Thread: 3phase motor

Simple answer ... NO.
There are devices that allow you to run certain 3 phase motors from a single phase supply but you cannot economically convert 3 phase to single phase.

The electronic black boxes that can allow you to run a 3 phase motor from single phase, usually with speed control, reversing and other niceties are called a variable frequency drive, or VFD, from memory.

Using the forum search at the top of the page should yield a fair bit of previous discussion & suppliers. Search done here.


Cheers.....................Sean

Thanks gents

Yeah found out it costs about $400 to get a converter - not very economical

Actually, you can run a 3 phase motor on single phase . Of course, it will have a higher starting torque with a fancy black box but even so,it will run quite nicely.
You only use 2 windings and a capacitor, as per the attaching drawing.
It depends on the application as to the suitability of using a 3 phase motor this way.

Apart from causing an increase in motor vibration & the potential of extremely high voltages being present across certain windings as a result of a possible incorrectly sized capacitor, which can both culminate in catastrophic motor failure & perhaps personal injury, there is nothing wrong with this idea. But what a waste of a perfectly good & efficient 3 phase motor, considering that they are significantly more efficient than single phase motors.

You'd be better off selling the 3 phase motor & buying an appropriate single phase motor for the job required.

Where I work,we had motorized valve actuators, using just this setup, that ran for 20 years that I know of, with no problems.

Originally Posted by Sterob

You only use 2 windings and a capacitor, as per the attaching drawing.
It depends on the application as to the suitability of using a 3 phase motor this way.

Please don't try this.

Originally Posted by elkangorito

Apart from causing an increase in motor vibration & the potential of extremely high voltages being present across certain windings as a result of a possible incorrectly sized capacitor, which can both culminate in catastrophic motor failure & perhaps personal injury, there is nothing wrong with this idea. But what a waste of a perfectly good & efficient 3 phase motor, considering that they are significantly more efficient than single phase motors.

You'd be better off selling the 3 phase motor & buying an appropriate single phase motor for the job required.

Well said El K

Hi Timbo,

A true 3 phase motor has its coils connected in star mode for accelaration and in triangle mode for operation and load. In triangle mode every coil receives the full 415 Volts, in star mode every coil receives 415 devided by the square root of 3 equals 240 volts. This lower voltage reduces the current surge which would be huge when starting from zero at full power. With a non-rotating rotor an induction motor behaves like a transformator with a short-circuited secondary winding, meaning that the power absorption can reach 5 to 6 times the nominal load current.

Feeding a 3 phase motor on 240 Volts single phase means that only one coil can be provided with mains power, and then at far too little voltage. With only one 415 Volt coil active on 240 Volts, the motor will have no power at all and will have to be started up by hand or by a capacitor connected to the other coils, providing the necessary time shift to give some sense of predominant rotation. The loading and unloading of the capacitor across these coils (other than the mains fed single one) gives a current lagging behind the sine peaks of the mains current, at approx. 90 degrees when measured as a rotational angle. But the capacitor cannot make 415 Volts out of 240 and therefore starting torque of a usuable level will not be there. So it's the same there as with the one coil fed on the mains. The resulting motor output will probably be as poor as 5 to 10% of the output value as stated on the motor type plate, which means: useless.

There are, however, smaller 3 phase motors (0,18 to 2 kW output) with coils operating on 240 Volts. Their design stems from the fifties, when power levels in most countries around the world were cranked up from around 135 to 240 Volts, and for a while old generators providing 135 single phase and 240 three phase, were kept operational along newly built generators for 240 single phase and 415 three phase. For circumstances where both three phase levels could be found (factories, boiler rooms, pump rooms), a three phase motor type was devised for both 240 volts and 415 Volts operation. Being of small to medium power, these motors had acceptable power peaks at startup and could be operated in triangle mode from standstill. The "square-root-three-rule" came in handy to operate the 240 Volt coils on 415 Volts, meaning that the motor didn't need to be replaced in boiler or pump rooms where the new higher three phase voltage was expected to be introduced soon.

Should your motor be of such type (the type plate than states "240 triangle/415 star" (it doesn't say so in letters, but as a triangle symbol or a Mercedes-Benz style star symbol), than you could try the Steinmetz alternative. In that way one coil is properly fed on 240 volts and the other two are capacitor fed. There is still considerable power loss as measured against true 3 phase feed (about 40 or 50% output loss) and efficiency and starting and stalling torque are very poor. With light use, this needn't be a problem. But with nominal load, the motor will probably suffer of rpm-loss and cooling problems and will eventually burn. Especially the single coil operated directly on mains current is at risk of overheating. When heat and cooling are critical, rpm loss during load must not be more than 5 to max. 10% of no-load rpm, especially during minutes or more. This is because reduction of fan capacity is the square of rpm loss (e.g. at half rpm the fan's air volume will only be a quarter of that at full rpm). Also, the poorer efficiency of single phase operation means that more heat is generated anyway.

Should Steinmetz mode be an option, than the rule of thumb for the capacitor value is approx. 80 micro-Farads for every kiloWatt of output power. The rise of heat in the coils during operation wil have to be watched and together with overall performance (50 to 60% of the performance when operated in true 3 phase), the best capacitor value can be determined and fine-tuned. I once adapted a 0,75 kW geared motor on a small conveyor belt to running in Steinmetz mode. Good care was taken that the belt transported about half the mass that it did before, since the motor was supposed to deliver 1/2 HP instead of 1 HP. It ran happily for days with very little overheating of the most loaded coil (the one fed directly by the mains) but was later converted back when three phase power was again available. So i wouldn't know about Steinmetz reliability in makeshift applications when run year after year.

About the trouble of going through finding and purchasing a static electronic phase converter, i would just say: when you consider the cost of such a solution compared to the availability of cheap 3 phase motors nowadays, than why choose that option?

I scribbled some diagrams in Microsoft Paint to go with this story.

As far as Steinmetz mode in modern induction motors is concerned; many single phase capacitor motors meant to run in both directions, are wound in a three phase lay-out, with their capacitor connected the Steinmetz way. That way, all coils can have the same wire gauge, making the winding job easier and cheaper. Also the way the power and the condensor are connected to the coils in triangle, can only result in a wrong sense of rotation but hardly in coil damage, should the mains wires be connected improperly. Capacitor motors with conventional starter coils not wound the triangle way, however, have thinner wire gauge in those starter coils, resulting in very quick burn-out when main and starter coils should be inadvertantly mixed up when connected to the mains.

Success and greetings!

gerhard

Hi Gerhard,
It's obvious to me that you are not a "native English speaker". This is not to say that I think your comments are in any way wrong, although I do wish to clarify some of your points, if I may. I find your comments to be very informative (I've never heard of Steinmetz) & have learnt something from your post. Please forgive me as I will edit your post as follows;

My comments in blue.

Originally Posted by gerhard

Hi Timbo,
Should your motor be of such type (the type plate than states "240 triangle(Delta)/415 star" (it doesn't say so in letters, but as a triangle symbol or a Mercedes-Benz style star symbol), than you could try the Steinmetz alternative. In that way one coil is properly fed on 240 volts and the other two are capacitor fed. There is still considerable power loss as measured against true 3 phase feed (about 40 or 50% output loss) and efficiency and starting and stalling torque are very poor. With light load use, this needn't be a problem. But with nominal load, the motor will probably suffer of rpm-loss and cooling problems and will eventually burn out. Especially the single coil operated directly on mains current is at risk of overheating. When heat and cooling are critical, rpm loss during load must not be more than 5 to max. 10% of no-load rpm, especially during minutes or more. This is because reduction of fan capacity is the square of rpm loss (e.g. at half rpm the fan's air volume will only be a quarter of that at full rpm). Also, the poorer efficiency of single phase operation means that more heat is generated anyway.

Should Steinmetz mode be an option, than the rule of thumb for the capacitor value is approx. 80 micro-Farads for every kiloWatt of output power. The current rating (Amps) of this capacitor is very important. If too small a capacitor (Farads) is used, resonance may result, which can mean (in a series circuit) VERY high & dangerous voltages. It is always best to "oversize" the capacitor. The rise of heat in the coils during operation wil have to be watched and together with overall performance (50 to 60% of the performance when operated in true 3 phase), the best capacitor value can be determined and fine-tuned. I once adapted a 0,75 kW geared motor on a small conveyor belt to running in Steinmetz mode. Good care was taken that the belt transported about half the mass that it did before, since the motor was supposed to deliver 1/2 HP instead of 1 HP. It ran happily for days with very little overheating of the most loaded coil (the one fed directly by the mains) but was later converted back when three phase power was again available. So i wouldn't know about Steinmetz reliability in makeshift applications when run year after year.

About the trouble of going through finding and purchasing a static electronic phase converter, i would just say: when you consider the cost of such a solution compared to the availability of cheap 3 phase motors nowadays, than why choose that option?
Absolutely, wholeheartedly & completely agreed.

I scribbled some diagrams in Microsoft Paint to go with this story.

gerhard

Gerhard, I'm glad you spelled "condensor" the same as I do. What is it with these American dictionaries in Australia?

I would also like to add that posts similar to these (electrical), be posted in the " HOME RENOVATION PLUMBING, ELECTRICAL, HEATING, COOLING, etc
section.

Hi Elkangorito!

The additions you made are valuable, please leave them just as they are, they make my story much clearer. Indeed English is not my strong point. So there's nothing wrong with the blue alterations between the black, they will benefit everyone.

Thanks & regards!

gerhard