With out properly balancing the phases with the correct value of capacitance you will just burn out the motor. The step up transformer will need to be big enough to support the motor as well. Power loss will be relative to the input circuit and transformers limitations more than anything.

If the balancing capacitance is right and the starting capacitance is close to optimal you can in fact run most unmodified three phase motors on single phase at their full rated power and with full rated starting torque without problems!

The catch is the starting amps are often very high which will limit how big of motor a particular circuit or power source can actually start.

That is what I said, the right capacitor is essential. Make sure you connect it in the right way too. There are six main schemes to make 3-phase motor work on 1-phase system without rewinding it. 3-phase motor has tree windings (they may be split into 2, 3, 4, 6...windings, but for simplicity I call them one winding for no matter how they are split, they work on one single phase). When you connect the motor to one phase, what you basically do, you put two windings on one phase (no matter in parallel or in series) and with the capacitor you create current phase shift for the third winding. It will work, but because this is not really 3-phase system, you will loose some power. 3-phase motor has the same maximum torch all the time, when 1-phase motor changes its power and torch 60 times for a second from its maximum to zero; we cannot notice that visually, but it does happen, that is why we essentially loose power. There is another way to get all the power from 3-phase motor on 1-phase system: you connect one winding directly to the phase, second winding through capacitor (it will make current shift forward), the third phase will be connected through inductive impedance, which will make current lag (we can not do it with the capacitor only, because we need 120 and 240 degrees of current shift; capacitor can do only up to 180 degrees, that is why we need to shift current in different direction). In such a manner we can create three real phases, and will achieve almost the same power. I say "almost" because in different modes the motor will need different capacitor size and inductive impedance. So, even

Do not use transformer solely to "step up" the motor. If you do it, you will increase not only voltage, but current as well, because the impedance remains the same. To increase the voltage safely, you have to increase the motor impedance proportionally. You can do it increasing frequency with VFD or reconnecting windings from parallel to series. Otherwise you may burn your motor. Variable Frequency Drive is the best to use... well, it is not as cheap as capacitor...

Quote

"There is another way to get all the power from 3-phase motor on 1-phase system: you connect one winding directly to the phase, second winding through capacitor (it will make current shift forward), the third phase will be connected through inductive impedance, which will make current lag"

It seems to be only possible on a "star" winding type induction motor:

one line to the central point,

the other line to the first winding directly, through a capacitor to the second winding, through an inductance to the third.

Dear Yuri B

I am not sure to obtain three phases power at one phase connection in such way.

(If I understand correctly ) One phase direct, the second phase via capacitor and the third phase via an inductor.

The phase angles for three phases is 120 degrees as well as known by yourself.

Windings for each phase are distributed in the motor frame in this configuration. They are 120 degrees to each other. (Not physically, but electrically 120 degrees).

However time lag at the capacitor or at the inductor can be maximum 90 degrees.

This means you are offering a system, of which one phase at 0 degree, second at +90 degrees and the third phase at -90 degrees.

Furthermore this assumption is only theoretic. The motor itself is also an inductor, and windings must be taken into consideration.. (Such as Capacitor + inductor(Windings), Inductor+inductor(Windings).

I am not sure how to compose the vector diagram for this complex situation. It is along my knowledge. (Better I go back to the school)

However it is worth to make a trial. Trial approves everything in doubt

Am also usure of too many things after having worked for so many years. Would occupy for a week a well equipped electro laboratory of a good technical college.

Best regards

Yes Yuri B You're very right.

laboratory opportunities erases many doubts. The machines do not make comments or do not offer anything. Correct is correct and wrong is wrong.

However it's not possible here to be both in the school and also to be in the industry at the same time.

Sometimes I found the possibility of working in limited laboratories in the facilities of some big factories. It's my good chance.

By the way where in the world you are?

Regards

Nezih

You know, Yuri, it seams that you are right. I did not even think about that. About 12 years ago I was working at rewinding unit of our plant, and I thought that if someone in America is dealing with connecting 3-phase motor to 1-phase system, he probably knows the motor construction well. Maybe I was wrong. If the motor is connected in triangle and only tree leads are out, you may get inside the motor and reconnect it in any way you want; just bring all ends out and you will have 6 leads. Make sure to mark them. If you find the wire coming out of the slot you can mark it "beginning 1" or B-1, then using Ohmmeter find the end of the winding and mark it E-1. You have to find two other beginnings. They are located 120 electrical degrees apart. Electrical degree is not the physical one. If you do not know what it means, it is OK. You want to find ends and beginnings. Here is a hint. Just count slots between out coming wires. You will find two groups of three wires with similar number of slots between them (between 1 and 2 X slots, between 2 and 3 also X slots, between 3 and 1 may be much more, Y slots, but this is 120 electrical degrees between them). Call one group BEGINNINGS and another group ENDS. Use your multimeter to find windings and you will find that your ANDS group has exactly the same pattern as the first group, I mean that between E-1 and E-2=X slots, between E-2 and E-3=X slots and between E-3 and E-1=Y slots. Having these 6 leads out, you can connect them in any way you want, just make sure to connect all the ends together for a star and B1-E2, B2-E3, B3-E1 for triangle. Hope it helps. Andrey. By the way, where do you live?

One of this forms great orators once said

"You can also use capacitors to create a "phantom" 3rd phase from the other 2. This may be what you have seen, but what you probably didn't know is that this severely robs the motor of power capacity. The motor runs severely unbalanced and has a lot of what are called "negative sequence currents" because of the 1 phase power and because the sine waves are 180 degrees apart on the 2 "real" phases, compared to being 120 degrees out on true 3 phase. Negative sequence current produces negative torque in the motor and causes heating without doing useful work, meaning that it will severely overheat under load. For this reason, you must derate the motor by at least 50%, some even say 66%. So that means if you have a 15HP motor, you had better not need more than 7-1/2 or even 5HP out of it if you are going to do this."

Who said that?

Exactly! When done wrong it creates a weak and very inefficient motor thats just waiting to let the smoke out!

Just tossing some motor run capacitors on them between one of the feed lines and the third leg does work but not well! Unfortunately that also seems to be the industry standard of design for making a phase converter system. Its half assed, inefficient, has very weak starting torque, and is generally just hard on the motor itself.

For whatever reason who ever came up with the original design stopped just short of having done it right. And it became the standard design and thusly assumed correct way of making one. Close and sort of functional but not yet right.

I personally developed a different method (similar parts though) and have built dozens of converters that do allow most three phase motors to work normally and efficiently at their full ratings on single phase without problems. Many have put in years of regular service operating numerous second hand commercial machines and equipment with out problems.

The problem is that if you want them to be efficient and reliable they have to be properly electrically balanced in regards to the natural working characteristics of the motor and what load it has to work with. That part is unfortunately what tends to make it harder for someone to copy and implement without first having a good practical working knowledge of three phase motors and alternating current power systems.

For me I can not give anyone a simple yes and this is how its done answer. I will give you a definite yes, but its not that simple, as an answer though!

One of this forms great orators once said

"You can also use capacitors to create a "phantom" 3rd phase from the other 2. This may be what you have seen, but what you probably didn't know is that this severely robs the motor of power capacity. The motor runs severely unbalanced and has a lot of what are called "negative sequence currents" because of the 1 phase power and because the sine waves are 180 degrees apart on the 2 "real" phases, compared to being 120 degrees out on true 3 phase. Negative sequence current produces negative torque in the motor and causes heating without doing useful work, meaning that it will severely overheat under load. For this reason, you must derate the motor by at least 50%, some even say 66%. So that means if you have a 15HP motor, you had better not need more than 7-1/2 or even 5HP out of it if you are going to do this."

JRaef

Should I give myself a grade of Good Answer or you?

If you know how to do that right, the motor will lose from 15% to 25% of its power.

While stepping up from 230V to 400V make sure you stepping up with your frequency as well, otherwise you may lose your motor. That is good if you use VFD. What do you mean saying "stepped up"? Did you reconnect the windings from parallel to series? If so, you stepped it up to 460V; it will not lose its power if you connect it to 460V.

Dear Yuri B

I am not sure if any calculation on this specific matter is submitted in the books. (I've checked and could not find)

However I did make many trials on the matter of driving a three phase motor, at the single phase, by using a capacitor..

I remarked that the motor gives the 1/3 of it's rated power, with a suitable capacitor. Startup moment is always remarkabily small.

It is not safe to start the motor under load, such as a compressor driver. Better is to apply the load after startup is completed.

Sorry I can't give you a mathematical explanation for approval.

The attempt for trying a three phase motor with single phase in compulsary situations is not an engineering work. (Pls. accept this is only my personal opinion)

The way I do it has some basic rules of thumb numbers that will get you close enough to be able to fine tune your system with very favorable results.

Since you guys like numbers and math here you go!

For a typical 1 horsepower 230 volt three phase motor to work well on single phase you will need two AC motor run (power factor correction capacitors) of around 10 micro farads each. Preferably with at least a 300 VAC rating. and one 80 - 100 uF motor start capacitor. A 250 VAC rating is fine for it. These values are proportional to any size of motor within reason. A 25 HP motor would need around 250 uF of motor run capacitors and around a 2000 - 2500 uF of motor start capacitors. (starting current would be around 300+ amps for a motor that size though!)

Obviously if other voltages, frequencies, and power rating systems are used the numbers will change as well. They still follow proportionally is whats important. 50Hz needs 6/5 th's the capacitance. 400 volts needs 230/400 th's the capacitance and KW's needs 1000/746 the capacitance. The voltage reference values for tuning follow proportionally as well. All of these capacitor values are typical but not set in stone. Depending upon the characteristics of the motor and what type of loads it powers they can vary from as low as 50% to as high as 150% of the typical suggested values. So if an exact capacitor value is unavailable just pick its nearest value, its likely going to be close enough. The recommended 10 uF per hp at 230 VAC 60 Hz with a 80 - 100 uF per HP starting capacitance just works out as the common average.

Now for the actual build up part.

The two motor run capacitors will go from each supply line (L1 and L2) to the third phase line L3. That creates two LC tank circuits which use the rotors spinning motion to create a simple phase shifting autotransformer effect in the motor itself once its up to speed. This is what makes it possible for the motor to run up to its full working power rating. This is also how the three phase windings are able to work at the proper phase angle relationships to each other so as far as the physics are concerned the rotating magnetic field sees three sine wave power sources with a 120 degree phase angle separation between them.

Because every motor brand, model, and speed has a different inductance and efficiency you may need to change the two capacitors values up or down for peak power and efficiency for matching the actual motor to its load. This is done by reading the voltage across the L1 - L3 and the L2 - L3 lines themselves while the motor is at its typical load range. If the voltage is higher than around 115% of the incoming line voltage the capacitor values need to be bigger. If its lower than 90% of the incoming line voltage they need to be smaller. And always keep them equal. This is keeping the LC tank circuits tuned to near the line frequency and is what keeps the current loads between each phase balanced using that phase angle shifting autotransformer effect.

The start circuit.

To start the motor use around a 100uF per HP capacitor on either leg between L1 - L3 or L2 - L3. To set the direction of rotation connecting to one leg (L1) starts it rotating one way and connecting to the other leg (L2) starts it rotating the other way. Obviously disconnect it once the motor is up to speed. This can easily be done with a common potential relay that is common to HVAC applications. However that relays release voltage likely will need to be adjusted as well in order to prevent it from either releasing too soon or not releasing at all. If it doesn't release when the motor is up to speed its voltage needs to be dropped . If it chatters or releases too soon it voltage needs to be raised. Most potential relays can be disassembled and adjusted internally. For setting the basic start circuit use a potential relay thats rated for a release voltage range near your motors third line running voltages. That is if both the L1 - L3 and the L2 - L3 voltages are around 230 - 250 volts when the motor is working normally the potential relay should have a release voltage of 240 volts or slightly less. Lowering the starting capacitance value will drop the starting current draw and torque and will affect the potential relays release voltage set point as well.

Assembling one is reasonably easy and thats why I give the information out for free! However tuning it to work efficiently and reliably is the hard part!

My money comes from knowing how to do the hard part.

In reply to comm. 13th and 14th

Hello Nezih from Turkey and Andrey (Russia? Ukraine?). Sorry to be late - thought new comments on this thread ended. I live in Latvia.