14-10 Mains Failure
Option:
Function:
down to about 373 V DC and the IGBTs cut off
and looses the control over the motor. When the
mains is restored, and the IGBTs start again, the
output frequency and voltage vector does not
correspond to the speed/frequency of the motor,
and the result is normally an overvoltage or
overcurrent, mostly resulting in a trip lock.
can be programmed to avoid
this situation.
Select the function to which the frequency
converter must act when the threshold in
14-11 Mains Voltage at Mains Fault
has been
reached.
cannot be changed while
motor is running.
[0]
*
No
function
The frequency converter will not compensate for a
mains interruption. The voltage on the DC-link will
drop quickly and motor control will be lost within
milliseconds to seconds. Trip lock will be the
result.
[1] Ctrl.
ramp-
down
The frequency converter will remain control of the
motor and do a controlled ramp down from
14-11 Mains Voltage at Mains Fault
level. If
is
[0] Off
or
[2] AC brake
, the
ramp will follow the Over Voltage Ramping. If
is
[1] Resistor Brake
the ramp
will follow the setting in
. This selection is particularly useful in pump
applications, where the inertia is low and the
friction is high. When the mains is restored, the
output frequency will ramp the motor up to the
reference speed (if the mains interruption is
prolonged, the controlled ramp down might take
the output frequency all the way down to 0 RPM,
and when the mains is restored, the application is
ramped up from 0rpm to the previous reference
speed via the normal ramp up). If the energy in
the DC-link disappears before the motor is
ramped to zero the motor will be coasted.
[2] Ctrl.
ramp-
down,
trip
This selection is similar to selection [1] except that
in [2] a reset is necessary for starting up after
power-up.
[3] Coasting Centrifuges can run for an hour without power
supply. In those situations it is possible to select a
coast function at mains interruption, together with
a flying start which occurs when the mains is
restored.
[4] Kinetic
back-up
Kinetic back-up ensures that the frequency
converter keeps running as long as there is
energy in the system due to the inertia from
motor and load. This is done by converting the
mechanical energy to the DC-link and thereby
14-10 Mains Failure
Option:
Function:
maintaining control of the drive and motor. This
can extend the controlled operation, depending
on the inertia in the system. For fans it is typically
several seconds, for pumps up to 2 seconds and
for compressors only for a fraction of a second.
Many industry applications can extend controlled
operation for many seconds, which is often
enough time for the mains to return.
130BC918.10
U
14-11*1.35
Ref
n [RPM]
t [S]
A
B
C
D
E
A
DC
UDC
[V]
t [S]
Illustration 3.54
A = Normal Operation D = Mains return
B = Mains failure
E = Normal Operation:
ramping
C = Kinetic back-up
Table 3.23
If the mains do not return U
DC
is maintained as
long as possible by ramping the speed down
towards 0 RPM. Finally the frequency converter
coasts.
If the mains return while in kinetic back-up U
DC
will increase above par. 14-11*1.35. This is
detected in one of the following ways.
1.
If U
DC
> par. 14-11*1.35*1.05
2.
If the speed is above the reference. This
is relevant if the mains come back at a
lower level than before, e.g. par.
14-11*1.35*1.02. This does not fulfil the
criterion in point one and the frequency
converter will try to reduce U
DC
to par.
14-11*1.35 by increasing the speed. This
will not succeed as the mains cannot be
lowered.
3.
If running motoric. The same mechanism
as in point two, but where the inertia
will prevent that the speed goes above
the reference speed. This will lead to the
motor running motoric until the speed is
above the reference speed and the
situation in point two occurs. Instead of
waiting for that criterion three is
introduced.
Parameter Descriptions
VLT
®
AutomationDrive Programmming Guide
130
MG33ME02 - VLT
®
is a registered Danfoss trademark
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