SIEMENS
2/14
Siemens MD 50.8 · 2017
Configuring guide
Configuring a brake
2
■
Overview
The brakes can be used as working brakes or holding brakes.
A holding brake is suitable for holding masses and loads at a
fixed position. A working brake is also capable of decelerating
masses and loads.
The brakes are designed as fail-safe spring-operated brakes.
When the brake is mounted, it increases the length of the motor.
The dimensions are shown in the dimensional drawings.
The spring-operated disk brakes are suitable for a standard
ambient temperature range of -20 to +40 °C.
Variables
■
Determining the braking torque
The braking torque must be selected in accordance with the
particular drive application.
The following criteria are decisive when it comes to making the
selection:
•
Static safety
•
Required braking time
•
Permissible brake delay
•
Possible braking distance
•
Brake wear
The braking torque is determined using the safety factor
k
, which
can be selected in the range from 1.0 to 2.5. As a general rule of
thumb, the factor for horizontal motion is approx. 1.0 to 1.5 and
for vertical motion approx. 2.0 to 2.5. However, the precise
braking torque depends to a large extent on the particular
operating conditions.
The rated braking torque is referred to a speed of
n
= 100 rpm
and decreases with increasing motor speed. When calculating
the braking torque, this is taken into account using the correction
factor
f
br
. This means that the rated braking torque is applicable
for most braking operations for inverter operation.
For line operation, braking is directly from the motor speed.
In addition, for vertical conveyors, the increased speed when
moving downwards must be taken into account.
■
Braking torques as a function of the speed and
permissible speed limits
The braking torque available decreases with increasing motor
speed.
The maximum permissible speeds from which emergency stops
can be made are listed in the
table on page 6/32
. These speeds
should be considered as guide values and must be checked for
the specific operating conditions.
The maximum permissible friction energy depends on the
switching frequency and is shown for individual brakes in the
diagram “Permissible operating energy”, page 6/32
. Increased
wear can be expected when the brakes are used for emergency
stops.
■
Braking energy per braking operation
The braking energy
W
per braking operation comprises the en-
ergy of the moments of inertia to be braked and the energy
which must be applied in order to brake against a load torque:
T
x
is positive if the load torque is working against the braking
torque (horizontal motion, upward vertical motion).
T
x
is negative if it supports the brake (downward vertical motion).
The permissible operating energy
Q
perm
must be checked
against the relevant switching frequency using the
diagram
“Permissible operating energy”, page 6/32
. This is of particular
importance for emergency-stop circuits.
■
Service life of the brake lining
The service life of the brake lining
L
n
until the air gap has to be
readjusted depends on various factors. The main influencing
factors include the masses to be braked, the motor speed, the
switching frequency, and, therefore, the temperature at the
friction surfaces.
This means it is not possible to specify a value for the friction en-
ergy until readjustment that is valid for all operating conditions.
However, a wear calculation can be made according to the fric-
tion energy, so that the service life can be defined in normal op-
eration.
■
Brake service life
The brake lining is subject to wear as a result of friction. As a
consequence, the air gap increases and the brake application
time lengthens. The air gap can be readjusted. The friction lining
should be replaced after a certain number of readjustments.
Service life of the brake lining until readjustment
Service life of the brake lining until replacement
Code
Description
Unit
f
br
Braking torque correction factor
-
J
AD
Moment of inertia of the adapter
kgm²
J
G
Moment of inertia of the gearbox
kgm²
J
mot
Moment of inertia of the motor
kgm
2
J
X
Moment of inertia of the load referred to the
motor shaft
kgm²
J
Z
Additional moment of inertia of a high inertia fan
kgm
2
k
Factor for taking into account operating
conditions
-
L
n
Service life of the brake lining until readjustment
h
L
nmax
Service life of the brake lining until replacement
h
n
br
Braking speed
rpm
Efficiency
%
Q
perm
Permissible operating energy
J
s
br
Braking distance
m
t
1
Application time of the brake
ms
t
br
Braking time
s
T
br
Rated braking torque
Nm
T
x
Load torque
Nm
v
Travel velocity
m/s
W
Friction energy per braking operation
J
W
tot
Friction energy until the brake lining is replaced
MJ
W
V
Friction energy until the brake is readjusted
MJ
Z
Switching frequency
1/h
T
br
T
x
k
f
br
W
T
br
T
br
T
x
------------------------------
J
G
J
AD
J
+
+
mot
J
z
J
x
+
+
n
br
2
182.5
---------------------------------------------------------------------------------------------------
=
W
Q
perm
L
n
W
V
W
Z
-------------
=
L
n
max
W
tot
W
Z
-------------
=
© Siemens AG 2017