7200 rpm motor on 60 Hz?

The normal induction motor is limited by the synchronous speed of the stator windings. Given 60Hz and the minimum number of magnetic poles possible is two, this means *induction* motors can't go above 3600 RPM

But there are other types of motors. The 'universal' motor (first called 'universal' because it can run on AC or DC) can reach higher speeds. A typical use is 'Dremel' tools for hobbiest and small work. Can reach speeds of 10 000 RPM. Common electric drill is another one. These take advantage of the speed control of universal motors, vary the voltage applied and you have varying speed (within limits of course). But universal are *usually* only fractional HP.

Did it look like it had 'brushes'?? Either can see them directly, or often you can see the 'caps' where they can be removed/replaced without disassembling the entire end-bell. If so, it is most definitely a universal.

daestrom

| | The normal induction motor is limited by the synchronous speed of the stator | windings. Given 60Hz and the minimum number of magnetic poles possible is | two, this means *induction* motors can't go above 3600 RPM | | But there are other types of motors. The 'universal' motor (first called | 'universal' because it can run on AC or DC) can reach higher speeds. A | typical use is 'Dremel' tools for hobbiest and small work. Can reach speeds | of 10 000 RPM. Common electric drill is another one. These take advantage | of the speed control of universal motors, vary the voltage applied and you | have varying speed (within limits of course). But universal are *usually* | only fractional HP. | | Did it look like it had 'brushes'?? Either can see them directly, or often | you can see the 'caps' where they can be removed/replaced without | disassembling the entire end-bell. If so, it is most definitely a | universal.

I didn't get a close enough look, and that was several years ago. I do not recall if there were any caps to access them. I do recall the nameplate saying the speed was 7200. I didn't state any slower speed as I would now expect from an induction motor (relative to its power speed, such as 3600 in US). I just don't recall (probably didn't read) what the exact HP is, nor the stated amps. I do remember it said 240 volts delta and had a 15 amp 4 wire twist-lock plug. It was not connected when I saw it. It was about 2 feet long with the cylindrical case about 1 foot diameter. There was a shaft on both ends. It was definitely a lot larger than the motor in a Dremel tool.

in article snipped-for-privacy@enews2.newsguy.com, snipped-for-privacy@ipal.net at snipped-for-privacy@ipal.net wrote on 2/7/04 1:27 AM:

I wonder if 7200 rpm is too close to double synchronous speed to be a fluke. If I had to come up with something, it would be a full wave (bridge) rectifier driving a 7200 rpm motor at the second harmonic of the line frequency. A capacitor could block the dc component. It would also be possible to use a monocyclic network to filter out other frequencies. The biggest component would be the capacitor that would have to carry the motor current. There may some tricky winding arrangements to avoid the need of large components, but I have not figured that out yet. That is not part of my job description. :=)

Bill

On Sat, 07 Feb 2004 19:47:58 GMT Repeating Rifle wrote: | in article snipped-for-privacy@enews2.newsguy.com, snipped-for-privacy@ipal.net at | snipped-for-privacy@ipal.net wrote on 2/7/04 1:27 AM: | |> I once saw a motor that was rated for 7200 rpm. I don't recall seeing any |> special electronics added. Now I'm wondering how that could be done. The |> one way I can think of is counteracting windings between stator and rotor |> where the field rotates in one direction on the stator, and in the other |> direction on the rotor (so that the rotor is repelling itself away from |> the rotating stator field). But this is only a guess and I don't know much |> about motors. Can this even be done? FYI, the motor was about 3 to 5 HP |> (just to guess based on the size of the power cord ... 15 A at 240 volts). |> It was originally used (but no longer part of when I saw it) some kind of |> rock grinding system. | | I wonder if 7200 rpm is too close to double synchronous speed to be a fluke. | If I had to come up with something, it would be a full wave (bridge) | rectifier driving a 7200 rpm motor at the second harmonic of the line | frequency. A capacitor could block the dc component. It would also be | possible to use a monocyclic network to filter out other frequencies. The | biggest component would be the capacitor that would have to carry the motor | current. There may some tricky winding arrangements to avoid the need of | large components, but I have not figured that out yet. That is not part of | my job description. :=)

Interesting idea. The control box on it wasn't all that large, so I suspect the capacitors were not there.

It's not my job either. I'm just doing some learning time and motors are what I picked on this week. If I had learned this stuff 20 years ago then I'm sure I'd have known exactly what to peek at to see what that motor was actually doing. Hell, I might have tried to even plug it in :-)

Since it did say 7200, and not 7000 or such, I did assume it was syncronous in some way. But at the time I didn't really think about that being hard to do because I didn't really understand different motor types. I knew there were high speed motors around (like my father's dental drills) so 7200 rpm just didn't jump out at me as unusual. Now I'm reading about different motor types, online, and nothing jumps out at me as capable of doing that, except for that idea I first suggested.

---------- First of all, was it an AC motor? While not typical for a DC motor of this size, it is certainly possible. A so called "universal motor" will do this but this seems to be too large for that.

How old was the motor? I have seen an old motor which had, instead of "60Hz", a frequency rating of 7200 "alts" which appears to be half cycles/minute. This was a 1200rpm motor built sometime before 1915 and was a beautiful piece of workmanship. As for the opposite direction of rotor and stator fields- No- If induction motor, the induced field will be in synchronism with the stator field-same speed and same direction (but not in phase). If not, the motor will stop and reverse. If it is a synchronous motor, the motor will stop after the breaker trips and hopefully there will be no damage.

-- Don Kelly snipped-for-privacy@peeshaw.ca remove the urine to answer

I have seen large universal motors on woodworking machinery, specifically spindle shapers. A rating of 2 hp at 10,000 rpm is not unusual. Other such pieces of equipment were often fed off 3 phase "high cycle," or 180 Hz. Many small shop tools, such as hand drills and saws were made for this service.

Another common large universal motor application was the on-car dynamic wheel balancer. A universal motor with a small pulley was placed in contact with the vehicle's tire to accelerate it up to highway speed. Due to the smaller diameter of the drive pulley a very high rpm was required.

at

| First of all, was it an AC motor? While not typical for a DC motor of this | size, it is certainly possible. | A so called "universal motor" will do this but this seems to be too large | for that.

I had the full impression at the time that it was an AC motor. But I did not know of a universal motor at the time, so I wouldn't have checked for any indication of that. In fact back then I didn't really understand an induction motor.

| How old was the motor? I have seen an old motor which had, instead of | "60Hz", a frequency rating of 7200 "alts" which appears to be half | cycles/minute. This was a 1200rpm motor built sometime before 1915 and was a | beautiful piece of workmanship.

It was not so old as to be considered an antique. From the look, I'd say it was manufactured somewhere between 1960 and 1975. I saw it around 1990. But it did look mothballed. It was in a corner of junk equipment and was rather dusty. The casing was painted dark green and looked like original.

| As for the opposite direction of rotor and stator fields- No- If induction | motor, the induced field will be in synchronism with the stator field-same | speed and same direction (but not in phase). If not, the motor will stop and | reverse. If it is a synchronous motor, the motor will stop after the breaker | trips and hopefully there will be no damage.

Suppose the rotor has permanent magnets and the stator has the rotating field and it is wired to rotate clockwise. The permanent magnets on the rotor are going to be pulled along with the field.

Now suppose the stator has permanent magnets instead, and the rotor has the rotating magnetic field (it will need brushes or some way to get the current into the rotor). The field rotates clockwise. It tries to keep the electro- magnetic field coupled with the permanent magnetic field. If the stator were free to rotate and you locked the rotor, then the stator would turn clockwise to follow the field. But because it is the field generator that is free to turn, and the stator is, by definition, locked, the rotor will turn in the opposite direction.

Reverse the direction of the magnetic field on the rotor to counter-clockwise. Now the rotor will spin clockwise.

Now, let's have electromagnetic fields on both stator and rotor. The field on the stator spins clockwise. The field on the rotor spins counter-clockwise. The stator is trying to make the rotor go clockwise. The rotor is trying to make the stator go counter-clockwise. That adds up to double syncronous speed as I see it.

This is not an induction motor, and certainly requires 3 phase power. Maybe it could be done with 1 phase and a capacitor to get a 2nd phase, but I'll leave that up to someone interested in it.

escreveu na mensagem news: snipped-for-privacy@enews2.newsguy.com...

For 60 Hz and 7200 RPM, synchronous, you can use the doubly-fed induction generator.

The doubly-fed is a wound rotor with 3-phase windings perfectly equal to the stator armature, via 3 brushes.

You fed the stator with 3-phase 60 Hz rotating clockwise and the rotor with the same 3-phase line (60 Hz) rotating counter-clockwise.

Et voilá, 7200 RPM synchronous.

If equal windings are used, both (rotor and stator) consumes the same amount of power, hence double power, meaning double speed by energy conservation Law.

JM Albuquerque

JM- You sent me an e-mail a short while ago and before I could look it over carefully and respond, I accidentally erased it. Could you resend and possibly clarify the construction you are thinking of?

| escreveu na mensagem | news: snipped-for-privacy@enews2.newsguy.com... | |> I once saw a motor that was rated for 7200 rpm. I don't recall seeing any |> special electronics added. Now I'm wondering how that could be done. | | For 60 Hz and 7200 RPM, synchronous, you can use the doubly-fed induction | generator. | | The doubly-fed is a wound rotor with 3-phase windings perfectly equal to the | stator armature, via 3 brushes. | | You fed the stator with 3-phase 60 Hz rotating clockwise and the rotor with | the same 3-phase line (60 Hz) rotating counter-clockwise. | | Et voil?, 7200 RPM synchronous. | | If equal windings are used, both (rotor and stator) consumes the same amount | of power, hence double power, meaning double speed by energy conservation | Law.

That sounds just like what I had speculated could be done. Of course another poster said it would just sit there and trip the breaker. I posted another followup with more detail. Since you gave it a name, I assumed it is a known thing and googled for it. I found some interesting links out there.