OT: Single Phase to 3 Phase Inverter

Slightly off topic, but I am scratching my head and trying to find a solution for this.

Basically a friend of mine wanted to set up a small workshop. He has some 3 phase equipment, but no 3 phase supply, so I suggested that he get a single phase inverter which would have a 3 phase output.

He bought the inverter and I installed it and set it up, so far no problem. He has a pillar drill, a band saw and a 3 phase motor he uses as a lazy susan. All of these work fine with the inverter.

He bought a new machine which rolls steel and we put this on the inverter but it won't work. When we try to run it, the motor sounds as if it is single phasing, so I checked it out with my meter and sure enough there is only 270v across the phases.

I thought it was a bad cable so I put my meter on the output of the inverter and only 270v across the phases again.

What baffles me is why everything else will work on the inverter but not this new machine.

How does the inverter work? how does it take single phase in and give out 3 phase and why am I only seeing 270v?

Any advice is appreciated.

Mark

Hi Mark,

What make / model?

What is the kW rating of the inverter vs. the kW rating of the machine?

The inverter is rectifying single phase AC into +- 400V DC , then using a couple of semiconductor switches (probably IGBTs) to generate 3 modified sine waves that are 120 degrees apart in phase and voila, 3 phase! Thats a very simplistic explanation though.

Some equipment is sensitive to "modified" sine wave output of inverters (the switching can generate all sorts of nasty byproducts, such as high dv/dt, short duration voltage spikes etc), although I wouldn't think a steel rolling machine would be, I assume it's just basically an induction motor?

Hi Saffa,

The inverter is a Eurotherm 2.2KW, and the motor on the rollers are 2.2KW, and you are right its just a standard induction motor.

The thing that I can't understand is why I am only getting 270v across phases and not the 415v I am expecting, and why would it work with the other machines and not this one, since all the motors are pretty much the same.

Mark

When you size an inverter to go from single phase to 3 phase you need to upsize it to account for the fact you're drawing more amperage from the single phase than the 3 phase. I doubt a 2.2kW drive will be able to start a 2.2kW motor with any kind of load attached.

What does the inverter display show for load %?

Mark
Any single phase inverters I've worked with gave 220v 3ph out not 415v. Most small motors are dual voltage 415v star, 220v delta. Check the motor connections because, from yor description the motor sounds starved of voltage.

Hey Mark,

Just a hunch, but what is your incoming single phase voltage. The rectified DC voltage is limited by the peak AC voltage, type of rectifier (full or half wave), the capacitor bank capacity, and the load on the inverter. That may be why you can only get 270 volts from the inverter. Try powering up the inverter with no load attached to see what the output voltage is.

The other motors may run fine on a lower voltage if they do not require as much torque.

First, your meter most likely can't measure voltage, amps, or hz on the drive output leads due to the complex and fast waveforms. Use the drive display to read voltage, amps, and hz. That will be accurate.

Second, the point above about the output probably being 230V three phase due to the input being 220-240V single phase is probably the answer to the problem.

Third, a drive rated for single phase input does not need to be oversized. It is built to power right up to its rated KW. Only a drive rated for three phase input being used on single phase needs to be derated by half.

Thanks for all the answers.

I haven't got the machine plugged in at the moment so can't get any readings from the inverter, but as soon as its back I'll plug it in and hopefully get the answers.

I suppose it makes sense that the inverter can't give me 415v when I am only putting 240v in.

Mark

Christoff84 said:

When you size an inverter to go from single phase to 3 phase you need to upsize it to account for the fact you're drawing more amperage from the single phase than the 3 phase. I doubt a 2.2kW drive will be able to start a 2.2kW motor with any kind of load attached.

Click to expand...

As DickDV said, You don't need to upsize your drive, if it's rated for 2.2kW output then it should deliver that (plus between 10-50% more for up to 30 sec, depending on the drive). What you do need to check is source side wiring and protection, because of course you're going to be drawing around 10A on your single phase, even if your motor is only drawing less than 4A per phase.

Another thing you need to be very careful about is your motor overload protection, especially if you're using this thing to run a number of different motors. the drive will have an internal thermal overload protection (software based), but that algorithm is based on whatever you've told the inverter the motor current was! There's a set up parameter for that.

This may also be what's causing your problem - if that current parameter is set lower than the nameplate rated current of the motor, the inverter may be limiting output voltage / frequency and running in a "current limiting" mode. Again, depends on what you've configured in the inverter. Read the manual and check the inverter display to see what frequency it's running at.

One last thing, If your motor size for the other loads is less than 50% of the drive rating (i.e. less than 1.1kW in your case), you need to ensure you've got appropriate thermal overloads fitted to protect the motors.

If you use a "true RMS" multimeter then you can measure the effective AC voltage even if voltage is not a clean sinus shape.

Anyway, the single phase VFDs I have seen, are for 3-phase 230V output.
So I am guessing you have to check the name plate of the motor, and hook it up for 230V. If the motor name plate says f.ex. 400V/690V then you are stuck. If it says 230V/400V, then it should work when connected in delta.

Q: How does the inverter work? how does it take single phase in and give out 3 phase and why am I only seeing 270v?

Drives take the incoming 60HZ (0r 50Hz) line voltage and rectify it to a DC bus. The drive then uses electronics to convert the DC voltage to 3 phase AC voltage.

If you have a drive designed to run from single phase source, then the drive should be able to supply its rated KW.

A trick that some use is to connect single phase (two wires) to a three phase drive (no wire on the third phase). The drive can still convert this to the DC bus but at a lower power output. The general rule of thumb is that the drive needs to be twice the KW of the motor when you coonect a three phase drive to a single phase source.

Question for you: What is the single phase voltage that you are connecting to the inverter? If you are connecting it to 220V, you will not get 480V out.

JesperMP said:

Anyway, the single phase VFDs I have seen, are for 3-phase 230V output.
So I am guessing you have to check the name plate of the motor, and hook it up for 230V. If the motor name plate says f.ex. 400V/690V then you are stuck. If it says 230V/400V, then it should work when connected in delta.

Click to expand...

What Jesper says

Your European motors at 2.2 Kw will most likely have the 230/400 volt rating with two current ratings and where the windings of the motor are most likely STAR (WYE) connected they need to be changed to DELTA connected.


Care needed
Now comes the next problem, if you shift this unit to where there is true 400 volt 3 phase supply, rather than the VFD inverter, (230 volt supply), then the winding connections will need changed back to Star connection

Gil47 said:

Care needed
Now comes the next problem, if you shift this unit to where there is true 400 volt 3 phase supply, rather than the VFD inverter, (230 volt supply), then the winding connections will need changed back to Star connection

Click to expand...

Gil,

This is one issue I have raised with my friend, and we are lucky enough to have 3 phase in the main factory, so we have decided to keep it as it is.

Thanks for all your advice.

Mark

JesperMP, I have personally witnessed testing of numerous "true RMS" and other PWM rated meters and have not found one that is anywheres near accurate. And that includes some of the best that Fluke offers.

In addition, at least in 480V and sometimes in 415V environments, the pulse voltages can easily exceed 1000V. Now, if you are using a CATIII 1000V rated meter, both you and the meter are at risk!

For those reasons, I have always taught that the output data should be read from the drive keypad rather than from the output leads. It's more accurate and a lot safer.

I agree, a good true rms meter no matter what the brand will not be able to read the voltage on a drive output of modern pwm drives. At times it may agree with what the drive displays at other times it will be off. however, it is useful to compare the 3 phase outputs to see if they are all the same. it may not read the exact voltage, but all 3 should read the same. and in most cases it will read withing 5% of the actual value which in most cases is fine. I do know a fluke 87 true RMS meter will do the job well enough to determine if it is a drive voltage or motor problem.