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Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
OK, I guess you are right about the reluctance motor operating mode, it just didn't cross my mind, but since that is how a lot of stepper motor work, I don't see why it should not work here. Rocketry76, why don't you try it and see how it goes?
Regarding the electrode mounting, I would trust a stack of FR4 PCBs etched free of copper a lot more than Lexan or whatever. If you have ever seen a CD (I believe they are lexan) explode when spun fast enough on a Dremel tool, I am sure you would agree Not only is FR4 a material with a high structural strength (it does not get much better, AFAIK) but it is also certified to take a lot of heat without breaking down.
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
IamSmooth wrote ...
How does the "grinding the flats" on the rotor actually convert it to a sychronous motor?
As Steve said, It converts the motor into a variable reluctance motor. Just like in a coil-gun, the iron tries to move in the position that maximizes the inductance, so whenever the field peaks, the rotor will allign itself to minimize the air-gap, i.e. move the flats out of the way. Each cycle of the field will turn the rotor exactly to the next "inductance maximum", i.e. the next hump on the rotor.
sparky wrote ...
I'm wondering if guys you know if Dayton 1/2 horse power 10,000 RPM AC/DC motors are synchronous... they are CCW. Anyone know???
Thats rather self-evident: They are AC/DC (read: universal) motors, so they are by definition not synchronous. The RPM support this notion, since a synchonous motor can only turn in fractions of the line frequency, nowhere near as fast as 10k rpm without a gearbox.
Registered Member #530
Joined: Sat Feb 17 2007, 07:56AM
Location: Victoria BC, Canada
Posts: 178
....but yet these brushless motors are used all the time in Tesla Coils - high powered ones at that....humm, I wonder if it is the number of breaks that forces near synchronous performance? I know several guys going with 11,000 RPM angle grinders as a ARSG and they are getting pretty decent sized outputs.... but would it be possible to make one of these in sync? Curious....
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
sparky wrote ...
....but yet these brushless motors are used all the time in Tesla Coils - high powered ones at that....humm, I wonder if it is the number of breaks that forces near synchronous performance? I know several guys going with 11,000 RPM angle grinders as a ARSG and they are getting pretty decent sized outputs.... but would it be possible to make one of these in sync? Curious....
Hang on! AC/DC motors are not brushless. BL motors allways need to be driven with AC, since, surprise, they don't have commutators (brushes) to make their own AC. BL motors come in two flavours, synchronous and asynchronous, and as Steve said one can be converted to the other.
I am by no means an expert in spark gap coiling, but from what I get you want your ARSG to be as far away from sync as possible, e.g. running at least twice sync speed. The closer you get to sync, the lower the beat frequency between mains and the gap, and the quicker your coil dies.
Angel grinders generally use universal (AC/DC) motors, since these are cheap, small, powerful and noisy. There is no way these can be made synchronous, but since they turn about 3x sync speed, they are OK to use for an ASRG.
(OMG how did I get into this discussion, I have never build an SGTC in my live, nor do I intend to!)
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Oh, go on, you know you want to build one. What Joe said is dead right. A synchronous rotary gap will give you the best efficiency. John Freau's efficiency benchmark equation, which is still the standard to beat, was derived from coils with sync gaps running at 120bps (ie, one spark per half cycle of the line)
A sync gap running at 240 or 360bps isn't nearly so efficient, but gives great looking and sounding results.
As Joe said, if you use an asynchronous rotary gap, you must keep the break rate high to avoid dangerous voltage surges. In practice this means at least 300bps. A static "safety" gap in parallel with the rotary is highly recommended too.
The high break rate leads to streamers that are fat, bright, noisy, and not as long for a given power input as with a 120bps SRSG. But still a hell of a show! :-o
Another reason to use an ASRG is when you have a large power supply like a pole pig, but can't afford a tank capacitor big enough to load the power supply fully with only one discharge per half-cycle.
My own personal preference in spark-gap coiling is a DC resonant charging power supply and a rotary gap driven by a variable speed DC motor.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
joe wrote ... As Steve said, It converts the motor into a variable reluctance motor. Just like in a coil-gun, the iron tries to move in the position that maximizes the inductance, so whenever the field peaks, the rotor will allign itself to minimize the air-gap, i.e. move the flats out of the way. Each cycle of the field will turn the rotor exactly to the next "inductance maximum", i.e. the next hump on the rotor.
Does this mean that the tendency to maximize inductance supercedes the induced current in the squirrel cage and it is this effect that drives the rotor around and not the magnetic field induced in the rotor?
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New synchronous motor?
What Joe said is dead right. A synchronous rotary gap will give you the best efficiency. John Freau's efficiency benchmark equation, which is still the standard to beat, was derived from coils with sync gaps running at 120bps (ie, one spark per half cycle of the line)