Schneider Altivar 61/71 Guía De Explotación página 44

Braking principle and calculation
4) Braking a downward vertical movement
P ˆ
⋅ ⋅
P = m g v
b b
–
Average braking power during time t
P
b
^
Peak braking power
P
b
m Mass
g Acceleration
a Deceleration
v Linear downward speed
J Moment of inertia
ω
Angular speed
t Downward stopping time
b
n Downward motor speed
All the braking power calculations are only true if it is assumed that there are no losses (η = 1) and that there is no resistive torque.
Since all these points are important, an accurate assumption must be made:
1 Losses in the system
The losses generated in the motor (working as a generator, quadrants II and IV) are of some help during the braking phase. Without
exception, efficiency must be calculated to the braking power squared.
2 Resistive torque
There may sometimes be resistive torque connected with mechanical friction, air and opposing quadratic torque of the fans.
These phenomena, which are rarely taken into consideration, reduce the braking power. The resistive torque or the power should be
deducted from the calculated braking power.
3 Driving torque
Additional phenomena, such as the wind, can cause an increase in the braking power.
The required braking power is calculated as follows:
P ˆ P ˆ P
2
( ) η ×
= –
total
bR
load
^
Max. actual braking power
P
bR
–
Max. actual continuous braking power
P
bR
η
Total efficiency
total
P Braking power connected with the resistive
load
torque
η
Drive efficiency = 0.98
drive
1757084 11/2009
2
J ω ⋅
⋅ ( ) v ⋅
------------- -
= m g + a +
t
b
[W]
b
[W]
[kg]
9.81 m/s
[m/s
[m/s]
[kgms
[rad/s]
[s]
[rpm]
( ) η ×
P = P P –
bR
load
[W]
[W]
[W]
⋅
2π n
ω --------------
=
60
2
2
]
2
]
2
η η
=
total
total mec
× η ×
0,98
mot
43