VARIAC SHORT TERM OVERLOAD RATINGS

Although STACO VARIAC Variable Transformers are small and light weight for the large power ratings which they handle, brush currents up to 10 times normal may be drawn for a brief time. The maximum on time curve of the figure below shows the duration of surge on transient currents which may be absorbed by a cold unit on motor starting or similar service. Protection for this level of service may be approximated by hydraulic-magnetic circuit breakers with trip coil in the brush output lead. If the unit is hot from previous loading, such as repetitive overloads, it must also be allowed sufficient off time as indicated on the minimum off time curve of below figure to prevent excessive temperatures. If the time on is less than allowed by the “on” curve, the minimum time off may be reduced in accordance with the following equation: