10.1 Selecting Motors and Inverters
10-2
Figure 10.1-2 Output Torque Characteristics (Base frequency: 60Hz)
(1) Continuous allowable driving torque
1) Standard motor (Curve (a1) in Figure 10.1-1 and Figure 10.1-2)
Curve (a1) shows the torque characteristic that can be obtained in the range of the inverter continuous
rated current, where the standard motor's cooling characteristic is taken into consideration. When the
motor runs at the base frequency of 60 Hz, 100 % output torque can be obtained; at 50 Hz, the output
torque is somewhat lower than that in commercial power, and it further lowers at lower frequencies. The
reduction of the output torque at 50 Hz is due to increased loss by inverter driving, and that at lower
frequencies is mainly due to heat generation caused by the decreased ventilation performance of the
motor cooling fan.
2) Motor exclusively designed for vector control (Curve (a2) in Figure 10.1-1 and Figure 10.1-2)
Curve (a2) shows the torque characteristic that can be obtained in the range of the inverter continuous
rated current, where the motor exclusively designed for vector control is connected. In the motor
exclusively designed for vector control, the attached forced-cooling fan reduces heat generation from the
motor, so that the torque does not drop in the low-speed range, compared to the standard motor.
(2) Maximum driving torque in a short time (Curves (b) and (c) in Figure 10.1-1 and Figure 10.1-2)
Curve (b) shows the torque characteristic that can be obtained in the range of the inverter overload
capability in a short time (HD mode: output torque is 150% for 1 minute and 200% for 3 seconds, LD
mode: output torque is 120% for 1 minute) when torque-vector control is enabled. At that time, the motor
cooling characteristics have little effect on the output torque.
Curve (c) shows an example of the torque characteristic when one class higher capacity inverter is used to
increase the short-time maximum torque. In this case, the short-time torque is 20 to 30% greater than that
when the standard capacity inverter is used.
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