On Tue, Mar 29, 2022 at 01:25 PM, Ed Breya wrote:
I'd suggest doing some experiments to get a good handle on what each part of
the system actually does. The big thing is the dynamotor, which can probably
be soft-started with some simple old-school techniques too. If you have a big
enough variable PS or other means, you can estimate the current (starting
torque) needed just to spin up the motor a little. Then you'll have some idea
of how low it can be limited, and still get going. The next is to consider the
load. Is the dynamotor output driving say, a bunch of tube equipment, with all
those heaters to light up? Actually, I'm wondering what is the form of its
output? I had a small one many years ago, that was only for making tube B+
from 28 VDC, but I think once you have the motor part, the "dyna" part could
be whatever you want.
Also, what is the control sequence? Do you juice up the 28 VDC, then throw a
big switch or contactor to start the motor? Or, does the line input get
switched, and everything goes? Another question is whether you have any taps
on the big transformer that may allow for reduced-voltage starting. Without
knowing more about the pieces, but presuming your 240 branch line wiring
includes the neutral, one alternative is start the whole works on 120 V, then
relay switch to the full 240 after some short delay, once the motor gets
going.
Since you have that nice current sensor, you can check out whatever you need.
It would be good to check the magnetizing current of just the transformer with
no load.
Thanks for the observations, Ed. In reverse order:
I actually checked the magnetizing current when I first got the transformer, and it doesn't even move the needle on my Amprobe clamp-on meter's 6 amp range.
The 240 volt outlet is line, line and ground only. No neutral and it's not code to use the ground wire as one. Besides, I already had the 12-2 (+ ground) Romex. Have you seen the prices of 12-2 lately... let alone 12-3???
No transformer taps, just two 120 volt primaries in series.
The rotary inverter I'm currently (pun intended) experimenting with is a 115 volt, 400 Hz, three-phase 2 KVA (or 1.5 kva single-phase) machine with carbon pile regulators for frequency (RPM) and output voltage. Applying +28v to the Switch On terminal actuates an internal contactor to turn on the motor and also a relay that connects the three-phase to the output terminals. No starter circuitry. As it turns out, the transfer relay does not engage until the DC is 21 volts or higher (probably to protect the generator in the event of low DC bus voltage) so the huge drop during "cranking" keeps it offline until it's spinning nearly full speed anyway. It's not a bug, it's a feature!
So my starting procedure is: Power supply AC breaker on, wait a second or two for inrush; DC breaker on; trigger the relay line and away she goes :) I can actually turn the DC on first with the relay line tied high, and flip the AC breaker, and it doesn't trip. But that's hard on the breaker contacts as well as adding the capacitor inrush AND transformer magnetizing surge to the heavy motor inrush. As I posted, once the inverter is at full speed it's only drawing 35 amps. Then I can turn on whatever 400 Hz load I wish to connect :)
I think this evening I'll do some experimenting with the Ultrastab current sensor and report back later :)
