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3.1 Energy balance
To be able to estimate the effect of an interconnection of drives via a
common DC bus, create an energy balance of the individual drives
over a movement cycle. A movement cycle typically consists of the fol-
lowing phases: acceleration, continuous movement and deceleration.
The energy generated during deceleration can be used by other
drives connected via a common DC bus. Excess energy can be
absorbed by the braking resistors.
The assessment of the energy balances of the individual drives per
movement cycle and of the cyclic sequence of the movement cycles
allows you to draw a conclusion regarding the efficiency of a common
DC bus.
3.1.1 Energy balance basics
The energy balance is influenced by the following factors:
• Energy absorption of capacitors E
var
in the drive
• Electrical losses of the drive system E
el
• Mechanical losses of the facility and the drive system E
mech
• Braking resistor E
B
Energy absorption of the capaci-
tors E
var
The higher the mains voltage, the lower the energy absorption of the
capacitors E
var
. In your calculation, use the values for the highest
mains voltage that is used in your application, see chapter
"2.3 Braking resistor".
The energy absorption of the capacitors E
var
is the square difference
between the voltage prior to the start of the deceleration and the
switch-on voltage of the braking resistor.
Electrical losses E
el
The electrical losses E
el
of the drive system can be estimated on the
basis of the peak power of the drive. The maximum power dissipation
is approximately 10% of the peak power at a typical efficiency of 90%.
If the current during deceleration is lower, the power dissipation is
reduced accordingly.
Mechanical losses E
mech
The mechanical losses result from friction during operation of the sys-
tem. Mechanical losses are negligible if the time required by the sys-
tem to coast to a stop without a driving force is considerably longer
than the time required to decelerate the system. The mechanical los-
ses can be calculated from the load torque and the velocity from
which the motor is to stop.
Braking resistor E
B
Two characteristic values determine the energy absorption of a brak-
ing resistor:
• The continuous power P
PR
is the amount of energy that can be
continuously absorbed without overloading the braking resistor.
• The maximum energy E
CR
limits the maximum short-term power
that can be absorbed.
3 Engineering
LXM32
22
Common DC bus
MNA01M001EN, V1.01, 08.2014