SIEMENS
2/15
Siemens MD 50.8 · 2017
Configuring guide
Configuring a brake
2
■
Brake control
Definition of switching times (VDI 2241)
Fig. 2/9
Brake switching times
Switching times:
Fast brake application
Disconnection on the AC side
If the brake is disconnected from the line supply, the brake is
applied. With AC brake voltages, the brake application time is
extended as a result of the inductance of the solenoid
(disconnection on the AC side). This results in a considerable
delay before the brake is mechanically applied. In order to
achieve short brake application times, the circuit must also be
disconnected on the DC side.
Rectifier for disconnection on the DC side
Electromagnetically released spring-operated disk brakes can
be disconnected on the AC side and the DC side. Disconnection
on the DC side means that the inductance and thus the
magnetic field in the brake solenoid are reduced very quickly.
For disconnection on the DC side, a wire jumper can be
removed from the rectifier and replaced by the contacts of an
external switch. This enables significantly shorter application
times to be achieved than those achieved for disconnection on
the AC side.
Function rectifiers for fast brake application
If function rectifiers are used for fast brake application, then
there is no need for an external switch and therefore less wiring
is required.
Disconnection on the DC side using current sensing
One option of disconnecting on the DC side is to sense the motor
current. If the motor current falls below the rectifier's sensor
current when disconnected from the three-phase line supply, the
brake solenoid is disconnected from the DC voltage electroni-
cally without any contacts.
Used in conjunction with disconnection on the DC side by
means of current sensing, rectifiers are generally suitable for
being connected in parallel with the motor connection, even in
applications involving moving loads or large moments of inertia.
Brakes controlled in this way are completely wired to the motor
terminal board. Inverter operation is not permissible.
Disconnection on the DC side using voltage sensing
Another option of disconnecting on the DC side is by sensing the
rectifier supply voltage.
An integrated switching transistor switches off the load if the
input voltage falls below a specified switching threshold. Used
in conjunction with disconnection on the DC side using voltage
sensing, rectifiers are generally suitable for operation with
separate AC-side brake control using an additional switching
contact.
Connection in parallel with the motor connection is also possible,
but it is not recommended, as the rectifier disconnection re-
sponse will be impaired by the influence of the motor winding.
In addition, many applications involve driving loads or large
moments of inertia. This can cause the no-load voltage gener-
ated when the motor coasts down to considerably delay brake
application if the switching threshold for voltage sensing is not
fallen below.
If connection in parallel with the motor connection is neverthe-
less desired or required, disconnection on the DC side using
current sensing is recommended.
Fast brake release
Function rectifiers for fast brake release
Rectifiers with overexcitation (high-speed excitation)
Rectifiers with overexcitation operate for approximately
300 ms ± 15 % with bridge rectification, i.e. when being
released, the brakes are supplied with twice the rated solenoid
voltage.
After this time the rectifiers automatically switch from bridge to
half-wave rectification and the brakes are operated with the
rated solenoid voltage. This results in shorter release times and
higher brake switching frequencies. The friction lining wear is
also reduced, the permissible friction energy until the air gap is
readjusted increases, and starting losses are reduced.
Rectifiers with overexcitation are generally suitable for being
connected in parallel to the motor connection or for a separate
circuit in the case of inverter operation (note connection informa-
tion for disconnection on the DC side).
t
1
Application time of the brake
t
2
Disconnection time
t
3
Slipping time
t
11
Response time
t
12
Rise time
t
11
t
12
t
3
t
1
t
2
Time
Time
G_D087_EN_00047a
0.9
M
0.1
M
OFF
ON
Excitation
Characteristic torque
© Siemens AG 2017