Rating - Mitsubishi Electric FR-A700 Technical Manual

6) Vector control
A700
This method divides the currents flowing in the
motor into a current for making a magnetic flux in
the motor and a current for causing the motor to
develop a torque, and controls each current
separately. Therefore very high response is
obtained and torque at low speed can be
generated stably.
It is necessary to know the motor speed to
perform this control and a motor with an encoder
and a plug-in option FR-A7AP or FR-A7AL are
required.
It responds to the load variation quickly (high
response) by torque current control, and torque
control is also enabled by giving torque command.
It has excellent control characteristics and
achieves the control characteristics equal to those
of DC machines.
To help explain vector control, the fundamental
equivalent circuit of an induction motor is shown
below:
r
im
1
1
i d
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 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. With
Vector control, inverter
controls the motor so that the
excitation current and torque
current to flow to the optimum
by calculating the voltage and
output frequency.
Control block diagram is
shown on the next page.
2
r
2
M iq
S
current
Torque
current
481
Motor-generated torque TM, slip angular velocity
s and the motor's secondary magnetic flux
can be found by the following calculation:
TM 2 iq
2 M id
s
where, L2 = secondary inductance
L2= 2 + M
Vector control requires a dedicated motor with
less constants variation and high accuracy
encoder (speed detector) to calculate with high
accuracy.
Although torque accuracy decreases, vector
control can be performed by fitting an encoder to
the standard motor.
Short time rating (approx. 1min)
150
100
Continuous
duty
range
1500
Speed r/min
(For dedicated motor 5.5K to 22K)
Output characteristic example of the vector control
dedicated motor
When magnetic flux observer is selected, the
magnetic flux of the motor is always detected with
high accuracy so that an excellent characteristic is
provided regardless of the change in the
temperature of the motor secondary resistance.
SELECTION
2
75
50
3000
3