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Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
It's not witchcraft, do the math yourself.
Consider adjacent turns have perhaps 0.5v rms between them, and the brush is contacting 3 turns, and has a resistance of 1ohm from each turn to a common terminal. That is valid, as variacs frequently specify an anisotropic block of carbon, with lower resistance turns to output than it does turn to turn. Anyhow, if using an isotropic brush with lower resistnace turn to turn, the basic maths is the same, but the losses have increased a bit.
Resistance to output, 3 off 1ohm in parallel for 330mohm output impedance, that's 1v drop for 3A load current, not bad, not likely to be the biggest series impedance in the system.
Now consider the power lost to currents through the brush, there are two 1ohm resistors each with 0.5v across them, losing a total power of 2x .5^2 = half a watt, not too bad, not likely to be the largest power loss in the system.
You can use this property to use FETs to switch between terminals of a finely spaced auto transformer to get fine control of output voltage. Again it's a case of once the turn to turn voltage is low enough, then there's b*g*all power lost in the "shorted" turns, and still a usefully low impedance to the output. Fets would be used make before break.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
The math does not describe what is going on at the carbon-copper interface. The losses can be measured but the contact is not stable and is affected by pollution. It is only an appoximate voltage at any point on the dial and resetting it precisely is impossible. once current flows from brush to metal, the copper surface develops a coating which in motors is essential.
Brush companies keep the formulations of their brushes to themselves. They work with motor users to provide brushes of a grade and type that will work with a particular machine in a partiicular setting. The coating on the copper (electrographitic) comes from the brush and appears after some use.
How does electric current flow from copper to carbon, precisely? Chemistry problem?
Registered Member #53
Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638
I was thinking about this thread today and I wondered would it be feasible to use a class D amplifier to run an isolation transformer (or mot or nst) to give you a 0 - 120 or 0 - 2K power supply? Class D's are not that complex and having it run at one frequency makes driving a transformer makes it all the more simple. Would the silicon be worth more then the effort? I think it would be kinda cool to have a class D dimmer that could run inductive loads.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
Nik wrote: Class D's are not that complex and having it run at one frequency makes driving a transformer makes it all the more simple.
60 Hz output would place limitations on the Variac. Many applications need a variable AC source but not necessarily 60 Hz. If VF was available the variac could be made with a core capable of operation over a range, say 60-800 Hz. There is test gear out there, pre-solid state, that can operate over these ranges. The reason is that it is used in aircraft servicing.
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
I don't think that's what he means, though radiotech. He means like a variac, but without the mechanical bits of the variac. A solid state variac, if you will, with pure 60 hz sine wave output LIKE a regular variac.
It would ....*probably* be cheaper to make than an actual variac, and not to mention, it would be considerably lighter than an iron variac as well.
Registered Member #53
Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638
Duck gets what I meant. Basically a power supply that goes from 0 to xxx Volts at line frequency so you can test and soft start tesla coils or what have you.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Those things are standard lab equipment in places that do power supply design. One reason is that you have to test your new design on both 50 and 60Hz if it's to sell worldwide. Maybe even 400, if you're building things for aircraft.
I believe Elgar is one brand. The old ones used what amounted to a huge audio amplifier driving a step-up transformer, and were massively big and heavy. 1kVA, 200lbs.
The new ones effectively use a Class-D amp as you said, and work more or less the same as true sine wave UPSs.
Either way you can set any voltage and frequency you want, vary the voltage and frequency while the thing is running, and so on.
Once you see how much work it is to get it working reliably, you'll wish you'd bought the variac. For a start, how do you power up your prototype AC source safely to test it?
(On some lesser forums, they would connect the output back to the input and hope for the best, if not over-unity.)
Those things are standard lab equipment in places that do power supply design. One reason is that you have to test your new design on both 50 and 60Hz if it's to sell worldwide. Maybe even 400, if you're building things for aircraft.
I believe Elgar is one brand. The old ones used what amounted to a huge audio amplifier driving a step-up transformer, and were massively big and heavy. 1kVA, 200lbs.
The new ones effectively use a Class-D amp as you said, and work more or less the same as true sine wave UPSs.
Either way you can set any voltage and frequency you want, vary the voltage and frequency while the thing is running, and so on.
Once you see how much work it is to get it working reliably, you'll wish you'd bought the variac. For a start, how do you power up your prototype AC source safely to test it?
(On some lesser forums, they would connect the output back to the input and hope for the best, if not over-unity.)
This is a quote directed to Steve. Is this the steve from Arc Attack? :D
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Steve McConner wrote ... Those things are standard lab equipment in places that do power supply design. One reason is that you have to test your new design on both 50 and 60Hz if it's to sell worldwide.
Long ago, my employer (in USA) made a product requiring about 50 kVA. For export, we needed to test-run it on 50 Hz. Rented a V-belt-coupled motor-generator set that must have weighed a ton!
[edit, re. solid state, sinusoidal variable voltage drive] >>It would ....*probably* be cheaper to make than an actual variac, and not to mention, it would be considerably lighter than an iron variac as well.
I agree with the lighter part. But the reason people still buy iron variacs is that the alternatives (with comparable performance in the parameters which matter) cost more. As a comp, how 'bout this 600W inverter for $219? I bet more of these are sold than variacs. Note: no voltage adjustment, 5% harmonic distortion, poorer tolerance of temporary overload... . But the solid state inverter probably has better line and load regulation, because of active feedback. Can anyone point to an industrial grade variable-voltage pure-sine-wave inverter that can generally replace a 6 amp variac, for less than $500? Is there a question about why this business opportunity has been overlooked for so long?
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I want to make a variac
