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What is vector control?
2.1 What is vector control?
Vector control is one of the control techniques for driving an induction motor. To help explain vector control, the
fundamental equivalent circuit of an induction motor is shown below:
In the above diagram, currents flowing in the induction motor can be classified into a current id (excitation current)
for making a magnetic flux in the motor and a current iq (torque current) for causing the motor to develop a torque.
.
Motor-generated torque (TM), slip angular velocity (ωs) and the motor's secondary magnetic flux (φ2) can be found
by the following calculation:
r1 : Primary resistance
r2 : Secondary resistance
1
: Primary leakage inductance
2
: Secondary leakage inductance
M : Mutual inductance
S: Slip
id : Excitation current
iq : Torque current
im : Motor current
In vector control, the voltage and output frequency are
calculated to control the motor so that the excitation
current and torque current (as shown in the left figure)
flow to the optimum as described below:
(1) The excitation current is controlled to place the
internal magnetic flux of the motor in the optimum
status.
(2) Derive the torque command value so that the
difference between the motor speed command and
the actual speed obtained from the encoder
connected to the motor shaft is zero. Torque current
is controlled so that torque as set in the torque
command is developed.
T
M
∝ φ
2
⋅ iq
φ
2
= M ⋅ id
ωs =
where, L2 = secondary inductance
L2 =
2 + M
Vector control provides the following advantages:
(1) Excellent control characteristics when compared to
V/F control and other control techniques, achieving
the control characteristics equal to those of DC
machines.
(2) Applicable to fast response applications with which
induction motors were previously regarded as
difficult to use. Applications requiring a wide
variable-speed range from extremely low speed to
high speed, frequent acceleration/deceleration
operations, continuous four-quadrant operations etc.
(3) Allows torque control.
(4) Allows servo-lock torque control which generates a
torque at zero speed (i.e. status of motor shaft =
stopped).
1
im
r
1
r
2
S
Mid iq
2
torque current
iq motor current im
excitation current
id
L2
r2
id
iq