Danfoss VLT 5000 FLUX, Руководство по эксплуатации, Страница: 56 / 158

VLT
®
5000 FLUX
■
Dynamic brake function(Brake resistor)
The brake function is to limit the voltage in the
intermediate circuit when the load drives the motor
and the power enters the intermediate circuit.
The dynamic brake is a chopper circuit with the
connection of an external brake resistor.
The brake is protected against short-circuiting of the
brake resistor, and the brake transistor is monitored to
ensure that short-circuiting of the transistor is detected.
By using a relay/digital output, the latter can be used
for protecting the brake resistor against overloading in
connection with a fault in the frequency converter.
In addition, the brake makes it possible to read out the
momentary power and the mean power for the latest
120 seconds, as well as to monitor that the power
energizing does not exceed a monitoring limit selected
in parameter 402. Parameter 403 selects the function
to be carried out when the power transmitted to the
brake resistor exceeds the limit set in parameter 402.
NB!:
Monitoring of the brake power is not a safety
function; a thermal switch is required for
that purpose. The brake resistor circuit is
not earth leakage current protected.
■
Selection of Brake Resistor
In order to select the right brake resistor, it must
be known how often to brake and by how much
power braking is effected.
The resistor ED is an indication of the duty cycle
at which the resistor is working.
The resistor ED is calculated as follows:
where tb is the braking time in seconds and T
cycle is the total cycle time.
The maximum permissible load on the brake resistor is
stated as a peak power at a given ED. The following
example and formula apply to VLT 5000 only. The
peak power can be calculated on the basis of the
highest brake resistance required for braking:
P
PEAK
= P
MOTOR
x M
BR(%)
x
η
MOTOR
x
η
VLT
[W]
where M
BR(%)
is a percentage of the rated torque.
The brake resistance is calculated as follows:
The brake resistance depends on the intermediate
circuit voltage (UDC).
With VLT 5000 frequency converters that have a
mains voltage of 3 x 380-500 Volts, the brake will be
active at 822 Volts (UDC); if the frequency converter
has a mains voltage of 3 x 200-240 Volts, the brake
will be active at 397 Volts (UDC).
NB!:
The brake resistor used must be rated to
850 Volts or 430 Volts.
R
REC
is the resistance recommended by Danfoss,
i.e. one that guarantees the user that the
frequency converter is able to brake at the highest
braking torque (M
br
) of 160%.
η
motor
is typically at 0.90, while
η
VLT
is typically at 0.98.
For 200 V and 500 V frequency converters, respectively,
R
REC
at 160% braking torque can be written as:
P motor in kW.
NB!:
The max. brake resistance selected should
have an ohmic value max. 10% lower than
that recommended by Danfoss. If a brake
resistor with a higher ohmic value is selected, the
160% braking torque will not be achieved and there
is a risk that the frequency converter will cut out for
safety reasons. For further information, please consult
Brake Resistor Instruction MI.90.FX.YY.
MG.55.A6.02 - VLT is a registered Danfoss trademark
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