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1.6 Maximum coil temperature tw and ΔT
Another value marked on the ballast is the coil temperature rise Δt. This is the difference between the absolute coil temperature and the ambient temperature in standard conditions and is measured by a method specified in IEC Publ. 920 (EN 60920). Common values for Δt are from 50 to 70 degrees in steps of 5 degrees.
The coil temperature rise is measured by measuring the ohmic resistance of the cold and warm copper coil and using the formula:
| Δt = {(R2 - R1)/R1} . (234.5 + t1) - (t2 - t1) | |
or: | tc = R2/R1 . (t1 + 234.5) - 234.5 (IEC | |
where | R1 | = initial cold coil resistance in ohm |
| R2 | = warm coil resistance in ohm |
| t2 | = ambient temperature at measuring R2 in Celsius |
| t1 | = initial ambient temperature at measuring R1 in Celsius |
| tc | = calculated warm coil temperature in Celsius |
| Δt | = tc - t2 in Kelvin |
The value 234.5 applies to copper wire; in case of aluminium wire, the value 229 should be used.
So a ballast marked with tw 130 and Δt 70, will have the specified 10 years average life in continuous operation at standard conditions at an ambient temperature of 130 - 70 = 60 ºC.When the ambient temperature around the ballast is higher, a shorter ballast life has to be accepted or sufficient air circulation or cooling has to be applied. The
Additionally, a third temperature figure can be mentioned on the ballast: the ballast temperature rise in abnormal conditions, again measured according to specifications like EN 60920. In short: it is the winding temperature rise at 110 per cent mains voltage when the
The marking of the three temperature markings should be :
Δt ** / *** / tw *** with * = figure
Example: Δt 70 / 140 / tw 130.
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Watt losses
Ballast losses normally are published as ‘cold’ values, meaning that the ballast is not energised or only very shortly before and the ballast winding is at ambient temperature (25 ºC). In practice the ballast will reach more or less the marked Δt value and then the copper resistance is approx. 25 per cent higher than in the ‘cold’ situation.Therefore the ‘warm’ losses in practice will be 10 - 30 per cent higher than the published values.
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