10.1 Motor Output Torque Characteristics
10-2
(1) Continuous allowable driving torque
1) Standard motor (Curve (a1) in Fig. 10.1-1 and Figure Fig. 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 Fig. 10.1-1 and Fig. 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 Fig. 10.1-1 and Fig. 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 (HHD specification: 150% for 1 minute, 200% for 3 seconds, HND specification:
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.
(3) Starting torque (around the output frequency 0 Hz in Fig. 10.1-1 and Fig. 10.1-2)
The maximum torque in a short time applies to the starting torque as it is.
(4) Braking torque (Curves (d), (e), and (f) in Fig. 10.1-1 and Fig. 10.1-2)
In braking the motor, kinetic energy is converted to electrical energy and regenerated to the DC link bus
capacitor (reservoir capacitor) of the inverter. Discharging this electrical energy to the braking resistor
produces a large braking torque as shown in curve (e). If no braking resistor is provided, however, only the
motor and inverter losses consume the regenerated braking energy so that the torque becomes smaller as
shown in curve (d).
When an optional braking resistor is used, the braking torque is allowable only for a short time. Its time
ratings are mainly determined by the braking resistor ratings. This manual and associated catalogs list the
allowable values (kW) obtained from the average discharging loss and allowable values (kWs) obtained
from the discharging capability that can be discharged at one time.
Note that the torque % value varies according to the inverter capacity.
Selecting an optimal brake unit enables a braking torque value to be selected comparatively freely in the
range below the short-time maximum torque in the driving mode, as shown in curve (f).
For braking-related values when the inverter and braking resistor are normally combined, refer to
Chapter 11 “11.8 Braking Resistors (DBRs) and Braking Units”.
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