WEG CFW-11 Programming Manual page 127

Vector Control
The Optimal Braking makes it possible braking the motor with a higher torque than the one obtained with
traditional methods, as for instance, the braking by the injection of direct current (DC braking). In the DC
braking case, only the losses in the motor rotor are used to dissipate the energy stored as the mechanic load
inertia, rejecting the total friction losses. With the Optimal Braking, in the other hand, the total losses in the
motor, as well as the total inverter losses, are used. It is possible to get a braking torque roughly 5 times greater
than with DC braking.
In the Figure 11.3 on page 11-10 the Torque x Speed curve of a typical 10 hp/7.5 kW IV pole motor is presented.
The braking torque obtained at the rated speed, for an inverter with a torque limit (P0169 and P0170) adjusted
in a value equal to the motor rated torque, is supplied by the TB1 point on the Figure 11.3 on page 11-10.
The value of TB1 is on the function of the motor efficiency, and it is defined by the following expression, being
despised the attrition losses:
1-η
TB1 =
η
Where:
η = motor efficiency
In the Figure 11.3 on page 11-10 case, the efficiency of the motor for the rated load is η = 0.84 (or 84 %),
which results in TB1 = 0.19 or 19 % of the motor rated torque.
The braking torque, starting from the TB1 point, varies in the inverse ratio of the speed (1/N). At low speeds,
the braking torque reaches the torque limit of the inverter. In the Figure 11.3 on page 11-10 case, the torque
reaches the torque limitation (100 %) when the speed is less than approximately 20 % of the rated speed.
It is possible to increase the braking torque by increasing the inverter current limitation during the optimal
braking (P0169) – torque in the forward speed direction or P0170 – reverse).
Generally smaller motors have lower efficiency because they present more losses. Therefore, comparatively
higher braking torque is obtained if they are compared to bigger motors.
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