GEA ARIETE, Инструкция по эксплуатации и техническому обслуживанию

Страница: 418 / 556

VLT
®
5000 Series
■
Control with brake function
The function of the brake is to limit the voltage in the
intermediate circuit when the motor is acting as a
generator. This occurs, for example, when the load
drives the motor and the power enters the intermedi-
ate circuit. The brake is built up in the form of a
chopper circuit with the connection of an external
brake resistor. Placing the brake resistor externally
offers the following advantages:
- The brake resistor can be selected on the basis
of the application in question.
- The brake energy is dissipated outside the con-
trol panel, i.e. where the energy can be utilized.
- The electronics of the VLT frequency converter
will not be overheated if the brake resistor is
overloaded.
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 de-
tected. 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 VLT fre-
quency 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 via parameter 402. In parameter 403 select
the function to be carried out when the power trans-
mitted 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 leak-
age protected.
■
Selection of brake resistor
In order to select the right brake resistor, the appli-
cation must be known through-and-through, i.e. it
must be known how often to brake and by how much
power braking is effected.
The resistor ED, which is often used by motor
suppliers when stating the permissible load, is an in-
dication of the duty cycle at which the resistor is
working.
The resistor ED is calculated as follows:
ED (dutycycle) =
tb
(t2 0 t1)
in which t2-t1 = cycle time in seconds and tb is the
braking time in seconds (of the cycle time).
The maximum permissible load on the brake resistor
is stated as a peak power at a given ED. That is why
the peak power for the brake resistor and the resis-
tor value must be determined.
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
MOTOR
x
VLT
[W]
where M
BR(%)
is expressed as a percentage of the
rated torque.
The brake resistance is calculated as follows:
R
REC
= U
DC
2
P
P EAK
[]
As can be seen, 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!:
Remember to check whether the brake resis-
tor used is able to cope with a voltage of 850
Volts or 430 Volts - unless Danfoss brake resistors
are used.
R
REC
is the brake resistance recommended by Dan-
foss, i.e. one that guarantees the user that the
frequency converter is able to brake at the highest
braking torque (M
br
) of 160%.
motor
is typically at 0.90, while
VLT
is typically at
0.98. For 500 V and 200 V VLT frequency convert-
ers, respectively , R
REC
at 160% braking torque can
be written as:
R
REC
= 478801
P
MOT OR
[]
R
REC
= 111684
P
MOT OR
[]
MG.51.A1.02 - VLT is a registered Danfoss trade mark
84