Principle of vector (field oriented) control of an
induction motor drive is :
• to decouple the control of flux, and
• torque of an induction machine
Able to control AC motors:
• steady-state
• transient/dynamic
• achieved
by controlling the
machine currents
• simultaneously control :
• frequency current
• amplitude current
• phase of the current
s
Is1
Ia1
Is2
Ia2
1
2
Im1
Im2
• can
be used in control of AC motors
• induction (asynchronous) machines
• synchronous machines
• field orientation in synchronous machines is simpler
than in induction machines, since the position of
the flux generated by:
• the rotor winding
•permanent magnets is easy to monitor.
• vector control schemes are derived from an
appropriate dynamic (transient) model of the
machine,
•co-ordinate transformation has to be used in order
to correlate the control system with the real physical
world.
• The co-ordinate transformation requires information
on the instantaneous position of the appropriate
flux space vector.
• Stator flux space vector
• Air-gap flux space vector
• Rotor flux space vector
Clarke and Park Transform
To implement the basic principle of FOC, i.e.
• to maintain a desired alignment between the stator flux
and rotor flux.
• It is necessary to control the stator currents that produce
the stator flux.
• An angle closer 90o produces more flux per unit current.
stator,
b
r
stator, b
rotating
rotor, d
rotor, q
rotor,
r
stator, a
b
rotor, a
stator, a r
Both stator and rotor
rotating or stationary
Clarke and Park Transform
• It is necessary to control the stator currents that produce
the stator flux.
• An angle closer 90o produces more flux per unit current.
Is1
Ia1
Is2
Ia2
1
2
Im1
Im2
The values of Ia or Im can be
Independently controlled by
adjustment of the magnitude
Of the current Is and the angle 
• To control three sinusoidal currents is considerably more
complex but this can be simplified by first using the Clark
transformation on the stator currents.
•The three phase currents are transform into two phase
system with the Clarke transform,
Clark
Transform
• and then translating them into the rotor reference frame
with the Park Transform.
Park
Transform
• The Park transform is used to convert the fixed coordinates
into 2-axis rotating coordinates (Id, Iq). The reference
coordinates d (flux) and q (torque) and the reference frame
align the d axis with the rotor flux position.
Inverse Park Transforms
Inverse Park
Transform
Two voltages (vd and vq) that have to be applied to the
motor windings to drive the phase currents toward the
required values. However, these values exist on the rotating
reference frame. To apply them to the stator windings, we
must jump off of the rotor now, and transform vd and vq into
three stator voltages.
Inverse Clarke
Inverse Clark
Transform
•Direct vector control method
•The air-gap flux and stator flux can be measured directly,
• while any flux can be estimated from the stator voltage
and current signals.
• The stator, air-gap or rotor flux components can be
directly computed from stator quantities.
•In the indirect vector control method,
• the rotor flux angle information between the stator and
rotor fields using known characteristics of the rotor and
integration of the rotor speed and reference value of the
slip frequency
• The location of the rotating reference frame must be
accurately determined.
• the a-b reference frame is considered to be fixed to the
stator
• the d-q reference frame rotates at a synchronous speed
s
• At any time, the angle, the angle between the stationary
and rotating frame is s
•the d-q reference frame rotates at a synchronous speed
s
• At any time, the angle between the stationary
and rotating frame is s
q
s
slip
s
r
Stationary axis
b
s is the sum of the rotor’s angular position r and the slip
angle slip
• these angles need to determined in the implementation
of indirect vector control.
• the slip of an IM can be determined from the demanded
rotor current
q
s
slip
s
r
Stationary axis
b
•can be realised by controlling the magnitude of the stator
current space vector and its position with respect to the
chosen flux space vector.
•The rotor flux vector is chosen as the vector with respect to
which stator current space vector is orientated, rotor flux
oriented control of induction machine is obtained.
•This scheme is the most popular one in practical drive
realisations, because of its relative simplicity.
•The stator current space vector can also be orientated
with respect to stator flux space vector to form stator flux
oriented control of an induction machine.
•Another alternative is air-gap flux oriented control and the
stator current space vector is then orientated with respect to
the air-gap flux.
•Although there are three types of vector control, the one
used in commercially available drives is the rotor flux
oriented control
o Two motor phase currents Ia and Ib are measured and feed to
the Clarke transformation module.
o The outputs are i Sa and iSb.
o These two components of the current are the inputs of the Park
transformation that gives the current in the d,q rotating reference
frame.
o The i Sa and iSb components are compared to the
references iSdref (the flux reference) and iSqref (the torque
reference). For, synchronous permanent magnet motors,
the rotor flux is fixed i should be set to zero.
Sdref
o IM need a rotor flux creation in order to operate, the flux
reference must not be zero.
o The torque command iSqref could be the output of the
speed regulator when we use a speed FOC.
o The outputs of the current regulators are vSdref and vSqref;
they are applied to the inverse Park transformation.
oThe outputs of this projection are vSaref and vSbref which
are the components of the stator vector voltage in the a,b
stationary orthogonal reference frame.
o These are the inputs of the Space Vector PWM.
o The outputs of this block are the signals that drive the
inverter.