5.1.5 Calculation of Peak Power of Brake Resistor
P
PEAK, MEC
is the peak power at which the motor brakes on the motor shaft. It is calculated as follows:
PPEAK
,
MEC
=
PMOTOR
×
MBR
(%)
100
W
P
peak
is the term describing the braking power that is applied to the brake resistor when the motor applies the brakes. P
PEAK
is smaller than P
PEAK, MEC
, as
the power is reduced by the efficiency of the motor and the frequency converter. The peak effect is calculated as follows:
PPEAK
=
PMOTOR
×
MBR
(%) ×
η
INV
×
η
MOTOR
100
W
If you select Danfoss' recommended braking resistor (R
REC
), you are certain that the braking resistance can generate a braking torque of 160% on the
motor shaft.
5.1.6 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:
Duty
−
cycle
=
Tb
× 100
Tp
%
T
p
= The process time in seconds.
T
b
= The braking time in seconds.
Danfoss sells brake resistors with variable duty-cycles up to 40%. For example, with a 10% duty-cycle, brake resistors can take up P
peak
in 10% of the
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:
Pavg
=
Ppeak
× 10 %
W
The mean power at 40% duty cycle can be calculated as follows:
Pavg
=
Ppeak
× 40 %
W
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.
5.1.7 Continuous Braking
For continuous braking, a brake resistor should be selected in which the constant braking power does not exceed the mean power P
AVG
of the brake
resistor.
Please contact your Danfoss supplier for further information.
VLT
®
Decentral FCD 300 Design Guide
5 All About FCD 300
MG.90.S1.02 - VLT
®
is a registered Danfoss trademark
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