Additional correction factors with frequencies other than 50/60 Hz for 3VA1 molded case circuit breakers
The temperature rise in the bimetal is greater at frequencies above 50/60 Hz. This is due to eddy-current losses and the reduction in the available conductor cross section as a result of the skin effect.
The rated operational current must therefore be reduced from the value used in 50/60 Hz applications.
In an application with 400 Hz, this means the following for the example from chapter Derating of 3VA1 molded case circuit breakers with a 3VA1 molded case circuit breaker when current Ir = 90 A is required:
Setting value Ir(400 Hz) = 90 A · 0.9 = 86 A
Where ambient temperatures differ from the calibration temperature, temperature compensation as described in chapter Derating of 3VA1 molded case circuit breakers must be performed.
In addition, a correction factor for setting the magnetic trip unit as a function of frequency must be applied.
A 3VA1 molded case circuit breaker 100 A with a TM240 trip unit is used for a 400 Hz application. The molded case circuit breaker is required to trip instantaneously in response to an overcurrent of 900 A.
The maximum permissible rated continuous operational current is calculated first:
100 A · 0.9 = 90 A
The setting value for instantaneous short-circuit protection (Ii) is then determined on the basis of the correction factor from the table above:
Ii = 900 A · 0.7 = 630 A