Metal Halide lamp in-rush current

[sabrepoint]

Hi

I'm no lighting engineer so please bear with me. I need to calculate the cable voltage drop during start and operating conditions of the Ruud CF3499 flood light fitted with 1000 W 240 V MH lamps.

I was under the impression that any light fitted with a choke would have a large in-rush current at cold start (perhaps similar to the 6-7 times full load current of squirrel cage motors at start). So I'm a little surprised to see the 'ballast line current' for the Ruud Metal Halide 240 V 1000 W 'starting amperes' = 3 A when the 'operating current' = 4.5 A.

Now I've little doubt the Rudd data is correct - is this typical for MH lamps? I would just like to understand how the start current can be less than the operating current, so any help would be most appreciated.

Thanks for this
Graeme

 

[itsmoked]

Let me give it a try.. If you watch a MH bulb start as I have because I have a 180W-MH hanging in my living room, you will see the bulb start very slowly with the arc weak and small. With time it grows hotter as more metal is vaporized as the temperature rises. The maximum current will be when the bulb is at full temperature with the most conductive metal in the arc.

This is the same as two wires in a glass pot full of water. No current until you start dribbling salt into the water. Then as more salt is added the more ions are available for charge transport, current increases. Same with the plasma ions in the arc of any HID lamp.

If you are thinking of the instant strike current being higher then let me explain the arc initiation. When the lamp/ballast is energized not much current is required because there is no arc. A small current runs thru a heater in the bulb. It is a bimetal strip. When it heats to a specific temperature it snaps open. This current was running thru an inductor. When the inductor current is interrupted the voltage spikes abruptly. The voltage is high enough to strike an arc across the tube electrodes. The main current then piggy-backs across the arc. The current then slowly increases as I previously described.

 

[itsmoked]

sabrepoint; I will point out that Tungsten lamps have a horrendous inrush. Because the filament resistance is very low when it is cold. Rising sharply as it heats up.

Motors have a large inrush because there is no back emf when they are not rotating.. the major resistance possessed by a motor.

An inductor doesnt mean "big inrush" they actually retard change in current. A capacitor can mean "large inrush". Since it starts out looking like a short before acquiring charge.

 

[sabrepoint]

Thanks itsmoked

I Googled following up your response (should have done this first!) and have a better understanding now. You're saying there is no large current flow for a few cycles during the magnetising of choke?

In a nutshell, I just need to ensure that I can use the operating current in my volt drop calcs and don't have to cater for anything larger.

Thanks again
Graeme

 

[itsmoked]

LOLO!

I don't believe you need to worry about any inrush on HID lamp circuits... You would if you were talking Tungsten.

BUT let someone else confirm that. I don't do much power circuit calcs for buildings etc.

There are some lighting guys here. They will confirm/disavow this.

 

[ScottyUK]

Sodium lamps - the SON series in Europe - typically have a big inrush and can awkward for breaker sizing. Some guidance on the starting current here:

http://www.light.ie/protection.htm

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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[itsmoked]

That's a lame web page... 25X for 3ms? 7X 2s?
No explaination..

Starting current 1.5A running current 1.3A Contradiction of sorts.

Most confusing.

ScottyUK can you shed any light on where that 25X is coming from? Especially given it lasts only 15% of a single power line cycle.

Do those ballasts have a capacitive front end?

 

[ScottyUK]

Is the 'shed light' pun intentional? No definitive answer but practical experience shows that SON luminaires are a pain when it comes to MCB protection. Short cables to SON luminaires usually end up with a Type D MCB; long cables seem to be self-limiting via the loop impedance of the cable.

Agree that 3ms is a really short period - not even sure if the breaker would notice it before it was gone. I was almost awake when I posted the link - perhaps not one of my best efforts!!


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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[itsmoked]

LOLOL. The "shed light" was not intentional, but I had to leave it in when I saw it.

Yeah that kind of number 25X 3ms just seems .. I don't know. Like what I might expect from energizing any cable with a wee bit of capacitance. 5000x for first 7us!!

 

[sabrepoint]

Thanks both

Well it seems to me there is an in-rush magnetising current decreasing over a few cycles, similar to what is found with transformers. Then will be the much-reduced 'starting' current for a few minutes prior to full output.

If this is the case I need to consider whether the volt drop during the in-rush is of such duration that it causes adverse effects on other lights which are running at the time. I will speak to the Ruud supplier here soon - maybe they can help too.

Hopefully I've got it right, but feel free to refute my findings!

Thanks again
Graeme

 

[itsmoked]

Let us know what the Ruud-ites say.

 

[sabrepoint]

Ruudites you say! But not Luddites I trust. The guy who gave us the quote is away for a few days but I'll come back after.