WEG CFW-11 Programming Manual page 111

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 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. 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:
Where:
η = motor efficiency
In the 11.3 figure 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 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.
Examples: 1 hp/0.75 kW, IV poles: η=0.76 resulting in TB1=0.32;
20 hp/15.0 kW, IV poles: η=0.86 resulting in TB1=0.16.
T
T
nom
1.0
TB1
0
0.2
0
Figure 11.3 - T x N curve for Optimal Braking with a typical 10 hp/7.5 kW motor, driven by an
inverter with the torque adjusted at a value equal to the motor rated torque
1-η
TB1 =
η
(a)
(b)
(c)
1.0
Vector Control
N
N
2.0
nom
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