Can you use a single phase motor to create three phase power.

[David345]

This might sound like a stupid question so apologies in advance.

So I had a random thought in my head.

So I was wondering if I was to use a single phase motor to spin a three phase generator to create three phase power, would you be able to create more power then the single phase motor is using?

I mean if this was the case then I'm pretty sure someone else would of thought of this idea and used it in the real world. So I guess it probably wouldn't work due to the laws of physics. Or maybe there's a loophole?

And again apologies for the strange question.

 

[James]

yes you can use a single phase motor to run a 3 phase generator.

you will not gain any free power though.
there will be losses in the motor, losses in the generator and you will end up with slightly less power than you are putting in.

it is a great way to generate 3 phase where it is not available and a cost effective way of doing it, so it is not worth ruing out if you are only using a small amount of equipment that requires 3 phase power.

 

[David345]

yes you can use a single phase motor to run a 3 phase generator.

you will not gain any free power though.
there will be losses in the motor, losses in the generator and you will end up with slightly less power than you are putting in.

it is a great way to generate 3 phase where it is not available and a cost effective way of doing it, so it is not worth ruing out if you are only using a small amount of equipment that requires 3 phase power.

So as the 3 phase power is being created and gets converted down to the correct voltage through a step down or step up transformer then I start to connect a load to the other end, would that then cause the magnetic field in the generator to increase making the turning force harder for the single phase motor to spin the three phase generator. Therefore causing the single phase motor to use more power in the end?

 

[plugsandsparks]

As mentioned its a very common way to derive 3 phase from a single phase supply. BUT it is impossible to get more out than you put in. Several Nobel Prize winning physicists proved this hundreds of years ago

 

[David345]

As mentioned its a very common way to derive 3 phase from a single phase supply. BUT it is impossible to get more out than you put in. Several Nobel Prize winning physicists proved this hundreds of years ago

Yeah that is true. Because you would get losses such as heat losses and vibration. And would the magnetic field around the generator increase when connected to a load causing the turning force to increase as well?

 

[rapparee]

I feel a Lucien answer coming on.

 

[Lucien Nunes]

So do I!

There are two rotary methods of producing 3-phase from single.

A single-phase motor driving a separate 3-phase generator is called (not surprisingly) a motor-generator. The 3-phase output can be of high quality, but because all the power must be converted from electrical to mechanical and back again, such units are large, expensive and relatively inefficient. For example, to generate 10kW electrical output might require 12kW mechanical input to the generator, for which the driving motor might require 13.5kW electrical input. Other advantages of the M-G are the ability to change voltage and frequency (with suitable drivetrain) since the motor and generator are independent.

A cheaper, more compact and efficient method is to power a 3-phase induction machine, which can be a regular motor or a purpose-built machine, with a single-phase supply. Such a device is commonly called a phase-converter. With the aid of capacitors to provide the necessary leading reactive current it can regenerate the third line and provide a 3-phase output with the same line voltage and frequency as the single-phase input. Not all the power needs to be converted, and the fraction that does needs converting only once, hence the overall efficiency can be higher. However, since the output circuit is not symmetrical either for real or reactive power, the output voltage regulation and symmetry and the phase angles are less accurate and significantly load dependent, leading to higher losses and possibly reduced performance in the application load motors.

Regarding the driving torque, yes you are correct that the torque needed to drive a generator is proportional to the electrical load taken from it. The shaft of a 10kW generator delivering 10kW is 10 times stiffer to turn than when the same generator is supplying only 1kW (ignoring losses). It is not due to an increase in magnetic field (although in a real machine that does have to increase to compensate for increased losses). Rather it is due to the force on the conductors of the output winding cutting through the field, which is proportional to the current flowing through them. In a practical AC generator it is actually the field system that revolves while the output winding remains stationary, but the reaction force on the rotor creates the same effect of producing a torque at the input shaft opposing the rotation.

 

[David345]

So do I!

There are two rotary methods of producing 3-phase from single.

A single-phase motor driving a separate 3-phase generator is called (not surprisingly) a motor-generator. The 3-phase output can be of high quality, but because all the power must be converted from electrical to mechanical and back again, such units are large, expensive and relatively inefficient. For example, to generate 10kW electrical output might require 12kW mechanical input to the generator, for which the driving motor might require 13.5kW electrical input. Other advantages of the M-G are the ability to change voltage and frequency (with suitable drivetrain) since the motor and generator are independent.

A cheaper, more compact and efficient method is to power a 3-phase induction machine, which can be a regular motor or a purpose-built machine, with a single-phase supply. Such a device is commonly called a phase-converter. With the aid of capacitors to provide the necessary leading reactive current it can regenerate the third line and provide a 3-phase output with the same line voltage and frequency as the single-phase input. Not all the power needs to be converted, and the fraction that does needs converting only once, hence the overall efficiency can be higher. However, since the output circuit is not symmetrical either for real or reactive power, the output voltage regulation and symmetry and the phase angles are less accurate and significantly load dependent, leading to higher losses and possibly reduced performance in the application load motors.

Regarding the driving torque, yes you are correct that the torque needed to drive a generator is proportional to the electrical load taken from it. The shaft of a 10kW generator delivering 10kW is 10 times stiffer to turn than when the same generator is supplying only 1kW (ignoring losses). It is not due to an increase in magnetic field (although in a real machine that does have to increase to compensate for increased losses). Rather it is due to the force on the conductors of the output winding cutting through the field, which is proportional to the current flowing through them. In a practical AC generator it is actually the field system that revolves while the output winding remains stationary, but the reaction force on the rotor creates the same effect of producing a torque at the input shaft opposing the rotation.

Thanks for the big reply. It's much appreciated. Just to clarify something though, I know I've mentioned this before I maybe mistaken sorry. So if the load increased on the output side of the generator would the turning force of the generator increase.

 

[Lucien Nunes]

Yes, in proportion.

For an ideal generalised generator or motor:
Torque is proportional to current
Speed is proportional to voltage
Mechanical power is equal to electrical power.

Run your car engine at idle and then switch on the rear demist and headlights together, and you will hear the engine note dip slightly, at least for a second while the ECU adjusts the idle valve to compensate. That is the effect of adding about 300 watts of load on the electrical side, absorbing 1/2 horsepower of extra mechanical power from the engine. (Car alternators are rather inefficient.)

It's hardly funfair season now, but go to a fairground, stand by the generator powering a large ride and hear and feel the engine come under load as the ride pulls away. As a generator man and vintage engine enthusiast I have to say there is nothing like the feel of a big old diesel opening its fuel rack to meet the demand. On a showmans steam road loco belted up and driving a ride with a high motor load like an ark, you can often see the governor move in response to every stud of the rheostat.

/dreamy

 

[David345]

Yes, in proportion.

For an ideal generalised generator or motor:
Torque is proportional to current
Speed is proportional to voltage
Mechanical power is equal to electrical power.

Run your car engine at idle and then switch on the rear demist and headlights together, and you will hear the engine note dip slightly, at least for a second while the ECU adjusts the idle valve to compensate. That is the effect of adding about 300 watts of load on the electrical side, absorbing 1/2 horsepower of extra mechanical power from the engine. (Car alternators are rather inefficient.)

It's hardly funfair season now, but go to a fairground, stand by the generator powering a large ride and hear and feel the engine come under load as the ride pulls away. As a generator man and vintage engine enthusiast I have to say there is nothing like the feel of a big old diesel opening its fuel rack to meet the demand. On a showmans steam road loco belted up and driving a ride with a high motor load like an ark, you can often see the governor move in response to every stud of the rheostat.

/dreamy

Ahh right makes sense now. Thanks for clearing that for me. Much appreciated.

 

[DPG]

Yes, in proportion.

For an ideal generalised generator or motor:
Torque is proportional to current
Speed is proportional to voltage
Mechanical power is equal to electrical power.

Run your car engine at idle and then switch on the rear demist and headlights together, and you will hear the engine note dip slightly, at least for a second while the ECU adjusts the idle valve to compensate. That is the effect of adding about 300 watts of load on the electrical side, absorbing 1/2 horsepower of extra mechanical power from the engine. (Car alternators are rather inefficient.)

It's hardly funfair season now, but go to a fairground, stand by the generator powering a large ride and hear and feel the engine come under load as the ride pulls away. As a generator man and vintage engine enthusiast I have to say there is nothing like the feel of a big old diesel opening its fuel rack to meet the demand. On a showmans steam road loco belted up and driving a ride with a high motor load like an ark, you can often see the governor move in response to every stud of the rheostat.

/dreamy

Oohh - rheostats, governors. Thank flip he didn't say knife switches or I may have got over excited. On slate panels. Mmmmm. Nice!

 

[StephenRowley]

BUT it is impossible to get more out than you put in. Several Nobel Prize winning physicists proved this hundreds of years ago

If there was possible to break this fundamental law the universe would just be one giant infinity hot fire ball.

 

[Lucien Nunes]

@DPG

Watt flyball guv'nors on Corliss-valve triples,
Dynamos whirring with tuneful slot ripples,
Carbon pile rheostats tensioned with springs;
These are a few of my favourite things...

 

[Edtwozeronine]

One of my lecturers did an experiment (not in the time I was at the college several years before) and they got a form of 3 phase out of a cars alternator. He did try and explain it to us students but I feel we all got rather lost...

 

[StephenRowley]

One of my lecturers did an experiment (not in the time I was at the college several years before) and they got a form of 3 phase out of a cars alternator. He did try and explain it to us students but I feel we all got rather lost...

Car alternator are three phase generators with a full bridge rectification.

So you just connect before the diodes.