Continuous braking
For continuous braking, a brake resistor should be se-
lected in which the constant braking power does not
exceed the mean power P
AVG
of the brake resistor.
Please contact your Danfoss supplier for further infor-
mation.
D.C. injection braking
If the three-phase winding of the stator is fed with direct
current, a stationary magnetic field will be set up in the
stator bore causing a voltage to be induced in the bars
of the cage rotor as long as the rotor is in motion. Since
the electrical resistance of the rotor cage is very low,
even small induced voltages can create a high rotor
current. This current will produce a strong braking ef-
fect on the bars and hence on the rotor. As the speed
falls, the frequency of the induced voltage falls and
with it the inductive impedance. The ohmic resistance
of the rotor gradually becomes dominant and so in-
creases the braking effect as the speed comes down.
The braking torque generated falls away steeply just
before standstill and finally ceases when there is no
further movement. Direct current injection braking is
therefore not suitable for actually holding a load at rest.
AC-braking
When the motor acts as a brake the DC-link voltage
will increase because energy is fed back to the DC-
link. The principle in AC-brake is to increase the mag-
netisation during the braking and thereby increase the
thermal losses of the motor. Using par. 144 in VLT
2800 it is possible to adjust the size of the generator
torque that can be applied to the motor without the in-
termediate circuit voltage exceeding the warning level.
The braking torque depends on the speed. With the
AC-brake function enabled and parameter 144 = 1,3
(factory setting) it is possible to brake with about 50 %
of rated torque below 2/3 of rated speed and with about
25 % at rated speed. The function is not working at low
speed (below 1/3 of nominal motor speed). It is only
possible to run for about 30 seconds with parameter
144 greater than 1.2.
NB!
If the value in parameter 144 is increased,
the motor current will simultaneously in-
crease significantly when generator loads
are applied. The parameter should there-
fore only be changed if it is guaranteed
during measurement that the motor cur-
rent in all operating situations will never
exceed the maximum permitted current in
the motor. Please note: The current can
not be read out from the display.
Optimal braking using resistor
Dynamic braking is useful from maximum speed down
to a certain frequency. Below this frequency DC brak-
ing is to be applied as required. The most efficient way
of doing this is by using a combination of dynamic and
DC braking. See the illustration.
NB!
When changing from dynamic to DC brak-
ing, there will be a short period (2-6 milli-
seconds) with very low braking torque.
How to calculate optimum DC-brake cut in frequency:
Slip S
=
n
0
−
n
n
n
0
× 100 %
S ynchronous speed n
0
=
f
× 60
p
1 /
min
f = frequency
p = no. of pole pairs
n
n
= speed of the rotor
DC
−
brake cut in frequency
= 2 ×
s
×
f
100
Hz
MG.27.E2.02 - VLT is a registered Danfoss trademark
25
In
tro
duc
ti
on
to
VLT 280
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