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Calculating Generator Requirements

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Re: Calculating Generator Requirements

Postby cyrano.mac » Mon Jun 08, 2020 2:57 pm

Dramatic Hammer wrote:It seemed rather large at the time but reading this I reckon the 40kva generator I hired for my wedding might have been more than a little overkill, even running toilets, bouncy castle and fridge trailer off it as well as sound! :bouncy:

Hard to tell, really. How big was the fridge?

Most generators are based on an electro-magnetic field winding. Especially the smaller ones. They need power to start producing output. That power comes from the battery. A 2 kW one will give you 2 kW; but no big current peaks allowed as they produce an avalanche effect because the field winding looses power.

Permanent magnet field generators do not need power to get going. They are bulkier, heavier and simpler. As such, they can easily support current peaks without even twitching.

For audio, it doesn't matter much as the current drawn is relatively constant. However, if you need to power electrical motors (or lights), you maybe want to look at permanent magnet types. Or you can get the electro-magnet type, but to run a 5 kW motor, you'll definitely need a 10 kW generator. Sometimes even more, depending on what the motor is driving and who's operating it.
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Re: Calculating Generator Requirements

Postby RMCL » Mon Jun 08, 2020 10:39 pm

Out of interest from an electrical safety point of view.

As many items of PA equipment have been designed to operate from a supply of 100 - 240 VAC 50 - 60 Hz, is a 110 VAC centre tapped earth (CTE) supply, as used on building sites, frequently used on PA equipment that is being used outdoors?

With this approach the potential electrical hazard is exposure to 55 VAC rather than 230 VAC.
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Re: Calculating Generator Requirements

Postby Sam Spoons » Mon Jun 08, 2020 10:59 pm

No experience of doing that (though it might make sense?). But you hit the nail on the head when you say "as many items of PA equipment.....".

The problem is that not all PA or backline gear runs off what Mackie call their "Worldwide power supply", much of it runs on fixed 230VAC or 120VAC supplies so it would be a potentially gig wrecking decision to go with such a plan.
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Re: Calculating Generator Requirements

Postby RMCL » Wed Jun 10, 2020 1:02 pm

In the UK the Mains electricity supply is single phase 230VAC, from which current (and hence power) is taken via a Live and Neutral connection. This is not a floating supply as the Neutral is connected to Earth (Ground).

On UK building sites, for safety reasons, a step down transformer drops the 230VAC down to 110VAC. On site step down transformers, the Earth from the 230VAC on the primary side of the transformer (input) is connected to the centre tap (mid point of transformer winding) on the secondary side of the transformer (output). Thus the output of the step down transformer is two Live outputs (which are in anti phase to each other) of 55VAC relative to the Earth connection.

If the step down transformer is connected to the Mains supply then Earth continuity will be maintained through the step down transform. However, if the step down transformer is connected to a generator then it will depend on whether the generator is Earthed or not. For smaller output generators the Earth connection as seen on the generator's output connector may only be connected to the generator's metal frame, so in effect the Earth is floating and the Live outputs are also floating. For higher output generators the generator is likely to be connected to an Earth rod.
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Re: Calculating Generator Requirements

Postby Hugh Robjohns » Wed Jun 10, 2020 4:00 pm

Oops :oops: I stand corrected RCML -- I was thinking of a true isolating transformer like wot we use in studios for dodgy bass amps, rather than a building-site step-down transformer with centre-tap. My bad -- and thanks for the correction -- Safety First! :D I've removed my earlier post to avoid confusion!

But to answer the question again, I think the answer is still no -- at least, not in my (limited) experience. I've never seen it done at any of the outdoor events I've attended, anyway.

You would have to be careful about stage mains distribution too because most standard 13A plug-boards only switch the live side -- so when switched off you would end up with 55V going to the switch mode power supply between neutral and ground which some really aren't going to like very much.... It's enough to get them excited but not enough to let them run properly!
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Re: Calculating Generator Requirements

Postby wireman » Wed Jun 10, 2020 8:38 pm

RMCL wrote:On UK building sites, for safety reasons, a step down transformer drops the 230VAC down to 110VAC. On site step down transformers, the Earth from the 230VAC on the primary side of the transformer (input) is connected to the centre tap (mid point of transformer winding) on the secondary side of the transformer (output). Thus the output of the step down transformer is two Live outputs (which are in anti phase to each other) of 55VAC relative to the Earth connection.
.

How is this better than a floating output?
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Re: Calculating Generator Requirements

Postby RMCL » Thu Jun 11, 2020 12:25 am

Hugh Robjohns wrote:But to answer the question again, I think the answer is still no -- at least, not in my (limited) experience. I've never seen it done at any of the outdoor events I've attended, anyway.

You would have to be careful about stage mains distribution too because most standard 13A plug-boards only switch the live side -- so when switched off you would end up with 55V going to the switch mode power supply between neutral and ground which some really aren't going to like very much.... It's enough to get them excited but not enough to let them run properly!
Thanks for advising that you have not seen 110 VAC CTE supplies, used on PA equipment outdoors and I would suspect many others would say the same thing.

There would be no issue if a 110 VAC CTE supply is used with SMPS and only one of the live inputs is switched, as both live inputs require to be connected for current to flow.
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