Danfoss VLT Brake Resistor MCE 101, Руководство по конструкции

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4.1.4 Calculation of the Brake Resistor Peak
Power
P
peak, mec
is the peak power by which the motor brakes on
the motor shaft. Calculate P
peak, mec
as follows:
P
peak, mec =
P
motor ×
M
BR %
W
P
peak
is the braking power dissipated to the brake resistor
when the motor brakes.
P
peak
is lower than P
peak,mec
since the power is reduced by
the efficiencies of the motor and the frequency converter.
Calculate P
peak
as follows:
P
peak =
P
motor ×
M
BR %
× η
motor
× η
VLT
W
When the brake resistor recommended by Danfoss is
selected (R
rec
) on the basis of the tables in
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
b
) transferred to the resistor in
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:
P
avg =
E
b
T
p
W
T
p
= period time in s, see Illustration 4.2 .
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:
Duty cycle =
Tb
× 100
Tp
%
where
T
p
= process period time in s
T
b
= braking time in s
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
peak
for 10% of the period
time. The remaining 90% of the period time is used on
deflecting excess heat.
Calculate the average power with 10% duty-cycle as
follows:
Pavg
=
Ppeak
× 10%
W
Calculate the average power with 40% duty-cycle as
follows:
Pavg
=
Ppeak
× 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,
Δ
ω,
Δ
t, see
Illustration 4.3 .
ω Start
ω Stop
∆t
∆ω/∆t
175ZA863.11
Illustration 4.3 Braking of High Inertia
Δ
t is determined by the ramp-down time.
NOTICE
The ramp-down time goes from the rated motor
frequency to 0 Hz.
P
peak
can be calculated as:
P
peak =
η
motor ×
η
VLT
×
ω
start ×
j
×
Δω
Δt
P
peak
=
η
motor
×
η
VLT
×
n
start
×
j
×
2 ×
π
60
2 ×
Δ
n
Δ
t
j is the motor shaft inertia.
System Integration Design Guide
MG90O202 Danfoss A/S © Rev. 05/2014 All rights reserved. 17
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