MG SET
Submitted: Tuesday, Nov 29, 2005 at 22:41
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Ray Bates
I am considering building a motor /generator set from an alternator and an electric motor but what is worrying me is the RPM I should use for the alternator
Reply By: kesh - Wednesday, Nov 30, 2005 at 07:52
Wednesday, Nov 30, 2005 at 07:52
The more output you require, the more revs. are needed. Most alternators are rated max. output at ~ 6000rpm.
Are you working on a perpetual motion machine, the electric motor drives the alternator which powers an invertor supplying 240v. to drive the motor which drives the alt..............?
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Reply By: joc45 - Wednesday, Nov 30, 2005 at 13:42
Wednesday, Nov 30, 2005 at 13:42
It begs the question, why are you doing this?
Gerry
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Follow Up By: Mr Fawlty - Wednesday, Nov 30, 2005 at 19:47
Wednesday, Nov 30, 2005 at 19:47
Yes I asked why? run a welder? perhaps to provide current to drive a stranded tram.....Back in the Old days motor generator sets were used to provide DC from DC at a different voltage. If you want to convert AC to DC why not use a rotary converter or a mercury arc rectifier...Both can be found in techology museums....sorry forgive my flippancy
Basil
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Follow Up By: joc45 - Wednesday, Nov 30, 2005 at 19:57
Wednesday, Nov 30, 2005 at 19:57
Even more exciting technological designs can be found in the works of Heath Robinson....
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Reply By: waynet - Wednesday, Nov 30, 2005 at 21:09
Wednesday, Nov 30, 2005 at 21:09
Ray
Don't take too much notice of the ''know it alls'' that have replied to your question and made no sensible contribution. A 12 volt auto alternator has a maximum speed of about 8,000 RPM but will give full output above 3,500 to 4,000 RPM. The problem you will have with making a set like this is you will need at least a motor of 3.5 to 5 H.P., depending on the size of the alternator you wish to drive. If you have only 240volts available a motor of this size is very hard to find and very expensive to set up and run, you could realistically only use 3 phase/415 volts. A better alternative is to spend your money on a good quality switchmode battery charger. If you want to make a good
camping power supply with your alternator get youself a 3.5HP petrol motor to drive the alternator to charge your auxiliary battery, this is the most efficient system for
camping. I have used this system for the last 15 years and have made several units for friends and some companies and find the system to be very good. If you need any more info on how to make a unit like this post your email adress on the
forum and I will give you any help that I can.
Good luck
Wayne
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Follow Up By: angler - Wednesday, Nov 30, 2005 at 22:41
Wednesday, Nov 30, 2005 at 22:41
I too have built a few of these using an engine around 3.5 Hp however one modification I found made things a lot easier was to have an adjustable regulator on the 12V side. Starting with a fairly flat battery the engine had to work quite hard and used heaps of fuel, we set the output lower for the first 30 minutes or so and then switch the regulator to the full 14.4 or so volts out to get the battery
well charged.
We ended up using a regulator with a constant current output rather than a constant voltage, this enables the output to remain constant right up to full charge.
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Reply By: Eric Experience. - Wednesday, Nov 30, 2005 at 22:36
Wednesday, Nov 30, 2005 at 22:36
Ray. As already mentioned this is stone age technology. But you must have a reason, a typical 240volt motor has an eficiancy of about 60 % and a automotive alternator is about 25 % so you are building a heater. 1kw in will give you about 100 watts out. if you use a generator instead of the alternator you will get about 200 watts for your 1kw in because the old generators were about 50% . as for revs use a 2 pole motor and direct couple it [2800rpm] Eric.
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