Current-type GTO inverter with surge energy restoration

In a current type GTO inverter, commutation surge voltage is inevitably generated from an inductive load whenever each GTO is turned off. The commutation surge voltage thus generated is once stored in a capacitor (C.sub.1) through a diode surge voltage rectifier (5) and then restored to the DC source terminals (3A, 3B) of the GTO bridge-connected inverter (3) through a pair of other GTOs (G.sub.7, G.sub.8) turned on during steady state intervals of inverter commutation. Magnetic energy stored in a reactor (Lr.sub.1, Lr.sub.2) in motor-driving operation is recharged to the capacitor (C.sub.1) through the diode surge voltage rectifier (5) after the GTOs (G.sub.7, G.sub.8) have been turned off; the motor kinetic energy stored in the capacitor (C.sub.1) through diodes (D.sub.8, D.sub.9) in motor-braking operation is regenerated to the AC source side of the inverter (3) through a pair of other GTOs (G.sub.9, G.sub.10) when the voltage across the capacitor (C.sub.1) exceeds a predetermined value, and magnetic energy stored in the reactor (Lr.sub.1, Lr.sub.2) in motor-braking operation is recharged to the capacitor (C.sub.1) through diodes (D.sub.12, D.sub.13) after the GTOs (G.sub.9, G.sub.10) have been turned off. The circuit operation is stable at higher frequency range because no vibration circuits are provided, and the energy conversion efficiency is high because every energy loss is effectively restored to the inverter or the power source side.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a current type GTO (gate 
turn-off thyristor) inverter, and more specifically to a surge voltage 
clamping circuit for clamping the surge voltage generated when each GTO 
incorporated in a current type GTO bridge-connected inverter is turned 
off. The clamped surge voltage is stored once in a capacitor and then 
returned to the terminals between the rectifier and the GTO inverter for 
energy restoration. 
2. Description of the Prior Art 
In a current-type gate turn-off thyristor bridge-connected inverter, since 
gate turn-off thyristors (referred to as GTOs, simply hereinafter) are 
incorporated in the inverter as the main switching elements, no 
commutating circuit including a commutation reactor, for instance, is 
required, because the GTO can be turned from on to off or vice versa in 
response to a control signal applied to the gate terminal thereof. Here, 
the terminology "commutation" means that the load current of one phase is 
switched to that of another plane or vice versa by thyristor switching 
operation. In the above-mentioned current-type GTO inverter, however, in 
the case where a load such as an induction motor having an inductance is 
coupled, commutation surge voltages are inevitably generated whenever each 
GTO is turned off. The generated surge voltages are superimposed upon the 
alternating output voltage of the GTO inverter, thus resulting in a 
problem in that some of the GTOs may be damaged by these commutation surge 
voltages. 
In order to overcome the above problem, a commutation surge voltage 
clamping circuit has been proposed, by which the commutation surge 
voltages generated whenever each GTO is turned off are absorbed or stored 
in a single electrolytic capacitor and thereafter returned to the load 
side through the GTO inverter for reducing the electric power loss. This 
function is called energy restoration. 
In the conventional commutation surge voltage clamping circuit used for a 
current type GTO inverter, however, there exist some disadvantages as 
follows. 
(1) Since a pair of ordinary thyristors are used for restoring the stored 
commutation surge voltage energy to the DC source terminals of the GTO 
inverter, two vibration circuits or thyristor turning-off circuits 
including a capacitor and an inductor respectively are necessary. Further, 
since the surge voltage energy is restored through these capacitors used 
for the vibration circuits, the capacity of these capacitors of the 
vibration circuits is determined to be relatively large. As a result, the 
turn-off operation of the ordinary thyristors often fails at higher 
frequency range. In other words, it is impossible to stably operate the 
commutation surge voltage clamping circuit when the GTO inverter operates 
at a high speed. 
(2) Since the commutation surge voltage energy is restored from the 
electrolytic capacitor to the DC source terminals of the GTO inverter 
through the vibration capacitors connected in series with the electrolytic 
capacitor, the capacitance of the restoring circuit is relatively large. 
Therefore, a reactor having a large inductance is necessary in order to 
smooth the current restored to the GTO inverter. In other words, the cost 
of the commutation surge voltage clamping circuit is relatively high. 
(3) Since the charging and discharging circuits of the capacitor are 
operable only when the motor is driven in the forward or the reverse 
direction, when the motor is being braked, it is impossible to regenerate 
the motor kinetic energy stored in the capacitor in motor-braking 
operation to the AC source side of the inverter or to charge the magnetic 
energy stored in the reactor in motor-braking operation in the capacitor. 
A more detailed description of the prior-art commutation surge voltage 
clamping circuit will be made with reference to the attached drawings 
under DESCRIPTION OF THE PREFERRED EMBODIMENT. 
SUMMARY OF THE INVENTION 
With these problems in mind, therefore, it is a primary object of the 
present invention to provide a surge voltage clamping circuit for a 
current type GTO inverter which can operate stably at high frequency 
range. 
It is another object of the present invention to provide a surge voltage 
clamping circuit for a current type GTO inverter in which no vibration 
circuit for turning off the energy-restoring thyristor is provided without 
use of a large-inductance reactor, and therefore the circuit configuration 
is simplified or reducing the manufacturing cost. 
It is still the other object of the present invention to provide a surge 
voltage clamping circuit for a current type GTO inverter which can also 
regenerate the motor kinetic energy while the motor is driven in the 
forward or reverse direction or being braked. 
To achieve the above-mentioned object, a surge voltage clamping circuit for 
a current type GTO inverter according to the present invention comprises 
(a) a GTO bridge-connected inverter, (b) a thyristor bridge-connected 
rectifier, (c) a diode bridge-connected commutation surge voltage 
rectifier, (d) a capacitor for storing commutation surge voltage energy, 
(e) a cumulative reactor, (f) a DC reactor, (g) a first GTO, (h) a second 
GTO, (i) a first diode, (j) a second diode, (k) a third GTO, (l) a fourth 
GTO, (m) a third diode and (n) a fourth diode. In the circuit 
configuration thus constructed, commutation surge voltage energy stored in 
said capacitor in motor-driving operation is restored to said inverter 
through said first and second GTOs during steady state intervals of 
inverter commutation; magnetic energy stored in said reactor in 
motor-driving operation is recharged to said capacitor through said first 
and second diodes after said first and second GTOs have been turned off; 
the motor kinetic energy stored in said capacitor in motor-braking 
operation is regenerated to the AC source through said third and fourth 
GTOs when the voltage across said capacitor exceeds a predetermined value; 
and magnetic energy stored in said reactor in motor-braking operation is 
recharged to said capacitor through said third and fourth diodes after 
said third and fourth GTOs have been turned off.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
To facilitate understanding of the present invention, a reference will be 
made to an example of prior-art surge voltage clamping circuits for a 
current type GTO bridge-connected inverter, with reference to the attached 
drawings. 
With reference to FIG. 1, the GTO inverter provided with a surge voltage 
clamping circuit is roughly made up of a thyristor bridge-connected 
current rectifier 1, a DC reactor having two inductive reactances 2A and 
2B magnetically connected each other, a GTO (gate turn-off thyristor) 
bridge-connected inverter 3, an induction motor 4, a diode 
bridge-connected commutation surge voltage rectifier 5, and a commutation 
surge voltage clamping circuit 6 including an electrolytic capacitor 
C.sub.1. The above clamping circuit 6 functions also as a commutation 
surge voltage energy restoring circuit. 
The thyristor bridge-connected rectifier 1 includes six rectifying 
thyristors RT.sub.1 to RT.sub.6, which are turned on sequentially in the 
order of RT.sub.1 and RT.sub.6, RT.sub.3 and RT.sub.2 and RT.sub.5 and 
RT.sub.4 so that each half cycle of the sine wave of a three-phase power 
source can be passed in sequence. Therefore, when the rectifying 
thyristors RT.sub.1 and RT.sub.6 are both turned on, a first-phase current 
R is supplied from the terminal A to the terminal C by way of thyristor 
RT.sub.1, reactor 2A, terminal 3A, GTO bridge-connected inverter 3, motor 
4, GTO bridge-connected inverter 3, terminal 3B, reactor 2B, and thyristor 
RT.sub.6. Similarly, when the rectifying thyristors RT.sub.3 and RT.sub.2 
are both turned on, a second-phase current S is supplied from the terminal 
B to the terminal A by way of rectifying thyristor RT.sub.3, reactor 2A, 
terminal 3A, GTO bridge-connected inverter 3, motor 4, GTO 
bridge-connected inverter 3, terminal 3B, reactor 2B and the rectifying 
thyristor RT.sub.2 ; when the rectifying thyristors RT.sub.5 and RT.sub.4 
are turned on, a third phase current T is supplied from the terminal C to 
the terminal B by way of the rectifying thyristor RT.sub.5, reactor 2A, 
terminal 3A, GTO bridge-connected inverter 3, motor 4, GTO 
bridge-connected inverter 3, terminal 3B, reactor 2B, and rectifying 
thyristor RT.sub.4. The rectified full-wave direct current is further 
smoothed through the DC cumulative reactor having two inductances 2A and 
2B magnetically connected each other. Therefore, the smoothed direct 
current Id is further converted into an alternate current of an 
appropriate frequency through the GTO inverter 3 to drive the induction 
motor 4 at any desired speed. 
The GTO inverter 3 includes six bridge-connected GTOs G.sub.1 to G.sub.6. 
When the GTOs are turned on in the order of G.sub.1 and G.sub.6 and then 
G.sub.3 with G.sub.6 on in sequence for each 60 degrees, an alternate 
square-wave U-phase current i.sub.u and a V-phase current i.sub.v with 
each pulse width of 60 degrees is first obtained by the GTO inverter. When 
the GTOs are turned on in the order of G.sub.3 and G.sub.6 and then 
G.sub.2 with G.sub.3 on in sequence for each 60 degrees, an alternate 
square-wave V-phase current i.sub.v with each pulse width of 120 degrees 
is obtained by the GTO inverter. When the GTOs are turned on in the order 
of G.sub.3 and G.sub.2 and then G.sub.5 with G.sub.2 on in sequence for 
each 60 degrees, an alternate square-wave V-phase current i.sub.v and a 
W-phase current i.sub.w with each pulse width of 60 degrees is obtained by 
the GTO inverter. 
In other words, when GTOs G.sub.1 and G.sub.6 are turned on, the rectified 
direct current Id flows as the U-phase current (the latter half of 120 
degrees) i.sub.u through the first U-phase winding having reactance 
X.sub.u and the third W-phase winding having reactance X.sub.w of the 
motor. When GTO G.sub.1 is turned off and GTO G.sub.3 is turned on with 
GTO G.sub.6 kept turned on, the current Id flows as the V-phase current 
(the first half of 120 degrees) i.sub.v through the second V-phase winding 
having reactance X.sub.v and the third W-phase winding having reactance 
X.sub.w. 
Similarly, when G.sub.6 is turned off and G.sub.2 is turned on with G.sub.3 
kept turned on, the current Id flows as the V-phase current (the latter 
half of 120 degrees) i.sub.v through X.sub.v and X.sub.u. When G.sub.3 is 
turned off and G.sub.5 is turned on with G.sub.2 kept turned on, the 
current Id flows as the W-phase current (the first half of 120 degrees) 
i.sub.w through X.sub.w and X.sub.u. 
In summary, GTOs are turned on or off in the order of G.sub.1, G.sub.6, 
G.sub.3, G.sub.2, G.sub.5 and G.sub.4 for each 60 degrees. The current 
passed through these three-phase windings having motor reactances X.sub.u, 
X.sub.v, and X.sub.w generates a rotational magnetic flux. 
Whenever each of these three-phase windings is switched off, commutation 
surge voltages are inevitably generated by the magnetic energy stored in 
the respective windings. In order to absorb these commutation surge 
voltages, there is additionally provided a surge voltage clamping circuit 
including a diode bridge-connected commutation surge voltage rectifying 
section 5 made up of fix diodes D.sub.1 to D.sub.6 and a surge voltage 
clamping section 6 made up of an electrolytic capacitor C.sub.1, two 
capacitors C.sub.2 and C.sub.3, two ordinary thyristors T.sub.1 and 
T.sub.2, four diodes D.sub.8 to D.sub.11, two turning-off inductors 
L.sub.1 and L.sub.2, and a reactor having two reactances Lr.sub.1 and 
Lr.sub.2, as shown in FIG. 1. In the above reactor, the positive side of 
the first reactance Lr.sub.1 is connected to the positive terminal of the 
DC rectifier 1 through the diode D.sub.10 ; the negative side of the 
second reactance Lr.sub.2 is connected to the negative terminal of the DC 
rectifier through the diode D.sub.11, respectively. 
The operation of the prior-art surge voltage clamping circuit will be 
described hereinbelow with reference to FIGS. 1 and 2. 
When mode I (G.sub.1 and G.sub.6 are on) is switched to mode II (G.sub.1 is 
off, G.sub.6 is on, G.sub.3 is on), for instance, as depicted in FIG. 2, 
the current i.sub.u flowing through windings X.sub.u and X.sub.w is 
commutated to the current i.sub.v flowing through windings X.sub.v and 
X.sub.w. In this transient state, the current i.sub.u does not immediately 
fall to zero level but decreases gradually and the current i.sub.v does 
not immediately rise to the current Id but increases gradually as depicted 
in FIG. 2. This is because there exists each inductance in each winding 
and thereby an induced surge voltage is inevitably generated across each 
winding. It is very important to suppress or eliminate these induced surge 
voltages for protection of GTO thyristors. 
An induced surge voltage V.sub.vw developed across the windings X.sub.v and 
X.sub.w in this transient state can be charged in the capacitor C.sub.1 as 
follows: When the surge voltage V.sub.vw exceeds the voltage across the 
capacitor C.sub.1, since the diodes D.sub.3 and D.sub.6 are both forward 
biased (the anode of D.sub.3 is high in voltage level; the cathode of 
D.sub.6 is low in voltage level), the major part of the current to be 
passed through the winding X.sub.v flows by way of GTO G.sub.3, diode 
D.sub.3, capacitor C.sub.1, diode D.sub.6 and GTO G.sub.6. In this 
transient state, the surge voltage V.sub.vw is suppressed by the capacitor 
C.sub.1 if the voltage e.sub.c1 across the capacitor C.sub.1 is 
sufficiently low. 
Simultaneously, when an induced surge voltage V.sub.uv developed across the 
windings X.sub.u and X.sub.v in this transient state falls to the voltage 
e.sub.c1 of the capacitor C.sub.1, since the diode D.sub.3 and D.sub.2 are 
both forward biased (the cathode of D.sub.2 is low in voltage level and 
anode of D.sub.3 is high in voltage level), the commutation energy 
generated across the windings X.sub.u and X.sub.v is charged into the 
capacitor C.sub.1 by way of GTO G.sub.3, diode D.sub.3, capacitor C.sub.1, 
diode D.sub.2, winding X.sub.u, winding X.sub.w and GTO G.sub.6. In this 
transient state, the surge voltage V.sub.uv is suppressed by the capacitor 
C.sub.1. As a result, the induced surge voltage V.sub.wu developed across 
the windings X.sub.w and X.sub.u becomes zero as shown in FIG. 2. The 
current i.sub.v increases gradually up to the direct current Id in 
accordance with a time constant determined by the circuit constant of the 
motor load. When the current i.sub.u reaches zero, the diode D.sub.2 is 
cut off. Simultaneously, no induced surge voltage is generated in the 
winding X.sub.u. When the induced surge voltage V.sub.vw falls below the 
capacitor voltage e.sub.c1, the diodes D.sub.3 and D.sub.6 are both cut 
off, so that the capacitor C.sub.1 is electrically disconnected from the 
inverter 3 and thus the commutation from GTO G.sub.1 to GTO G.sub.3 is 
completed. 
The above-mentioned mode II corresponds to the overlapped (transient) 
period in a series-connected diode type current inverter. However, there 
still exists a difference between the GTO inverter shown in FIG. 1 and the 
series-connected diode type current inverter in that two transient 
currents flow through the each-phase winding in the directions opposite to 
each other being superimposed upon each other. 
When the charged-up voltage e.sub.c1 in the capacitor C.sub.1 increases 
sufficiently, the two reverse blocking ordinary thyristors T.sub.1 and 
T.sub.2 are turned on in response to a pulse applied to each gate 
terminal. Therefore, the surge voltage energy stored in the capacitor 
C.sub.1 is discharged to the DC source terminals 3A and 3B of the GTO 
inverter 3 by way of inductor L.sub.1, thyristor T.sub.1, reactor 
Lr.sub.1, diode D.sub.10, reactor 2A, GTO inverter 3, motor 4, GTO 
inverter 3, reactor 2B, diode D.sub.11, reactor Lr.sub.2, thyristor 
T.sub.2, and inductor L.sub.2. The above-mentioned discharge is called 
commutation surge voltage energy restoration or energy rebound. In this 
initial state of discharge, the capacitors C.sub.2 and C.sub.3 are also 
charged up with the polarity as shown in FIG. 1. These two capacitors 
C.sub.2 and C.sub.3 form two vibration circuits independently. The first 
vibration circuit is made up of the capacitor C.sub.2 and the inductor 
L.sub.1 ; the second vibration circuit is made up of the capacitor C.sub.3 
and the inductor L.sub.2, each having a relatively high frequency or a 
relatively small time constant. An example of the voltage wave form 
e.sub.c2 or e.sub.c3 across the capacitor C.sub.2 or C.sub.3 is also shown 
in FIG. 2. When the polarity of the capacitor C.sub.2 or C.sub.3 of the 
vibration circuit is reversed, the thyristor T.sub.1 or T.sub.2 is 
automatically turned off, because a positive potential is applied to the 
cathode of the hyristor T.sub.1 to T.sub.2. In this state, it should be 
noted that the polarity of the charged-up voltages of the three capacitors 
C.sub.1, C.sub.2, C.sub.3 are the same, that is, three charged-up voltages 
are added to each other. Therefore, when the addition of these three 
charged-up voltages exceeds the DC source voltage across the thyristor 
bridge rectifier 1, the energy stored in these three capacitors C.sub.1, 
C.sub.2 and C.sub.3 are returned to the DC source terminals 3A and 3B by 
way of the reactor Lr.sub.1, diode 10, reactor 2A, GTO inverter 3, motor 
4, GTO inverter 3, reactor 2B, diode D.sub.11 and the reactor Lr.sub.2. In 
this energy restoration operation, since the inductance Lr.sub.1 or 
Lr.sub.2 of the reactor is so determined as to be sufficiently greater 
than that of the turning-off (vibration) inductor L.sub.1 or L.sub.2, the 
two vibration circuits L.sub.1 .multidot.C.sub.2 and L.sub.2 
.multidot.C.sub.3 can stably vibrate and provide an sufficient turn-off 
time for the thyristor T.sub.1 or T.sub.2. In other words, the reactor 
Lr.sub.1 or Lr.sub.2 functions as a smoothing element. After the surge 
voltage energy has been discharged, the capacitors C.sub.2 and C.sub.3 are 
charged again in the direction as shown in FIG. 1, to the voltage level 
roughly the same as the voltage e.sub.c1 across the capacitor C.sub.1, 
because three capacitors C.sub.1, C.sub.2, and C.sub.3 are connected in 
series. 
In this state, since the capacitance of capacitor C.sub.1 is determined to 
be sufficiently great as compared with that of the capacitors C.sub.2 or 
C.sub.3, after the capacitors C.sub.2 and C.sub.3 have been charged up, 
the magnetic energy stored in the reactor Lr.sub.1 or Lr.sub.2 is 
recharged into the capacitor C.sub.1 by way of diode D.sub.11, reactor 
Lr.sub.2, diode D.sub.8, capacitor C.sub.1, diode D.sub.9, reactor 
Lr.sub.1 and diode D.sub.10. This energy is unavailable reactive power by 
nature. After the reactor energy has been recharged into the capacitor 
C.sub.1, the two diodes D.sub.10 and D.sub.11 are both returned to its off 
state, respectively, that is, to the initial conditions. Therefore, the 
electric discharge of the capacitor C.sub.2 or C.sub.3 is prevented for 
being ready for the succeeding commutation of the GTO bridge-connected 
inverter 3. After the GTO G.sub.1 has been turned off and the GTO G.sub.3 
has been turned on, that is, the commutation has been completed from 
G.sub.1 to G.sub.3, the operation mode shifts to the mode III in which the 
driving current is supplied from GTO G.sub.3, through windings X.sub.v and 
X.sub.w, to GTO G.sub.6. 
In the prior-art surge voltage clamping circuit for the current-type GTO 
inverter described above, however, there exist some disadvantages as 
follows. 
(1) The turning-off inductors L.sub.1 and L.sub.2 and the turning-off 
capacitors C.sub.1 and C.sub.2 are required for forming two vibration 
circuits in order to turn off the ordinary thyristors T.sub.1 and T.sub.2 
after the surge voltage energy stored in the capacitor C.sub.1 has been 
restored to the DC source terminals of the GTO inverter 3. Additionally, 
the commutation energy is restored to the GTO inverter 3 mainly through 
the capacitors C.sub.2 and C.sub.3. Therefore, in order to sufficiently 
restore the stored surge voltage energy even under a heavy load, the 
capacitance of C.sub.2 or C.sub.3 should be relatively large. When 
capacitors having a large capacitance are used, the vibration frequency 
becomes low, thus resulting in turn-off failure of the ordinary thyristors 
T.sub.1 and T.sub.2. In other words, it is impossible to stably operate 
the surge voltage clamping circuit at a high frequency range when a heavy 
load is applied to the induction motor. 
(2) Since the added charged-up voltage of the series-connected capacitors 
C.sub.1, C.sub.2, and C.sub.3 is restored to the GTO inverter 3 through 
the cumulative reactor having reactances Lr.sub.1 and Lr.sub.2, a large 
inductance is required for this reactor. Otherwise, current overshoot may 
be generated. In other words, the cost of the reactor Lr.sub.1 and 
Lr.sub.2 is relatively high. 
By the way, in order to drive an induction motor in the same manner as in a 
DC motor, four-quadrant operation is indispensable. This four-quadrant 
operation will be described below. As depicted in FIG. 3A, when the rotor 
angular frequency .omega..sub.r is taken as abscissa and the motor torque 
T is taken as ordinate, the first quadrant indicates that a motor is 
driven in the normal rotational direction; the second quadrant indicates 
that the motor is braked while rotating in the normal direction; the third 
quadrant indicates that the motor is driven in the reverse rotational 
direction; the fourth quadrant indicates that the motor is braked while 
rotating in the reverse direction. 
In other words, in the first quadrant, the motor torque T is positive and 
the rotor angular frequency .omega..sub.r also is positive; in the second 
quadrant, T is negative but .omega..sub.r is positive; in the third 
quadrant, T is negative and .omega..sub.r is also negative; in the fourth 
quadrant, T is positive but .omega..sub.r is negative, as depicted in FIG. 
3B. 
FIG. 3C shows an example in which a motor rotating in the normal direction 
is switched to the reverse direction at time t.sub.1. In more detail, when 
a motor is rotating in the 1st quadrant operation (T&gt;0, .omega..sub.r &gt;0), 
if the reference frequency (speed) +w*.sub.r is switched to -w*.sub.r, the 
motor rotates in the 2nd quadrant operation (T&lt;0, .omega..sub.r &gt;0) (the 
motor is braked or the motor torque is adsorbed). The instant the rotor 
frequency reaches zero, the motor begins to rotate in the 3rd quadrant 
operation (T&lt;0, .omega..sub.r &lt;0) (the motor is driven in the reverse 
direction). 
Further, in the above description, it should be noted that while the 
induction motor is being braked for stopping the motor or for reversing 
the rotational direction of the motor, the induction motor operates as a 
generator which can return the motor rotational kinetic energy to the AC 
source side. 
Therefore, the prior-art surge voltage clamping circuit shown in FIG. 1 has 
the following third disadvantages: 
(3) Since the charging and discharging circuits for the capacitor is 
provided only for motor-driving operation, it is impossible to discharge 
or regenerate the motor kinetic energy stored in the capacitor in 
motor-braking operation (motor operates as a generator in brake) to the AC 
source side or to charge the magnetic energy stored in the reactor in 
motor-braking operation in the capacitor. 
In view of the above description, reference is now made to an embodiment of 
a surge voltage clamping circuit for a current-type GTO inverter according 
to the present invention with reference to FIG. 4. In this embodiment, 
GTOs are incorporated in the surge voltage clamping circuit, without 
providing turning-off (vibration) circuits, in order to operate the 
circuit stably at a high speed. 
In FIG. 4, the points different from the prior-art surge voltage clamping 
circuit shown in FIG. 1 are that (1) a single direct-current reactor 7 is 
incorporated in place of the cumulative direct-current reactors 2A and 2B 
and (2) four gate turn-off thyristors (GTOs) G.sub.7, G.sub.8, G.sub.9 and 
G.sub.10 and four diodes D.sub.8, D.sub.9, D.sub.12 and D.sub.13 are 
incorporated without providing the vibration circuits including two 
inductors L.sub.1 and L.sub.2 and capacitors C.sub.2 and C.sub.3. 
A surge voltage clamping circuit 6 or a surge energy restoring circuit 
according to the present invention comprises an electrolytic capacitor 
C.sub.1 for absorbing the commutation surge voltage energy, four GTOs 
G.sub.7, G.sub.8, G.sub.9 and G.sub.10 for restoring the surge voltage 
energy stored in the capacitor C.sub.1 to the GTO inverter 3, a cumulative 
reactor having two inductive reactances Lr.sub.1 and Lr.sub.2 magnetically 
connected to each other for smoothing the current restored from the 
capacitor C.sub.1 to the GTO inverter 3, and four diodes D.sub.8, D.sub.9, 
D.sub.12, and D.sub.13 for transferring the magnetic energy stored in the 
reactor Lr.sub.1, Lr.sub.2 to the capacitor C.sub.1 after the surge 
voltage energy stored in the capacitor C.sub.1 has been restored. 
The first pair of GTOs G.sub.7 and G.sub.8 serve to discharge or restore 
the surge voltage energy generated in motor-driving operation to the DC 
source terminals of the GTO inverter 3; the second pair of GTOs G.sub.9 
and G.sub.10 serve to discharge or regenerate the motor kinetic energy 
generated in motor braking operation to the AC source side of the GTO 
inverter 3. 
The electrolytic capacitor C.sub.1 is connected in parallel with the diode 
bridge-connected commutation surge voltage rectifier 4. The first two GTOs 
G.sub.7 and G.sub.8 are connected between the reactor Lr.sub.1 and 
Lr.sub.2 and the capacitor C.sub.1 as follows: the positive side of the 
first winding Lr.sub.1 is connected to the positive terminal of the DC 
source; the negative side of the first winding Lr.sub.1 is connected to 
the cathode of the first GTO G.sub.7 ; the anode of the first GTO G.sub.7 
is connected to the positive side of the capacitor C.sub.1 ; the negative 
side of the second winding Lr.sub.2 is connected to the negative terminal 
of the DC source; the positive side of the second winding Lr.sub.2 is 
connected to the anode of the second GTO G.sub.8, and the cathode of the 
second GTO G.sub.8 is connected to the negative side of the capacitor 
C.sub.1, respectively, respectively. 
Further, the two diodes D.sub.8 and D.sub.9 are connected between the 
reactor Lr.sub.1 and Lr.sub.2 and the capacitor C.sub.1 as follows: the 
cathode of the first diode D.sub.8 is connected to the positive side of 
the capacitor C.sub.1 ; the anode of the first diode D.sub.8 is connected 
to the positive side of the second winding Lr.sub.2 ; the cathode of the 
second diode D.sub.9 is connected to the negative side of the first 
winding Lr.sub.1 and the anode of the second diode D.sub.9 is connected to 
the negative side of the capacitor C.sub.1. 
The additional second two GTOs G.sub.9 and G.sub.10 for regenerating the 
motor rotational energy produced in motor braking operation to the AC 
source side are connected between the reactor Lr.sub.1 and Lr.sub.2 and 
the capacitor C.sub.1 as follows: the anode of the third GTO G.sub.9 is 
connected to the positive side of the capacitor C.sub.1 ; the cathode of 
the third GTO G.sub.9 is connected to the positive side of the second 
winding Lr.sub.2, the anode of the fourth GTO G.sub.10 is connected to the 
negative side of the first winding Lr.sub.1 ; and the cathode of the 
fourth GTO G.sub.10 is connected to the negative side of the capacitor 
C.sub.1. 
Further, additional two diodes D.sub.12 and D.sub.13 are connected between 
the reactor Lr.sub.1 and Lr.sub.2 and the capacitor C.sub.1 as follows: 
the anode of the third diode D.sub.12 is connected to the negative side of 
the first winding Lr.sub.1 ; the cathode of the third diode D.sub.12 is 
connected to the positive side of the capacitor C.sub.1 ; the anode of the 
fourth diode D.sub.13 is connected to the negative side of the capacitor 
C.sub.1 ; and the cathode of the fourth diode D.sub.13 is connected to the 
positive side of the second winding Lr.sub.2. 
In order to distinguish between the four GTOs and four diodes, the GTOs 
G.sub.7 and G.sub.8 are referred to as in-drive energy restoring GTOs; the 
GTOs G.sub.9 and G.sub.10 are referred to as in-brake energy regenerating 
GTOs; the diodes D.sub.8 and D.sub.9 are referred to as in-drive energy 
restoring diodes; the diodes D.sub.12 and D.sub.13 are referred to as 
in-brake energy regenerating diodes, hereinafter. 
As already described with reference to FIG. 2, when the GTO G.sub.1 is 
turned off and the GTO G.sub.3 is turned on with the GTO G.sub.6 kept 
turned on, the current i.sub.v cannot rise immediately and the current 
i.sub.u cannot fall to zero immediately because a commutation surge 
voltage is developed. The time interval within which two currents i.sub.u 
and i.sub.v overlap each other corresponds to a transient state. The time 
interval within which a single current i.sub.u or i.sub.v exists 
corresponds to a steady state. 
The above-mentioned energy restoring GTOs G.sub.7 and G.sub.8 are turned on 
only in the driving state and GTOs G.sub.9 and G.sub.10 are turned on only 
in the braking state but turned off in the transient state, in response to 
each gate signal generated by each gate circuit (not shown), in order to 
realize the energy restoration or regeneration function. 
The operation of the embodiment of the surge voltage clamping circuit for a 
current-type GTO inverter according to the present invention will be 
described hereinbelow with reference to FIGS. 5(A), 5(B), 6, 7(A) to 7(D). 
Further, the operation is described only during a one-sixth period (60 
degrees) of the inverter and during the commutation from GTO G.sub.1 (U 
phase) to GTO G.sub.3 (V phase). 
FIG. 5(A) shows a steady state (single current period) where GTOs G.sub.1 
and G.sub.6 are both turned on, so that a load current Id flows from the 
positive terminal P to the negative terminal N by way of DC reactor 7, GTO 
G.sub.1, reactance X.sub.u, reactance X.sub.w and GTO G.sub.6 as a U-phase 
constant load current i.sub.u as depicted in FIG. 2. 
In general, since no time delay exists due to inductive elements in the 
gate circuit of GTOs, it is possible to instantaneously turn off GTO 
G.sub.1 and to instantaneously turn on GTO G.sub.3 in response to gate 
signals. However, the instant GTO G.sub.1 is turned off and GTO G.sub.3 is 
turned on with GTO G.sub.6 kept on, a transient state occurs in the 
inverter circuit 3 as shown in FIG. 2 and FIG. 5(B), in which both the 
U-phase current i.sub.u and the V-phase current i.sub.v flow (overlap 
current period). In more detail, upon turning-on of GTO G.sub.3, the 
V-phase current i.sub.v to be passed through the reactances X.sub.v and 
X.sub.w cannot rise immediately due to the presence of the inductance, as 
depicted in FIG. 2. When this transient induced surge voltage V.sub.vw 
developed across the induction motor reactances X.sub.v and X.sub.w 
(positive at X.sub.v and negative at X.sub.w) is applied to the capacitor 
voltage e.sub.c1, since the diodes D.sub.3 and D.sub.6 are both 
forward-biased, the major part of current to be passed through the 
reactance X.sub.v is bypassed by way of GTO G.sub.3, diode D.sub.3, 
capacitor C.sub.1, diode D.sub.6 and GTO G.sub.6. However, this transient 
induced surge voltage V.sub.vw is charged into the capacitor C.sub.1 when 
the voltage across the capacitor C.sub.1 is sufficiently low. 
Simultaneously, another transient induced surge voltage V.sub.uv is 
developed across the induction motor reactances X.sub.u and X.sub.u 
(positive at X.sub.v and negative at X.sub.u). While this surge voltage 
V.sub.uv rises up to the capacitor voltage e.sub.c1, since the diodes 
D.sub.3 and D.sub.2 are both forward-biased, the surge voltage V.sub.uv is 
restored to the capacitor C.sub.1 by way of diode D.sub.3, capacitor 
C.sub.1, diode D.sub.2, reactance X.sub.u, reactance X.sub.w, and GTO 
G.sub.6. As a result, the surge voltage V.sub.wu across the motor 
reactances X.sub.w and X.sub.u is reduced to zero. The V-phase current 
i.sub.v gradually increases up to the direct current Id in accordance with 
a time constant determined by the circuit constants dependent upon the 
capacitor voltage e.sub.c1 at that moment. When the U-phase current 
i.sub.u reaches zero, the diode D.sub.2 is off; the induced surge voltage 
V.sub.vw is no longer produced lower than the capacitor voltage e.sub.c1, 
so that the diodes D.sub.3 and D.sub.6 are both off. In this state, the 
capacitor C.sub.1 is isolated perfectly from the GTO bridge-connected 
inverter 3, thus the commutation from GTOs G.sub.1 to G.sub.3 being 
completed with the GTO G.sub.6 kept turned on. 
FIG. 6 shows charging and discharging paths of the capacitor C.sub.1 in the 
surge voltage clamping circuit 6 shown in FIG. 4, by which it is possible 
to better understand the operation to charge energy to the capacitor 
C.sub.1 or the operation to discharge the charged energy to the GTO 
inverter 3. 
Further, these charging and discharging paths shown in FIG. 6 are 
separately depicted in FIGS. 7(A) to 7(D), being classified into four 
states. 
(1) Surge energy restoration in motor-driving operation: 
Since the GTOs G.sub.7, G.sub.8 and G.sub.9, G.sub.10 are all turned off in 
the transient interval (commutation period or overlap current period), the 
surge voltage energy is charged as pulsive current i.sub.DBR (shown in 
FIG. 12) from the inverter 3 to the capacitor C.sub.1 through the diode 
bridge-connected rectifier 5 as already described with reference to FIG. 
5(B), so that the capacitor voltage e.sub.c1 increases gradually. When the 
energy restoring GTOs G.sub.7 and G.sub.8 are turned on in the steady 
state interval (single current period), the energy stored in the capacitor 
C.sub.1 is discharged (restored) to the DC source terminals 3A and 3B of 
the GTO inverter 3 by way of capacitor C.sub.1, GTO G.sub.7, reactor 
Lr.sub.1, reactor 7, GTO inverter 3, motor 4, GTO inverter 3, reactor 
Lr.sub.2, GTO G.sub.8, and capacitor C.sub.1, as depicted in FIG. 7(A). 
Further, since the two GTOs G.sub.7 and G.sub.8 are kept turned off in the 
transient state interval, the magnetic energy stored in the reactor 
Lr.sub.1, Lr.sub.2 is charged into the capacitor C.sub.1 by way of reactor 
Lr.sub.2, diode D.sub.8, capacitor C.sub.1, diode D.sub.9, reactor 
Lr.sub.1, as depicted in FIG. 7(B). Therefore, the capacitor C.sub.1 is 
charged up in the same porality as in the surge voltage. 
(2) Motor kinetic energy regeneration in motor-braking operation: 
When the induction motor is braked, the frequency of the inverter current 
is lowered; that is, each phase of gate signals applied to the gate 
terminals of the GTO inverter 3 is delayed from the motor speed. As a 
result, the polarity of the voltage generated from the inverter 3 is 
reversed because the motor operates as a generator. In other words, the 
direction of the current flowing through the reactor Lr is reversed. 
However, the polarity of the capacity C.sub.1 should be kept in a 
predetermined direction. 
The voltage generated by the motor kinetic energy is charged into the 
capacitor C.sub.1 by way of reactor Lr.sub.2, diode D.sub.8, capacitor 
C.sub.1, diode D.sub.9 and reactor Lr.sub.1 as depicted in FIG. 7(B). In 
this state, it should be noted that regenerating GTOs G.sub.9, G.sub.10 
are both reverse biased by the diodes D.sub.8 and D.sub.9. Under these 
conditions, these GTOs will not be turned on even if a gate signal is 
applied to each of the gate terminals. However, after the current supplied 
from the reactor Lr.sub.1 and Lr.sub.2 to the capacitor C.sub.1 has 
decreased to near zero, the energy regenerating GTOs G.sub.9, G.sub.10 can 
be turned on, so that the energy charged in the capacitor C.sub.1 is 
discharged or regenerated to the AC source side by way of capacitor 
C.sub.1, GTO G.sub.9, reactor Lr.sub.2, the thyristor bridge-connected 
current rectifier 1, reactor LR.sub.1, GTO G.sub.10 and capacitor C.sub.1 
as depicted in FIG. 7(C). In this state, since the voltage e.sub.c1 is 
high, the diodes D.sub.12 and D.sub.13 are reverse biased. However, since 
the regeneration energy in motor braking operation becomes great in a 
moment and therefore the capacitor C.sub.1 is immediately charged up. It 
is preferable to turn on the GTO G.sub.9 and G.sub.10 whenever the voltage 
e.sub.c1 across the capacitor C.sub.1 exceeds a predetermined reference 
value. 
Further, when the voltage e.sub.c1 drops below a predetermined value and 
therefore the GTOs G.sub.9, G.sub.10 are turned off, the magnetic energy 
stored in the reactor Lr in motor-braking operation is recharged into the 
capacitor C.sub.1 by way of reactor Lr.sub.1, diode D.sub.12, capacitor 
C.sub.1, diode D.sub.13, and reactor Lr.sub.2 as depicted in FIG. 7(D). 
The above-mentioned regenerative braking operation is completed while the 
motor is being braked. When the induction motor is switched from braking 
operation to reverse operation, the phase order of the GTO inverter 3 is 
reversed, as compared with that when the induction motor is driven in the 
forward direction. In this case, since only the phase order is reversed, 
the operation is quite the same as that when the motor is driven in the 
forward direction as shown in FIGS. 7(A) and 7(B). 
As described above, the surge voltage clamping circuit of the present 
invention provides the first function to absorb or charge the surge 
voltage energy generated in transient state interval (commutation period) 
and to restore or discharge the surge voltage energy to the DC source 
terminal of the GTO inverter in the steady state interval in the 
motor-driving operation, the second function to absorb or charge the 
magnetic energy stored in the reactor in the transient state interval and 
to restore or discharge the magnetic energy to the DC source terminal of 
the GTO inverter in the steady state in both the motor-driving and 
-braking operation, and the third function to absorb or charge the motor 
kinetic energy and to regenerate or discharge the energy to the AC source 
side in the motor-braking operation. 
Hereupon, the above surge voltage absorbing function is greatly dependent 
upon the voltage e.sub.c1 developed across the capacitor C.sub.1. This 
voltage e.sub.c1 can be controlled by adjusting the off-time interval 
.tau. of the GTOs G.sub.7 and G.sub.8. 
FIG. 8 shows the relationship between the off-time interval .tau. of the 
GTOs G.sub.7 and G.sub.8 and the capacitor voltage e.sub.c1 with the 
capacitance C.sub.1 as parameter at a fixed dc source voltage Ed and a 
fixed frequency f of the GTO inverter 3. This graphical representation 
indicates that the longer the off-time interval .tau. of GTOs G.sub.7 and 
G.sub.8, the higher the voltage e.sub.c1, because the discharge time 
interval of the capacitor C.sub.1 decreases. However, there exists little 
influence of the capacity of the capacitor C.sub.1 upon the capacitor 
voltage e.sub.c1. 
FIG. 9 shows the relationship between the frequency f of the GTO inverter 3 
and the capacitor voltage e.sub.c1 with the off-time interval .tau. as 
parameter at a fixed dc source voltage Ed and a fixed capacitance C.sub.1. 
This graphical representation indicates that the higher the frequency f, 
the higher the voltage e.sub.c1, because the commutation energy per unit 
time increases. However, the shorter the off-time interval .tau., the 
capacitor voltage e.sub.c1 undergoes little influence of change in the 
frequency f. 
FIG. 10 shows the relationship between the dc source voltage Ed and the 
capacitor voltage e.sub.c1 at a fixed frequency f of the inverter 3, a 
fixed off-time interval .tau. of the GTOs G.sub.7 and G.sub.8 and a fixed 
capacitance C.sub.1. This graphical representation indicates that the 
capacitor voltage e.sub.c1 increases with increasing dc source voltage Ed 
and further e.sub.c1 always exceeds Ed. 
As explained above, the off-time interval .tau. of the GTOs G.sub.7 and 
G.sub.8 incorporated in the surge voltage clamping circuit 6 according to 
the present invention exerts a serious influence upon the clamping 
operation of surge voltage. 
Further, in the case where the DC source voltage Ed is low, it should be 
noted that since the capacitor voltage e.sub.c1 increases with increasing 
the off-time interval .tau. as depicted in FIG. 8, the clamping operation 
is deteriorated, thus resulting in sharp waveform change in the motor load 
current. 
FIG. 11(A) shows an equivalent circuit diagram corresponding to the current 
path shown by thicker lines in FIG. 5(A). That is, FIG. 11(A) shows the 
steady state where both GTOs G.sub.1 and G.sub.6 are turned on. However, a 
discharging loop of the capacitor C.sub.1 is neglected. FIG. 11(B) shows 
an equivalent circuit diagram coresponding to the current paths shown by 
thicker lines in FIG. 5(B). That is, FIG. 11(B) shows the transient state 
GTO G.sub.1 has been turned off and GTO G.sub.3 has been turned on with 
GTO G.sub.6 kept on. 
With reference to FIG. 11(A), it is possible to obtain a steady-state 
circuit equation as follows: 
EQU (L.sub.d +2L)dI.sub.d /dt+(R.sub.d +2R.sub.1)i.sub.d +V.sub.u -V.sub.w 
=E.sub.d 
EQU V.sub.u =E sin (.omega.t+.psi..sub.1 2.pi./3) 
EQU V.sub.w =E sin (.omega.t+.psi..sub.1 -2.pi./3) 
Similarly, with reference to FIG. 11(B), it is possible to obtain 
transient-state circuit equations as follows: 
##EQU1## 
where R.sub.1 : induction motor stator resistance 
L: sum of stator and rotor leakage inductance 
V.sub.u, V.sub.v, V.sub.w : motor's counter electromotive force (CEMF) 
generated by fundamental component of input current 
e.sub.c1 : initial capacitor voltage 
E: peak phase voltage of motor CEMF 
.psi.: phase angle between fundamental component of input current and 
fundamental component of CEMF 
V.sub.u ', V.sub.v ', V.sub.w ': each phase terminal voltage 
.omega.: inverter angular frequency. 
I.sub.d : dc current flowing from dc source Ed 
i.sub.d1 : current flowing through phase u 
i.sub.d2 : current flowing through phase v 
i.sub.d3 : current flowing through diode D.sub.6 
E.sub.d : average voltage of dc source Ed 
R.sub.d : resistance of dc reactor 7 
L.sub.d : inductance of dc reactor 7 
FIG. 12 shows a timing chart of waveforms of the inverter shown in FIG. 4, 
in which the time interval enclosed between two dashed lines roughly 
corresponds to FIG. 2. In FIG. 12, the output current i.sub.u rises and 
falls relatively gradually and surge voltage are sufficiently clamped or 
suppressed. Further, the output current i.sub.DBR of the diode 
bridge-connected circuit 5 is generated for each commutation in pulsive 
waveform state to sequentially charge up the capacitor C.sub.1. 
The surge voltage clamping circuit according to the present invention has 
the following various features: 
(1) The circuit operates stably when driving an induction motor at a high 
speed, because there are provided no vibration or commutation circuits to 
turn off ordinary thyristors. 
(2) The circuit is high in energy conversion efficiency, because no 
commutating capacitors are provided. 
(3) The circuit cost is reduced, because an ordinary DC reactor can be used 
in place of a cumulative DC reactors, because the energy stored in the 
capacitor C.sub.1 is discharged frequently through the GTOs, and no 
commutating thyristors to generate sharp transient current are 
incorporated. 
(4) The circuit can protect the GTOs (G.sub.1 to G.sub.6) of the inverter 
circuit from induced surge voltages in dependence upon relative-small GTOs 
(G.sub.7 to G.sub.10) of the clamping circuit. 
(5) The magnitude of the surged voltage and the time interval of the 
transient state can be adjusted by adjusting the off-time intervals of the 
GTOs of the clamping circuit. 
(6) The circuit can restore or regenerate surge voltage, magnetic or 
kinetic energy both in motor-driving operation and motor-braking 
operation. 
It will be understood by those skilled in the art that the foregoing 
description is in terms of a preferred embodiment of the present invention 
wherein various changes and modifications may be made without departing 
from the spirit and scope of the invention, as set forth in the appended 
claims.