Calculating glow plug draw and voltage drop

BabaYaga

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There is no problem, I just have a few questions to try and improve my understanding...

Engine is a three cylinder Beta 722. Unlike on some other engines, there is no relay for the glow plugs. They are fed directly from the engine panel by a 2.5mm sq wire within the engine harness.
The current draw of the plugs is not to be found in the engine manual, but recently another forumite posted a link to the workshop manual for Kubota diesel engines, including the D722, upon which my Beta supposedly is based.
This manual says the glow plug should have a resistance of approx. 0.9 Ohms.
Since there are three plugs in parallel, I calculate the total current draw to be 40A (12V nominal, disregarding the wiring).

1. This current is a lot more than I expected, is my calculation correct?
2. The engine panel itself is also fed by a 2.5mm sq wire within the engine harness. There is also a 40A fuse in this wire. Provided that the answer to question 1. is yes, doesn't the wiring appear to be a bit marginal?
3. The engine harness has a length of about 3 metres, so the total length of 2.5mm sq wire supplying the glow plugs is some 6 metres. When I try to calculate voltage drop over this run. I get 3.33V or 27 percent. Does this sound normal/reasonable?

Thanks for any input.
 

lw395

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Go and measure the cold resistance of a light bulb.
The resistance of a glow plug will be a lot more when it's glowing.
 

BabaYaga

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Thanks... so without knowing the material of the glowing tip or how hot it gets there is no way to calculate the current draw only from cold resistance?
I would hope it triples when glowing hot, at least.
 

rogerthebodger

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Thanks... so without knowing the material of the glowing tip or how hot it gets there is no way to calculate the current draw only from cold resistance?
I would hope it triples when glowing hot, at least.

Why not just measure the current flow with a clamp amp meter.
 

DownWest

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If worried about the wiring, why not put a relay close to the engine? Might even prolong the life of the (expensive) keyswitch.
Faced with duff switch on a four banger Kubota, I fitted a push button and a 50amp relay. After some six months of starting probs, the owner thought it was magic, as it started straight away. Finding a keyswitch would have taken time and loot.
 

matthewriches

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Since working with a Beta dealer in Southwold, I have seen that the 2.5mm cable can get warm after a few failed cold starts using the glow plugs in anger. When you actually consider the ampacity of a 2.5mm cable, small bundled, it isn't actually a huge number!

I've done the relay modification on a few of the larger engines as the ends had crisped up nicely on the first connection to the glow plugs. I tend to use the 100 amp big relays as they're not much more expensive than the smaller ones and you know you've got good capacity.
 

Matata

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I've just been working on this very thing...Beta 20 ...So I made a proper rig with the glow plugs set up as per the engine and an ammeter....They initially draw 35amps and over the next couple of secs this falls to 20 amps and finally settles to 15 amps....They glow impressively hot! i'd say the resistance cold is less than an ohm each...hope this helps. Nik
 

lw395

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I've just been working on this very thing...Beta 20 ...So I made a proper rig with the glow plugs set up as per the engine and an ammeter....They initially draw 35amps and over the next couple of secs this falls to 20 amps and finally settles to 15 amps....They glow impressively hot! i'd say the resistance cold is less than an ohm each...hope this helps. Nik

Proper Job!
 

BabaYaga

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I've just been working on this very thing...Beta 20 ...So I made a proper rig with the glow plugs set up as per the engine and an ammeter....They initially draw 35amps and over the next couple of secs this falls to 20 amps and finally settles to 15 amps....They glow impressively hot! i'd say the resistance cold is less than an ohm each...hope this helps. Nik

Thanks for that, good to know.
Workshop manual says 0.9A resistance cold, so everything in order then.
 

rogerthebodger

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I've just been working on this very thing...Beta 20 ...So I made a proper rig with the glow plugs set up as per the engine and an ammeter....They initially draw 35amps and over the next couple of secs this falls to 20 amps and finally settles to 15 amps....They glow impressively hot! i'd say the resistance cold is less than an ohm each...hope this helps. Nik

From those figures its more like 1/3 ohm resistance cold an 0.8 ohm at final settle (hot)
 

BabaYaga

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Sorry, my mistake.
Workshop manual says 0.9 Ohms resistance.

I understood Matatas figures of 35 – 20 - 15 A to mean current draw for the three plugs together.
Cold resistance estimate of less than 1 ohm for each plug.
 
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