Question for the sparkies

[wookie]

Hi

I'm trying to work what power rating some resistors in series with a contactor coil need to be.

I've measured the coil at 4k7ohm, the actual resistors are 6k8ohm and a variable of 1k5ohm all in series with the coil.

minimum resistance = 4k7 +6k8 + 0 = 11k5
maximum resistance = 4k7 +6k8 + 1k5 = 13k
and using 240Vdc (yes DC as measured with my multimeter)

Following ohms law's W=(V x V) / R

I get...

so using minimum resistance (240 x 240) / 11500 = 5.009 watts
and using maximum resistance (240 x 240) / 13000 = 4.4307 watts

Does that look right or is that not the correct way to calculate the power drawn in a circuit with a coil ?

 

[piman]

Hello Wookie,

that figure seems OK, but is the coil going to be operating on DC rather than AC? If it's on AC then the figure will be much different.

Alec

 

[rtbcomp]

That looks about right for DC. You've calculated the total power dissipation, including that dissipated by the coil, so the power dissipated in the resistors will be slightly less.

To be more accurate you need to calculate the current (V = IRt, I = V/Rt) and then use W = I^2 R1 and W = I^2 R2 to calculate the power dissipated by each resistor.

Rt is the total resistance, R1 & R2 are the resistances of each resistor in turn.

You'll need to do the calc twice, once for the variable resistor = 0 and once for 1k5.

On AC the current drawn will be less due to the impedance of the coil.

 

[wookie]

That looks about right for DC. You've calculated the total power dissipation, including that dissipated by the coil, so the power dissipated in the resistors will be slightly less.

I know I didn't state this in my original post, but the reason I'm trying to work this out is because both resistors have failed open circuit, I know the resistance value (printed on the resistor) but not the power rating.
I know I calculated total power dissipation but as I only want to know the power rating so I can buy suitable replacements I thought that would be safe way of getting a maximum figure for the resistors.

I'm trying to undo a previous owners meddling and get an old generator set fully working, this part of the circuit controls the contactor coil that allows it to automatic switch the output of the generator to the house when power is required. If I work the contactor by hand then power is sent to the house as expected, it's purely that at the moment it doesn't do this by itself. Everything else about the control circuit works as it should now after the repairs I've done.

To be more accurate you need to calculate the current (V = IRt, I = V/Rt) and then use W = I^2 R1 and W = I^2 R2 to calculate the power dissipated by each resistor.

Rt is the total resistance, R1 & R2 are the resistances of each resistor in turn.

You'll need to do the calc twice, once for the variable resistor = 0 and once for 1k5.

On AC the current drawn will be less due to the impedance of the coil.

I know that impedance affects the AC current (reduces it) but I don't know how to calculate it, DC is a lot easier to understand

One final question and I think/hope that it doesn't make too much difference, the actual supply in this case isn't pure DC, it's half-wave DC with a capacitor across the power rails to smooth it out, with my limited knowledge I don't think it'll throw my calcs out too much, if anything it should reduce the current flowing ?

Thanks for the advise.

 

[Dee J]

I wonder why the AC is half-wave rectified before the dropper circuit and the contactor coil...is it part of the original desgn? If not then I think I'd remove the rectifier and dropper resistors and replace the contactor with one with a 230v coil.

 

[wookie]

I wonder why the AC is half-wave rectified before the dropper circuit and the contactor coil...is it part of the original desgn? If not then I think I'd remove the rectifier and dropper resistors and replace the contactor with one with a 230v coil.

Yes it's the original design, my generator was built in 1958 but the design was from late '40s early '50s and my feeling is "If it ain't broke don't fix it", I like old designs as they are generally built to last, my avatar is me doing some welding on my WWII Willys MB chassis

 

[Dee J]

I too like old tools and equipment- just that aspect of the design puzzled me - especially that for 58 it's probably using a metal plate rectifier - which wasn't a cheap option at the time. Just for curiosities sake can you post make/model?

 

[Cyberprog]

Why not just use a modern contactor set and electronics to do this?

 

[wookie]

I too like old tools and equipment- just that aspect of the design puzzled me - especially that for 58 it's probably using a metal plate rectifier - which wasn't a cheap option at the time. Just for curiosities sake can you post make/model?

Lister Start-o-matic 1.5kva 240V ac.

Why not just use a modern contactor set and electronics to do this?

To quote myself... "If it ain't broke, don't fix it" plus the cost of a couple of resistors is a few quid at most, whilst a new contactor will cost a lot more and that probably won't be as good a quality as the original , unless it's a quality make and that will cost an arm and a leg

 

[Cyberprog]

Lister Start-o-matic 1.5kva 240V ac.


To quote myself... "If it ain't broke, don't fix it" plus the cost of a couple of resistors is a few quid at most, whilst a new contactor will cost a lot more and that probably won't be as good a quality as the original , unless it's a quality make and that will cost an arm and a leg

Fair point; but what the older stuff can't do is monitor the incoming mains for x minutes and check that both frequency and voltage is stable, before doing a switchover, and then keeping the set running for x minutes for a combined cooldown/immediate reaction to further fluctuations. And/or control multiple sets

 

[Dee J]

Ah... It's the line contactor ballast... only fitted to 230v versions. Presumably to maintain a common build over a range of output voltage variants. I guess the contactor voltage rating is around 110v maybe? Not sure what the common generator voltages were back then.

 

[wookie]

Ah... It's the line contactor ballast... only fitted to 230v versions. Presumably to maintain a common build over a range of output voltage variants. I guess the contactor voltage rating is around 110v maybe? Not sure what the common generator voltages were back then.

You don't have a copy of the manual do you, I've been working off the diagram inside the control box but it's a bit tatty and parts are missing.

Fair point; but what the older stuff can't do is monitor the incoming mains for x minutes and check that both frequency and voltage is stable, before doing a switchover, and then keeping the set running for x minutes for a combined cooldown/immediate reaction to further fluctuations. And/or control multiple sets

To be honest, I'm not after a 100% automatic solution, the way I have it wired is via a manual changeover switch feeding my consumer unit. So if the power goes out and I need power I just have to flick the changeover switch to generator, and then the start-o-matic detects the load and fires up automatically. It then stays running as long as required, once the mains is back on and staying up, I can just flick the changeover back to mains and then the start-o-matic detects the lack of load and shuts down.

The benefits to me of this setup are that at night I can switch off the changeover switch if I don't want to keep the generator running all night and most importantly I don't have to trek up to my garage to start & stop the generator when required.

 

[Cyberprog]

The benefits to me of this setup are that at night I can switch off the changeover switch if I don't want to keep the generator running all night and most importantly I don't have to trek up to my garage to start & stop the generator when required.

The newer systems are ethernet and GSM enabled too, so you can just remote into them from your phone either via Wifi or via the GSM network directly.

 

[wookie]

The newer systems are ethernet and GSM enabled too, so you can just remote into them from your phone either via Wifi or via the GSM network directly.

Erm, slight problem, say hello to someone who doesn't have/own a mobile phone having said that the last time that we had a 3 day power cut you couldn't get a mobile signal other than emergency calls only, whether your modern system would work under those circumstances I don't know.

 

[rtbcomp]

Calc 5W fit 10W and mount them so air can circulate freely.

 

[wookie]

Calc 5W fit 10W and mount them so air can circulate freely.

Brilliant I knew I could count on the brains on this site, I'll get some ordered a.s.a.p.

 

[Dee J]

You don't have a copy of the manual do you, I've been working off the diagram inside the control box but it's a bit tatty and parts are missing.

No, but google is your friend... google images for Lister Startomatic wiring diagram.I found this page
http://freespace.virgin.net/stephen.kelly7/pageengines.html
But if that's not the exact variant there are many more out there.
HTH
Dee

 

[wookie]

My new resistor and variable resistor have arrived, only took 10 days to come from Hong Kong and all for under a fiver, don't know how they can do it so cheep anyway I could only get 25watt versions, they should be man enough for the job