Mitsubishi Electric 800 Series Instruction Manual page 135

Control method
5.2.1
Vector control and Real sensorless 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:
im
r
1
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 torque.
In vector control, the voltage and output frequency are calculated to control the motor so that the excitation current and torque
current flow to the optimum as described below:
• The excitation current is controlled to place the internal magnetic flux of the motor in the optimum status.
• The torque command value is derived so that the difference between the motor speed command and the actual speed
(speed estimated value for Real sensorless vector control) 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.
Motor-generated torque (TM), slip angular velocity (ωs) and the motor's secondary magnetic flux (Ф2) can be found by the
following calculation:
 Ф
T
• iq
M 2
Ф = M • id
2
iq
r2
ω =
L2 id
where, L2: secondary inductance
L2= +M
2
Vector control provides the following advantages:
• Excellent control characteristics when compared to V/F control and other control techniques, achieving the control
characteristics equal to those of DC machines.
• 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.
• Allows torque control. (When induction motors are used.)
• Allows servo-lock torque control which generates a torque in the motor shaft while stopped. (Not available under Real
sensorless vector control.)
134
PARAMETERS
1 2
id M iq
iq
torque current
r1: Primary resistance
r2: Secondary resistance
1: Primary leakage inductance
2: Secondary leakage inductance
r
2
M: Mutual inductance
S
S: Slip
id: Excitation current
iq: Torque current
im: Motor current
motor current im
excitation current
i d