Inverter question.

[MrBean]

I have a single to three phase Inverter. What would happen if I were to connect one terminal of a single phase AC motor to one leg of the inverter output, and the other terminal were connected directly to the neatral wire of the mains supply?

Basically, can I run a single phase motor from a 3 phase VFD?
Would it unbalance the loading on the Drive?
Would it matter if it did?

Thanks.

Regards Terry.....

[Al_The_Man]

Definately not worth trying, in all probability (hopefully) there is no reference from the 3ph output back to the neutral, as the way VFD's are constructed, the input is a three phase rectified to high capacity DC, then the three phase variable frequency out is produced by solid state switching, which may or may not be isolated from the incoming main supply. If it is isolated you may be lucky and nothing happens, if not isolated, you blow the VFD.
In the case of your VFD it is the same principle but single phase in.
Al

[TinkerDJ]

Probly not going to work with the 3 phase vfd. Try and find a 3 phase motor. If you can give the specs on the vfd, are you using an inverter to operate the vfd or is it a combo system. What voltage is the vfd output? I've seen 3 phase motors go really cheap on ebay. As well if you find local motor winding companies have them all the time, some very cheap.

The idea i had for a single phase to 3 phase vfd system was to run my lathe with a 208 3 phase motor and have the nice control that an ac vfd can give. The one system i ran across even has a nice setup that can be controlled from a computer program.

[ViperTX]

MrBean,

The question is Why would you want to do that?? Makes no sense at all since you have a single phase source (your wall outlet).

[unterhaus]

You would want to put connect both sides of the motor winding to the inverter output. It could be made to work by the drive's developers, but I think the way the control systems are designed the inverter would go nuts trying to figure out what the motor was doing and fault. Single phasing will kill three phase motors, so I would think most VFD's would refuse to function.

[MrBean]

Originally Posted by ViperTX MrBean, The question is Why would you want to do that?? Makes no sense at all since you have a single phase source (your wall outlet).

The wall outlet doesn't have a built in speed control. I dropped my pulley shaft from my OLD lathe. The 5 step pulley wheel (one piece, cast), cracked. I've got a Eurotherm 601 drive from work. I guess I'll have to hunt around for a three phase motor to go with it.

Thanks.

[Al_The_Man]

Incidentally there is another thing to consider, if the motor is a standard split phase induction motor that is non-permanent split phase connected, i.e. has a centrifugal switch for the starting winding, these motors are not even suitable for VFD that are made for single phase use, a single phase VFD requires a motor that is connected permanent split phase. And even these have a problem of dropping out of run when operated at low speed. Definitely the way to go with a VFD is a 3ph motor.
Al

[HuFlungDung]

Single phase motors are wound differently than three phase, there is no way to substitute the type of power source without converting it correctly.

Without a starting winding, a single phase motor will behave like a solenoid, twitching from one pole to the next, as there are only two states to the magnetic field, North and South.

A three phase motor has three different currents flowing in three sets of windings. "Phasing" means that the relationship between the three currents is one of timing: they are still alternating AC, but the waveform of each current is 1/180 of a second behind its neighbour. There is a natural progression to the way the magnetic field builds and dies between the three magnetic poles which induces the rotor to begin rotation in a particular direction. Thus the motor self starts and runs.

The single phase motor needs to be tricked into running in a particular direction, because its two field poles do not induce a particular direction for rotation, until the rotor has been spun a particular way. An extra starting winding is part of a single phase motor and provides the delayed action required to make it seem as though the magnetic fields are rotating in a certain direction around the stator. The start winding cuts out after a while through means of a mechanical switch which is activated by centrifugal force when the rotor is near running speed.

My dad used to have a small power brush running on a single phase motor without a functional start winding. Just plugging it in and giving it a spin in either direction would start the motor running.

How does varying the frequency of the current supplied to a 3 phase motor change the speed? Well, you'd have to visualize the magnetic fields in the stator housing like a Moire pattern. Of course, the fields are not moving at all, they just appear to be moving because of the sequential build and dying off of the fields (think of the strobe effect of a movie camera shooting a wagon wheel). So, changing the frequency changes what appears like the speed of "rotation" of a Moire pattern.

Single phase windings are more like a binary state: a red and a green light flashing on and off if you will. Changing the frequency changes the speed at which the lights flash, but there is no "moire effect" in the stator fields.

[Al_The_Man]

Originally Posted by HuFlungDung but the waveform of each current is 1/180 of a second behind its neighbour. .

Or 1/150 where mr Bean comes from.
Al

[HuFlungDung]

That's right, too, Al. I've often wondered if North America has a 16% productivity advantage because of the difference in standard powerline frequencies

[kong]

Originally Posted by MrBean The wall outlet doesn't have a built in speed control. I dropped my pulley shaft from my OLD lathe. The 5 step pulley wheel (one piece, cast), cracked. I've got a Eurotherm 601 drive from work. I guess I'll have to hunt around for a three phase motor to go with it. Thanks.

I recently picked up a decent Crompton motor on ebay for £15, just make sure you can run it on 240v three phase. I think the wiring term is star-delta for 240v but I could be wrong!

[Al_The_Man]

Originally Posted by kong just make sure you can run it on 240v three phase. I think the wiring term is star-delta for 240v but I could be wrong!

The term star-delta or (wye delta in N.amer.) refers to two connection arangements. i.e. the motor is either star or delta connected, in the star mode is the low voltage, lower rpm mode. delta will be the the higher rpm mode, British motors tend to bring out all 6 leads on motors which offer the choice of either.
Star-delta starting arrangement is also used on very large motors, especially when started under load, the motor control has either a manual change over switch or a relay or other method of switching the motor over to Delta once it is up to the Star fed speed.
Crompton is a very old manufacturer and used to build excellent motors.
Al