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RE: soooo a few questions

Posted by twystd (A) on April 15, 2016 at 17:31:04

In Reply to: soooo a few questions posted by Penguin on April 15, 2016 at 15:07:50:

First question: The power supplies put out a theoretical 150 watts, and yes theoretically, one supply could put out the theoretical 90 watts to run the loads. However that is theoretical, as the supplies are showing early signs of instability, when I'm running the motor at higher voltages (above 100 VRMS), more testing and tweaking are in order soon.

The second question: I have all those power resistors there to give the amp a load at start up. If I was to just use the transformer winding as a load at start up, the amps would see the transformer as pretty much a dead short, until the cores magnetize, and the amps see the load connected to the secondary reflected back through the transformer. To my knowledge, everyone who has tried that has failed, due to toasting their amp. and or supply, from inrush current, or causing either the amp, and or supply, to shut down because of overload protection. I'm basically burning 90 watts to get 30 (all three amps combined), but have lots of headroom, as I have a theoretical 450 watts available. Soon I'll be burning 60 to get 30.

Third question: I'd bet a 3 phase transformer would cost a lot more than three little 120v:6V torroid filament transformers ($30 delivered to my door). I'd need a 30VA three phase transformer with a 20:1 step up, I'd probably have to have something like that custom wound. The 6V to 120V step up is great, because that step up ratio gives a reflected impedance from the 1800 ohm motor leg, as a 4.5 ohm reflected load to the amp. With the 8.2 ohm resistor load. The actual impedance shown to the amp is about 9.3 ohms, because the reflected 4.5 ohm motor load is mostly inductive. That 9.3 ohm load is pretty much in the sweet spot for solid state amps (good for stability). The filament transformers are 10VA, which work out about perfect for the output needs. Any larger sized cores would add more inrush current, while they are magnetizing, a big reason other attempts have failed. It's that initial and instant power demand, into low loads, from these solid state amps, that are the problem, causing too much inrush current demand, and toasting things. The input to the amps are wired in a 3 phase Y configuration, and the output from the amps is wired that way too, to the primaries. The secondaries are wired in delta configuration, to run the motors, which are also delta wound.

You've got to understand that building a 3 phase, 30 watt plus, 120V supply, is quite different than building a 2 phase supply for a 5-10 watt 24V motor. These papst motors are probably the best possible motor for a TT, however they are hard to drive. Building this three phase variable frequency 120V 30 plus watt supply was one of the most challenging things I've ever tried, especially knowing nothing of solid state amps.

Since power is so cheap, I paid 16 dollars and change for each amp, and 20 for each supply. It's one thing to use a supply that is designed to deliver 150-200 watts, but to run them at anything near that for continuous duty, is not a good idea. This is kind of a Jethro Bodine brute force approach. All this reserve power is useful for stability issues. Stability has been a problem in other attempts I've seen. The extra power is also useful from a longevity viewpoint as well, and is so cheap, why not use it. Sure I might (but doubtful) be able to run these with little 25 watt chip amps for 8 bucks apiece, but if I can get a 200 watt amp for 16 why take the chance. If 20 dollars of supply might do it, why not spend 60 to have a much better chance of success. It's become evident to me, that these amps and supplies are awfully tweaky, just because they are rated at a certain power, that doesn't mean that in all conditions they will work as advertised, especially when used with highly inductive loads at relative low frequencies.

I used 8.2 ohm resistors for the load, because that was what I had laying around. After initial testing, I've decided to order 4 ohm resistors, and use that as the resistive leg of the load. That should give me about 50% more efficiency, and about a 6 ohm Z load to the amps. So they'll start out at 4 ohms, and move to a 6 ohm load once the cores are magnetized. That way, I will only be throwing away half the power. instead of two thirds. That will also lesson the demand on my supplies. More about the supplies later, as issues in further testing have come up. I'll post another post when I get some more parts in, and go through further testing.

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