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# What is the criteria for braking resistor selection? Is there certain min and max value of resistance?

Published date: 07 June 2019

Selection of braking resistor must respect 3 criteria:
1. Resistance of braking resistor must be equal or greater then minimal allowed resistance Rmin
2. Resistance of braking resistor must be equal or less then maximal resistance Rmax

1. Each ATV requires that resistance of braking resistor must be above min value. The purpose is to prevent braking transistor from damage because when transistor is conductive, it is only the resistance of braking resistor that limits the current via braking transistor. Min values for each ATV size can be found in dedicated ATV document, i.e. installation manual.
Example ATV930D22N4 says that Rmin is 15 Ohms

2. The resistor's resistance has also to be below certain value, otherwise when braking transistor is permanently conductive, the resistor will not be able to absorb peak power. There is no risk of damage if resistance of braking resistor is above the max value, but only the braking power will be reduced.
Information about maximal resistance is sometimes missing in ATV document. However it can be easily calculated. Considering the level of DC bus voltage when braking transistor operates (785V for ATV...N4 drives) and by defining that peak power must be at least equal (or greater) then ATV power including overload, we can write:
peak power Pmax=Udc^2 / R > ATV max power
If we have drive that allows 1.2 times overload
we will get:
Rmax = Udc^2 / ATV max power = 1.2* ATV rated power.

Example: In case of ATV930D22N4 we get Rmax= 23 Ohms

3. This criteria has nothing to do with resistance, but with the power. Power of braking resistor must be selected with respect to the load and load cycle. Usually it requires calculation of average braking power or you can use values defined for some typical load cycles in catalog.

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