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240 volt to 24 volt


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Output power1) : @ Ta=25°C

Pnom 850VA

P30 1000VA

Psurge 1800VA

 

I'm gonna fire it up shortly and listen to the beeps when it shuts down as I think it only does it once which means low power from the batteries.

 

850Va could actually be as low as 650 Watts.

 

VA is not the same as watts - but manufacturers quote it so you think you are getting more that you really are

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Wooooosh.

 

That's all this electrickery stuff going over my head.

Put simply... It could feed an 850W bar fire or maybe a 650W motor, or a 700W charger. There's no way of knowing in advance unless the equipment either states its consumption in VA or quotes a power factor.

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Wooooosh.

 

That's all this electrickery stuff going over my head.

 

 

Just to explain, with a DC load the power is always V x A. So W = VA.

 

With AC, it gets more complicated. There is something called inductance which explained simplicistically, causes the current in a circuit to lag behind the voltage that caused the current. The bigger the delay, the less work gets done by the voltage and current. The amount of the delay is measured using 'power factor'. When the current and voltage are perfeftly aligned the power factor is 1. When they are totally out of phase the power factor is 0.

 

The thing is, whether the power factor is 0 or 1 or something in between, the voltage and current are still using the same energy to generate them. So if you have say a battery charger with a power factor of 0.8, the voltage and current multiplied together give you the power needed by the genny, but the maximum power delivered by the charger can only be the (input) voltage x current x 0.8.

 

You may need to read that a few times!

 

 

 

(Edit to correct a fundmental error!)

Edited by Mike the Boilerman
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So if you have say a battery charger with a power factor of 0.8, the voltage and current multiplied together give you the power needed by the genny, but the maximum power delivered by the charger can only be the (input) voltage x current x 0.8.

To put that in a possibly more useful form, you divide the wattage of an appliance by the power factor to find out the VA.

 

So an 800W charger with a power factor of 0.8 would demand (800/0.8)=1000VA.

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To put that in a possibly more useful form, you divide the wattage of an appliance by the power factor to find out the VA.

 

So an 800W charger with a power factor of 0.8 would demand (800/0.8)=1000VA.

 

 

Yes, exactly.

 

This is what most explanations of power factor tell people, but I thought explaining why the power factor exists in the first place would help Dave understand what its all about.

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So returning to the OP question, why not use a 12v-24v converter of sufficient capacity to run the pump motor? Efficiency not really an issue as it would only need to be switched on when the loo's in use.

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So returning to the OP question, why not use a 12v-24v converter of sufficient capacity to run the pump motor? Efficiency not really an issue as it would only need to be switched on when the loo's in use.

 

 

 

There was a longgggg previous thread discussing this IIRC

 

Probably because the transient start current of the motor will be in the order of 30A, and such things are not easily found. Or maybe they are!

 

Can anyone remember and point to the thread?

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OP suggests specs give 7.5A locked rotor current, most Jabsco non-macerators seem to be 7.5 or 8A rated with a 15A fuse, so not a large motor, about the same as my domestic water pump which runs happily off a 360W dc-dc converter.

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There was a longgggg previous thread discussing this IIRC

 

Probably because the transient start current of the motor will be in the order of 30A, and such things are not easily found. Or maybe they are!

 

Can anyone remember and point to the thread?

I can't be arsed to go looking for the thread but put simply a couple of small batteries and a small charger would be much cheaper than a very hefty psu.

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I can't be arsed to go looking for the thread but put simply a couple of small batteries and a small charger would be much cheaper than a very hefty psu.

You might even get away with a big capacitor to supply the surge current, but as above couple of small batteries would be best, and you can even use a small charger.

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Just to explain, with a DC load the power is always V x A. So W = VA.

 

With AC, it gets more complicated. There is something called inductance which explained simplicistically, causes the current in a circuit to lag behind the voltage that caused the current. The bigger the delay, the less work gets done by the voltage and current. The amount of the delay is measured using 'power factor'. When the current and voltage are perfeftly aligned the power factor is 1. When they are totally out of phase the power factor is 0.

 

The thing is, whether the power factor is 0 or 1 or something in between, the voltage and current are still using the same energy to generate them. So if you have say a battery charger with a power factor of 0.8, the voltage and current multiplied together give you the power needed by the genny, but the maximum power delivered by the charger can only be the (input) voltage x current x 0.8.

 

You may need to read that a few times!

 

 

 

(Edit to correct a fundmental error!)

Excellent explanation. I remember when power factor was first explained to me, years ago back at college, the lecturer rambled on about the companies running the power stations finding that they needed to burn more coal than they thought and eventually found that some customers had very inductive loads which needed more power to support.

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