Engineering
10/05 AWB8230-1541GB
24
Bypass operation
If you want to have the option of operating the motor with the
frequency inverter or directly from the mains supply, the incoming
supplies must be mechanically interlocked:
Braking
Motor braking shortens unwanted deceleration distances and
times. Braking can be mechanical or electrical.
Mechanical brakes act directly on the motor’s rotating shaft and
experience mechanical wear. The type of friction surface used
depends on the brake’s purpose:
• Emergency-Stop braking
• Normal braking during operation
• Holding brakes
Electrical braking – which does not cause wear – can be
implemented with frequency inverters:
•DC braking
• Dynamic braking
DC braking
In DC braking – also called induction braking – the frequency
inverter supplies three-phase current to the motor’s three-phase
stator winding. This creates a stationary magnetic field, which in
turn induces a voltage in the moving rotor. Because the rotor’s
electrical resistance is low, even small induction voltages can
cause a high rotor current and therefore a strong braking action.
As the rotor slows down, the frequency of the induced voltage and
with it the inductive resistance drops. The braking effect of the
resistive load increases, but the resulting braking torque abruptly
drops shortly before the rotor stops and disappears completely as
soon as the rotor is stationary. DC braking is therefore not suitable
for holding loads or for intermediate braking: once DC braking has
been activated, the motor comes to a standstill. Note also that DC
braking causes increased heat dissipation in the motor.
Dynamic braking
Dynamic braking allows a controlled speed reduction from a high
motor speed to a specific lower speed. During dynamic braking,
the motor operates in regenerative mode. The generated power is
fed into the frequency inverter’s internal DC link, which results in
an excessive DC link voltage.
Combined with a braking resistor, frequency inverters with built-
in braking transistors (also called braking choppers) can dissipate
the braking energy in the form of heat.
Caution!
Changeover between the frequency inverter and the
mains supply must take place at zero voltage.
Warning!
The frequency inverter outputs (U, V, W) must not be
connected to the mains voltage (destruction of the device,
risk of fire).
Figure 14: Bypass motor control
Caution!
Switch S1 must switch only when frequency inverter T1 is
at zero current.
h
Contactors and switches (S1) in the frequency inverter’s
output and for DOL starting must be dimensioned for
AC-3 and the motor’s rated power.
Q1
I
>
I
>
I
>
Q11
S1
M1
T1
M
3h
L1
L2
L3
L1 L2 L3
UVW
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