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Old 08-29-2013, 09:52 PM   #16
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1) DC motors will vary speed with voltage. AC motors require change in frequency to change speed.
2)The motor that you suggested is rated from 2350-3600 RPM. If your desired speed range is 500 RPM - 6840 RPM, than this motor will not do it. Your research has likely found motors that will achieve this speed range, but they are far more expensive than what you find in a Grainger catalog.
3) A 3/4 HP motor makes 3/4 HP at rated speed, not at any speed. Assuming torque is constant (fair assumption for reasonable changes in speed), cutting the speed by x% will decrease the HP a similar amount. The alternator is likely not rated to supply 360W at any speed, it is rated to supply 360W at a particular speed. Probably a pretty high speed on a bike.
4)See note 2. If 500-6840 RPM is what you need, then you need to spend more money. Get a quote from a motor distributor for a matched set to meet your specs, but the cost is going to higher than out of the Grainger catalog.
5) If you want simple, there is always the old fashioned way:
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Old 08-30-2013, 11:12 AM   #17
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I did not read all of your post as it was to long.

The VFD varies motor speed by regulating volts and hertz proportionally. Example with a 4 pole motor (1800 RPM). Example.

460 volts at 60 Hz = 1800 RPM
230 Volts at 30 Hz = 900 RPM
115 Volts at 15 Hz = 450 RPM
And everything in between.

This is the principle of the AC inverter or VFD.

Do not buy an expensive AB drive for this application.
Buy something from KB electronics. Something inexpensive.

Pick your motor as close to the output max speed you need. The drive can over speed and under speed with caution to torque.
The closer you match motor base speed to actual working speed, the better off you are.

With a simple KB chassis mount VFD, there are no parameters to concern yourself with. The drive is set up with dip switches.
Very simple and very inexpensive.

Try www.automationdriect.com
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Old 08-30-2013, 11:19 AM   #18
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Originally Posted by J. V. View Post

Do not buy an expensive AB drive for this application.
Buy something from KB electronics. Something inexpensive.

Try www.automationdriect.com
Not sure what you consider expensive....I can get an AB PowerFlex 4 for less than $500.

Automation Direct? You couldn't give me one....yea...it's cheap.....and cheap....one warranty repair and any money you have saved is gone....and with the stuff I do....one failure during production....and the production loss is magnitudes greater than the cost of the Automation Direct.

Basically, Automation Direct is the Harbor Freight of PLC's and related hardware.
Even if you are on the right track, you will still get run over if you just sit there.

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Old 08-30-2013, 12:01 PM   #19
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Originally Posted by ddawg16 View Post
Basically, Automation Direct is the Harbor Freight of PLC's and related hardware.
Sometimes Harbor Freight is all you need. :-)
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Old 08-30-2013, 12:10 PM   #20
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Originally Posted by silvercbx View Post
2) The particular motor I am looking at has a shaft speed of 3420.
That's at 60Hz. Speed is determined by frequency (hence "variable frequency drive"). The actual theoretical speed at 60Hz is 3600RPM - 3420 is under full load. That's a two-pole motor. The other common motor configuration you'll find is four-pole, which runs at 1750RPM (1800 theoretical). If you crank a two pole motor up to 300Hz on a VFD, it would try to spin at 18,000 RPM and probably self destruct. As long as your VFD can hit 120Hz, you can get over 6000RPM. The motor should handle that just fine for short periods of time. However, due to the relationship between speed and voltage, you will have limited torque available at above rated speed.

3) I was told that 750 Watts = 1 HP, so the Alternator at 360 Watts will require just a little less than 1/2 HP to drive it (under full 30 amp load). I kind of figure that doubling the speed of the motor (with pulleys) will place extra load on the motor but I think a 3/4 HP motor can handle it (besides, their is a HUGH jump in costs when I move into the 1 HP level - both in the VFD and the Motor).
If the alternator is rated for 30A, that's more like 441W of electrical output (30A * 14.7V). It's probably not more than about 50% efficient - so you will need about 1HP of mechanical power to drive it, or a little more. Your motor will produce rated power at its rated speed and torque values. Torque is proportional to current, and power is the product of torque and speed. Current is limited by the VFD's capabilities and the motor's back-EMF (the voltage it produces internally when spinning, which opposes the voltage applied to it). So you cannot really get rated power at less than rated speed. You can exceed rated power at higher speeds, for a short time, if the VFD will handle it and if you have enough available voltage.

If your VFD has a 120V input, you will not have enough voltage available to get good torque at high speed. A 240V input VFD with a 208V rated motor is much better at this because it can provide up to 300V to the motor, thus supporting full torque at speeds up to around 5200 RPM.

4) What little contact I had with 3 phase motors during my years with IBM did let me agree that they are better than single phase motors for what I'm trying to do. But I have no technical experience with them. I only picked that motor because it had the right rpm and the price was good. As I am big on reliability, if anyone has a better motor (than doesn't pick my pocket), I'd be obliged.
For variable speed applications, three phase induction motors with VFDs are the only good solution aside from DC motors and brushless motors - both of which are likely to be more expensive here. You could get a big DC motor cheaper (military surplus usually), but good constant-speed controllers for them are more expensive than VFDs.

5) To my uneducated eye (very), this is a simple speed control project; no programming, no remote. All I want to do is mount the Alternator, select the pulley range and turn it on, varying the speed by just turning a knob (pot, rheostat, whatever). I'll rig up a photo tach on the shaft to see a visual of the Alt rpm at any given setting.
All VFD applications require programming. You have to make it do what you want. Parameters like minimum and maximum frequencies, number of motor poles, maximum current, overload response, braking behavior, acceleration profiles, etc. must be set by the user to match your installation's needs. The voltage/frequency curve is user configurable too, which is very useful for your kind of application where you are operating outside the normal frequency range of the motor. You will not need a photo tach though. Good VFDs come with a display that can be set to show the motor speed (or any number of other parameters).

If I were doing this, I would use a 1HP 208V two-pole motor and a 1HP VFD with a 240V input, like this one:

I'd probably start with a 1:1 drive ratio, and see if you can maintain full test speed with the alternator loaded down. If not, then try a 1.5:1 overdrive. You should have enough voltage overhead that you can push this drive/motor combination beyond 1HP for short periods of time, which may be necessary to reach full alternator output.
I am a lawyer, but not your lawyer. And who cares anyways? We're here to talk construction. This is DIY advice, not legal advice.

Last edited by mpoulton; 08-30-2013 at 12:20 PM.
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Old 08-31-2013, 11:17 AM   #21
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Take a look at these VFD's. The chassis mount, nema 1 and nema 4 enclosure controls have no programming function. it is accomplished with dip switches. No programming language or even a screen to look at.
Plug and play. Simple and cheap. Cheaper than the $500 AB drive.

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