Report on UIE infrequent starts, 3.5% rule, (Action 14)

Roger Bergeron

The UIE/IEC standardized limits for the flicker produced by infrequent motor starts. Standard gives also design guide line to avoid flicker problem when connecting motors on the power system. The guide line uses the shape factor to translate the voltage fluctuation produced by a motor start to the standardized rectangular voltage fluctuation.

As shown in the graphic of the attached document (Figure 7 of IEC 1000-3-3), the shape factor is function of the front and tail times of the voltage fluctuation. The front time (ms) is the abscissa of the graphic and each line is related to each tail time (ms) of the voltage recovery. The voltage drop predicted during the start is estimated. This predicted voltage drop multiplied by the factor F obtained from Figure 7 of IEC 1000-3-3 gives the equivalent rectangular voltage drop that could produces the same flicker. IEC 1000-3-3 and IEC 1000-3-7 limits are based on this equivalent rectangular voltage drop.

The flicker is function of the number of motor starts per week. IEC 1000-3-7 and IEC 1000-3-3 disregard the phenomenon if the number of starts is less then 9 per week and if the equivalent rectangular voltage drop is less then 4%. No equivalent rectangular voltage fluctuation shall exceed 4% (section 5 IEC 1000-3-3).

Each infrequent start is considered as 1 change per 10 min during that particular 10-min interval and the flickermeter gives the corresponding Pst value for that particular 10-min interval. If only 9 starts occur during 9 different 10-min intervals of the week, the resulting Pst that matches 99% probability to not be exceeded will give the background flicker value. If the background flicker is low, IEC1000-3-7 disregards these 9 motor starts.

I hope this clears the issue of infrequent starts and IEC standard.

Roger Bergeron

Last updated 12-8-97 - L. Conrad