AC variable speed driving apparatus and electric vehicle using the same

An AC variable speed driving apparatus including an AC motor and an inverter for driving the motor. The AC motor includes a synchronous motor and an induction motor. The synchronous motor includes first stator windings and a first rotor having a permanent magnet. The induction motor includes second stator windings and a second rotor. The first and second stator windings are disposed so that they do not magnetically interfere with each other. The first and second rotors are mounted on a common axis of rotation. The inverter supplies AC power to the stator windings so that the synchronous motor and the induction motor are driven independently. A highly efficient, large output and low cost system can be realized in a wide speed range.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an AC variable speed driving apparatus and 
an electric vehicle using the same. The electric vehicle generally employs 
a battery as a power supply and an inverter for converting the DC voltage 
of the battery to the AC voltage to be supplied to its driving apparatus. 
2. Description of the Related Art 
FIG. 1 is an electric system for an electric vehicle known in the art, 
which comprises a battery as a power supply, and drives wheels by AC 
motors via inverters. In FIG. 1, the reference numeral 1 designates a 
battery comprising a sufficient number of unit cells 100 connected in 
series. The reference numeral 4 denotes an inverter that supplies AC power 
to an AC motor 5 for driving wheels 81 and 82. The output shaft of the 
motor 5 is coupled to a differential gear 7 via a reduction gear 6, and 
drives the wheels 81 and 82. A protective fuse 3 is used as needed and a 
main switch 2 connects or disconnects the battery 1 to or from the 
inverter 4. 
The performance of an electric vehicle must be comparable to that of an 
internal combustion engine car. 
FIG. 2 illustrates an example of the torque-rotational frequency 
characteristics of a driving motor. As shown in FIG. 2, the torque is 
maintained constant over the range of rotation frequencies 0-N.sub.1, 
whereas the output power becomes constant beyond the rotation frequency 
N.sub.1. In this figure, [1] indicates the characteristic curve when an 
amount of depression of an accelerator pedal is maximum, [3] indicates the 
characteristic curve when it is minimum, and [2] indicates the 
characteristic curve when it is intermediate. 
The following requirements must be met in order to have electric vehicles 
used as often as internal combustion engine cars. 
(1) Having good acceleration characteristics. 
(2) Having high mileage per charge. 
(3) Providing high reliability and ease of maintenance. 
(4) Comprising a battery of good performance in both output density (W/kg) 
and energy density (Wh/kg). 
(5) Comprising a mechanism that is highly efficient, small in size, light, 
and easy to maintain. 
(6) Low cost. 
FIG. 1 shows a typical system of such an electric vehicle, which employs a 
lead acid battery or nickel-cadmium cells as the battery 1, a brushless AC 
motor as the motor 5, and a transistor inverter as the inverter 4. 
Next, the AC motor for the driving wheels will be described in more detail. 
First, let us suppose that an induction motor is used as the AC motor 5. 
As is known in the art, an induction motor generates its main magnetic flux 
from the primary current, and allows the magnetic flux and the torque to 
be independently controlled by a vector control. Thus, by employing a 
magnetic flux weakening control, the induction motor can provide a wide 
range of speeds in the driving system of an electric vehicle, in which the 
primary voltage is restricted by the voltage of the battery. 
However, since the induction motor generates torque by inducing a current 
to flow through the secondary side, its efficiency reduces owing to the 
copper loss at the secondary side. This requires a large capacity cooling 
device. In addition, there is another problem in that the input efficiency 
of the motor reduces because the exciting current is supplied from the 
primary side and this increases copper loss and eddy current loss. In 
particular, the efficiency is greatly reduced in a low output power range, 
and this presents a great problem in the field of the electric vehicle. 
Next, let us suppose that a synchronous motor is used as the AC motor. 
Synchronous motors are generally divided into a revolving-armature type and 
a revolving-field type, both of which employ slip ring brushes or a rotary 
transformer in order to supply currents to the rotor windings. This, 
however, not only increases the size of the motor, but also reduces the 
efficiency thereof. Accordingly, a permanent magnet synchronous motor 
whose rotor is made of permanent magnets, and which is widely used as an 
AC servo-motor, is suitable for an electric vehicle. 
This motor makes it possible to increase the power factor because it has no 
secondary copper loss, and hence provides high efficiency. 
The permanent magnet synchronous motor has a constant field flux generated 
by the permanent magnets. In addition, the number of turns of the primary 
windings of the motor cannot be increased beyond a certain number because 
the voltage of the power supply is limited in the electric vehicle. 
Accordingly, it is very difficult for the motor to increase the rotation 
frequency and to provide required output power without increasing its 
currents. In other words, it is difficult to achieve high speed and large 
output power simultaneously under the condition that the currents are 
restricted to a certain amount. 
Furthermore, high performance magnets that are used as permanent magnets 
are generally expensive, and hence the total cost of the system increases. 
In Summary, requirements for AC motors for driving wheels of an electric 
vehicle are as follows: 
(1) Having high efficiency, particularly in a low output range. 
(2) Providing large output power in acceleration. 
(3) Having a wide speed range. 
(4) Small in size, light, and inexpensive. 
Next let us consider the battery. 
Although there are various types of batteries for an electric vehicle as 
mentioned above, there is no battery, for the present, that satisfies the 
output density (W/kg) and the energy density (Wh/kg) at the same time at 
reasonable cost. Accordingly, the type and capacity of the battery is 
decided considering the performance of the car, cost, and the like. 
As an inverter, a transistor inverter is mainly used. This is because it is 
enough for the inverter for an electric vehicle to have a capacity not 
more than one hundred kVA, and an input voltage range of 100-300 V. The 
maximum output of the inverter takes place during acceleration, and in 
this case, the output current of the inverter reaches several hundred 
amperes. Thus, a plurality of power transistors are usually connected in 
parallel in the inverter. 
FIG. 3 shows an example of a conventional AC variable speed driving 
apparatus using an AC motor and an inverter. 
In this figure, a main circuit comprises an AC power supply 101, an 
inverter 102, an AC motor M, a speed sensor 12, and a position sensor 12'. 
The inverter includes a rectifier portion that performs AC/DC conversion, 
and an inverter portion that performs DC/AC conversion. 
A control circuit, on the other hand, comprises an adder 103, a PI 
(Proportional-Integral) controller 104, and a voltage-current computing 
circuit 105. The adder 103 computes a speed difference .DELTA.n from an 
actual speed value n, which is detected by and fed from the sensor 12, and 
a speed command value n*. The PI controller 104 produces a torque command 
.tau.* such that the difference .DELTA.n becomes zero. The voltage-current 
computing circuit 105 computes from the torque command the voltage or 
current applied to the stator windings of the AC motor, and supplies it to 
the inverter 102 as a command value. In FIG. 3, the voltage-current 
computing circuit 105 provides the inverter 102 with a current command 
value i*. 
The operation of the voltage-current computing circuit 105 varies in 
accordance with the type of the AC motor and the motor control scheme. 
When a permanent magnet synchronous motor is employed as the AC motor and 
the vector control like that used for an AC servo motor is adapted, the 
phase of the current whose amplitude is proportional to the torque command 
is made perpendicular to the position of the permanent magnet detected by 
the position sensor 12'. 
On the other hand, when an induction motor is used as the AC motor, a 
vector control system as shown in FIG. 4 is widely employed. Details of 
the vector control system is described in 4th edition of "An AC servo 
motor and control of the same by a microcomputer" Sogou Denshi Publishing 
Ltd., Japan, Jun. 10, 1989, from which FIG. 4 is cited. 
In the conventional system, the sensors 12 and 12' are required to detect 
the rotation speed of the AC motor and the magnetic position of a rotor. 
The sensors may sometimes cause faults in the system, and increases cost. 
In view of this, various systems are proposed which drive a synchronous 
motor or an induction motor without using sensors. These systems, however, 
require a complicated control circuit and a complicated computing circuit. 
In summary, the electric system of an electric vehicle must meet the 
following requirements. 
(1) It can achieve a great output torque in acceleration. 
(2) It should have high total efficiency in a low output power range. 
(3) It should have high availability of a battery, thereby reducing the 
size and weight of the battery. 
(4) It should be of low cost. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an AC variable speed 
driving apparatus, which is highly efficient in a wide speed range, having 
high output power, easy to cool, and inexpensive, as well as having 
redundancy against faults in an inverter, by combining an inverter and a 
motor comprising an induction motor and a synchronous motor. 
Another object of the present invention is to provide an electric system 
for an electric vehicle which is highly efficient, small in size and 
light, and of low cost. 
Still another object of the present invention is to provide an AC variable 
speed driving apparatus which can detect the magnetic position of the 
rotor of a motor, which incorporates as its integral part a permanent 
magnet synchronous motor and an induction motor, without using a sensor in 
order to control the rotation speed, that is, which can perform sensorless 
drive, by a simple circuit. 
In a first aspect of the present invention, there is provided an AC 
variable speed driving apparatus including an AC motor and an inverter 
which drives the AC motor, 
the AC motor comprising: 
an axis of rotation; 
a first rotor which includes a permanent magnet, and is mounted on the axis 
of rotation; 
first stator windings constituting a synchronous motor in conjunction with 
the first rotor; 
a second rotor which is mounted on the axis of rotation; and 
second stator windings constituting an induction motor in conjunction with 
the second rotor, the first and second stator windings being disposed in a 
manner that they do not magnetically interfere with each other; 
wherein the inverter supplies the first and second stator windings with AC 
power independently, thereby driving the synchronous motor and the 
induction motor separately. 
Here, the inverter may comprise a first inverter which is connected to a DC 
power supply via a first disconnecting switch, and is used to drive the 
synchronous motor, and a second inverter which is connected to the DC 
power supply via a second disconnecting switch, and is used to drive the 
induction motor. 
The number of poles of the synchronous motor may equal that of the 
induction motor, and the first stator windings and the second stator 
windings may be shifted in a direction of rotation of the first and second 
rotors by a predetermined electric angle. 
The AC variable speed driving apparatus may further comprise torque control 
means for controlling the synchronous motor and the induction motor via 
the inverter, the torque control means performing on the synchronous motor 
a constant torque control in which constant torque is outputted in an 
entire speed range, and on the induction motor a constant torque control 
in a range below a predetermined speed, and a flux weakening control in a 
range above the predetermined speed, an output torque of the AC motor 
being the sum of output torque of the synchronous motor and output torque 
of the induction motor. 
The synchronous motor may have larger overload capacity in low and middle 
speed ranges than in a high speed range. 
A torque command value .tau..sub.s * of the synchronous motor and a torque 
command value .tau..sub.i * of the induction motor may be determined as 
follows where .tau.* is a total torque command value of the AC motor, and 
.tau..sub.smax is a maximum output torque of the synchronous motor: 
(1) .tau..sub.s *=.tau.*, and .tau..sub.i *=0, when 
.tau.*.ltoreq..tau..sub.smax ; and 
(2) .tau..sub.s *=.tau..sub.smax, and .tau..sub.i *=.tau.*-.tau..sub.smax, 
when .tau.*&gt;.tau..sub.smax. 
A torque command value .tau..sub.s * of the synchronous motor and a torque 
command value .tau..sub.i * of the induction motor may be determined as 
follows where .tau.* is a total torque command value of the AC motor, 
.tau..sub.scont is continuous rating torque of the synchronous motor, 
.tau..sub.icont is continuous rating torque of the induction motor, and 
.tau..sub.smax is a maximum output torque of the synchronous motor: 
(1) when .tau.*.ltoreq..tau..sub.scont, .tau..sub.s *=.tau.*, and 
.tau..sub.i *=0; 
(2) when .tau..sub.scont &lt;.tau.*.ltoreq..tau..sub.scont +.tau..sub.icont, 
.tau..sub.s *=.tau..sub.scont, and .tau..sub.i *=.tau.*-.tau..sub.scont ; 
(3) when .tau..sub.scont +.tau..sub.icont &lt;.tau.*.ltoreq..tau..sub.smax 
+.tau..sub.icont, .tau..sub.s *=.tau.*-.tau..sub.icont, and .tau..sub.i 
*=.tau..sub.icont ; and 
(4) when .tau.*&gt;.tau..sub.smax +.tau..sub.icont, .tau..sub.s 
*=.tau..sub.smax, and .tau..sub.i *=.tau.*-.tau..sub.smax. 
The AC variable speed driving apparatus may further comprise a filter 
circuit to which the torque command value .tau..sub.i * of the induction 
motor is inputted, the filter circuit having a time constant sufficiently 
larger than a secondary circuit time constant of the induction motor, 
wherein an exciting current supplied to the induction motor is stopped 
when the torque command value .tau..sub.i * after passing through the 
filter is substantially zero. 
In a second aspect of the present invention, there is provided an electric 
system for an electric vehicle comprising: 
a first battery; 
a second battery; 
an AC motor for driving wheels of the electric vehicle, the AC motor 
including a synchronous motor and an induction motor; 
a first inverter connected between the first battery and the synchronous 
motor; and 
a second inverter connected between the second battery and the induction 
motor. 
The synchronous motor and the induction motor may have a common axis of 
rotation. 
The synchronous motor and the induction motor may be separately 
constructed, and wherein the first battery, the first inverter and the 
synchronous motor may constitute a first main system, and the second 
battery, the second inverter and the induction motor may constitute a 
second main system. 
The synchronous motor may comprise a permanent magnet rotor. 
The first battery may have greater energy density or greater energy than 
the second battery, and the second battery may have greater output power 
density or greater output power than the first battery. 
Only the synchronous motor may be operated in a low output range, and only 
the induction motor or both the synchronous motor and induction motor may 
be operated in a high output range. 
In a third aspect of the present invention, there is provided an AC 
variable speed driving apparatus comprising: 
an AC motor including a first rotor which has a permanent magnet and is 
mounted on a rotor shaft, a second rotor which is mounted on the rotor 
shaft, and first stator windings and second stator windings which are 
disposed in a manner such that they do not magnetically interfere with 
each other, and that they correspond to the first rotor and the second 
rotor, respectively, the first rotor and the first stator windings 
constituting a synchronous motor, and the second rotor and the second 
stator windings constituting an induction motor, the synchronous motor and 
the induction motor being integrally constructed into one body; 
an inverter supplying the first stator windings and the second stator 
windings with AC power, independently; 
first computing means for computing a rotation speed of the first rotor on 
the basis of actual values of voltages and currents of the synchronous 
motor or on the basis of command values of voltages and currents of the 
synchronous motor; and 
control means for controlling the induction motor by using the rotation 
speed computed by the first computing means as a feedback value 
corresponding to the speed of the induction motor. 
The AC variable speed driving apparatus may further comprise second 
computing means for computing command values of voltages or currents to be 
supplied to the second stator windings by using secondary flux command 
values and a torque command value to the induction motor, and electric 
constants of the induction motor, wherein the first computing means 
computes a magnetic position of the first rotor or a rotation speed of the 
first rotor on the basis of actual values of voltages and currents of the 
synchronous motor or on the basis of command values of voltages and 
currents of the synchronous motor, and the second computing means uses the 
magnetic position of the first rotor or the rotation speed of the first 
rotor outputted from the first computing means as a position or a speed 
signal of the rotor of the induction motor. 
The AC variable speed driving apparatus may further comprise a command 
circuit which outputs to the induction motor voltage command values that 
have a predetermined voltage-to-frequency ratio and is used to drive only 
the induction motor during a starting time period of the induction motor, 
and switching means for switching command values to be supplied to the 
induction motor, from the voltage command values to current command values 
computed on the basis of the torque command value, after a predetermined 
time has elapsed from the start of the induction motor, or after the 
induction motor has reached a predetermined speed. 
In a forth aspect of the present invention, an electric vehicle driving 
apparatus comprising: 
an AC motor including a permanent magnet synchronous motor and an induction 
motor which are integrally constructed into a single body, the synchronous 
motor having a rotor including a permanent magnet, and the synchronous 
motor and the induction motor having a common axis of rotation joined to a 
shaft for driving one or more wheels; 
a first inverter supplying AC power to windings of the synchronous motor; 
a second inverter supplying AC power to windings of the induction motor; 
a main battery supplying the first inverter and the second inverter with a 
DC voltage; 
first disconnecting means for electrically disconnecting the first inverter 
from the main battery; 
second disconnecting means for electrically disconnecting the second 
inverter from the main battery; and 
third disconnecting means connected to AC output lines of the first 
inverter for electrically disconnecting the synchronous motor from the 
first inverter. 
The electric vehicle driving apparatus may further comprise means for 
connecting the AC output lines of the first inverter, which is 
electrically disconnected from the synchronous motor by the third 
disconnecting means, to an external AC power supply so that the main 
battery is charged through the inverter. 
According to the first aspect of the present invention, the AC motor which 
is driven by the inverter comprises a permanent magnet synchronous motor 
and an induction motor, which have a common axis of rotation. As a result, 
in applying the AC variable speed driving apparatus of the present 
invention to the driving of an electric vehicle, the capacity of each 
motor can be reduced in such a way that both synchronous motor and 
induction motor are used to accelerate the vehicle when the maximum output 
power is required as in the maximum acceleration of the vehicle. 
Furthermore, the total efficiency of the system can be improved by using 
the synchronous motor prior to the induction motor when a constant output 
power is required as in the crusing speed driving which occupies a large 
part of the driving pattern of the vehicle. 
According to the second aspect of the present invention, since the 
synchronous motor and the induction motor can be driven independently, 
only the induction motor or both synchronous motor and induction motor are 
operated when a high output power is required as in the acceleration of 
the vehicle. Generally, such an operation mode occupies only a small part 
of the operation. In contrast, only the synchronous motor is operated in a 
low output range as driving on a flat road. As a result, high output power 
is obtained during acceleration, whereas high efficiency is achieved in 
the low output power range. In addition, the availability of batteries can 
be improved by employing different types of batteries such as a high 
energy type or a high output type in accordance with the types of motors, 
and by changing modes of using the motor and battery in accordance with 
the speed range. 
According to the third aspect of the present invention, the AC motor, 
integrally constructed of a synchronous motor and an induction motor by 
mounting the motors on a common rotor shaft, is controlled on the basis of 
the speed difference between the desired and the actual speed values. The 
actual speed of the AC motor is detected by a position-speed computing 
circuit that computes the speed from the detected voltage and current or 
from the desired voltage and current of the synchronous motor. And the 
detected speed is fed back to the control loops for the synchronous and 
the induction motors. The position-speed computing circuit also detects 
the actual magnetic position of the rotor on the basis of the voltage and 
current of the synchronous motor. The detected magnetic position of the 
rotor is employed in the vector control of the synchronous motor. Thus, 
according to the third aspect of the present invention, sensorless control 
of the AC motor is realized in which the AC motor is driven on the basis 
of the desired speed by the aid of the position-speed computing circuit 
and the vector control of the synchronous and the induction motors. 
According to the fourth aspect of the present invention, even if the 
inverter that drives the permanent magnet synchronous motor fails, the 
motor disconnecting means can prevent the velocity electromotive force 
generated in the synchronous motor from being applied to the inverter by 
breaking the electric connection between the inverter and the synchronous 
motor. In addition, by connecting an external power supply to the AC 
output side of the inverter, with the inverter being disconnected from the 
permanent magnet synchronous motor by the motor disconnecting means, DC 
power is supplied from the external AC power supply to the main battery 
through the freewheeling diodes of the inverter and the DC disconnecting 
means, thereby charging the main battery. 
The above and other objects, effects, features and advantages of the 
present invention will become more apparent from the following description 
of the embodiments thereof taken in conjunction with the accompanying 
drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The invention will now be described with reference to the accompanying 
drawings. 
EMBODIMENT 1 
FIG. 5 shows the construction of an AC motor M used in a first embodiment 
in accordance with the present invention. 
In the AC motor M, the reference numeral 10 designates a frame in which a 
permanent magnet synchronous motor 20 and an induction motor 30 are 
incorporated having a common axis of rotation 11. In addition, a rotary 
sensor 12 is attached to an end portion of the axis of rotation 11. The 
rotary sensor 12 detects the speed and position of the axis of rotation 
11, which are used by the motors 20 and 30. 
The permanent magnet synchronous motor 20 comprises a first rotor 21 which 
includes permanent magnets 22 attached to the surface of its poles, three 
phase windings 23 as first stator windings, and three phase terminals 24 
connected to an inverter. 
The induction motor 30 comprises a second rotor (a squirrel-cage rotor) 31 
containing conductors formed in a cage, three phase stator windings 33 as 
second stator windings, and three phase terminals 34 connected to another 
inverter. 
FIG. 6 shows a driving circuit of the AC motor M. 
In FIG. 6, the main battery 1 is connected to voltage type inverters 28 and 
38 through DC disconnecting switches 27 and 37, respectively. Each 
disconnecting switch is composed of a breaker or the like. The inverters 
28 and 38 comprise electrolytic capacitors 26 and 36, and semiconductor 
device groups 25 and 35, respectively. The capacitors 26 and 36 function 
as filters for removing spikes on the DC voltage caused by harmonic 
currents generated by the inverters 28 and 38. Since the configuration of 
the semiconductor device group 25 or 35 of the voltage type inverter is 
known in the art, the description thereof is omitted here. 
The AC output terminals of the inverter 28 are connected to the stator 
windings 23 via the terminals 24 of the permanent magnet synchronous motor 
20, and the output terminals of the inverter 38 are connected to the 
stator windings 33 via the terminals 34 of the induction motor 30. 
Not only in voltage type inverters but also in any electric power 
converters that employ semiconductor devices, in general, various 
quantities such as currents, voltages, temperatures and the like may 
exceed allowable values because of an overload, an overvoltage, noise, 
misoperation, heat, oscillation or the like. Although various protective 
functions are added in order to prevent the devices from being damaged, 
they can never be perfect, and hence some devices might be damaged in the 
worst case. 
In particular, when a voltage source is employed as a power supply as shown 
in FIG. 6, the voltage source might be short-circuited in which case 
damage is caused such that the semiconductor devices, are short-circuited. 
This might result in a fault in the entire apparatus. 
To prevent such a problem, when a fault in a semiconductor device or a 
fault which might lead to damage of a semiconductor device is detected in 
the inverter 28 or 38, the disconnecting switch 27 or 37 is opened by a 
control system 45 so that the main battery 1 and an unimpaired inverter 
are protected. 
In such a case, the disconnecting switch 27 or 37 connected to the 
unimpaired inverter may be closed again if necessary so that the motor 20 
or 30 will continue to rotate the AC motor M. 
Next, the arrangement of the stator windings 23 and 33 of the motor 20 and 
30 will be explained with reference to FIGS. 7A and 7B. FIG. 7A shows the 
stator windings 23 of the synchronous motor 20, and FIG. 7B shows the 
stator windings 33 of the induction motor 30. The windings 23 and 33 are 
shifted by a predetermined electrical angle .theta.. 
The number of poles of the motor 20 and 30 are supposed to be identical. 
Although the waveforms of currents flowing through the windings 23 or 33 
may vary in accordance with a control method of the inverter 28 or 38, the 
voltage type square waveform inverter includes in its voltage waveform a 
large amount of fifth and seventh harmonics of the fundamental frequency. 
In general, the output voltage of the inverter is modulated by the PWM 
technique known in the art. Although this method will remove low-order 
harmonics, higher order harmonics will not be completely removed unless 
the modulation frequency is set at a sufficiently high value. 
In addition, the dead time, which is set in the voltage type inverter to 
prevent a short-circuit between the semiconductor devices of the upper and 
lower arms in each phase, will cause a distortion in the output voltage. 
Thus, fifth, seventh, eleventh, thirteenth . . . harmonics are usually 
included in the input current to the AC motor driven by the inverter. In 
particular, the fifth and seventh harmonic currents cause torque ripples 
whose frequency is six times the applied frequency. This causes not only 
rotation distortion, but also strange sounds in the driving system of an 
electric vehicle which comprises a great number of gears and the like, and 
shortens the life of the system, as well. 
For this reason, the stator windings 23 and 33 of the motor 20 and 30 are 
displaced by an electrical angle .theta. as shown in FIGS. 7A and 7B, and 
the phases of the voltages or currents supplied to the windings 23 and 33 
are also shifted by .theta.. This makes it possible to shift the phases of 
the torque ripples which are generated in both motors 20 and 30. 
For example, considering the ripples whose frequency is six times the 
fundamental frequency, the shift angle of .theta.=30.degree. (electrical 
angle) will double the frequency of the torque ripples synthesized, and 
can reduce the amplitude thereof. Thus, the adverse effect of the rotation 
distortion and the torque ripples on the mechanical system can be reduced. 
FIG. 8A illustrates the relationship between the maximum torque and speed 
that is required of the motors 20 and 30 and the driving system of the 
electric vehicle. 
In general, the maximum torque required by the electric vehicle exhibits 
constant torque characteristics in the range less than a certain speed 
(fundamental speed) N.sub.B, whereas it shows constant output power 
characteristics in the range above N.sub.B, wherein the torque is 
inversely proportional to the speed. 
Since the permanent magnet synchronous motor has a constant magnetic flux, 
it is supposed that the synchronous motor has a constant torque 
characteristic in the entire speed range, and that the value of the 
constant torque is less than the required torque at the maximum speed. 
Accordingly, in designing the synchronous motor and the inverter that 
drives it, the maximum output torque of the synchronous motor may be 
determined to take such a value as .tau..sub.20 of FIG. 8A. 
In contrast, the output torque required of the induction motor is the 
difference .tau..sub.30 between the maximum torque and the constant torque 
that the synchronous motor can provide. 
When the maximum torque required is inversely proportional to the speed in 
the high speed range as shown in FIG. 8A, the maximum torque of the 
induction motor can be approximated as inversely proportional to the 
square of the speed. In this case, the voltage applied to the induction 
motor may be constant in the range of magnetic flux weakening control, 
that is, in the range beyond N.sub.B. Therefore, the maximum output 
voltage of the inverter can be set at the voltage corresponding to the 
fundamental speed N.sub.B. 
Thus, the rated voltage of the induction motor at N.sub.B can be selected 
at a high value. This makes it possible to decrease the current to produce 
the required torque, and hence to reduce the copper loss, which presents 
an advantage in that the total efficiency is improved. 
FIG. 8B also illustrates the relationships between the maximum torque and 
speed that is required of the motors 20 and 30 and the driving system of 
the electric vehicle. 
The maximum torque required by an electric vehicle has, in general, 
characteristics as shown in FIGS. 8A and 8B. Such maximum torque, however, 
is required for a relatively short time such as during passing. Usually, 
the torque required during the cruising speed driving is less than half 
the maximum torque. 
In the driving system comprising the motor and the inverter, the size, 
weight and cost thereof, particularly those of the cooling device vary 
depending on the selected rate of the continuous output torque to the 
short term output torque for each of the motor, even when the required 
maximum torque is identical. 
Accordingly, short term ratings are defined in a manner that the overload 
capacity of the permanent magnet synchronous motor in the high speed range 
is set less than that in the low and middle speed ranges as shown in FIG. 
8B. The benefit of this will be described referring to FIGS. 9A and 9B. 
FIG. 9A shows an equivalent circuit of a synchronous motor, and FIG. 9B 
shows a vector diagram thereof. The equivalent circuit shows that the 
synchronous motor can be approximated by the counter-electromotive force 
vector E (the dot to be attached over the top of the character E will be 
omitted in the specification for the purpose of convenience), and the 
synchronous reactance X. This is particularly so in the high speed range. 
In this figure, the voltage vector V of the motor (which is equal to the 
applied voltage vector) and the current vector I are also shown. 
Generally speaking, in controlling the permanent magnet synchronous motor, 
the current vector I is controlled to coincide in phase with the 
counter-electromotive force E so that the amplitude of the current vector 
I is proportional to the torque. That is, the synchronous motor outputs 
torque in proportion to the current flowing through the motor. FIG. 9B 
shows the vector diagram in such a case. If we want to use a synchronous 
motor with high efficiency, the counter-electromotive force of the motor 
should be increased. If we assume in FIG. 9B that the 
counter-electromotive force E.sub.a1 is generated at the maximum speed of 
the synchronous motor by applying the terminal voltage V.sub.a1 
corresponding to the maximum voltage V.sub.max which the inverter can 
output, and that the current I.sub.a1 flows through the synchronous motor 
by the application of the voltage V.sub.a1, we can not force a current 
greater than I.sub.a1 to flow through the motor. The output torque at the 
maximum speed is determined by the current I.sub.a1. If we want to force 
the current I.sub.a2 greater than I.sub.a1 to flow through the motor, the 
counter-electromotive force should be decreased to E.sub.a2 by weakening 
the magnetic flux of the motor. This is especially so in the high speed 
range where the voltage drop, caused by the counter-electromotive force or 
by the synchronous reactance, increases with the increase in the frequency 
of the voltage applied to the synchronous motor. 
Thus, in the high speed range, increase in the output torque of the 
synchronous motor causes a shortage of the output voltage from the 
inverter. If the counter-electromotive force of the synchronous motor is 
decreased to avoid the shortage of the output voltage from the inverter, 
the motor current must be increased to a level necessary for generating 
required torque. This will increase the loss (particularly copper loss), 
and decrease the efficiency of the synchronous motor. 
For this reason, the counter-electromotive force E is set as large as 
possible by reducing the short term output torque, as shown in FIG. 9C, 
which illustrates speed-torque characteristics in the high speed range, in 
which the voltage drop is great and the voltage margin is small. 
FIG. 10 is a flowchart showing a calculation method of the torque command 
values for the permanent magnet synchronous motor 20 and the induction 
motor 30. More specifically, the torque command value .tau..sub.s * of the 
permanent magnet synchronous motor 20 and the torque command value 
.tau..sub.i * of the induction motor 30 are computed from the total torque 
command value .tau.* supplied to the AC motor M of the electric vehicle. 
First, the total torque command value .tau.* is read at step S1, and is 
compared with the maximum torque .tau..sub.smax of the synchronous motor 
20 at step S2. If .tau.*.ltoreq..tau..sub.smax, the torque command value 
.tau..sub.s * of the synchronous motor 20 is set equal to the total torque 
command value .tau.*, and the torque command value .tau..sub.i * of the 
induction motor 30 is set at zero at step S3. 
In contrast, if .tau.*&gt;.tau..sub.smax, the torque command value .tau..sub.s 
* of the synchronous motor 20 is set equal to the maximum value 
.tau..sub.smax, and the torque command value .tau..sub.i * is set equal to 
the difference (.tau.*-.tau..sub.smax) between the total torque command 
value .tau.* and the maximum value .tau..sub.smax at step S4. 
By such operations, the torque command value for the synchronous motor is 
determined prior to that of the induction motor, thereby improving the 
efficiency in the low torque drive. 
FIG. 11 is a flowchart illustrating another method for computing the torque 
command values to the permanent magnet synchronous motor 20 and the 
induction motor 30. 
In this method, the total torque command value .tau.* is read at step S11, 
and is compared with the continuous rating torque .tau..sub.scont of the 
synchronous motor 20 at step S12. If .tau.*.ltoreq..tau..sub.scont, the 
torque command value .tau..sub.s * of the synchronous motor 20 is set 
equal to the total torque command value .tau.*, and the torque command 
value .tau..sub.i * of the induction motor 30 is set at zero at step S13. 
On the other hand, if .tau.*&gt;.tau..sub.scont, the torque command value 
.tau.* is compared with the sum of .tau..sub.scont and the continuous 
rating torque .tau..sub.icont of the induction motor 30 at step 14. If 
.tau.*.ltoreq..tau..sub.scont +.tau..sub.icont, the torque command value 
.tau..sub.s * of the synchronous motor 20 is set equal to the continuous 
rating torque .tau..sub.scont, and the torque command value .tau..sub.i * 
of the induction motor 30 is set equal to the difference 
(.tau.*-.tau..sub.scont) between the total torque command value .tau.* and 
the continuous rating torque .tau..sub.scont at step S15. 
Next, if the torque command value .tau.* satisfies the relationships 
.tau.*&gt;.tau..sub.scont +.tau..sub.icont (negative at step S14), and 
.tau.*.ltoreq..tau..sub.smax +.tau..sub.icont (positive at step S16, that 
is, the torque command value .tau.* is equal to or less than the sum of 
the maximum torque .tau..sub.smax of the synchronous motor 20 and the 
continuous rating torque .tau..sub.icont of the induction motor 30), the 
torque command value .tau..sub.s * of the synchronous motor 20 is set at 
the difference (.tau.*-.tau..sub.icont) between the total torque command 
value .tau.* and the continuous rating torque .tau..sub.icont of the 
induction motor 30, and the torque command value .tau..sub.i * of the 
induction motor 30 is set at the continuous rating torque .tau..sub.icont 
at step S17. 
Furthermore, if .tau.*&gt;.tau..sub.smax +.tau..sub.icont (negative at step 
S16), the torque command value .tau..sub.s * of the synchronous motor 20 
is set at the maximum torque .tau..sub.smax, and the torque command value 
.tau..sub.i * of the induction motor 30 is set at the difference 
(.tau.*-.tau..sub.smax) between the total torque command value .tau.* and 
the maximum torque .tau..sub.smax of the synchronous motor 20 at step S18. 
Thus determining the torque command values .tau..sub.s * and .tau..sub.i * 
makes it possible to realize efficient driving because the synchronous 
motor 20 is used prior to the induction motor 30 in the continuous rating 
ranges of the motors 20 and 30, and in the overload ranges and their 
borders. 
FIG. 12 shows an arrangement of a torque control system for the motors 20 
and 30. 
The torque control system shown in FIG. 12 comprises a torque command value 
distribution circuit 40, filter circuits 51 and 61, and torque control 
circuits 50 and 60. The torque command value distribution circuit 40 
computes the torque command values .tau..sub.s * and .tau..sub.i * on the 
basis of the total torque command values .tau.* in accordance with the 
flowchart of FIG. 10 or FIG. 11. The filters 51 and 61 remove high 
frequency components from the torque command values .tau..sub.s * and 
.tau..sub.i * produced by the distribution circuit 40. The torque control 
circuits 50 and 60 control the torque of the motors 20 and 30 in 
accordance with the torque command values .tau..sub.s * and .tau..sub.i *, 
respectively. The control method of the torque control circuits 50 and 60 
is known in the art, and hence details thereof are omitted here. 
The torque command value .tau..sub.i * of the induction motor 30 passing 
through the filter circuit 51 is obtained as the torque command value 
.tau..sub.i ** at the output of the filter circuit 51. This command value 
.tau..sub.i ** is inputted to a decision circuit 52 that judges whether 
the value of the command value .tau..sub.i ** is zero or not. A switch 53 
operates in response to the result of the decision such that a flux 
command value .phi..sub.i ** is set at a predetermined value .phi..sub.i * 
or at zero. 
According to the method describe above, in the range where the total torque 
command value .tau.* is rather small, the electric vehicle is driven only 
by the synchronous motor 20 which requires no exciting current, and the 
induction motor 30 is not used. As a result, the iron loss and copper loss 
of the induction motor 30 are kept zero in that range. Thus, highly 
efficient driving is realized. 
In addition, it is easy to prevent transitional torque fluctuations which 
might occur in the switching of the flux, by setting the time constant of 
the filter circuit 51 larger than the time constant of the equivalent 
second circuit of the induction motor. This is possible because the 
response time of the torque of the electric vehicle can be set much 
greater than the second-order time constant of the equivalent second 
circuit of the induction motor. 
Although the AC motor M comprises a single synchronous motor 20 and a 
single induction motor 30 mounted on the common axis of rotation 11 in the 
embodiment described above, the AC motor M may comprise two or more pairs 
of a synchronous motor and an induction motor. 
This embodiment has the following advantages. 
(1) It can distinguish between the use of a permanent magnet synchronous 
motor which is highly efficient by nature and the use of an induction 
motor which is readily subjected to flux-weakening control and has a wide 
speed control range. As a result, the AC motor and its driving apparatus 
can be reduced in size and cost to a greater extent than when either a 
synchronous motor or an induction motor is used individually. 
Furthermore, when one of the two inverters fails, the other inverter can 
drive the motor connected thereto after disconnecting the failed inverter. 
This redundancy provides the system with high reliability and convenience 
of operation. 
(2) Torque ripples and irregularity of rotation can be reduced by 
displacing the positional angle of the stator windings of each motor as 
shown in FIGS. 7A and 7B. 
(3) The efficiency of the induction motor can be improved by employing 
constant torque control for the synchronous motor and a flux-weakening 
control for the induction motor. 
(4) The efficiency of the synchronous motor can be improved by reducing the 
overload capacity of the synchronous motor in a high-speed range. 
(5) The efficiency of the total system in the low torque range can be 
improved by mainly driving the synchronous motor in that range. 
(6) The efficiency in the entire torque range is improved because the 
torque command value for each motor is computed separately in the 
continuous rating range and in the overload range. 
(7) When the total torque command value is small and only the synchronous 
motor is driven, and hence the torque command value to the induction motor 
is zero, the exciting current is not supplied to the induction motor. As a 
result, the loss of the induction motor can be reduced, and hence the 
efficiency of the system can be improved. Furthermore, the cooling device 
of the induction motor can be reduced in capacity. 
EMBODIMENT 2 
FIG. 13 shows a second embodiment in accordance with the present invention. 
This embodiment comprises an AC motor for driving wheels, which includes a 
synchronous motor and an induction motor. The two motors have a common 
axis of rotation, and are each connected to driving circuits for 
separately driving the two motors. 
In this figure, reference numerals 111 and 112 designate batteries, which 
are connected to inverters 141 and 142 via main switches 121 and 122 and 
fuses 131 and 132, respectively. 
A motor 150 comprises a permanent magnet synchronous motor 151 and an 
induction motor 152 having a common axis of rotation 157. The synchronous 
motor 151 comprises a stator 211 and a permanent magnet rotor 212. The 
induction motor 152 comprises a stator 221 and a rotor 222. 
The output terminals of the inverter 141 are connected to the stator 211 of 
the synchronous motor 151, and the output terminals of the inverter 142 
are connected to the stator 221 of the induction motor 152. 
The inverter 141 drives the synchronous motor 151 under the control of a 
control system, and the inverter 142 drives the induction motor 152 under 
the control of another control system. These control systems are not shown 
in this figure. Further in this figure, the devices from the reduction 
gear 6 and onward of FIG. 1 are omitted. 
As the battery 111 for driving the synchronous motor, one that has great 
energy density (Wh/kg) (that is, a high energy type), or great energy (Wh) 
is employed so that it is suitable for low output power and long time 
travel. 
On the other hand, as the battery 112 for driving the induction motor, one 
that has great output power density (W/kg) (that is, a high output power 
type), or great output power (W) so that it is suitable for a short term 
operation such as acceleration and deceleration. 
FIG. 14 illustrates the operation of the electric system of FIG. 13. 
In FIG. 14, the solid lines represent the characteristics of the maximum 
output power operation of the electric vehicle, where Tm denotes torque 
and Pm designates output power. The dashed-and-dotted lines represent the 
characteristics of the maximum output power operation of the synchronous 
motor 151, where T.sub.1 designates torque and P.sub.1 denotes output 
power. The broken lines represent the characteristics of the maximum 
output power operation of the induction motor 152, where T.sub.2 indicates 
torque and P.sub.2 denotes output power. 
This figure illustrates the load sharing of the motors 151 and 152. The 
torque T.sub.1 is the maximum torque assigned to the synchronous motor 
151, and is constant regardless of the rotation rate. Accordingly, the 
torque exceeding T.sub.1 is assigned to the induction motor 152. 
In the operation range of the electric vehicle in which the required torque 
is less than T.sub.1, such as in a low output power drive on a normal flat 
road, the inverter 142 for the induction motor is stopped or the 
electrical torque of the induction motor is controlled to zero, and only 
the synchronous motor 151 is used. 
When high output power is required as in acceleration and deceleration, 
only the induction motor 152 is driven, or both synchronous motor 151 and 
the induction motor 152 are driven. 
Since the rotors 212 and 222 of the motors 151 and 152 are mounted on the 
common rotor shaft 157, the output frequency of the inverter 142 for the 
induction motor is different from that of the inverter 141 for the 
synchronous motor 151 by the slip frequency. 
The selection of the operation mode from the three modes, that is, a first 
mode in which only the synchronous motor is driven, a second mode in which 
only the induction motor is driven, and a third mode in which both 
synchronous and induction motors are driven, is determined in advance in 
accordance with the torque value required with respect to each number of 
revolution. Thus, the inverters 141 and 142 are controlled. 
The operation mode is selected in such a manner that the optimum total 
efficiency of the system is obtained for each operation point. More 
specifically, an amount of depression of the accelerator pedal is 
detected, and the inverters 141 and/or 142 are operated in accordance with 
the required torque value commanded in response to the detected signal in 
a manner similar to the operation shown in FIG. 10. 
EMBODIMENT 3 
FIG. 15 shows a third embodiment in accordance with the present invention. 
In this figure, the same elements are designated by the same reference 
numerals as in FIG. 13. 
The third embodiment comprises a first system including a battery, an 
inverter and a synchronous motor, and a second system including a battery, 
an inverter and an induction motor. The third embodiment differs from the 
second embodiment in that the synchronous motor and the induction motor 
are separately provided. 
More specifically, although in the second embodiment shown in FIG. 13, the 
two motors 151 and 152 have rotors 212 and 222 which are mounted on the 
common axis of rotation 157, and are incorporated in a common motor frame, 
in the third embodiment shown in FIG. 15, the synchronous motor 153 and 
the induction motor 154 are separately installed, and the axes of rotation 
253 and 254 of the motors 153 and 154 are coupled by a reduction gear 161. 
The axes of rotation 253 and 254 may be joined directly to each other 
within the reduction gear 161, or be coupled via a gear. 
The motors 153 and 154 are controlled in the same manner as the motors 151 
and 152 in FIG. 13. 
EMBODIMENT 4 
FIG. 16 shows a fourth embodiment in accordance with the present invention. 
In this figure, the same elements are denoted by the same reference 
numerals as in FIGS. 13 and 15. The fourth embodiment of FIG. 16 comprises 
two electric system, one for a synchronous motor and the other for an 
induction motor. 
In FIG. 16, reference numeral 171 denotes a differential gear having two 
input shafts. Each of the input shafts is joined to each one of reduction 
gears 162 and 163. The reduction gear 162 is joined to a synchronous motor 
155, and the reduction gear 163 is joined to an induction motor 156. The 
motors 155 and 156 are controlled in the same manner as the motors in 
FIGS. 13 and 15. 
According to the embodiments 2-4 described above, the AC motor for driving 
the wheels are divided into the permanent magnet synchronous motor having 
high efficiency, and the induction motor having high torque, great output 
power, and a wide output range. In addition, the synchronous motor is 
driven by the high energy type battery via the inverter, and the induction 
motor is driven by the high output power type battery via the other 
inverter. With this arrangement, in the low output power range, only the 
synchronous motor is operated, and in the high output power range, only 
the induction motor or both synchronous and induction motors are operated. 
As a result, the following advantages are obtained. 
(1) Since the availability of the batteries increases, the size, weight, 
and cost of the batteries mounted on the vehicle can be reduced. 
(2) Since only the synchronous motor is operated in the low output power 
range, which occupies the longest time in the operation of the electric 
vehicle, the efficiency of the system can be increased. 
(3) As a result, the mileage per charge of the electric vehicle increases. 
EMBODIMENT 5 
FIG. 17 shows a fifth embodiment in accordance with the present invention. 
In FIG. 17, the AC motor M comprises the permanent magnet synchronous motor 
20 and the induction motor 30, which have a common rotor shaft, and are 
integrally constructed in a single frame. The synchronous motor 20 is 
connected to an inverter 412, and the induction motor 30 is connected to 
an inverter 422. The inverters 412 and 422 are connected to an AC power 
supply (a commercial power supply) 101. Although the construction of the 
AC motor M is similar to that of FIG. 5, the sensor 12 shown in FIG. 5 is 
not provided here. 
Each of the inverters 412 and 422 includes a rectifying portion that 
performs AC/DC conversion, and an inverting portion that performs DC/AC 
conversion. When the inverters 412 and 422 receive only direct voltage 
source, that is, perform only DC/AC conversion, two DC power supplies are 
connected separately, or a DC power supply is connected in common, in 
place of the AC power supply 101. 
A control circuit of the motors 20 and 30 comprises adders 413 and 423, PI 
controller 414 and 424, voltage-current computing circuits 415 and 425 and 
limiter circuits 416 and 426. The adder 413 (423) outputs the difference 
between the speed command value n* and the actual speed n. The PI 
controller 414 (424) outputs a torque command value such that the speed 
difference is reduced to zero. The voltage-current computing circuit 415 
(425) outputs the voltage command value or the current command value to 
the inverter 412 (422). The limiter circuit 416 (426) limits the torque 
command value. 
The actual speed (the number of rotations) n and the magnetic position 
.PSI. of the rotor needed for driving the permanent magnet synchronous 
motor in the speed control of the motors 20 and 30 are obtained from a 
position-speed computing circuit 200. 
The computing circuit 200 is necessary for performing a sensorless driving 
of the permanent magnet synchronous motor 20, which is known in the art. 
For example, see, Watanabe, et al., "A Sensorless Detecting Strategy of 
Rotor Position and Speed on Permanent Magnet Synchronous Motor", The 
Journal of the Institute of Electric Engineers of Japan, D-110, No. 11, 
pages 1193-1200. 
The principle of this method is as follows: First, the voltage of each 
phase winding and instantaneous value of the current of the synchronous 
motor are detected on the inverter side; second, the positional angle of 
the rotor and the rotation speed are computed on the basis of the detected 
values, by a DSP (Digital Signal Processor) under the control of a 
microprocessor. 
In a similar way, the computing circuit 200 detects the magnetic position 
.PSI. of the rotor and the rotational speed n by the digital computation 
based on the voltage and current applied to the synchronous motor 20. 
Instead of using the actual values of the voltage and current applied to 
the synchronous motor 20, the command value of the voltage, which can be 
calculated in the voltage-current computing circuit 415, and that of the 
current fed to the inverter 412 from the voltage-current computing circuit 
415 may be used. 
As is known in the art, in order to control the permanent magnet 
synchronous motor 20, it is necessary to control the phases of the applied 
currents in response to the magnetic position of the rotor. In the present 
embodiment, the computation of the computing circuit 200 makes it possible 
to detect the absolute position .PSI. of the rotor and the rotation speed 
n in the speed control loop without using a sensor. 
With regard to the induction motor 30, the rotation speed obtained by the 
computing circuit 200 is used as the feed back value in the speed control 
loop, and the torque command value is produced from the PI controller 424. 
On the other hand, the rotation speed n is used as relative angle 
information of the rotor, which is necessary to carry out the coordinate 
transformation in the vector control as shown in FIG. 4. 
According to this embodiment, the position-speed computing circuit 200 is 
used at least as a speed computing circuit, and the rotation speed n 
computed by the circuit is used as the feedback value of the speed to the 
speed command value to the induction motor 30. Thus, the rotation speed n 
produced from the computing circuit 200 is used in the speed control loop 
of the induction motor 30. 
In addition, the voltage-current computing circuit 425 is provided for the 
purpose of obtaining the voltage or current command values to be fed to 
the stator windings of the induction motor 30 by using the command value 
of the secondary flux, the torque command value, and the electric 
constants of the induction motor 30. The torque command value is fed from 
the controller 424. The command value of the secondary flux and the 
electric constants are pre-set in the circuit 425 as the inherent 
constants of the induction motor 30. Thus, the magnetic position .PSI. of 
the rotor or the rotation speed n outputted from the position-speed 
computing circuit 200 is used by the voltage-current computing circuit 425 
as the position of the rotor or the speed signal of the induction motor 
30. Accordingly, although the rotation speed n is inputted to the 
computing circuit 425 in FIG. 17, the magnetic position .PSI. of the rotor 
computed by the computing circuit 200 may be used instead of the rotation 
speed n. 
According to this embodiment, the speed control and the torque control of 
the induction motor 30 which is integrally constructed with the 
synchronous motor 20 can be carried out by using the absolute magnetic 
position of the rotor or the rotation speed of the synchronous motor 20 
which is obtained by the computation during the sensorless drive of the 
permanent magnet synchronous motor 20. 
EMBODIMENT 6 
FIG. 18 shows a sixth embodiment in accordance with the present invention. 
This embodiment differs from the fifth embodiment shown in FIG. 17 in that 
two command values v.sub.1 * and I.sub.2 * are switched by a switch 428 so 
that one of them is applied to the inverter 422. The voltage command value 
v.sub.1 is issued from a command circuit 427, and the current command 
value I.sub.2 is calculated by the computing circuit 425 on the basis of 
the torque command value from the PI controller 424. 
According to this embodiment, the induction motor 30 can be started in the 
open-loop operation by applying the voltage command value v.sub.1 * from 
the command circuit 427. In this case, the command circuit 427 regulates 
the voltage command value v.sub.1 * to satisfy the condition of the 
constant V/F (Voltage-to-Frequency ratio) control which matches the 
characteristics of the induction motor. The constant V/F control of the 
induction motor 30 is continued for a predetermined period of time until 
the AC motor M gains a certain speed at which the synchronous motor 20 
generates a counter-electromotive force large enough to accurately detect 
therefrom the magnetic position of the rotor and the speed of the 
synchronous motor. Once the predetermined period of time has elapsed, the 
switch 428 is switched to supply the current command value I.sub.2 * 
produced from the computing circuit 425, and the control scheme of the 
induction motor is switched to the vector control. 
Thus, adding the command circuit 427 and the switch 428 makes it possible 
for the position-speed computing circuit 200 to employ a method that uses 
the counter-electromotive force of the permanent magnet synchronous motor 
20 to compute the magnetic position of the rotor or the rotation speed of 
the synchronous motor. 
The first embodiment described before with reference to FIG. 6 poses the 
following problems: 
(1) Let us consider the case where the inverter 25 for the permanent magnet 
synchronous motor 20 in FIG. 6 fails, and only the induction motor 30 is 
driven by the other inverter 35, while disconnecting the inverter 25 from 
the main battery 1 by the DC current disconnecting switch 27. In this 
case, the permanent magnet synchronous motor 20 produces a velocity 
electromotive force because the flux of the permanent magnet moves across 
the stator windings, and the electromotive force is applied to the 
inverter 25. The velocity electromotive force may act in such a manner 
that it further impairs the failure of the inverter 25 depending on the 
type of that failure. 
(2) In addition, in the electric system as shown in FIG. 6, effective 
measures to charge the main battery 1 by utilizing the inverter have not 
yet been proposed, and a separate charging circuit attached thereto is 
generally complicated in circuit arrangement and is expensive. 
A seventh embodiment in accordance with the present invention is proposed 
to prevent the velocity electromotive force from being applied to the 
inverter for the synchronous motor even if that inverter fails and so only 
the induction motor is operated. An eighth embodiment in accordance with 
the present invention is proposed to provide a simple, inexpensive 
charging system of the main battery of the electric vehicle driving 
system. 
EMBODIMENT 7 
FIG. 19 show the seventh embodiment in accordance with the present 
invention. In FIGS. 19 and 6, the same reference numerals designate the 
same elements, and the description thereof is omitted here. 
The seventh embodiment differs from the first embodiment shown in FIG. 6 in 
that it comprises a three-phase breaker 528 which is inserted in the power 
lines (AC output lines of the inverter 25) connecting the permanent magnet 
synchronous motor 20 to the inverter 25 that supplies power to that motor. 
Thus, the breaker 528 functions as a motor disconnecting means for 
disconnecting the electrical connection between the inverter 25 and the 
motor 20. 
If the inverter 25 fails, it is disconnected from the main battery 1 by the 
DC disconnecting circuit 27. 
In this case, when the operation is continued by the inverter 35 and the 
induction motor 30 which are not injured, the permanent magnet synchronous 
motor 20 produces a velocity electromotive force because the flux of the 
permanent magnet moves across the stator windings during the rotation of 
the axis of rotation 11. This electromotive force might be applied to the 
inverter 25 as a DC voltage through a freewheeling diode, and might act in 
such a manner that it further impairs the failure of the inverter 25 
depending on the type of the failure. 
To prevent this, the present embodiment operates the breaker 528 to 
disconnect the synchronous motor 20 from the inverter 25 before driving 
the induction motor 30. Thus, the velocity electromotive force generated 
in the permanent magnet synchronous motor 20 has no adverse effect on the 
inverter 25. 
If the inverter 35 for the induction motor 30 fails, it is unnecessary to 
break the lines between the inverter 35 and the induction motor 30 because 
the velocity electromotive force is not generated in the induction motor 
30. 
According to this embodiment, even if the inverter for the synchronous 
motor fails, the induction motor 30 can operate without fear of further 
impairing that inverter. This makes it possible to increase the redundancy 
of the driving apparatus because the induction motor can be used when the 
synchronous motor fails or vice versa. 
EMBODIMENT 8 
FIG. 20 show an eighth embodiment in accordance with the present invention. 
In this embodiment, a motor disconnecting means for disconnecting the 
synchronous motor 20 from the inverter 25 comprises a three-phase transfer 
switches 529 (although only one transfer switch element is shown in FIG. 
20, there are actually three transfer switch elements, each of which is 
for each one of the three phases). The switch 529 changes the connection 
of the AC output lines of the inverter 25 to either the synchronous motor 
20 or to AC input terminals 540 for an AC power supply. 
The transfer switch 529 is connected to the synchronous motor 20 at its 
terminals b, and to the AC input terminals 540 at its terminals c via 
reactors 541. The AC input terminals 540 are connected to a three-phase or 
a single phase commercial power supply. 
In a normal operation mode, the transfer switch 529 is connected to the b 
terminals, that is, to the synchronous motor 20, and the inverter 25 
supplies AC power to the synchronous motor 20. In case where the inverter 
25 fails, the transfer switch 529 is changed to the a terminals, and the 
DC disconnecting means 27 is turned off. Thus, the inverter 25 is 
disconnected from the battery 1 and the synchronous motor 20. 
This state is substantially equal to the operation during a fault of the 
inverter 25 of FIG. 19. Thus, the inverter 25 is protected from the 
velocity electromotive force that would be applied to the inverter 25 in 
failure. 
As is well known, the main battery 1 of the electric vehicle must be 
recharged after it travels a predetermined time or distance. 
This embodiment is provided with a charging means as shown in FIG. 20. In 
the charging mode, the DC disconnecting means 27 is closed, and the AC 
input terminals 540 are connected to a three-phase or single phase 
commercial power supply while the transfer switch 529 is connected to the 
a terminals. With this arrangement, the AC power passes through the 
reactors 541, the transfer switch 529 and the freewheeling diodes in the 
inverter 25, and thus the DC power is supplied to the main battery 1 via 
the DC disconnecting means 27. 
The reactors 541 are used to limit the AC current, and may be inserted 
between the AC input terminals 540 and the commercial power supply. 
According to this embodiment, the charging circuit of the main battery 1 
can be arranged by only adding simple elements such as the transfer switch 
529 and the reactors 541. These elements can be mounted on the vehicle 
with little increase in the size and weight of the vehicle. 
In addition, using the freewheeling diodes serves to increase the 
availability of the inverter 25. 
Although only two inverters each of which corresponds to each one of the 
motors 20 and 30 are employed in the seventh and eighth embodiments, the 
present invention can also be applied to a system in which a plurality of 
inverters are connected to each one of the motors 20 and 30. 
The present invention has been described in detail with respect to various 
embodiments, and it will now be apparent from the foregoing to those 
skilled in the art that changes and modifications may be made without 
departing from the invention in its broader aspects, and it is the 
intention, therefore, in the appended claims to cover all such changes and 
modifications as fall within the true spirit of the invention.