Thread: single to 3 phase vsd confusion.

RC.
basically half way through the process of building a prototype device, which through my hobby, had discovered a relationship that deals with force, balance and resistance,
won't say much more for obvious reasons.

The fact is that perhaps it was the experience with using my previous lathe, that has scared me off purchasing another lathe around a similar size.
no longer have the lathe due to defects causing premature wear and tear.
a DOC of 2mm (4mm o.d.) would do.

not having a lathe at this point is frustrating, and would purchase the al-1000c, though their out of stock.

wasted enough time in searching for a S/H lathe or thinking about importing a lathe and decided against it,
the main options at the moment, is either waiting for the al-1000c to arrive, or purchase the cl-38a.

prefer a cheaper lathe, though with limited choice, the al-1000c would be the only other option with a 40mm bore, noticing the price difference from that lathe to the cl-38a, makes the cl-38a look tempting.
now when adding up all the extra costs involved with the cl-38a, funds for the project would be reduced.

need to decide if the time the cl-38 saves in pissing about when performing certain jobs is worth it.

Originally Posted by Stustoys

I'd thought about that, but at full load isnt most of the power is going to the spindle and being turned into heat there? The heat in the motor is some % of the input(depending on the efficiency of the motor)?

Stuart

Motor manufacturer's seem to go to some trouble to cool your bog stock common, cap start, cap run, single phase motor with fans and fins on the side, or in the case of older motors blowing air through the windings.

Originally Posted by RayG

Hi Graziano, with multipole switching it also depends on how you do the switching, you can do constant torque or constant horsepower.

If you are interested I could dig out some reference books and scan the relevant pages.

Did you have a copy of "Electric Motor Repair" by Rosenthal?, that's about the only text I have access to at the moment.

Originally Posted by RayG

Hi PDW,
I suspect PeteF was talking about soft starters, the supply authority sets a limit of what size motor you can start dol, I think above 15-22 kw you will find soft starters are the preferred option these days.

Doesn't a VFD function as a soft starter anyway with it's slow starting and programmable current limits?. At least a lathe won't need huge starting currents to get going compared with, say an air compressor.

Originally Posted by RayG

PS Stuart, efficiency of cap start single phase motors is something I vaguely remember... I'll see what I can find.

I'd be interested also, as I can't find anything on the net that's specific to single phase induction motors.

Originally Posted by RayG

Hi PDW,
I suspect PeteF was talking about soft starters, the supply authority sets a limit of what size motor you can start dol, I think above 15-22 kw you will find soft starters are the preferred option these days.

Yeah, that explains the big cabinet full of electronic thingies that came attached to my 25kW air compressor motor. The seller told me it ran off a 20A 3 phase supply. Fortunately I didn't believe him as the cabinet was full of stuff with 32A breakers when I had a look. I gave it its own dedicated 6mm line to the sub board. Needless to say I don't fire it up to put air in the wheelbarrow tyre.

PeteF, there are all sorts of rules ) I do my best to balance load across the phases, never had any problems from the supply authorities. Using a 32A single phase line to feed a big single phase VFD *might* technically be breaking the rules but there are some big heat pumps/air conditioners out there fed via a dedicated line (in fact 1 good excuse for getting 3 phase installed is to tell them you want to but in a 3 phase air con plant so as to balance the load). I can't see that there is any difference. Main thing is to comply with *all* the rules governing wiring and over current protection. For anything over a domestic GPO, I typically run wire 1 size up from the standard ie 4mm for a dedicated 15A circuit, same for 20A and then 6mm for 32A. Wire & properly sized breakers are cheap in comparison with a fire.

PDW

Peter, yes I agree. The trouble also is, as I understand it, the supplier themselves will dictate what can and can't be put on their supply, and under what conditions. The brochure I attached after posting that comment was simply the first that I could find to support the statement. I'm not suggesting that would apply everywhere, maybe just something members may want to look into before rushing out and buying an XYZ lathe etc with a big motor and simply presuming they will be allowed to drop it on a single phase. On that note, I must confess all the larger VFDs I've seen were 3 phase in anyway, maybe I just need to get out more and just haven't seen others, but I'm not sure if all this is a moot point.

Likewise I over-specify the wiring when I do it. As you say, the difference is peanuts when considering the overall cost of things.

That's one big compressor you have there! It's a shame Anna and the family have already done our "Tassie Experience" recently as I'd have loved to call in to see your "humble" workshop. Do you have any idea of the CFM of a compressor that size? I checked mine and it free-flows 12 cfm (I'd previously thought it was only 8!), but it's too small. I think I may have to be happy with 16 cfm and simply look on at others like yourself with air-to-spare with admiration. I do like air tools!

Pete

Originally Posted by Graziano

A single phase motor under no load has poor efficiency compared to full load, in other words load or no load it sucks the same power, I could be wrong but I understand that under no load one winding fights the other winding to keep the RPM constant. An example I have are some Ebay single phase 2Hp motors, under no load they consume 6.8A current which works out to 1632 Watts. This is under no load for a motor that should use 1500W when loaded.

I believe that when a single-phase motor is not loaded the power factor drops significantly. So, while the current drawn may be near full rating, it is significantly out of phase with the voltage waveform. Thus the "real power" consumed by the motor is quite low. As Stuart pointed out, if this were not the case the motor would basically become a heater and get very hot. The "reactive power" is stored in the magnetic field and released back to the power distribution system each cycle. This causes problems for the power supplier and is the reason why some single-phase motors have a 'run' capacitor to increase the PF.

How all of this affects VFD rating I have no idea!

Originally Posted by jack620

How all of this affects VFD rating I have no idea!

Surely you'd be running a 3 phase motor off the VFD?

Originally Posted by jack620

I believe that when a single-phase motor is not loaded the power factor drops significantly. So, while the current drawn may be near full rating, it is significantly out of phase with the voltage waveform. Thus the "real power" consumed by the motor is quite low. As Stuart pointed out, if this were not the case the motor would basically become a heater and get very hot. The "reactive power" is stored in the magnetic field and released back to the power distribution system each cycle. This causes problems for the power supplier and is the reason why some single-phase motors have a 'run' capacitor to increase the PF.

How all of this affects VFD rating I have no idea!

Hi Jack,

Yes, that makes sense, and seperates the magnetizing current (no load current) from the torque producing current, a bit similar to vector drives (see attached ppt)


Hi Graziano,

I can't find anything in Rosenberg that relates directly to single phase induction motor efficiency, but the multipole switching is in table 3-143.

Here's a power point presentation on vector drives, the vector stuff is not strictly relevant to this discussion, but the slides relating to basic ac motor principles are.

The forum won't let me upload ppt as an attachment, so I've made it downloadable from here.

http://www.etech.net.au/TorqueProdACDrives.ppt

Regards
Ray

Pete F,
I don't understand your question.

Sorry, now I get it. Correct, you wouldn't be running a single phase motor off a vfd. What I meant to say was the single phase power discussion is unrelated to the vfd question.

Originally Posted by RayG

The forum won't let me upload ppt as an attachment, so I've made it downloadable from here.

http://www.etech.net.au/TorqueProdACDrives.ppt

Regards
Ray

Nice find Ray, it explains things very well.

Pete

Originally Posted by jack620

This causes problems for the power supplier and is the reason why some single-phase motors have a 'run' capacitor to increase the PF.

Isn't the run capacitor providing enough phase delay to get the motor running as a two phase motor?. I'm pretty sure it's to provide a higher torque motor than any concern about power factor, although they do have a higher power factor than a cap start only motor.

Originally Posted by jack620

How all of this affects VFD rating I have no idea!

Before things went off topic into single phase motors, the original poster mentioned that the lathe likely had a two speed three phase motor. If that is the case then at the lower speed it's likely to be running at half the rated horsepower. It would save a lot of trouble if he just ran the lathe at the lower speed and used less current. I was also wrong about the current demand halving, apparently as the RPM decreases with the switch from two pole to four pole and higher the motor's efficiency drops off as well so the current doesn't decrease as expected.

I'm attaching a couple of three phase motor data plates that illustrate the loss of horsepower as the speed goes down. One is off a Bantam lathe and the other is off a Wadkin power saw motor.

Originally Posted by Graziano

I was also wrong about the current demand halving, apparently as the RPM decreases with the switch from two pole to four pole and higher the motor's efficiency drops off as well so the current doesn't decrease as expected.

Hallelujah! Thank you

Originally Posted by Pete F

Hallelujah! Thank you

Ugh, so instead of the 50% I originally claimed, it's a 33% reduction in current demand. If the original poster's lathe has a similar motor, that's still a significant reduction without any modification to the lathe.

Originally Posted by RayG

Hi Graziano,

I can't find anything in Rosenberg that relates directly to single phase induction motor efficiency, but the multipole switching is in table 3-143.

Regards
Ray

Hi Ray, what I was looking for in Rosenberg's book yesterday was a table of expected current draws for single phase motors vs HP when under no load, if I find it I'll pass it on.

Cheers

Correct, my comments were not especially directed at you. My previous point was that motors are actually quite complex little buggers from an electrical perspective, one reason being that they're interacting between the physical environment and the electrical environment, and the interaction is two way. Distilling that down to explain some things with simple, albeit inappropriate, formula may well be quite convenient, however as it turns out it's also quite wrong. But hey, what would I know

Sorry for the confusion.

Pete