Control With Brake Function - Danfoss VLT 5000 Series Operating Instructions Manual

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
intermediate 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 control
panel, i.e. where the energy can be utilized.
- The electronics of the 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 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
via parameter 402. In parameter 403 select 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 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.
80
®
VLT 5000 Series
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:
x η
P = P x M
PEAK MOTOR BR(%)
where M is a percentage of the rated torque.
BR(%)
The brake resistance is calculated as follows:
The brake resistance depends on the intermediate
circuit voltage (UDC).
The brake will be active at the following voltages:
• 3 x 200-220 V: 397 V
• 3 x 380-500 V: 822 V
•
3 x 525-600 V: 943 V
• 3 x 525-690 V: 1084 V
NB!:
The brake resistor used must be rated to 430
Volts, 850 Volts, 960 Volts or 1100 Volts, unless
Danfoss brake resistors are used.
R is the resistance recommended by Danfoss,
REC
i.e. one that guarantees the user that the
frequency converter is able to brake at the highest
braking torque (M ) of 160%.
br
η is typically at 0.90, while η
motor
R at 160% braking torque can be written as:
REC
P motor in kW.
MG.51.A9.02 - VLT is a registered Danfoss trademark
x η
[W]
MOTOR VLT
is typically at 0.98.
VLT