Motor Characteristics - Mitsubishi Electric FR-A700 Technical Manual

3.2 Motor characteristics

3.2.1 Characteristics of the induction motor
There are the following relationships between the speed,
voltage, frequency, magnetic flux density, torque and other
factors of an induction motor:
120f
N = (1-S)
p
V
B = K
1
f
V V
2
T = K
I K ( )
2
3
f f
P = K TN K Tf = K VI
4 5 6
N: Speed
f : Frequency
p : Number of motor poles V : Terminal voltage
S : Slip
B : Magnetic flux density
When a three-phase standard motor is used, the value of
magnetic flux B is maximum at 50Hz and cannot be
increased beyond that value. In addition, motor current
cannot be flown continuously at more than the rated
current value, in principle. To run the standard motor
without burnout, V/f must be controlled to be constant or
below the constant value.
3.2.2 Torque generated by a motor driven by an inverter
When the motor is driven with variable voltage/variable
frequency power supply, the torque curve is as shown on
page 486 (example of 1.5kW). When the inverter is used
as a power supply, however, the overload capacity of the
inverter suppresses the torque of the motor to be less than
the maximum value of the motor. This value is the
maximum torque for short time. Hence, the use of a large-
capacity inverter raises the overload capacity, leading to
the increase in maximum torque for short time. For the
values for the standard combinations, refer to the Technical
note No.30 (data part).
3.2.3 Continuous motor output characteristic
When the motor is driven from the inverter, its power factor
and efficiency are lower than those of the motor driven with
a sine wave (commercial power supply) due to the
influence of harmonics included in the inverter output.
Therefore, since the motor current increases and the motor
temperature rises to generate the same load torque, the
motor must be used with its output torque reduced.
Because the self-cooling fan of the motor has a less effect,
P:Output
T : Torque
I : Motor current
K to K Constants
1 6 :
common
(common)
As indicated by the above formula, making V/f constant
causes the motor torque at the rated current to be
constant. Therefore, by controlling the voltage and
frequency to be kept constant, the torque characteristic is
made constant. However, a voltage drop due to the primary
impedance cannot be ignored in the low frequency range
and the torque lowers as shown below. Refer to page 486.
The Advanced magnetic flux vector control and Real
sensorless vector control compensate for a voltage drop
due to the primary impedance and therefore provide an
ideal constant-torque characteristic.
Constant torque characteristic
Real sensorless vector control
Advanced magnetic flux vector control
Combination of
standard motor and
inverter (V/F control)
Base frequency
Motor/torque characteristic
Due to the inverter characteristic, a voltage above the
power supply voltage cannot be developed at the
frequency of higher than 50Hz or 60Hz or higher. For this
reason, the voltage is kept constant, only the frequency is
changed, and the torque is inversely proportional to the
frequency, i.e. the constant output characteristic is
achieved with the voltage kept constant.
common
(common)
In the low frequency range, the torque value changes
according to the V/f pattern of the inverter output and the
electrical constant of the motor. Selecting General-purpose
magnetic flux vector control, Advanced magnetic flux
vector control or Real sensorless vector control minimizes
the torque reduction. In the V/F control, the torque boost
function ([Pr. 0]) also allows the torque to be raise
especially in the low frequency range.
Refer to page 486 starting torque boost. For the value
increased by the torque boost function, refer to the
Technical note No.30 (data part).
common
(common)
especially at lower than 30Hz, the load torque must be
reduced for continuous use.
Refer to the corresponding inverter series catalog for the
permissible continuous operation torque of the three rates,
200V/50Hz, 200V/60Hz, 220V/60Hz standard motor driven
from the inverter.
When the motor is run continuously with the rated torque
down to the low speed range, consider the use of a
constant-torque motor.
491
SELECTION
Constant output characteristic
(base frequency or higher)
Frequency (Hz)
3