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Choosing the brake

Calculating possible load thresholds:
the number no-load starts possible is considered to be wo, listed in the motor specification tables to remain within the peak temperature limits posed by the “IC. F” insulation class of the brake, and the maximum peak temperature admissible for maintaining the rated braking torque of the lining.
This makes it possible to determine the number of starts per hour under load through the following experimental formula:

where:
ξ and γ are determined from the following experimental graphs, based respectively on the torque [Nm] and mass [kg] in question.
The γ-dimensional coefficient is a function of the ratio between the inertia moments of the applied load Jc [kg m2] and the rotating masses of the first motor Jm [kg m2] γ = f (Jc/Jm), while the adimensional coefficient ξ is a function of the ratio between the resistance torque Cr [Nm] and the starting torque of the first motor Ca [Nm] ξ = f (Cr/Ca).

Jc = load inertia moment [kg m2]
Jm = first motor inertia moment [kg m2]
Cr = resistance torque of the load [Nm]
Ca = starting torque of the motor [Nm]
γ = f (Jc/Jm)
ξ = f (Cr/Ca)

 

For masses with cylindrical symmetry, the inertia moment J is calculated according to the formula:

where: M [kg] is the mass of the rotating assembly, while R [m] is the radius of the cylindrical symmetry volume.
A classical example is that of the rotor and shaft of an asynchronous electric motor.

If we consider the inertia moments of the shaft J1 and the rotor J2, these are added algebrically to determine the total inertia moment J=J1+J2 [kg m2] as they rotate around the same rotation axis.
If the rotation axis is not the same (a typical example is that of transmission belts and pulleys), it is necessary to consider a transport end.

Calculating the braking time tf [s]
To arrive at an approximate braking time, the following formula may be used:

where:
Jtot = Overall inertia moment at the motor shaft [kg m2]
n = Motor rotation speed [min-1]
Cf = Braking moment [Nm]
Cr = Resistance moment of the applied load [Nm] with a + sign if the sign agrees with the braking moment, or – if not
tB = Electrical brake response time [s]
        - 7 ms AC brake
        - 20 ms DC brake (rapid)
        - 80 ms for DC brake (normal)

Then select the brake based on the two variables ωc and tf.

Lining break-in
The nominal brake operation is achieved after a few cycles, to allow the lining to settle.
The braking torques indicated are static average and may vary slightly Technically the range to be considered is ± 20% for run-in Brake.