Success! I found the problem. It just seemed like SOMETHING was
being told to draw way too much, so I started looking at the
control circuitry.
There is a small (10 watt) variable resistor (remember the
cigar-sized Ohmite "Dividohm" with the exposed resistance wire and
a sliding clamp)? A small version of one of those. It was
completely open circuit! And its function is to set the voltage
output via a carbon pile regulator which drove the 400 Hz
generator field. So with it open, the field (rotor) was driving
wide-open all the time. Which means the RPM was excessively high
and likewise lots of power going somewhere it shouldn't. Glad I
didn't fry it.
Given the clamp position, it looked like it should be somewhere
around 350-400 ohms. So I tacked in a handy 383 ohm power
resistor, and left the 150 ohm wirewound adjustment pot (which
also had some oxidation but improved after working it back and
forth) in the center where it had been.
Fired it up with the usual horrendous starting surge, near 20 amps
primary transformer current on the Amprobe, transformer ratio
18:210 taps = 11.7:1. That 220 amps I measured on the transformer
secondary wasn't so far off after all! Primary current dropped to
10 amps as the machine spun up, as before... but suddenly, there
was an audible entry to closed-loop operation just like letting
off the gas, and the primary current immediately dropped to just
3.5 amps. That translates to 41 secondary amps which is (estimated
41/1.8)* = 23 DC amps... not bad for nameplate 115 full load amps.
*(Signal Transformer's power supply design catalog says the RMS
secondary current is 1.8 amps the DC current in a capacitor-input,
bridge rectifier system).
DC power supply volts under that load now up to 23 with less
ripple, RPM sounding more reasonable too. Should be much better
once my 220,000 uF filter cap gets here later this week.
And without even tweaking the pot, there is exactly 115 VAC
measured with my Triplett 630-A on all three line-to-lines
(A-B/gnd, C-B/gnd and A-C)! So it really is a 115 delta with
corner ground. I still can't tell if that will work with a Collins
618T-2 or not. The schematic says it should, showing a 115 delta
input to the HVPS, but the literature keeps referring to 115/208Y
which is the current aircraft standard...
Haven't loaded it yet because I don't have the bridge rectifier
properly cooled (just a 3x4" heat sink that might not be enough
for frequent or prolonged use).
I will check the frequency too, although it's not adjustable
(fixed parallel-resonant LC).
About the resistor, I don't have an exact replacement (750
ohm)...does anyone have one on hand? 500 ohm would also work fine.
Or I'll just mount the 383 fixed resistor in there! :)
It's not the capacitor on phase A. Oh pooh. Now I have to look for zebras rather than horses...
Yes, I am aware of the basic motor physics ;) That "about a second" is with a stiff supply. Mine is apparently not stiff enough <that's what she said> :D
My original plan was to acquire a 618T with the 400 Hz three-phase supply - usually much cheaper than the 28 vdc or the 400 Hz single phase versions. But this inverter is becoming a real white elephant. It would be running all the time, not PTT like the ART-13.
Anyway I am tempted to go buy a pair of car batteries and find out, just because I'm stubborn that way! (My two "ARC-5" receivers have their dynamotors, but I only run one at a time, and the 28v supply I built with a handy Stancor RT-204 transformer has no trouble).
Charles, WB3JOK/0 since '76
On 2/26/22 15:13, Christopher Bowne wrote:
No, just a an Ames (Harbor Freight) 80 buck multimeter with a clamp on AC or DC ammeter.
Peak current is always at initial 0 RPM, as the motor winds up the counter EMF brings the current down in about a second. BasicDC motor physics. Considering the instrument pedigree I take no responsibility for the accuracy of the reported result.
As an alternative to floating batteries on the output of a DC supply, one could implement a step starter circuit, but you would needsome hefty dropping resistors and contactor relays. Wouldn’t lend itself to quick PTT operation on voice.
Chris AJ1GStonington CT
Sent from my iPhone
On Feb 26, 2022, at 15:38, Charles <[email protected]> wrote:
Thanks, there's a data point of 5 times the nameplate.. What was the decay time of that 160 amp peak (did you use a scope)?
If I HAD a couple of spare car batteries I'd certainly try them ;) but have you seen the prices lately? they start at $55 plus core plus tax at Wally World...
On 2/26/22 13:37, Christopher Bowne wrote:
I’ve measured the peak inrush current on my ART-13’s DY-17A dynamotor, it was a nominal 160 amps. Just float a couple of 12 batteries across the output of your supply, make sure that they are fully charged and full sized automotive or deep cycle marine type, small one may not deliver enough peak current to preclude tripping the output breaker in on your high current supply.
Chris AJ1G
Stonington CT
Sent from my iPhone
On Feb 26, 2022, at 13:57, Charles via ARC5 <[email protected]> wrote:
I am sure I would have no trouble starting a DY-17 with a 33 amp full load rating. This beast is 3.5 times that full load draw and consequently any surge current would likely also be 3.5 times that...
Anyway I am going to investigate shorts on the AC side, and my prime suspect is the (noise reduction?) capacitor on A phase. Hope it's not the armature. It does have a repair request tag from the 80's or 90's (don't remember at moment) and an unreadable "Serviceable" tag. The electromechanical regulator (carbon piles!) will get a look after I check the cap. Something else I have no experience with.
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