This invention relates to a novel current hysteresis controller for three phase inverters to drive an induction motor for variable speed applications.
In conventional three phase current hysteresis controllers, the current error along the
three phase axes are independently controlled. This result in random selection of switchinq vectors causing
high inverter switching losses. Further such type of controllers will go into 3 imit. cycle hiqh frequency
switchinq at low back emf voltages. Also the current error reaches twice the value of hysteresis hand.
Predictive current controller, with optimised PWH switchinq and with reduced switching frequency, require
complicated off line computations and complex imp 1 emeritat i on schemes.
Therefore the main object of the present invention is to propose a novel current hysteresis
controller for three phase inverters and wherein the computation of machine voltage any limit !Vm! is not
required,

Another object of this invention is to propose a novel space phasor based current hysteresis
controller for three phase inverters and wherein adjacent vectors close to the machine voltage space
phasor (irrespective of the machine voltage amplitude IVm! and position), in a sector, for PMW current
hysteresis control are selected.
According to this invention there is provided a novel current hysteresis controller for three phase
invertors to drive an induction motor comprising a plurality of analog covertor being connected a first set
of operational amplifiers, the output of said first set of operational amplifiers are connected to a second set
of operational amplifier being provided at the right hand side of said first set, the other input of said
first set being connected to the output of a third set of amplifiers provided on the left hand side of said set
of amplifiers, said third set of amplifier being adapted to be connected with the outputs of a three
phase inverter provided to receive the signals produced by the getting signal generator, the output of said
three phase invertors being connected to an induction

motor to drive the same at the required speed, said
motor being connected to a techo provided to convey speed feedback to an adder, the input of said adder
being connected to the soft start provided to receive the signals of the speed reference, the soft star being
connected to a summer which is connected to a voltage control oscillator being connected to the respective
E—PROM sign waves connected to said analog convertor, the adder being connected to a PI controller which is
connected to a functional generator being connected to said analog convertors characterized in that the outputs
of said first and second set of operational amplifiers being connected to fourth and fifth sets of amplifiers
being connected to programmable array logic, the inputs of said third set of operational amplifier being
connected to a set of amplifiers provided for current references, the outputs of said current references
amplifiers are connected to sixth set of amplifiers being connected to said programmable array logic through
the respective set of amplifiers interconnected with each other, the inputs of current references amplifiers
being connected to the said three phase invertor, said programmable array logic being provided to convey the
gate drive signals to said three phase invertor such that to drive said induction motor at the variable
speeds..

In accardance with this invention a current
hysteresis controller for three phase inverters comprises a plurality of digital to analog converters,
the output of said digital analog converters being connected to the input of a set of operational amplifier
whose outputs are connected to another set of operational amplifier provided on the right hand side
thereof. The output of the set of operational amplifier provided on the left. side of the first set of
operational amplifier are connected to the input of the first set of operational amplifier and also connected to
the inputs of a fourth set of operational amplifier whose outputs are connected to a fifth set of
operational amplifier. The outputs of the fifth set of operational amplifier are connected to a programmable
arrmy logic. The inputs of third set of operational amplifier are provided on the left hand side of the
first set of operational amplifier aire connected to the outputs of third phase inverters. Further the outputs
of third phase inverters a.re connected to a tacho through a IM block. The output of the tacho is
connected to speed feedback device which is connected to

the low and also connected to a slip speed signal. The
slip speed signal device is connected to a voltage control oscillator which is connected to E-PROMs signal
blocks. The E-PROlis signal blocks aire connected to the analog converters, The speed back device is connected
to the output of soft slip which is connected to a motor-speed reference device. The outputs of the programmable
array logic are connected to the three phase inverters. The outputs, of first set of operational amplifier are
also connected to the inputs of a sixth set of operational amplifier whose outputs are connected to a
seventh set of operational amplifier. The outputs of the third set of operational amplifiers are connected to
the input of eighth set of operational amplifier. The output of seventh and eighth set of operational
amplifier are connected to the programmable array logic.
A current hysteresis controller for three
phase inverters according to a preferred embodiment is
herein described and illustrated in the accompanying
drawings wherein:
Fig.l shows the circuit diagram of the conventional current hysteresis,
Fig.2 shows the circuit diagram of the current hysteresis of this invention.

Referring to the drawings particularly fig.
1, the current control hysteresis controller comprises a plurality of analog converters 0.1,02 and C3 for
converting digital signals into the analog signals. The analog converters are connected to a set of operational
amplifiers A1,A2,A3 respectively. The operational amplifier A1,A2 and A3 are connected to another set of
operational amplifiers A4,A5 and A6 which are connected to a getting signal generator which conveys the signals
to a three phase inverter I connected with induction motor IM. The motor is connected with a tacho T
provided for supplying speed feedback to the adder A. Adder A is connected with a functional generator G
through a PI controller C. The generator G is provided
for controlling the gain of the sine wave amplitude
for defend speed and loading conditions of the induction motor. The function generator G is connected
to the analog converters C±,C2 and C3. The PI controller is also connected to a summer S which is
connected to a voltage control oscillator 0 being connected to the E PROM sign waves E1,E2 and E3, The E
PROM sign waves E1.E2 and E3 are connected to analog converter Ci,C2,C3 respectively. A soft star- S

adapted to be connected to a speed reference SR is connected to the adder A and also to the summer S. The
speed reference is passed through the soft star S and the output of the soft star is compared with the speed
feed hack signal from the tacho in the P.! controller C. The output soft, the three phase inverter I are
connected to the operational amplifiers A7,A8 and A9 which arfi connected to the operational amplifier A1»A2
and A3 being connected to the operational amplifier A4,A5 and A6 provided for getting signal generation.
The three phase inverter is adapted to be connected with a three phase inverter.
Reference is now made to fig. 2, the three phase current reference Cj^ , jL , j 3^ ) are: the output of the analog converter 28,29 and 30, The speed
reference is passed through a soft star circuit (22) and the output of which is compared with the speed feed back
signal from a tacho (T) or synthesised from machine model or derived from encoders in a PI controller (23).
The output of the PI controller is the slip speed signal. The slipspeed signal is added to the motor

speed reference to synthesise the synchronous speed and fed I ■::■• a voltage controlled oscillator (31). The
voltage controller oscillator output is fed to three E-PROiis (25,26 and 27). In the E-PROMs the sine wave
pah terns Are stored with proper phase shifts. The output of the E—PROMs are three symmetrical sinewaves
with unity amplitude, stored digitally the digital sine wave amplitudes are converted to analog form by passing
through the digital to analog converter blocks 28,29 and 30,. The gain of the sine wave amplitudes as required by
the motor phase currents far different speed and loading conditions are controlled by the output of the function
generator block 24„ The function generator is based on the Jnduction motor steady state relation between slip
speed and the motor phase current amplitudes. Thus the output of the converter's 23,29 and 30 provides the
reference currents for the multi axis space phasor based current hysteresis controller. The reference current
can also he obtained from controllers for high dynamic performance applications such as vector controlled
drives. Input to the operational amplifier 1,2,3 in fiq„2 are the motor phase currents, from current sensors

output from blocks 1,2,3 are the orthogonal current components f |I ^ jl g, jl j-0 . Inputs to the operational
amplifier 4,5,6 are the orthogonal current components
from the operational amplifier 1,2,3 and the reference inputs from the controller output inverter 28,29,30,
The output of the blocks 4,5,6 are the orthogonal (jA,
iB% iC a>!Pt,) current differences between the reference
components and the motor feedback currents. Block 7,8,9, .1.0,11,12 ^r& the hysteresis comparators along the
orthogonal (jA, jB, jC axes) current axes synthesized. There are two hysteresis comparators along each axis,
one at the current error distance of Zb.1/2 with the hysteresis band of '&' on the positive side and the
other on the negative side at a distance of /\.l/2 with • he hysteresis band of 'U' . So from the three axis
i.ilf ■> i 1R * J ^ ) there is a total of six hysteresis comparators. The combined effect of all these
comparators from the hexagonal current error boundary. These comparators outputs and its various combinations
select the appropriate inverter output vectors, depending on the sector and also as implemented using a

programmable array logic in fig. 2 Blocks
13,1.4, 1.5, .1.9,20,21 ans comparators for level shiftinq the hysteresis comparators output, swing of + 15V to +5V and -
.1.5V to logic signal level of zero volts. So these voltaqe levels &r& in accordance with the requirements
for memory look up tables using E-PROMs or with other
loqic qat.es. The sector selection depending on the
current error space phasor direction. based on the location of the motor hark emf vector is achieved using
three more hysteresis comparators along the (jA, jB, jC axes) a>tes with a hysteresis hand, which are achieved by
the hysteresis comparators blocks 16,17 and IS. This hysteresis band is centered around the zero current
error point. Blocks 32,33,24 aire again comparator blocks for level transition of the hysteresis
comparators (Blocks 16,17,18) output level +15V to +5V and -15V to zero volt. The output of the blocks
32,33,34 besides the sector selection. The logic level, outputs from blocks 13,14 ,1.5,19,20,21,32 ,33,34 and its
combinations are used for a programmable arrsiy logic structure as shown in fig.2 for the inverter switching
vectors for the six inverters switches. The six outputs

of the programmable array logic unit are used to drive the inverter switches. The numbers 1,2,3,4,5,6 in fiq.2
inside the programmable array logic unit represents the three phase inverter output vectors. The number 8 in
prnqrammau]e array logic represents the zero vector 'Z'«

WE CLAIM: 1. A novel current hysteresis controller for three
phase invertors to drive an induction motor comprising a plurality of analog covert.or being connected a first set
of operational amplifiers, the output of said first. set of operational amplifiers are-- connected to a second set
of operational amplifier being provided at the right hand side of said first set, the other input of said
first set being connected to the output of a third set of amplifiers provided on the left hand side of said set
of amplifiers, said third set of amplifier being aUapterl to be connected with the outputs of a three
phase inverter provided to receive the signals produced by the getting signal generator5 the output of said
three phase invertors being connected to an induction motor to drive the same at the required speed, said
motor being connected to a techo provided to convey speed feedback to an adder, the input of said adder
being connected to the soft start provided to receive the signals of the speed reference, the soft star being
connected to a summer which is connected to a voltage control oscillator being connected to the respective
E—PROM sign waves connected to said analog convertor.

the adder being connected to a PI controller which is connected to a functional generator being connected to
said analog converters characterized in that the outputs of said first and second set of operational amplifiers
being connected to fourth and fifth sets of amplifiers being connected to programmable array logic, the inputs
of said third set of operational amplifier being connected to a set of amplifiers provided for current
references, the outputs of said current: references amplifiers are connected to sixth set of amplifiers
being connected to said programmable array logic through the respective set of amplifiers interconnected with
each other, the inputs of current references amplifiers being connected to the said three phase inverter5 said
programmable array logic being provided to convey the gate drive signals to said three phase invertor such
that. to drive said induction motor- at the variable
!~z, i"i i.-i i^s i i i_t
... |„. J... .._. wi ... fl
2„ A novel current hysteresis controller substantially as herein described and illustrated.