Brake Resistors 10; Brake Resistor; Special Conditions; Manual Derating - Danfoss VLT Decentral Drive FCD 302 Design Manual

System Integration
frequency converter. Code numbers can be found in
5.2.1 Ordering Numbers: Accessories.

3.4.2.1 Brake Resistors 10%

For frequency converters equipped with the dynamic brake
option, one brake IGBT along with terminals 81 (R-) and 82
(R+) is included in each inverter module for connecting a
brake resistor(s).
For internal brake resistor use:
Brake resistor 1750 Ω 10 For mounting inside installation box
W/100% below motor terminals
Brake resistor 350 Ω 10 W/ For mounting inside installation box
100% below motor terminals
Table 3.10 Brake Resistors 10%

3.4.2.2 Brake Resistor 40%

Placing the brake resistor externally has the advantages of
selecting the resistor based on application need,
dissipating the energy outside of the control panel, and
protecting the frequency converter from overheating if the
brake resistor is overloaded.
No. 81 (optional 82 (optional
function) function)
R- R+
Table 3.11 Brake Resistors 40%
•
The connection cable to the brake resistor must
be screened/armoured. Connect the screen to the
metal cabinet of the frequency converter and to
the metal cabinet of the brake resistor with cable
clamps.
•
Dimension the cross-section of the brake cable to
match the brake torque.

3.4.3 Special Conditions

Under some special conditions, where the operation of the
drive is challenged, derating must be taken into account.
In some conditions, derating must be done manually.
In other conditions, the drive automatically performs a
degree of derating when necessary. This is done in order
to ensure the performance at critical stages where the
alternative could be a trip.

3.4.3.1 Manual Derating

Manual derating must be considered for:
•
Air pressure – relevant for installation at altitudes
above 1 km
•
Motor speed – at continuous operation at low
RPM in constant torque applications
®
VLT Decentral Drive FCD 302
Contact Danfoss for the application note for tables and
elaboration. Only the case of running at low motor speeds
is elaborated here.

3.4.3.2 Automatic Derating

The drive constantly checks for critical levels:
As a response to a critical level, the frequency converter
adjusts the switching frequency. For critical high internal
temperatures and low motor speed, the drive can also
force the PWM pattern to SFAVM.
NOTE
The automatic derating is different when 14-55 Output
Filter is set to [2] Sine-Wave Filter Fixed.

3.4.3.3 Derating for Running at Low Speed

Brake resistor
terminals
When a motor is connected to a frequency converter, it is
necessary to check that the cooling of the motor is
adequate.
The level of heating depends on the load on the motor, as
well as the operating speed and time.
Constant torque applications (CT mode)
A problem may occur at low RPM values in constant
torque applications. In a constant torque applications a
motor may over-heat at low speeds due to less cooling air
from the motor integral fan. Therefore, if the motor is to
be run continuously at an RPM value lower than half of
the rated value, the motor must be supplied with
additional air-cooling (or a motor designed for this type of
operation may be used). An alternative is to reduce the
load level of the motor by choosing a larger motor.
However, the design of the frequency converter puts a
limit to the motor size.
Variable (Quadratic) torque applications (VT)
In VT applications such as centrifugal pumps and fans, the
torque is proportional to the square of the speed and the
power is proportional to the cube of the speed. In these
applications there is no need for additional cooling or de-
rating of the motor. In Illustration 3.17, the typical VT curve
is below the maximum torque with de-rating and
maximum torque with forced cooling at all speeds.
®
MG04H102 - VLT is a registered Danfoss trademark
•
Ambient temperature – relevant for ambient
temperatures above 50 °C
•
Critical high temperature on the control card or
heatsink
•
High motor load
•
High DC link voltage
•
Low motor speed
3 3
39