Danfoss VLT FC 102 Series Design Manual page 56

Introduction to VLT® HVAC D...
2.12.2 Brake Resistor Calculation
2 2
The brake resistance is calculated as shown:
2
U dc
Rbr Ω =
Ppeak
where
P = P
peak motor
Table 2.24 Brake Resistor Calculation
As can be seen, the brake resistance depends on the
intermediate circuit voltage (U
The brake function of the adjustable frequency drive is
settled in three areas of the line power supply:
Size [V]
3x200–240
3x380–480
3x525–690
Table 2.25 Brake Function Settled in Three Areas of Line
Power Supply
NOTICE!
Make sure that the brake resistor can cope with a
voltage of 410 V, 820 V or 975 V, unless Danfoss brake
resistors are used.
Danfoss recommends the brake resistance R
that guarantees that the is able to brake at the highest
braking torque (M
written as:
R
Ω =
rec
P
motor
η is typically at 0.90
motor
η is typically at 0.98
For 200 V, 480 V and 600 V adjustable frequency drives,
R at 160% braking torque is written as:
rec
107780
V : Rrec =
200
Pmotor
375300
480 V : Rrec =
Pmotor
428914
480 V : Rrec =
Pmotor
630137
V : Rrec =
600
Pmotor
832664
V : Rrec =
690
Pmotor
1) For adjustable frequency drives ≤ 7.5 kW shaft output
2) For adjustable frequency drives > 7.5 kW shaft output
54
x M x η x η[W]
br motor
).
DC
Brake Warning before
active [V] cut-out [V]
390 (U ) 405
DC
778 810
1084 1109
) of 110%. The formula can be
br(%)
2
U dc x 100
x Mbr x x motor
%
Ω
Ω 1
Ω 2
Ω
Ω
Danfoss A/S © Rev. 06/2014 All rights reserved.
Design Guide
NOTICE!
The brake resistor circuit resistance selected should not
be higher than that recommended by Danfoss. If a brake
resistor with a higher ohmic value is selected, the
braking torque may not be achieved because there is a
risk that the adjustable frequency drive cuts out for
safety reasons.
NOTICE!
If a short circuit in the brake transistor occurs, power
dissipation in the brake resistor is only prevented by
using a line switch or contactor to disconnect the line
power for the adjustable frequency drive. (The contactor
can be controlled by the adjustable frequency drive).
WARNING
Cut-out (trip)
[V]
Do not touch the brake resistor, as it can get very hot
410
during/after braking.
820
1130
2.12.3 Control with Brake Function
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. A relay/
digital output can be used for protecting the brake resistor
against overloading in connection with a fault in the
adjustable frequency drive.
In addition, the brake enables reading out the momentary
power and the mean power for the latest 120 s. The brake
, i.e., one
rec can also monitor the power energizing and ensure that it
does not exceed the limit selected in 2-12 Brake Power
Limit (kW). In 2-13 Brake Power Monitoring, select the
function to carry out when the power transmitted to the
brake resistor exceeds the limit set in 2-12 Brake Power
Limit (kW).
NOTICE!
Monitoring the braking energy is not a safety function; a
thermal switch is required for that purpose. The brake
resistor circuit is not ground leakage protected.
Overvoltage control (OVC) (exclusive brake resistor) can be
selected as an alternative brake function in 2-17 Over-
voltage Control. This function is active for all units. The
function ensures that a trip can be avoided, if the DC link
voltage increases. This is done by increasing the output
frequency to limit the voltage from the DC link. It is a
useful function, e.g., if the ramp-down time is too short
since tripping of the adjustable frequency drive is avoided.
In this situation, the ramp-down time is extended.
MG11BC22