Danfoss VLT 2800 Manual page 24

420139
volt R
400 =
REC
P
motor
105035
volt R
200 =
REC
P
motor
NB!
The brake resistance selected should
have an ohmic value no more than 10%
lower than that recommended by Dan-
foss. If a lower brake resistance is selec-
ted there is a risk of overcurrent, which
can destroy the unit.
Calculation of braking power
When calculating the braking power, it must be ensur-
ed that the mean and peak powers can be dissipated
to the brake resistor. The mean power is determined
by the period time of the process, i.e. for how long the
brake is applied in relation to the period time of the
process. The peak power is determined by the braking
torque, which means that during braking the brake re-
sistor must be able to dissipate the energy input. The
figure shows the relation between mean power and
peak power.
Calculation of peak power of brake resistor
P is the peak power at which the motor brakes
PEAK, MEC
on the motor shaft. It is calculated as follows:
P
MOTOR
P
=
PEAK , MEC
P is the term describing the braking power that is
peak
applied to the brake resistor when the motor applies
the brakes. P is smaller than P
PEAK
power is reduced by the efficiency of the motor and the
frequency converter. The peak effect is calculated as
follows:
P
×
MOTOR
P
=
PEAK
24
Ω
Ω
M
×
( % )
BR
W
100
, as the
PEAK, MEC
M
× η × η
( % )
BR INV
100
If you select Danfoss' recommended braking resistor
(R ), you are certain that the braking resistance can
REC
generate a braking torque of 160% on the motor shaft.
Calculation of mean power on brake resistor
The mean power is determined by the period of the
process, i.e. how long you brake in relation to the pe-
riod of the process.
Duty-cycle for braking is calculated as follows:
Duty cycle
− =
T = The process time in seconds.
p
T = The braking time in seconds.
b
Danfoss sells brake resistors with variable duty-cycles
up to 40%. For example, with a 10% duty-cycle, brake
resistors can take up P
riod. The remaining 90% of the period time is spent on
redirecting surplus heat.
The mean power at 10% duty cycle can be calculated
as follows:
P P
= × 10 %
avg peak
The mean power at 40% duty cycle can be calculated
as follows:
P P
= × 40 %
avg peak
These calculations apply to intermittent braking with
period times of up to 120 seconds.
NB!
Period times longer than 120 sec. may
lead to overheating of the resistor.
MOTOR
W
MG.27.E2.02 - VLT is a registered Danfoss trademark
T
× 100
b
%
T
p
in 10% of the process pe-
peak
W
W