Single phase power to 3 phase motor

[Rick Christopherson]

That would be hard to believe since Besoeker both designs and manufactures VFDs.

In his garage or for a major brand-name manufacturer? There is a HUGE difference between making a few one-off devices and being a full manufacturer. That would explain why he is trying to source composite rectifiers in limited ranges.

 

[mivey]

In his garage or for a major brand-name manufacturer? There is a HUGE difference between making a few one-off devices and being a full manufacturer. That would explain why he is trying to source composite rectifiers in limited ranges.

From some of the pics I've seen, it would be a very nice garage.

However, I'll agree there is a difference between custom manufacturing devices and having an assembly line process.

 

[Rick Christopherson]

From some of the pics I've seen, it would be a very nice garage.

However, I'll agree there is a difference between custom manufacturing devices and having an assembly line process.

OK. I'm starting to get a better picture on where his comments were comming from and why. His comments would make a lot more sense coming from a one-off fabricator. However, this discussion and my comments are geared toward manufactured VFDs, not one-off designs. If that was the basis for his comments, then he should have clarified this critical information at the time he made them. No one in this discussion previously was suggesting a one-off VFD design. We were discussing off-the-shelf VFDs.

 

[texie]

You really think single phase motor and starter will cost less than three phase motor and VFD?

What is so unreliable about single phase motors?

You can probably purchase 3 to 5 pressure switches with integral unload valve for cost of a solenoid type valve.

IMO unless soft start is a necessity it is not worth trying to use the three phase motor.

If you want to see severe voltage drop problems at motor start come to some of the grain storage bin sites around here and see how starting a 10 hp direct coupled fan with a fan impeller that is maybe 30 inches diameter and weighs maybe 150 -200 pounds drops the voltage as compared to the 7.5 compressor that is up to speed within a second or two. I'm talking single phase here, and it is a very common install around here. Worse yet we are starting to see 15 and 16 HP rated single phase motors on some of these fans. I replace start capacitors on these fans more often than any other motor, they take a lot to get started

Well your last paragraph seems to me to be tough duty for a single phase motor that has inherently poor starting characterisics.

I'm just trying to have simple, civil discussion on ideas without everybody blasting each other and beating thier technical chest. In my OP and my follow up post #33 I was just trying to debate the pros and cons and maybe have better solution than simply going the easy way.

 

[Besoeker]

I get the impression that you are thinking these are discrete components. Possibly even thinking that the end-user needs to supply their own bridge rectification (but I may be misinterpreting what you said).

You are misunderstanding it. No, I do not expect the end used to supply the rectifier.

High power VFDs won't use composite (all-in-one) full wave bridge rectifiers. They will use discrete diodes that can individually be mounted to an effective heatsink.

Something like this you mean?

It's one limb of a rectifier we shipped out last week.

so they are not limited to a few ranges of diode power ratings.

Completely misses the point. The rectifier components, whether a single module with six diodes, three double diode modules or six individual diodes come in specific ratings. I can buy a DD121N or a DD151N. But not a 123N, a 124N.....etc.

As a matter of fact, your notion that there are only a few discrete sizes of power diodes makes this all the more important.

Now where where did I say there were only a few?
My point was that they have discrete ratings, not a continuous spectrum from zero to infinity. See my point about the DD121N

If, as you suggest, there are only a couple sizes, then it would be very likely that a 10hp and 15hp VFD both have the same size input rectifier.

Maybe some do.

I asked you whether the input rectifier was the limiting factor for using a 3-phase VFD driven by 1-phase input. If you believe that is not what limits the available amperage for the VFD under these conditions, I would like to hear about it.

I think it unlikely in most cases for the reasons given above.

However, I think you misunderstand how VFDs are manufactured.

Um.......well here's one we made earlier:

The largest we made was a bit over 6,600 kW. From component level up.
We make, also from component level, high performance drives and motors mainly for the machine tool industry. These are typically up to about 150kW and 20,000 rpm. And have to stop within one degree positional accuracy within one second. Awesome to observe by eye.

 

[Rick Christopherson]

Um.......well here's one we made earlier:

Well, the rest of the forum is talking about off-the-shelf VFDs for a small compressor.....you're not. You're talking about one-off designs and will have different answers from the same questions than off-the-shelf.

Back up and reread the discussion. Do your answers apply to standard off-the-shelf VFDs that someone would use for a 7.5 hp compressor driven from a 3-phase VFD supplied with 1-phase power?

 

[Rick Christopherson]

Preface: I didn't do a lot of homework on this because I am not getting paid to do so. However, I pulled up an Allen Bradley Powerspec 40 brochure. If I was getting paid to research this, I would spend more than the 2 minutes I did spend on it. I don't guarantee the specifications are accurate.

A 10 hp AB PowerFlex40 is rated for 33 amps, 3? at the input. This means the input diodes are capable of 33 amps per phase even when driven 1?. The input of 33 amps at 1? converts to 19 amps at 3? at the output, minus losses. This is well within the range of a good quality 3? 7.5 hp motor's nameplate FLA rating. (That nameplate rating should be confirmed from the actual motor.) By the way, these ratings are capable of 200% overage for 3 seconds, which is more than sufficient for start-up spikes.

The DC bus is going to be rated at or above 33x√3=57A, so at 1?, it will be operating at √3 below its rating. The output transistors are rated at 33 amps per phase, so at the 19 amps, they too will be operating well below their rating.

The bottom line is that all solidstate components will be operating below their rating, except for the input diodes. They will be operating at their maximum rating. That is why they are the limiting factor when choosing a 3? VFD that is being powered from 1? power.

Yes, the DC bus will likely have a ripple (depending on how good the input filter is), but that will remain the same regardless whether this was a 10 hp VFD or 15hp VFD. Contrary to Besoeker's suggestion, this is not the limiting factor in an off-the-shelf VFD selection.

 

[Besoeker]

Back up and reread the discussion. Do your answers apply to standard off-the-shelf VFDs that someone would use for a 7.5 hp compressor driven from a 3-phase VFD supplied with 1-phase power?

Of course they do.

 

[Rick Christopherson]

Regarding my original post:

Clean sheet of paper-the compressor is not yet purchased, Cast in stone is 240 volt 1 phase power available. 7.5 or 10 HP TBD

With the previous silliness out of the way, let's look back at your original question.

The only time this question should be of concern is when you don't have a choice of motor (i.e. a used compressor). In nearly all cases, when you have single-phase supply, you stick with single-phase motors where that option exists.

I'm confused when you speak of the cost of a starter. I have never heard of a packaged compressor that required an external contactor between the pressure switch and the motor. A 7.5 hp compressor is somewhere between retail and light industrial, so it should be ready to run right out of the crate. Nevertheless, the cost of a contactor is way below the cost of a VFD or any other form of phase conversion.

 

[iwire]

Well, the rest of the forum is talking about off-the-shelf VFDs for a small compressor.....you're not. You're talking about one-off designs and will have different answers from the same questions than off-the-shelf.

Back up and reread the discussion. Do your answers apply to standard off-the-shelf VFDs that someone would use for a 7.5 hp compressor driven from a 3-phase VFD supplied with 1-phase power?

Rick, really?

First you question if he knew how VFDs were made, now that he showed you what he does you say it does not count because it is too big.

Come on, just lower the testosterone and try to be helpful.

 

[Rick Christopherson]

Rick, really?

First you question if he knew how VFDs were made, now that he showed you what he does you say it does not count because it is too big.

Come on, just lower the testosterone and try to be helpful.

As a moderator, please don't comment on what you don't understand. Are you honestly suggesting that what Besoeker showed is comparable to off-the-shelf VFDs like an Allen Bradley? They are not the same, and that is why neither Mivey nor Besoeker have commented on that aspect of the topic. Unless of course you're suggesting the OP power his $2000 compressor with a $200,000 VFD. :dunce:

 

[iwire]

As a moderator, please don't comment on what you don't understand. Are you honestly suggesting that what Besoeker showed is comparable to off-the-shelf VFDs like an Allen Bradley? They are not the same, and that is why neither Mivey nor Besoeker have commented on that aspect of the topic. Unless of course you're suggesting the OP power his $2000 compressor with a $200,000 VFD. :dunce:

Rick as a moderator I am asking you nicely to tone it down. You can disagree with a view point without the condescension. If you choose to ignore this request expect to see your posts vanish.

 

[kwired]

Well your last paragraph seems to me to be tough duty for a single phase motor that has inherently poor starting characterisics.

I'm just trying to have simple, civil discussion on ideas without everybody blasting each other and beating thier technical chest. In my OP and my follow up post #33 I was just trying to debate the pros and cons and maybe have better solution than simply going the easy way.

Most customers I have worked for would rather spend $1000 to change the motor than to spend $3000 to install phase conversion equipment. Nothing too technical about that. Being in a farming area - especially grain farming a lot of work I do involves motors for grain handling or storage. I am around motors quite a bit from fractional HP up to 100 HP quite frequently. I don't claim to be a motor expert, but am around them enough that I have learned a lot about them. On motors in the 5 - 10 hp range I get about as many service calls for single phase as I do 3 phase. I don't feel the design of single phase is that poor that they should be avoided. I do have preference of some manufacturers over others because of designs of aux start winding switches. I have a bigger problem with some of the OEM's and their selection of the motor used. Open drip proof motors on equipment to be outdoors is my biggest complaint. I have had much more trouble with them than totally enclosed - moisture, debris, and insects get into these motors and cause trouble with the aux start switch. I bet at least half of my my service calls for motors involves this situation.

 

[Besoeker]

A 10 hp AB PowerFlex40 is rated for 33 amps, 3? at the input. This means the input diodes are capable of 33 amps per phase even when driven 1?.

Not necessarily so.
With the three phase arrangement each diode carries DC link current for one third of the time. For single phase each carries DC link current for half the time. It's also a different shape and, as you are probably aware, the forward characteristic of a diode is non-linear. Most semiconductor manufacturers of power diodes give sufficient information to calculate this for various operating modes including different conduction angles. Designers of power electronics, myself included, pay good attention to those details.

The input of 33 amps at 1? converts to 19 amps at 3? at the output, minus losses.

You need to take account of input and output power factor. You can't assume they will be the same.

The DC bus is going to be rated at or above 33x√3=57A,

Depends on the shape of the current. For level DC it is Iac*sqrt(3/2)

The bottom line is that all solidstate components will be operating below their rating, except for the input diodes. They will be operating at their maximum rating.

Not a given. There is not an infinite choice of diode ratings. It is very unlikely that the 33A is exactly the maximum rating of the diodes.

Yes, the DC bus will likely have a ripple (depending on how good the input filter is), but that will remain the same regardless whether this was a 10 hp VFD or 15hp VFD. Contrary to Besoeker's suggestion, this is not the limiting factor in an off-the-shelf VFD selection.

Ripple current in the DC link capacitor will depend upon, among other things, the value of that capacitor. And it's not at all likely that a 10hp drive will have the same value as a 15hp drive.
Whether it's a limiting factor is difficult to determine without actual figures. But the increased ripple current and decreased will do is reduce the L_op of the capacitor. It won't last as long.

 

[Besoeker]

that is why neither Mivey nor Besoeker have commented on that aspect of the topic.

I did in post #48.
There is no fundamental difference between say, a 2.2kW variable frequency inverter and a 220kW variable frequency inverter. It's just bigger.
Both convert fixed frequency to DC and invert that to produce variable frequency.
For the most part, both will use an uncontrolled rectifier to convert from the fixed frequency AC to DC, a bucket capacitor to smooth the DC and provide a low impedance source for an IGBT inverter bridge operating in PWM mode to produce variable frequency AC.

 

[GeorgeB]

With the three phase arrangement each diode carries DC link current for one third of the time. For single phase each carries DC link current for half the time.

I think I disagree here and would like you to explain further. Each 120 degree separated line is positive for 50% of the period (period 20ms in your case, positive (or negative) for 10ms. Current will flow when the line voltage (minus forward drop) is greater than load voltage, almost certainly less than the 50% per diode element in operation.

It's also a different shape ...

I completely agree with the different shape ... if the capacitor bank is connected to a diode element powered by a 100VAC RMS supply, ignoring forward drop I would be supplying current when the waveform is above the capacitor bank voltage at that instant. Current will flow until the peak of the waveform is reached, approximately sqrt(2) * 100V or approximately 140V. The primary ripple frequency will be (assuming a bridge) 100 Hz with single phase, 300 Hz if 3 phase ... but percent of conduction for each diode will not vary ... WILL IT?

and, as you are probably aware, the forward characteristic of a diode is non-linear.

My thick skull only sees that as significant while the line-load difference is in the approximately 0-2V range ... being very generous on real device characteristics.

 

[Besoeker]

I think I disagree here and would like you to explain further. Each 120 degree separated line is positive for 50% of the period (period 20ms in your case, positive (or negative) for 10ms.
True. For continuous DC current, the conducting path from say, L1 to L2 will change when L2 voltage exceeds L1 voltage. That's where the 120deg comes from.

Current will flow when the line voltage (minus forward drop) is greater than load voltage, almost certainly less than the 50% per diode element in operation.I completely agree with the different shape ... if the capacitor bank is connected to a diode element powered by a 100VAC RMS supply, ignoring forward drop I would be supplying current when the waveform is above the capacitor bank voltage at that instant. Current will flow until the peak of the waveform is reached, approximately sqrt(2) * 100V or approximately 140V. The primary ripple frequency will be (assuming a bridge) 100 Hz with single phase, 300 Hz if 3 phase ... but percent of conduction for each diode will not vary ... WILL IT? My thick skull only sees that as significant while the line-load difference is in the approximately 0-2V range ... being very generous on real device characteristics.

Good points.
I'm about to have dinner - stuffed Shitake mushrooms with ground beef, Bulgar wheat herbs....etc
Mrs B is an amazing cook.
So not much time to answer all of your points.
Just one. At 300Hz the diode is likely to be forward biased for more of the conduction period. It starts at 0.866 of peak voltage.
With single phase it starts at zero.