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System Integration
4.1.4 Calculation of the Brake Resistor Peak
Power
P
is the peak power by which the motor brakes on
peak, mec
the motor shaft. Calculate P
peak, mec
P peak, mec = P motor × M
BR %
P
is the braking power dissipated to the brake resistor
peak
when the motor brakes.
P
is lower than P
since the power is reduced by
peak
peak,mec
the efficiencies of the motor and the frequency converter.
Calculate P
as follows:
peak
P peak = P motor × M
BR % ×
η motor × η
When the brake resistor recommended by Danfoss is
selected (R
) on the basis of the tables in
rec
chapter 8 Selection Guide, the brake resistor is certain to
provide a braking torque of 160%/150%/110% on the
motor shaft.
4.1.5 Calculation of the Brake Resistor
Average Power
The average power is determined by the length of the
braking time in relation to the process period time.
When the kinetic energy (E
) transferred to the resistor in
b
each braking sequence is known (see chapter 6.1 Conveyor
Belt and chapter 6.2 Centrifuge), calculate the average
power of the brake resistor as follows:
E
b
P avg =
W
T
p
T
= period time in s, see Illustration 4.2.
p
When the kinetic energy transferred to the resistor in each
braking sequence is not known, calculate the average
power on the basis of the process period time and the
braking time.
Calculate the duty-cycle for the braking sequence as
follows:
T b × 100
Duty cycle =
%
T p
where
T
= process period time in s
p
T
= braking time in s
b
MG90O202
Design Guide
as follows:
W
W
VLT
Danfoss A/S © Rev. 05/2014 All rights reserved.
Danfoss offers brake resistors with a duty-cycle of max.
10% and 40%. If a 10% duty-cycle is applied, the brake
resistors are able to absorb P
time. The remaining 90% of the period time is used on
deflecting excess heat.
Calculate the average power with 10% duty-cycle as
follows:
P avg = P peak × 10%
W
Calculate the average power with 40% duty-cycle as
follows:
P avg = P peak × 40%
W
The calculations apply to intermittent braking using a
period time of 30 s.
NOTICE
Exceeding the specified intermittent braking period time
may result in overheating the resistor.
4.1.6 Braking of Inertia
When braking high inertia values on the motor shaft, base
the brake resistor values on the inertia, Δω, Δt, see
Illustration 4.3.
ω Start
ω Stop
Illustration 4.3 Braking of High Inertia
Δt is determined by the ramp-down time.
NOTICE
The ramp-down time goes from the rated motor
frequency to 0 Hz.
P
can be calculated as:
peak
P peak = η motor × η
ω start × j ×
×
VLT
P peak
η motor × η
n
×
×
=
VLT
start
j is the motor shaft inertia.
for 10% of the period
peak
∆ω/∆t
∆t
Δω
Δt
Δ
π
n
2 ×
2
j ×
×
Δ
60
t
17
4
4
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