"generating" 3 phase with an engine?

[Henrik]

So I've been looking at rotary 3 phase converters this morning, and I have some dumb questions to ask regarding using a motor as a generator.

Is the 3rd phase generated "automatically" by the rotation of the motor?

I.e. if I spun a 3 phase motor to its rated RPM, would it generate voltage on all three legs, or does the rotary converter trick require 220v applied to two legs on the motor?

I.e. could e.g a petrol engine be used with a 3 phase motor to generate 3 phase electricity, or is it not that simple?

 

[rtbcomp]

Slightly off topic, you could derive 3 phase from an ordinary car alternator, you'd have to faff around with the regulator but it may be an option.

 

[madkayaker]

It's not quite as simple as spin a motor and you get voltage! You need to excite the windings to get the whole thing going this can be done by sticking a car battery onto windings before you spin it up and once going it self excites. Or you get a pernimant magnet motor but there often expensive.

That's as far as my thinking got as I had an old Landy engine not doing anything and a 10hp motor at work but I gave the idea up as bad one for now

 

[awemawson]

There are many third world village electricity supplies derived from a large three phase motor spun by an engine or in some cases a water turbine. If I remember correctly to get it going you need a capacitor across two legs of the motor

http://www.smallhydropower.com/induction.htm

http://www.youtube.com/watch?v=gYwGvSYpn8Q

 

[madkayaker]

It's worth mentioning at this point the supply company's take a very dim veiw of generators that are capable of back feeding onto there equipment as many of there staff have been killed that way

 

[@Turbo]

surely isolating it would prevent it going back down the lines?

 

[madkayaker]

Yes but that also includes isolating the earth as many generators have a floating earth and on pme system that can be a problem when the electric board are working on the system.

 

[brightspark]

surely isolating it would prevent it going back down the lines?

by rights if the generator is wired in to the mains as a standby it would have a changeover switch to enable the use of one or the other . no chance of both being conected at any one time . quite a lot of farmers were using generators using a 13 amp plug on genny and backfeeding through a socket to power the buildings or house and just turn the fuse box off only sometimes they didnt turn the fuse box off and backfed power down the supply lines and zapped fitters working up a pole miles away the board sent everyone a notice on the safe use of gennys to help stop it

 

[madkayaker]

by rights if the generator is wired in to the mains as a standby it would have a changeover switch to enable the use of one or the other . no chance of both being conected at any one time . quite a lot of farmers were using generators using a 13 amp plug on genny and backfeeding through a socket to power the buildings or house and just turn the fuse box off only sometimes they didnt turn the fuse box off and backfed power down the supply lines and zapped fitters working up a pole miles away the board sent everyone a notice on the safe use of gennys to help stop it

And I have heard of people being cut off for failing to comply with regulations for generator use

 

[wheresmejumper]

wire 1 or 2 specific outlets for the genny,easiest and safest

 

[the snooper]

It's worth mentioning at this point the supply company's take a very dim veiw of generators that are capable of back feeding onto there equipment as many of there staff have been killed that way

did i miss something ? cant see any metion of back feeding

backfeeding is illegal


even if you could get the motor to produce three phase power it may well not produce enough amperage to run very much at all, you also have the headache of finding a suitable engine and then need a way of governing the engine to keep a constant 50hz frequency while maintaining the correct voltage, id say its a non starter really.
much easier to look out for a cheap 3 phase genny and even then it depends on your power requirments

 

[madkayaker]

did i miss something ? cant see any metion of back feeding

backfeeding is illegal


even if you could get the motor to produce three phase power it may well not produce enough amperage to run very much at all, you also have the headache of finding a suitable engine and then need a way of governing the engine to keep a constant 50hz frequency while maintaining the correct voltage, id say its a non starter really.
much easier to look out for a cheap 3 phase genny and even then it depends on your power requirments

The website that was linked to was talking about connecting to the mains so I felt it was worth talking about. Also many generators get pulled out of the shed during mains outage and "plugged in" to a house with little understanding of the issues

 

[normspanners]

Slightly off topic, you could derive 3 phase from an ordinary car alternator, you'd have to faff around with the regulator but it may be an option.

Fequency would be far to high for normal mains ( to get any power out of it )
I have rewound a few stators to get 240v but use dc, they are suprisingly powerful

 

[CompoSimmonite]

What you need is a tractor PTO powered three phase generator. Then it'd simply be a case of making an adapter to connect the flywheel of the diesel engine to the PTO input of the generator.

Paul H

 

[madkayaker]

What you need is a tractor PTO powered three phase generator. Then it'd simply be a case of making an adapter to connect the flywheel of the diesel engine to the PTO input of the generator.

Paul H

Tractors run at 560 rpm you might struggle to run the engine that low

 

[the snooper]

The website that was linked to was talking about connecting to the mains so I felt it was worth talking about. Also many generators get pulled out of the shed during mains outage and "plugged in" to a house with little understanding of the issues

ahh right i didnt see or look at the link

 

[KemppiFrog]

Where I used to live, power outages were a daily problem. They turned it off if it rained(!) and every Sunday it was off from 8 to 12 in the morning for 'maintainance'. Result was, loads of standby gensets. Most had switches that were either mains or genset. Problem was: when the power came back on, there was no indication in the house. I got to mod several with contactors that where on the mains side and when that quit, the genset powered up. As soon as the mains came back on again, the contactor isolated the genny and apart from the odd blink, life went on. No idea what the company thought about this, but at least it wasn't feeding back.
KF

The link was probably about feed in tarifs in the US where the power utilites have to accept your home grown volts (& amps).

 

[awemawson]

Tractors run at 560 rpm you might struggle to run the engine that low

540 actually to be pedantic !

 

[wyn]

tractor PTO's can also be 1000 rpm

 

[Henrik]

Hmm, whilst I can easily control my engine to run within a small span (e.g. I can pin it at 1500 rpm or whatever), I've just read somewhere that these induction generators are not efficient at starting induction motors (i.e. what controls most machinery), so based on that it's a non starter for me, as I'd want it to produce "real" 3 phase.

I was essentially hoping to strap a great big squirrel cage motor (~ 25hp?) to a petrol car engine, so that I'd be able to run e.g. 3 phase welders and "real" lathe's etc off it, but it seems it'd be easier just to run a big rotary converter and not go crazy on the power requirements.

I've got a 16 amp socket to play with, which in my world gives me about 3840W to play with, which obviously isn't enough for a welder on a rotary, but should be enough for most lathes, pillar drills, mills etc.