Brake Functions; Selection Of Brake Resistor - Danfoss VLT AutomationDrive FC 301 Design Manual

System Integration
All control terminals and relay terminals 01-03/04-06
comply with PELV (Protective Extra Low Voltage), with the
exception of grounded Delta leg above 400 V.
Galvanic (ensured) isolation is obtained by fulfilling
requirements for higher isolation and by providing the
relevant creepage/clearance distances. These requirements
are described in the EN 61800-5-1 standard.
The components that make up the electrical isolation, as
described below, also comply with the requirements for
higher isolation and the relevant test as described in EN
61800-5-1.
The PELV galvanic isolation can be shown in 6 locations
(see Illustration 5.7):
To maintain PELV, all connections made to the control
terminals must be PELV, e.g. thermistor must be
reinforced/double insulated.
1. Power supply (SMPS) incl. signal isolation of DC-
link.
2. Gate drive that runs the IGBTs (trigger
transformers/opto-couplers).
3. Current transducers.
4. Opto-coupler, brake module.
5. Internal inrush, RFI, and temperature
measurement circuits.
6. Custom relays.
7. Mechanical brake.
7
5 4
6
a
Illustration 5.7 Galvanic Isolation
The functional galvanic isolation (a and b on drawing) is
for the 24 V back-up option and for the RS-485 standard
bus interface.
WARNING
Installation at high altitude:
At altitudes above 2,000 m, contact Danfoss regarding
PELV.
®
VLT AutomationDrive FC 301/FC 302 Design Guide, 0.25-75 kW
3
M
1 2
b
MG33BF02 - Rev. 2013-12-20
WARNING
Touching the electrical parts could be fatal - even after
the equipment has been disconnected from mains.
Also make sure that other voltage inputs have been
disconnected, such as load sharing (linkage of DC
intermediate circuit), as well as the motor connection for
kinetic back-up.
Before touching any electrical parts, wait at least the
amount of time indicated in Table 2.1.
Shorter time is allowed only if indicated on the
nameplate for the specific unit.

5.5 Brake Functions

Braking function is applied for braking the load on the
motor shaft, either as dynamic braking or mechanical
braking.

5.5.1 Selection of Brake Resistor

The brake resistor ensures that the energy is absorbed in
the brake resistor and not in the frequency converter. For
more information see the Brake Resistor Design Guide.
If the amount of kinetic energy transferred to the resistor
in each braking period is not known, the average power
can be calculated on the basis of the cycle time and
braking time also called intermittent duty cycle. The
resistor intermittent duty cycle is an indication of the duty
cycle at which the resistor is active. Illustration 5.8 shows a
typical braking cycle.
NOTICE
Motor suppliers often use S5 when stating the
permissible load which is an expression of intermittent
duty cycle.
The intermittent duty cycle for the resistor is calculated as
follows:
Duty cycle = t /T
b
T = cycle time in s
t is the braking time in s (of the cycle time)
b
5 5
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