These braking resistors are designed for applications which occasionally require a high braking power,
e.g. for emergency stopping. The braking resistor stores the braking energy generated and heats up as a
result. The resistor can store an energy value W corresponding to power P20 for 20 seconds.
Approximately the same energy is produced if the resistor is loaded with power P3 according to the time
characteristic shown below. After this type of load is applied, the thermal storage capacity of the resistor
is exhausted.
The braking resistor can only slowly release the stored energy (heat) to the environment. The drive must
be planned in such a way that a no-load pause appropriate to the braking resistor used is maintained
according to the following diagram.
Before the resistor is loaded again with P20 or P3, it must be allowed to cool down to ambient
temperature again. This state is reached after an interval corresponding to about 4 times the thermal
time constant τ.
P20
P3
t
Braking
power
T 3s 20s 23s T+20s
W = P20 x 20s
T = 120 min
A temperature switch is mounted on the resistor. This responds if the ambient temperature is so high that
the resistor is thermally overloaded. The temperature switch cannot protect the resistor if it overheats as
a result of prolonged braking with P3 or P20! Reason: There is a long delay before the surface
temperature reaches the internal temperature of the resistor, i.e. the temperature switch detects the
overload too late. The overloaded resistor reaches a high ohmic value due to an internal protective
mechanism and must then be replaced. Tables 1 and 2 apply when the braking resistor is loaded with
continuous power.
The surface temperature exceeds the response temperature of the switch as a function of load. In this
case, the switch must be unscrewed from the resistor and replaced if necessary by a switch with a higher
response temperature.