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In order to obtain the necessary power rating, it will usually be necessary to build up a series/parallel network of
resistors, as shown in Figure 3.5.
Each resistor = Eurotherm CZ057146, 56 ohms, 220W continuous
is the same as
One resistor of 28 ohms
Resistors in parallel
rated at 2 * 220 W = 440 W continuous
One resistor of 112 ohms
rated at 2 * 220 W = 440 W continuous
Resistors in series
is the same as
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By connecting resistors in series and in parallel the braking capacity can be selected for the application. Always use
identical resistors in series/parallel combinations for braking applications.
The formula to calculate the effects of series and parallel combinations are as follows.
Resistors in series: Total resistance = the sum of all the resistances (i.e. R1 + R2 + R3 +R4 etc.).
Resistors in parallel: Total resistance =
resistor value
total number of resistors
Power dissipation: the number of resistors times the individual power dissipation of each resistor.
For example, four Eurotherm CZ057146 56
Ω
220W continuous resistors in series:
Total resistance = 56
Ω
+ 56
Ω
+ 56
Ω
+ 56
Ω
= 224
Ω
Four Eurotherm CZ057146 56
Ω
220W continuous resistors in parallel:
Total resistance =
56
Ω
= 14
Ω
4
Continuous power ratings in both cases are 880W (four times 220W). Peak powers are similarly multiplied by four.
Series and parallel networks can be combined as shown in Figure 3-5. The calculations are then simply combined:
add up the series resistances first, then calculate the effect of having the appropriate numbers in parallel.
Each resistor = Eurotherm CZ057146, 56 ohms, 220W continuous
is the same as
One resistor of 56 ohms rated at 16 * 220 W = 3.25 kW continuous
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A special case is for 'square' series/parallel networks where the number of series elements is the same as the number
of parallel elements, as in Figure 3.6. In such an array the total resistance is always the same as one resistor; the
power rating is the rating of one resistor multiplied by the number of resistors.
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The resistor(s) must be specified for the maximum DC link voltage (800V for the 380-460V version, 405V for the
208-240V version).
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