 electrical degrees is 
graphically depicted as a function of such displacement in FIG. 3c. 
Although the slope of the magnetic restoring force is negative for relative 
displacements between 45 and 90 electrical degrees, the slope of the 
overall restoring force remains positive. The positive slope is desirable 
from a control standpoint because it guarantees direct proportionality 
between the loading of auxiliary machine 14 and the resulting displacement 
of the flux diversion member 34 relative to the rotor 32. 
The sharp increase in the resilient force of the springs beyond 90 degrees 
of relative displacement is due to the special characteristics of wrap 
springs 44, 46. In the rest position depicted in FIG. 1, there are small 
gaps between individual turns of the springs 44, 46. Such gaps shrink as 
the springs 44, 46 are loaded by relative displacement of the rotor 32 and 
flux diversion member 34. At maximum relative dsplacement (approximately 
90 degrees) adjacent turns of the springs 44, 46 are in engagement, and 
the springs 44, 46 define a substantially rigid coupling between the rotor 
32 and flux diversion member 34. 
Since the rotor 52 of the auxiliary machine 14 rotates with the flux 
diversion member 34, a voltage is induced in the windings of the stator 
50. Loading of the auxiliary machine stator windings produces a load 
torque on the rotor 52 which is connected to the flux diversion member 34. 
Such load torque is balanced by the overall restoring torque (defined in 
FIG. 3c) with relative displacement of the flux diversion member 34 with 
respect to the rotor 32. 
As the windings of stator 50 are loaded, the flux diversion member vanes 60 
rotate relative to the magnets of rotor 32 and variably bridge adjacent 
magnets to shunt magnetic flux therebetween and away from the stator 30. 
FIG. 3d graphically depicts the variation in stator flux as a function of 
the relative displacement. FIG. 2b depicts a maximum loading condition in 
which the flux diversion member 34 is angularly displaced from the rotor 
32 by 90 electrical degrees (30 mechanical degrees). Minimum stator flux 
occurs in this orientation, as substantially all of the flux produced by a 
respective rotor magnet is shunted to an adjacent rotor magnet 32 via the 
vanes 60 of flux diversion member 32. 
The variation of stator flux with displacement of the flux diversion cage 
member 34, as depicted in FIG. 3d, produces a corresponding variation in 
the output voltage of the main machine 12 stator winding 25. Thus, the 
stator windings 55 of the auxiliary machine 14 are loaded in relation to 
the desired output voltage. When the windings 55 are open-circuited, the 
output voltage is at a maximum; as the loading increases from zero, the 
output voltage decreases. 
Referring to FIG. 4, the load control unit 56 comprises a full-wave diode 
bridge rectifier 70 connected to the three-phase output 54 of auxiliary 
(control) machine 14, a filter capacitor 71, and the serial combination of 
a load resistor 72 and a solid state switch 74 connected across the DC 
output of the bridge rectifier 70. The switch 74, which may be a power 
bi-polar or field-effect transistor, is pulse-width-modulated by a 
conventional PWM CIRCUIT 76. 
The output duty cycle of PWM CIRCUIT 76, and hence the resistive loading of 
the auxiliary machine 
output 54, is controlled by adjusting the resistance of PWM CIRCUIT 
resistor 78. When the duty cycle is 0%, the auxiliary machine stator 
windings 55 are unloaded, the flux diversion member 34 is in the position 
shown in FIG. 2a and the output voltage of the main machine stator 
windings 25 is at its maximum. When the duty cycle is 100%, the effective 
load on the auxiliary machine stator windings 55 is determined by the 
resistance of the load resistor 72 so that the torque thereby produced is 
sufficient to rotate the flux diversion member 34 by 90 electrical degrees 
(30 mechanical degrees in the illustrated embodiment) with respect to the 
rotor 32 as in FIG. 2b. In such case, the output voltage of the main 
machine stator windings 25 is at its minimum. 
Alternative arrangements for causing relative angular displacement of the 
flux diversion member with respect to the rotor 32 such as a fan, for 
example, may be employed in combination with the auxiliary machine 14. In 
this regard, it will be understood that although this invention has been 
described in reference to the illustrated embodiment, various 
modifications may be made thereto. By way of further example, springs 44, 
46 may be omitted if the relative displacement of the flux diversion 
member 34 and the rotor 32 is mechanically limited to less than 45 
electrical degrees; this, of course, correspondingly limits the stator 
flux variation and is not a preferred mechanization. It should be 
recognized that systems incorporating these and other modifications may 
fall within the scope of this invention which is defined by the appended 
claims.