It is not "required" for warm up, but is required for stability - to stop the warm up from continuing when the lamp already is at its intended power
On a magnetic ballast (here i mean a 230V choke ballast, but HX will also have similar behavior), this is naturally achieved by the way how the magnetic ballast determines the current :
The current depends directly on the voltage drop across the inductive impedance. For a choke this simply means the choke's impedance, for HX this means the impedance of the secondary
The voltage drop on the impedance is a vector subtraction of the lamp arc voltage from the Voc. The voltage on the ballast and the lamp are approximately 90deg apart. (It is not exact because of additional less significant factors, like the discharge power factor and ballast wire resistance)
Simplifying things, the voltage drop on the ballast impedance will be lower when the lamp arc voltage is higher, and the effect will be more significant the higher the lamp arc voltage is (due to the angle between the vectors)
When the HPS lamp is fully warmed up the discharge voltage is higher, so the voltage drop on the ballast is lower, so the current is lower. (Compared to the current when the lamp is warming up)
The lamp and magnetic ballast are designed so, that the lamp's full power rating is near the max power which the ballast can provide. If the lamp arc voltage rises more, the current must be dropped significantly, in order for the overall effect to be power reduction and not power rise. This behavior will stabilize the lamp to the correct current/voltage/power and prevent runaway
During the initial warm up it is less important how exacty the current behaves (within some reasonable limits), as it will start and warm up anyway. The magnetic ballast's behavior is fairly simplistic, and cannot be magically divided into ranges of different behavior for lamp warm up vs. full power. So the same relation between lamp voltage and current which is essential for stable lamp operation, continues also into the warm up area
And this behavior is good, as the higher current during warm up accelerates the warm up
With electronic ballast, there is complete freedom to design the ballast to behave in any wanted way, including : - Imitating a magnetic choke/HX ballast - Current source (imitating CWA) - Power source (imagine what a REAL CWA would be which its name implies, but magnetic CWA isn't) And many more options
But, unlike magnetic ballasts, with electronic it is possible to detect the lamp warm up stage, and provide piecewise combinations of the different behaviors for different stages of the lamp warm up
This way, an electronic ballast may accelerate the warm up, reduce electrode wear, etc. And, independently from this, provide the correct behavior to keep the lamp stable when it is running at its full power
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