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#201192 - 05/17/11 12:55 PM help needed with a nano dc to ac rotary converter
odbob Offline
New Member

Registered: 05/17/11
Posts: 8
Loc: england
I am trying to build a tiny 40 watt rotary converter, components :- 12 volt 60 watt dc motor, driving a 240 volt 40 watt squirrel cage motor connected to 240 volt ac, in the past when I have driven large 3 phase motors above sync speed, they have produced power into the grid and yet this little set up won't, principal seems to be the same, am I missing something ?
And yes, before you ask, I do have the necessary protection relays in place, should the grid fail

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#201193 - 05/17/11 02:50 PM Re: help needed with a nano dc to ac rotary converter [Re: odbob]
LarryC Offline
Member

Registered: 07/05/04
Posts: 775
Loc: Winchester, NH, US
Quote:
I am trying to build a tiny 40 watt rotary converter, components :- 12 volt 60 watt dc motor, driving a 240 volt 40 watt squirrel cage motor connected to 240 volt ac,


Is the small motor single or three phase?

What is the ultimate electrical load? Would a suitably sized UPS be a more economical choice?

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#201194 - 05/17/11 03:55 PM Re: help needed with a nano dc to ac rotary converter [Re: odbob]
odbob Offline
New Member

Registered: 05/17/11
Posts: 8
Loc: england
Larry, the motor is single phase, the intention is for the motor to simply feed into the electrical system of the house.
The reason for not choosing UPS is that, should I get this to work, then my next project is to apply the same understanding to a nano hydro system, I have an absolute constant water flow and head, but that asside, I need to get this motor to function first.

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#201198 - 05/17/11 09:32 PM Re: help needed with a nano dc to ac rotary converter [Re: odbob]
LarryC Offline
Member

Registered: 07/05/04
Posts: 775
Loc: Winchester, NH, US
What type of single phase motor?

Split phase, capacitor start, capacitor run, synchronous, etc.

I suspect the single phase induction motor will not make a adequate generator.

Not knowing much about your installation or intended use, I still recommend using the ouput of the water to charge up a battery to drive a load like like a water cylinder or an outbuilding lamp or something like that.

What draws 40 W 24/7 or 80 W for 12/7 ? Landscape lighting? A grow light in the basement? Ornamental water fountain?

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#201200 - 05/18/11 03:06 AM Re: help needed with a nano dc to ac rotary converter [Re: odbob]
odbob Offline
New Member

Registered: 05/17/11
Posts: 8
Loc: england
Larry, the motor is a squirrel cage induction motor, no capacitor, 4 pole 1400 rpm @50Hz.
As I said earlier, it is not to drive any specific item, simply to feed into the house electrics 12/7, over a period of one year 150kW, minimal saving, but none the less, a saving, if I could get this to work, the the hydro would add a further 300 kW/ year.
This is simply me experimenting, on a larger scale it works and so I need someone to offer reasons, why on this small scale, it doesn't, same principle, grid acts as exciter, rotor magnetises, above sync speed, motor outputs to the grid, but for some reason this is not happening ?

regards Odbob


Edited by odbob (05/18/11 03:07 AM)

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#201204 - 05/18/11 02:11 PM Re: help needed with a nano dc to ac rotary converter [Re: odbob]
LarryC Offline
Member

Registered: 07/05/04
Posts: 775
Loc: Winchester, NH, US
Bob,

Stepping back for a minute to basic motor theory.

Three phase induction motors produce a rotating magnetic field that the rotor tries to follow. The phases are separated by 120 degrees.

Single phase induction motors produce a rotating magnetic field by altering one of the two magnetic fields by either changinging the stength / phase by adding capacitance or inductance to the field. When the rotor gets up to speed, a centrifigul switch removes the added reactance and the motor keeps running.

A BRUSHED "universal" motor that can run on AC or DC may be a better generator. Get the motor up to synchronous speed, if required, pulse DC through the rotor and I believe you will get a useful output.

Quote:
the motor is a squirrel cage induction motor, no capacitor, 4 pole 1400 rpm @50Hz


Isn't synchronous speed for a 4 pole 50 HZ motor 1500 rpm?

Good luck.

Larry

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#201212 - 05/19/11 02:51 AM Re: help needed with a nano dc to ac rotary converter [Re: odbob]
odbob Offline
New Member

Registered: 05/17/11
Posts: 8
Loc: england
Larry, thanks for your input, the theory, I agree with, in this instance, the motor in question, as I said, has no capacitor and no centrif switch, very simple, very tiny (40 watt) squirrel cage motor, presumably as you say, with a resistance winding to create the moving force, the resistance winding remaining in use whilst the motor is running, this is quite common practice with tiny motors, all that said, I am still at a loss as to why this motor will not output power, when taken above sync speed. Sync speed, you are quite right is 1500 rpm, but the motor quotes 1400 presumably, this is the expected slip speed.
Anyway, I will keep trying, many thanks again for your input
Regards Bob

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#201214 - 05/19/11 11:53 AM Re: help needed with a nano dc to ac rotary converter [Re: odbob]
LarryC Offline
Member

Registered: 07/05/04
Posts: 775
Loc: Winchester, NH, US
Bob,

Try to find a suitable motor from a older kitchen appliance or a vacuum cleaner.

Good luck.

Larry

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#201405 - 05/31/11 07:04 AM Re: help needed with a nano dc to ac rotary converter [Re: odbob]
Alan Belson Offline
Member

Registered: 03/23/05
Posts: 1801
Loc: Mayenne N. France
You need a 3 phase induction motor. A 1-ph motor of any sort will not work, IMHO, due to the "trick" way it produces rotation. 3-ph I motors exist in fractional sizes down to 1/3HP, [new about $70] - old well pumps may be a good place to start looking for a salvaged 3-ph model. Your set up is similar to the methods used to create 3-ph from 1-ph to drive old machinery, using a motor to drive a 3-ph slave induction motor as a generator. In such setups, the 3-ph motor needs to be BIG. For experimental purposes it won't be necessary to rate the 3 ph motor against your drive motor- a 40W dc motor will easily spin up a 3hp 3-ph, as long as you don't expect it to make 2kw crazy
To operate anything in your home at anywhere near your domestic voltage, you must control the motor/generator speed. With variable demand, a series wound dc motor will be useless, as at no-demand it could spin up the assembly to dangerous voltages. You need a shunt-wound dc motor which you can speed regulate efficiently with a variable pot. on the field windings. [Such small shunt w. dc motors are used to start autos, motorcycles or lawntractors].
With this set up you will be lucky to get within 10% of poco volts and hertz and will need a tacho to set up the machine rpm. With these limitations, you can't use your juice to run anything electronic, so it's best use would probably be auxiliary power on domestic hot water, where you can make year round savings. I assume you will make a larger machine once you are au fait with the technical challenges, because the only 40W use will be 3 x 15W filament lamps. Best of luck!
_________________________
Wood work but can't!

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#201474 - 06/03/11 03:52 PM Re: help needed with a nano dc to ac rotary converter [Re: Alan Belson]
odbob Offline
New Member

Registered: 05/17/11
Posts: 8
Loc: england
Thanks Alan, seems I am trying to do the impossible! the reason for keeping it small was two fold, 1:- I just wanted to send a small amount of single phase surplus power from my battery into the existing house single phase electrical system and, at 24/7 operation this would release in the order of 360 kW per year
2:- I had in mind to use a constant natural water supply, by adding a tiny hydro system of similar size and type, again supplying electricity to the house.
From what you have said, it seems that this is simply not possible to do with a single phase motor and so the project unfortunately stops here, not a happy chappy, but thanks for explaining
Best regards Robert

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