Danfoss VLT HVAC Drive FC 100 Series, Руководство по конструкции

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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 known as the intermitted duty cycle.
The resistor intermittent duty cycle is an indication of the
duty cycle at which the resistor is active. The below figure
shows a typical braking cycle.
The intermittent duty cycle for the resistor is calculated as
follows:
Duty Cycle = t
b
/ T
T = cycle time in seconds
t
b
is the braking time in seconds (as part of the total cycle
time)
ta tc tb to ta tc tb to ta
130BA167.10
Load
Time
Speed
Figure 2.37
Danfoss offers brake resistors with duty cycle of 5%, 10%
and 40% suitable for use with the VLT
®
HVAC Drive
Adjustable frequency drive series. If a 10% duty cycle
resistor is applied, it is capable of absorbing braking
energy up to 10% of the cycle time, with the remaining
90% being used to dissipate heat from the resistor.
For further selection advice, please contact Danfoss.
2.12.2 Brake Resistor Calculation
The brake resistance is calculated as shown:
Rbr Ω =
Udc
2
Ppeak
where
P
peak
= P
motor
x M
br
x η
motor
x η[W]
Table 2.17
As can be seen, the brake resistance depends on the
intermediate circuit voltage (U
DC
).
The brake function of the Adjustable frequency drive is
settled in three areas of line power supply:
Size Brake active Warning
before cut-
out Cut-out (trip)
3 x 200–240V 390V (UDC) 405V 410V
3 x 380–480V 778V 810V 820V
3 x 525–600V 943V 965V 975V
3 x 525–690V 1084V 1109V 1130V
Table 2.18
NOTE!
Make sure that the brake resistor can cope with a voltage
of 410V, 820V or 975V, unless Danfoss brake resistors are
used.
Danfoss recommends the brake resistance R
rec
, i.e., one
that guarantees that the Adjustable frequency drive is able
to brake at the highest braking torque (M
br(%)
) of 110%.
The formula can be written as:
Rrec Ω =
Udc
2 x 100
Pmotor x Mbr (%) x x motor
η
motor
is typically at 0.90
η is typically at 0.98
For 200V, 480V and 600V adjustable frequency drives, R
rec
at 160% braking torque is written as:
200
V : Rrec =
107780
Pmotor
Ω
480
V : Rrec =
375300
Pmotor
Ω 1)
480 V : Rrec =
428914
Pmotor
Ω 2)
600 V : Rrec =
630137
Pmotor
Ω
690
V : Rrec =
832664
Pmotor
Ω
1) For adjustable frequency drives ≤ 10 hp [7.5 kW] shaft
output
2) For adjustable frequency drives > 10 hp [7.5 kW] shaft
output
NOTE!
The resistor brake 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.
Introduction to VLT® HVAC D...
VLT
®
HVAC Drive Design Guide
MG11BB22 - VLT
®
is a registered Danfoss trademark 2-41
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