AC Motor Drive

[Lenp]

OK, here's the problem!
I have a small FHP motor, 120VAC single phase 2 amp load, and I need to increase it's speed. This is a 'powered conveyor roller' in that the motor is physically inside the conveyor roller so no gear/pulley change is possible.

Since it's an AC motor, and the speed is related to the power frequency, I need to increase the frequency

I am well acquainted with the VFD (Variable frequency Drive) and know there are single and three phase input units, but, they all produce 3 phase output. I need single phase output. I have been unable to find a simple (inexpensive) VFD for single phase output. I really do not need all the bells and whistles that are usually found on a VFD, but could accept them if it would work.

Don't be sidetracked by an AC 'Speed Control" like used on fans. They run from 0 to 95% of full motor speed, are triac based much like a lamp dimmer and alter the 60hz waveform through switching. I need to increase the frequency!

Several suppliers say a three phase VFD will not drive a single phase motor since there is some correlation between the output phases that is lost (feedback maybe?) if it is not a three phase motor. I asked if a three phase/single phase transformer would work and got the typical Duh... answer.
Right now I do not have access to a VFD or I would have tried this already

So, I am open to suggestions, or opinions. This would seem to be a simple design, and since only a few of these are needed, I would consider designing the oscillator/power stage but I would prefer to not reinvent the wheel if possible, and I'm wary there may be a red herring in the mix since none seem to be available!.

Thanks
Len

[Joseph]

Reminds me that one of my first projects was tinkering with a cassette deck that used an AC motor. I knew a lot less back then, and I was hoping to fix its slow playback problem by raising the frequency of the motor drive. Like what often happens when knowledge is lacking, my hope was not very well founded.

None of that story is meant to imply anything at all on your part. I think my experience there explains why your project interests me. I'm wondering, though, will your motors' drive torque drop as the supply power frequency is raised, and would that matter?

[Dean Huster]

I didn't think of the fact that using just one phase of a 3-phase VFD might not work right because of the lack of feedback (current vs. voltage phase, etc.), but I suppose that would be true. Trouble is, since it's a FHP motor, you probably can't sub a 3-phase motor because of the size, even with the smallest which are what ... one horse? Darned shame. And maybe a sub-horse 3-phase motor would be to small to work correctly with a VFD anyway. Interesting problem. Do you know anyone with a VFD who'd let you "play" with a single-phase motor to see what it's do? If you have a fairly constant load, it may not be a problem. In the same vein, I wonder if three single-phase motors, each hooked to one of the phases and the shafts connected together would operate properly with a VFD. Not suggesting that either since space is a problem.

[Lenp]

Hi All,
The motors could not be changed since they are physically inside the conveyor roller. The roller has two shafts, one on each end that are rigidly mounted and the wires pass through the shaft ends. In fact there are 2 motors, one connected CW and one CCW in the same roller but at opposite ends. I assume to improve the roller load rating.
These AC powered rollers are similar to the units in place. http://www.mcmaster.com/#powered-convey ... rs/=f8b3tj

I don't have shop access to a powered roller but I think that any FHP motor should perform in a similar manner. So maybe an oscillator that drove an amplifier stage would work and I'm not sure if a 'dirty' sine wave would be of any real importance.

Maybe I'll lash up my freq generator, someplace around 60-200Hz to an old 200W PA amplifier. If the amplifier doesn't roll off at the low end it should work. If voltage is too low, I'll try stepping it up since I think 200 Hz through a 60Hz transformer may work. This may, at least, be proof of concept.

Years ago I needed temporary 50 Hz power for some testing. I modified a 12 Volt Tripp-Lite inverter with a switch to select 50/60 Hz. output. That would work if the frequency could be increased as much as needed but the 120 V 60Hz AC to 12V DC to 120V 200Hz AC setup would be bulky and pretty inefficient!

Len

[dacflyer]

not sure how much you wanna adjust the speed on the motors, but if they have a run capacitor you might be able to change it slightly.
i worked on a fan motor before, needed to replace the run cap/ in it, could not find the exact one. but i had one that was close.. the fan actually a lil bit faster.
it did not seem to overheat any.

Just an idea....

[MrAl]

Hi Len,

I was involved with the design of one of these to power a somewhat lower power single phase motor many years ago.

The main idea is to use an H bridge to power the motor, and control the H bridge with a sine synth pattern but even a simple pattern would work. The line power is first rectified and filtered, which powers the bridge. The frequency is varied as the voltage is varied.
It's actually simpler than a full blown sine converter because there is no output filtering required just to drive a motor. You could probably rig this up yourself. If you dont want to be bothered with sine synth patterns, try using a pulse wave pulsing once per half cycle, i could look up the optimum pulse width if you think you'll build this thing. For variable voltage then you would simply cut that pattern back to shorter pulse widths each half cycle.
The H bridge would probably be done in MOSFETs these days, but you might use bipolar if you like. The firing is just a little bit tricky to get a delayed turn on for each pulse, but not too involved really, like first year electronics. The drivers for the MOSFETs are available in chip form these days too.

[Bygar]

You could use a small motor/generator set, depending on your gearing system you can get
different frequencies out. Caution! About 80 hertz should be the upper frequency limit
otherwise you could have over speed problems.
Hope this gives you some ideas. My experience was with somewhat higher power variable speed drives.
Good luck.

[Lenp]

Bygar,

Unfortunately, an M/G set is not really practical for this application.
The size and costs are not warranted for the power required.
Thanks!
Len

[Bygar]

It is just a matter size. What power is required?
A small DC motor and small AC generator would work.
Of course some losses will occur due to efficiency.
What power is required?

[haklesup]

I'm thinking along the lines of a modified inverter (DC-AC). The kind typically used to convert automotive 12V into useful AC in a car or camper.

Assuming you start with one that can deliver the required current (<300W) and a DC source (either a battery bank or a Rectified AC supply) then it should be possibe to hack into the clock circuit and change the triggering rate. I doubt you can drive it at 200kHz but 30 to 120HZ (maybe 240) should be possible while maintining the power you need. The motor should be fine with the modified sine unless you want to drive it real slow then it will probably get jittery.

Both motors not only need to see the same frequency but they probably need to be in synch (or at least well controlled to the same frequency) so that means using an inverter big enough to power both motors or an external clock for two inverters that runs both at the same time.

[Lenp]

Thanks haklesup!

I did that some time ago to test some equipment being shipped overseas with 50 Hz power. The inverter was a Trip-Lite and we installed a 50/60 hz switch. I think 120 HZ (2x motor speed) would be more than adequate for this application but I'm just not keen with the the AC to DC to AC conversion chain for a permanent installation. There must be a simpler and more economical way. I'm thinking about just chopping the rectified line AC at 60-120 Hz rate. The motor might make noise but if it runs without overheating it may be acceptable!
Len

[jwax]

Here's an out-of-the-box thought: Can you increase the diameter of the roller?

[Lenp]

jwax,

Good 'out of the box' thinking but 'in the box' there are other rollers adjacent to the powered roller.

Since this is a low torque application some new thinking is to machine a groove in the end of the roller and drive it with an 'O' ring from a DC motor. That is possible only if the roller wall is thick enough for a groove. The powered roller is epoxy sealed on the ends and some excavating with a Dremel will be required to expose the roller wall itself. I'll have to wait for down time on the machine before I can attempt that.

Thanks
Len

[Bygar]

I suggest a small D.C. motor of sufficient H.P.
turning a small generator.
Since the power required seems to be about 1/3 H.P.
Use a suitable D.C. motor and an automobile alternator
and a suitable transformer to get the voltage.
Automobile starter motor might do.
And use only 1 phase of alternator.
Suitable coupling will be required.
Once again maximum over speed RPM about 30%.
Good luck.
Bygar

[Externet]

Even not being the correct way; I would dare to try a variac capable of 150% voltage. What percentage of speed are you after and how close to the motor rating is the effort put to move the conveyor ?