Motor VFD's

Hi
I know this is not a PLC type of question but I'd appreciate if you could answer this.
At our work we have a mixture of 1 phase and 3 phase VFD's each gives out 240 and 415 volts 3 phase respectfully (UK voltages). I came across a 1 phase inverter with the motor connected in star, in this configuration the motor would normally be connected delta.(delta 200 to 240 volts, star 380 to 415 volts)When I connected the motor in delta it did not appear to run with any difference. My questions are:

What difference would it make to the motor's running with 240v 3 phase connected to the motor in star or delta?

Are there ant advantages/disadvantages using 1 phase inverters over 3 phase inverters?

Thanks in advance for reading/replying to this.

Balls Ache

As you correctly mention, the star connection is the higher voltage connection. Running this motor at the lower voltage in the higher voltage configuration would be the same as starving any three-phase induction motor for voltage. You could expect that the motor would turn the same sync speed at no load since shaft speed is determined by frequency, not voltage.

The ability of the motor to produce torque when loaded is drastically affected by voltage, however. Being starved for voltage would cause the motor to slip down excessively in speed and soon enter the breakdown region where the current rises dramatically and torque deteriorates even further.

As for single phase and three phase input inverters, clearly, as output KW goes up, the input KW must also go up. A single phase supply must provide current that is equal to the KW divided by the voltage. In a three phase supply, the needed current is equal to the KW divided by the voltage and then divided again by the square root of 3 or 1.732. You can see that three phase current is almost half that of single phase and is therefore cheaper to provide. For that reason, drives over about 1.5KW are almost always three-phase sources. Also, at least in the US, while 240V single phase is quite available, 460V is not. I don't know about availability of your 415V single phase but I suspect that it isn't common either.

460 single phase

Dick

I am confused. If someone asked me to supply 460 single phase, I would supply one phase from 460 volt 3 phase drop. Why wouldn't this work? Certainly most plants in the United States have 460 3 phase.

Steve

Remember that 460V three phase is 460V from one phase lead to another. If you measure from a lead to ground on a properly balanced service, you will read 277VAC.

Since single phase power is normally arranged with one side at ground (the neutral side), single phase 460V isn't nearly as available as it would first seem.

Of course, you could install a single phase 460/460V transformer with the primary on the three phase lead to lead, and the secondary with one side grounded, but that requires a transformer.

On the other hand, at least in the US, split-phase 240V single phase is readily available from the very common residential service transformers. Their secondary is 240V end to end but the secondary center tap is grounded giving 120V from ground to each end.

Clever, isn't it! And potentially confusing!

We have another thread about grounding that also touched on this issue.

On the other hand, at least in the US, split-phase 240V single phase is readily available from the very common residential service transformers. Their secondary is 240V end to end but the secondary center tap is grounded giving 120V from ground to each end.

Click to expand...

BUT in commercial/industrial applications 240v transformers are used to supply single phase for motors etc with no center tap involved and neither side attached to ground. It amazed me you said that about 460v single phase being a motor specialist, alot of places use 480v single phase coils in motor starters eliminates the need for secondary wiring in many cases.

When it comes to across-the-line motor controls, I have very little experience, rs. My work background has been almost entirely with VFD's and soft-starts on induction motors.

In fact, that's mostly why I'm on this BBS. I know so little about PLC's and related controls that I joined in the hope that I might learn something.

It looks like you might have just done that for me. I would have never guessed that there are magnetic motor starters out there with 460V coils.

One other thing, rs. I don't believe I've ever seen a VFD that was designed for 460V single phase input. I know it can be done with derating, etc. but, 460V single phase is so uncommon that no one that I'm aware of even offers a drive for that service

As far as standard industrial applications go I have not actually used a single phase AC inverter but I think they exist, I know that there are single phase inverters that convert DC input to AC output. I have lived and learned there are alot of things out there being used I dont know about.

My comments were based on the fact that there are single phase applications using 460/480, for that matter up to 600v. Low voltage is understood to be less than 600v.

What is happening nowadays with the use of more automation and solid state electronic devices it is more or less becoming the standard to have 120vac or 24vdc as the control voltage. In the past, and in alot of cases still, the 3 phase is needed to drive motors etc and to minimize cost they used line voltage as control voltage...ie if its a 575/600vac motor they would use line to line for control. This isnt as common now (I personally never liked it either, be in a panel and short out or have a PB short out at 480...it can be scary). I think you mentioned being familiar with ABB, I was going to show you there manuals with coil listings but couldnt locate it. I can offer an AB catalog that shows coil voltages for their NEMA contactors ranging from 24v to 600v and the IEC from 24 to 380v:http://www.ab.com/catalogs/a113/tab3.pdf

The Bulletin 500, 505, 509 starters are still very common in straight run applications like pumps, compressors etc that run at a continuous speed all the time.

Hi
Thanks for all your replies.That answers the problem - motor slows down, lack of torque, due to 240volt 3 phase supply with the motor configured for 415volt 3 phase!
Here in the UK you can purchase 1phase VFD's up to 2.2kw above are all 3phase. I don't suppose you can have a 1phase VFD with 415volt 3phase out??? I am not sure if you can get a higher voltage out than you put in.
Anyway thanks again to you all.

Balls Ache

You can't get higher voltage out of a VFD than you put in. You can put a transformer ahead of the VFD to boost the voltage, but you have to make sure your VFD is rated for that.

The best bet is to pull a three phase supply of the proper voltage to the VFD and get the VFD and motor matched.

Tom Jenkins said:

You can't get higher voltage out of a VFD than you put in.

Click to expand...

Watch those "blanket statements" Tom...

Most drive manufacturers make small drives that will run a 230V 3-phase motor on 120V single-phase. Take A-D's GS1 Drives for example...

beerchug

-Eric

Well, Eric, this hyar old dog is ready to learn a new trick. I can't git the durn manual to download (breakin' in a new browser) but I'll check it out when I git back home.

I'd be interested in Dick's take on how this neat trick is dun, cuz on three phase drives I've always been told max voltage out is max voltage in!

On page 1-5 of the manual, in the specs for the 100 volt class drives. the output voltage is given as "Three-phase: 200-240 VAC (x2 of input voltage)".

You can't get higher voltage out of a VFD than you put in. You can put a transformer ahead of the VFD to boost the voltage, but you have to make sure your VFD is rated for that.

Click to expand...

Looks like AD included the transformer in the drive.

I don't have detailed knowledge of AB's drives but, in general, the drives I know of with 120V inputs and 240V outputs use a voltage doubler circuit in their front ends rather than a transformer.

As hp goes up, this rapidly becomes impractical due to the rapidly increasing requirement for current at only 120V, single phase at that!