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roydm (Industrial)
28 Jul 08 9:02
     I have been following another thread "help slowing down electric motor" which got me wondering why single phase VFDs are not available. Given that there are millions of single phase motors why has it taken so long for someone to manufacture a universal 2 phase (90 degrees apart) VFD to match the start and run windings of single phase motors. (I have looked at INVERTEK)
Why not discard the start capacitor and switch and power both windings 90 degrees apart?
waross (Electrical)
28 Jul 08 9:13
After reading your other post, you may have a point. I suspect that the idea may be technically feasible but not economically feasible. I don't see small motor manufacturers spending any money to produce VFD friendly motors when the majority of their production will be used for fixed speed applications. I am sure that a VFD could be developed that will accommodate a wide range of single phase motors by taking over the starting control. I doubt that the small market for single phase VFDs will support the development costs.
But, with small developers looking for "Niche" markets, we may someday see VFDs such as you describe.
OEMs wanting to incorporate VFDs into their products may use 3 phase motors at about the same price or less than single phase motors.
You may have a good idea whose time has not yet come  

"Why not the best?"
Jimmy Carter

LionelHutz (Electrical)
28 Jul 08 9:13
Well, the first thing that comes to mind is that you would need to know the start winding capability to be able to control it with a VFD output instead of the capacitor and switch and this type of information is not generally released by the manufacturer. Basically, doing this is so open to misapplication that most manufacturers just don't want to even bother trying.

Also, once you add a VFD then there are much better motor types that can be used. Once you create the DC buss you can create any type of output you want so, for example, you can create a 3-phase output and use a 3-phase motor.
roydm (Industrial)
28 Jul 08 11:10
In Response.
Waross "I don't see small motor manufacturers spending any money to produce VFD friendly motors" I don't either, it would take a VFD manufacturer, perhaps a 3 phase drive could be adapted with different firmware. You are correct OEMs will always pick a 3 phase, so would I but what about the millions of single phase motors out there, home hobby market etc.
LionelHutz "you would need to know the start winding capability" OK, so you need to take a few measurements, I immagine that there is some sort of formula motor manufactures use to figure out the wire size / turns for any given motor size.
Wouldn't the motor develop more torque with the additional field energised rather than just taking up space?
Thank you both
itsmoked (Electrical)
28 Jul 08 14:21
I think most VFD makers dream of and seek big buyers.  Then you have less questions to answer.  Any big buyer is going to select a 3P VFD and a 3P motor over a 1ph motor that will be a little bigger and an Invertek that costs twice as much as the 3p VFD.

This means a lot of buyers of 1p VFDs are going to be end users like our doughboy.  This would mean lots of phone calls, miss applications, warranty issues, lawsuits, etc.

Technically you are correct. I think it's the non-technical that is preventing it.

Keith Cress
kcress -

roydm (Industrial)
28 Jul 08 14:43
         Yes I looked at the Invertek manual, their drive applies to a limited number of VFDs Shaded pole and capacitor run. The output is single phase, not 2 phase.
I sketched out the output for a typical 3 phase drive (6 transistors). I think ideally you would need 4 transistors per phase however the regular 3 phase configuration might be workable. It's a while since I opened a split phase motor but from memory the start winding has approx the same No of turns.
roydm (Industrial)
28 Jul 08 17:35
Oops I meant to say "their drive applies to a limited number of MOTORS Shaded pole and capacitor run
jraef (Electrical)
29 Jul 08 0:10
When last I was involved in a new VFD development project (over 5 years ago now), the projected cost estimate to design and prepare a product for production was almost $1 million, not counting cost of sales and marketing (since you can't do that without a product first). That's a lot of NRE that needs to be recovered in sales, something that banks and/or other financiers are loathe to cough up without a proven market. I'm not sure you could convince anyone that the market is there on this kind of drive.

Then you would have some product logistical issues to contend with. Small VFDs are no longer made with discrete power components. Now they are all made with what are called "IPMs", Integrated Power Modules, which contain the diode bridge, DC bus, and 6 output transistors complete with firing circuits ready for TTL connection directly to your mP. To offer a different configuration would mean trying to do it with discrete IGBTs again, which I'm not even sure you can do any longer in a cost effective way. So if you end up with a 1 phse - 1 phase VFD that ends up costing the OEM even 10% more than a 3 phase unit, why would he buy it? But without the volume of the OEM market, your product would be relegated to the end-user / retrofit market. Although you see it as "millions" of installations, they are not all applicable to variable speed, in fact only a small percentage would be. So the market is severely limited before you even get started.

Just because you can, doesn't mean it's a good idea.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Forum Policies  

ozmosis (Electrical)
29 Jul 08 1:41
There is a constant quandary in the development of VFD's: do you make the VFD suitable for the motor or do you change the motor and make it suitable for variable speed?

Look at standard squirrel induction motors used today. The IM is designed to start and run at a fixed speed and designed to withstand the rigours of a DOL start. It means the physical size and material content (iron, copper etc)have to be considerably more than actually needed.
Fit an VFD on a standard IM and you suddenly realise the losses in the motor could be avoided.
Motor manufacturers such as Siemens, ABB, Emerson etc manufacture motors that can only be operated by VFD. The base frequency will be anywhere between 50-70Hz, the material used in the motor is considerably less compared to a standard IM of equivalent power. It means the overall efficiency is higher.
High Efficient motors you see today are only High Efficient when running on fixed speed at full load.
Taking this on further, the development of Permanant Magnet motors means the overall efficiency of the system is improved considerably but this means that the motors are designed in conjunction with the VFD.
The IM hasn't changed much in nearly 100yrs but what has changed has been the technologies surrounding it. All technologies are predominatly restricted by the base design of the standard induction motor.
I'm not really sure where I'm going with this little sermon but to say that the technology of varying the speed of a motor does not just lie with the VFD.
itsmoked (Electrical)
29 Jul 08 2:00
Had no idea ozmosis.  Thanks.

Keith Cress
kcress -

Skogsgurra (Electrical)
29 Jul 08 3:03
Have been (very remotely) witnessing the development of an OEM product where a variable speed pump is involved. The resulting package is a compact cost optimized set of VFD, motor and pump plus piping and valves - all designed for one purpose.

I think that Roy's idea could be of good use in such an application where all components are cut to size and the reduction of motor complexity and number of power components can have a deep effect on cost and marketability.

I'll forward your thinking to those guys, if you don't mind Roy?

Gunnar Englund
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

roydm (Industrial)
29 Jul 08 8:41
Thank-you all for the input.
Some of you seem to suggest OEM as the main user of VFDs, is that really the case?
Is efficiency a factor when buying a VFD for a small motor? (I am thinking 1/4 - 1 HP).
I guess I was thinking about the 10s of millions of single phase motors already out there that might benefit from variable speed or soft start ability.
I hear you Jraef on the high cost of development, so I guess the answer is to use a 3 phase drive with a firmware change to suit 2 coils. I tried to sketch this out, the run winding is easy but start winding is a problem except that as the run winding approaches zero the transistors are on less and less whereas the start winding would be on longer, any thoughts on if that's workable?
It was an interesting idea anyway!

The evolution of VFDs has certainly been interesting, I remember being blown away the first time I saw one over 40 years ago, before that it was either DC or very expensive Schrage motors. Is it just me or are the modern VFDs much harder on motors than the early SCR based designs, I don't recall special motors or cable length as an issue even 20 years ago.

BTW, how many 3 phase VFD motors do you have in your house? I have a F&P washing machine with no gearbox. The 3 phase 28 pole permanent magnet motor directly coupled to the agitator swishes back and forth just like a regular washer only without all the mechanical clanking.

Skogsgurra (Electrical)
29 Jul 08 9:44
Yes, OEMs are volume users of VFDs. Washing machines and HVAC - especially air conditioners are huge applications. And, a whole new generation of jacuzzis is emerging.

If VFDs are tough on motors? You bet. The earlier thyristor designs and GTO designs had slow switches. The new IGBT ones are much faster and kill windings and bearings. It is so bad that that is my main occupation nowadays. I will even be running a seminar on bearing problems (EMD) caused by VFDs this fall (sorry, in Sweden - and in Swedish, so it won't of any use to you fellers).

You don't seem to have anything against my mentioning about the two-phase thing? Actually, that is how induction motors started. Tesla's first system was a two-phase, 90 degree system. So the thought isn't really new. But newish.

Gunnar Englund
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

roydm (Industrial)
29 Jul 08 10:51
           I expect IGBTs must have some advantage over thyristors otherwise why are we putting up with them "killing windings & bearings".
Yes I read several books on Tesla, what a facinating character. I wonder how he would utilize the advances we have today.
Go ahead on the 2 phase thing if it's got any merit at all.
jraef (Electrical)
29 Jul 08 12:50
VFDs are being incorporated into a lot of home appliances now. They are just using small 3 phase motors. Most appliance motors are custom made anyway, so they just make them 3 phase. Not a big deal really.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Forum Policies  

roydm (Industrial)
29 Jul 08 13:01
       Thats very interesting, I expect they will be recycled also by the home hobby market. The washing machine motor I mentioned is very popular for wind powered projects.
ozmosis (Electrical)
29 Jul 08 23:16
In Europe there are huge restrictions on the high volume domestic use of the traditional concept of VFD's due to EMI and harmonic controls. Single phase into a 3-phase diode bridge rectifier introduces very high triplen harmonic distortion and this is a key area for the power supply companies and general "energy Efficient building' trend to focus on. It requires large inductors or other mitigation techniques to reduce the harmonics and then there are the filter techniques for the High Frequency interference levels. All the restrictions add huge amounts of cost into the traditional methods of frequency control.
Skogsgurra (Electrical)
30 Jul 08 1:48
You are right Patrick. Still, I am right now witnessing VFDs being used in jacuzzis. I think you can guess of what make. VFDs use in Heat Pumps like the Panasonic Inverter types is massive and I see them in washing machines as well.

The harmonics issue is solved either by using a reactor (single phase - on reactor) or, better, with PFC circuitry. The EMI part is not an issue with the technology used. That also takes care about the insulation and bearing issues.  

Gunnar Englund
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

ozmosis (Electrical)
30 Jul 08 3:31
I really hope you are not in the jacuzzi writing this Gunnar...winky smile

the goal of the 'old' company a few years ago was to have (in the future) all motors with variable speed control.
Quite a challenge when you consider the motors that Roy is talking about.
Skogsgurra (Electrical)
30 Jul 08 4:31
We got better things than jacuzzis over here - we have litterally hundreds of thousands of lakes - and rapids. I prefer them over artificial water experiences. At least in the summer...

Gunnar Englund
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

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