Danfoss VT2800 Series Design Manual page 25

η is typically 0.90 and η
motor
Volt and 200 Volt frequency converters, respecti v ely,
R at 160% braking torque can be written as:
REC
NB!:
The brake resistance selected should have
an ohmic value no more than 10% lower
than that recommended by Danfoss. If a
lower brake resistance is selected there is a risk of
overcurrent, which can destroy the unit.
Calculation of braking power
When calculating the braking power, it must be
ensured 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 resistor 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
PEAK, MEC
brakes on the motor shaft. It is calculated as follows:
P is the term describing the braking power that
peak
is applied to the brake resistor when the motor
applies the brakes. P
PEAK
, as the power is reduced by the efficiency
MEC
MG.28.E9.02 - VLT is a registered Danfoss trademark
is typically 0.98. For 400
INV
is smaller than P
PEAK,
®
VLT 2800 Series
of the motor and the frequency converter. The
peak effect is calculated as follows:
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 period of the process.
Duty-cycle for braking is calculated as follows:
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
process period. 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:
The mean power at 40% duty cycle can be
calculated as follows:
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.
Continuous braking
For continuous braking, a brake resistor should be
selected in which the constant braking power does
not exceed the mean power P
in 10% of the
peak
of the brake resistor.
AVG
25