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Old 10-01-2009, 09:25 PM   #16
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240 volt motor


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Originally Posted by micromind View Post
I've never seen a VFD designed for a single phase motor. I wonder just exactly how they deal with speed regulation when the start switch engages or disengages?

I didn't look at it any too closely, possibly it's designed only for motors without centrifugal start switches. Like PSC type, or A/C compressors, or submersible well pumps. Controlling the speed of these motors would be a cinch.

Interesting.

Rob
I have not looked into them much either but I suspect your suspicions are quite likely correct.

In fact, I do know of one use where a sub well pump is controlled this way to match flow volume with needs.

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Old 10-01-2009, 09:48 PM   #17
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240 volt motor


Come to think about it, I've heard of those variable speed well pumps. There are a lot of wells around here, and they were heavily advertised for a while a few years ago. I don't know if any of them actually were installed though.

Since a well pump or an A/C compressor has all 3 wires are brought out, it would be easy to cause a phase shift on the start winding electronically. 100% torque would be obtainable at any speed.

I could easily see an A/C compressor using a VFD to control its refrigerant output to match the cooling load of the building. That would result in exact temperature control, continuous de-humidification, and quite likely, energy savings.

Amazing what these modern high-tech gizmos can do these days.

Rob
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Old 10-01-2009, 09:58 PM   #18
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240 volt motor


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Originally Posted by LanterDan View Post
In principle, you should be able to find a VFD (variable frequency drive) that can take a 120 V input and drive a 240 V motor. In practice, I suspect it will be more economical to replace the motor with one intended for a 120 V input.

In grad school I inherited an experiment set up by a previous student. I noticed that pump motor on one the chilled water lines was running rather hot. A commented on it to one of technicians a couple of times about it was told that many motors do run very hot to the touch. After about a year I some how discovered the motor was configured for 240V but connected to 120V (like many motors it could run off both depending on how you make the connection). Correcting this did make it run _much_ cooler. I never did notice any difference in performance though, although I wasn't measuring flow rates or anything.
I would have read the Nameplate (on the motor) before a year was up... Probably before I'd start the motor the first time!
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Old 10-01-2009, 10:20 PM   #19
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240 volt motor


I did check the nameplate. As indicated previously, the motor was intended for operation at either 120V or 240V, so it wasn't until I opened the connection box that I discovered the error. I was handed what I beleived to be a working experiment setup and run by someone else. I don't generally go disassembling things looking for problems until I have a reason to think they exist. Yes, I'm dumb for not have figured that one out sooner once I knew it was overheating, but I'm not completely stupid. Realize, this was first real project in school and I was still learning a lot of things, so cut be a little slack.
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Old 10-02-2009, 11:32 AM   #20
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240 volt motor


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Originally Posted by nap View Post
the frequency is what alters the speed with an AC motor unless you want to consider the slippage caused by a below rating voltage as speed control.

The formula to determine motor speed (without considering slip) is:
120X freq/ number of poles in the motor.
the freq is what determines the speed. The voltage is reduced as the freq is reduced to accommodate the reduced inductive reactance. If you did not reduce the voltage and still reduced the freq, you would smoke the motor due to high(er than designed) currents.

To further support my point, what do you think happens when you run over 60 Hz? You can't increase the voltage but you will still speed up the motor. As a result, you will get an increase in speed but a reduction in HP output so you will have a greater resulting slip so you cannot determine the exact speed simply by calculating the freq and # of poles
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Originally Posted by nap View Post
really. I wonder what this is for then:

http://www.anaconsystems.com/text/eagle1.html

certain motor types do not do well on a VFD but there are most definately single phase output VFD's for single phase motors.
You are saying exactly what I am saying about motor speed. You must adjust the voltage and the frequency to accomplish speed control. Frequency adjustment alone will not accomplish speed variation.
Now, you got me on the single phase motor VFD. I have never seen one or used one. Did not know they existed. I wonder how the VFD handles the centrifugal switch and the caps.


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Originally Posted by LanterDan View Post
1) You'll note, I began my post with the words "in principle" a VFD could do what the OP wanted, but then suggested another course of action. But I do stand by statement this is entirely possible. Here (http://www.abb-vfd.com/files/manuals...l-Brochure.pdf) you'll note a drive which will happily take a 120V input and output up 240V 3ph. If you can produce 240V 3ph, then in principle you can make drive than produces 240 1 ph.

2) I can't think of any reason why can't have a single phase VFD. In fact I seem to see one here http://www.anaconsystems.com/data/eagledrive11.pdf.

3) If you were going to a VFD, it really would make sense to go with a three phase motor (and one invertor rated), which is probably why you don't see a lot of them designed a single phase one.

4) Its been about five years since I had anything to do with any VFD installations, but I do remember the ABB drives I used to use had a couple of different options in terms of scaling the output voltage with frequency, so its not always quite as simple of halving the frequency and halving the voltage. You are right that they are typically increased and decreased together. I'm sure there are those on this board that can shed much more light on the reasons for this than I care to get into at the moment.
Dan, Your links have provided me with enough information to apologize for doubting your statement. I did take note your comment "In Principle". These must be newer controls as I have never used a VFD that could increase output voltage that much. The most I have ever seen in the field is about 10-15%. I doubt (I have not looked) that you will find 240 volt inputs that will output 460. It looks to be only in the small 120 volt controls, as three phase 120 is not something one would need or require in the US.
The example was, well just an example. Most DIY'ers on this forum are not familiar with scaling freq and voltage, and quite frankly this discussion. This really is not the place anyway.

Like I said above to nap, I had no idea a single phase motor VFD existed, even though it looks like the only one in existence. I remember when the first VFD's came out.

None the less, it's good to meet you and look forward to further conversations.

Last edited by J. V.; 10-02-2009 at 11:48 AM.
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Old 10-02-2009, 03:18 PM   #21
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240 volt motor


Quote:
J. V.;335472]You are saying exactly what I am saying about motor speed. You must adjust the voltage and the frequency to accomplish speed control. Frequency adjustment alone will not accomplish speed variation.
yes it will. The voltage reduction is not used to slow the motor. It is reduced to avoid smoking the motor because the current draw will shoot up with a lower counter EMF created.


Quote:
Now, you got me on the single phase motor VFD. I have never seen one or used one. Did not know they existed. I wonder how the VFD handles the centrifugal switch and the caps.
I do not believe a motor with a centrifugal switch would work well on a freq drive. They are only used on certain types of single phase motors successfully.






Quote:
Like I said above to nap, I had no idea a single phase motor VFD existed, even though it looks like the only one in existence. I remember when the first VFD's came out.
I can find more if you really want (or doubt me).

Quote:
Lantern Dan:

1) You'll note, I began my post with the words "in principle" a VFD could do what the OP wanted, but then suggested another course of action. But I do stand by statement this is entirely possible. Here (http://www.abb-vfd.com/files/manuals...l-Brochure.pdf) you'll note a drive which will happily take a 120V input and output up 240V 3ph. If you can produce 240V 3ph, then in principle you can make drive than produces 240 1 ph.
Now, this is a new one to me (the voltage increase). Any idea how they are providing an output of double the input?
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Old 10-03-2009, 10:34 AM   #22
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240 volt motor


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Originally Posted by nap View Post
yes it will. The voltage reduction is not used to slow the motor. It is reduced to avoid smoking the motor because the current draw will shoot up with a lower counter EMF created.

I have no idea why we are arguing this point. Would we sacrifice burning up a motor by just adjusting frequency. We both know you must adjust both. For the sake of the motor if you will.

This is the principle of the VFD. Volts/Hz proportionally.
You are talking in theory, I am talking in practical terms. As I can vary speed with voltage (no load) as well as you can vary speed with Hz. Not for long, but I can do it. But it is not practical.


I do not believe a motor with a centrifugal switch would work well on a freq drive. They are only used on certain types of single phase motors successfully.

Me either. First time I have ever saw one. Looks like only one manufacturer makes it. I saved the link for others who also do not know about it.

I can find more if you really want (or doubt me).

Friend, I have always respected your comments and post's. I sometimes wonder if I might have said something that you did not like. If so, lets get it corrected right here and now. Of course I want to know more. I consider us colleges.
I used to install, repair and implement VFD's and DC controls for a living. I worked in a motor shop in my last job. I have since retired in 2005. So I may not be up to speed on these new products as you are. I am not doubting you. You showed me the web site. Please tell me if you have an issue with me. It sure sounds like it.

Now, this is a new one to me (the voltage increase). Any idea how they are providing an output of double the input?
I checked some motor and drive catalogs and saw this is done only on the small 120/240 volt drives. Mostly chassis type drives like the KB brand.
I do remember seeing plenty small 120 volt single phase input drives that would output 240 volt 3 phase. But the motor sizes are very limited as would be expected. I should have remembered that when talking to the other poster.


I look forward to further discussions with you.......John Valdes


Last edited by J. V.; 10-03-2009 at 10:43 AM.
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