Inverter help

[737NUT]

Hi all,
I have a older DC to AC inverter, specifically a Collins T426-1 that outputs 115Vac @ 400Hz from a 28Vdc input. My problem is that as soon as I hook up a load to it the output frequency jumps to 3.4Khz! I tried a second Inverter and got the same result. Here is the exact set-up, 28Vdc to Inverter, 115Vac 400Hz out to a step down xfmr which is 28Vac 400Hz out to an aircraft instrument. The instrument only has a 1amp draw and the Inverter is rated at 650Va. Any idea what could cause the frequency to be so high and what should I check.
Thanks
Rob

 

[MikeMl]

Maybe it doesn't like the light load? What freq does it run at unloaded. Make a 600W test load out of 120V light bulbs, and try again...

 

[737NUT]

Maybe it doesn't like the light load? What freq does it run at unloaded. Make a 600W test load out of 120V light bulbs, and try again...

It is 400.1Hz no load I'll try a heavier load and report back. Thanks

 

[737NUT]

Ok, started with light load, less than .5 amp, frequency went from 401Hz to 1.2Khz, added another load of approximately 1 amp, went to 2.14Khz So this tells me the Load or lack thereof is not directly causing the issue. Maybe internal or external wiring? Dunno.
Rob

 

[cct314]

Question: How are you measuring the frequency?

If the output waveform is something other than a nice, low-distortion sine wave, your frequency counter may be reading something other than the actual (fundamental) frequency of the sine wave.

If you have a dual-trace scope with probes, do a differential measurement of the output and measure the frequency using that.

If you don't have that - or don't want to mess with setting it up, measure the frequency again using a 100k resistor in series and a 0.1uF capacitor (of suitable voltage rating!) across the frequency counter leads as a quick-and-dirty low-pass filter to see if you get the same results.

 

[tcmtech]

If its jumping from 400 Hz to several KHz there should be a very obvious change in the inverters sound tone. If not its a measurement glitch.

 

[737NUT]

Question: How are you measuring the frequency?

If the output waveform is something other than a nice, low-distortion sine wave, your frequency counter may be reading something other than the actual (fundamental) frequency of the sine wave.

If you have a dual-trace scope with probes, do a differential measurement of the output and measure the frequency using that.

If you don't have that - or don't want to mess with setting it up, measure the frequency again using a 100k resistor in series and a 0.1uF capacitor (of suitable voltage rating!) across the frequency counter leads as a quick-and-dirty low-pass filter to see if you get the same results.

I am using a Fluke87 I set up the dirty low pass filter. No change in readings, 400.2Hz no load 1.3K and higher depending on load. I tried isolating it from house ground, no change. Could the transformer cause this? I know you can use 60Hz xfmr on 400Hz but not the other way. Harmonics? Feedback Loop? I just cant see 2 different inverters doing this but I guess its possible. By that I mean identical internal faults causing this.
Thanks for the ideas so far. I do have a dual trace scope but it is PC based Mega scope from po labs.

Rob

 

[737NUT]

If its jumping from 400 Hz to several KHz there should be a very obvious change in the inverters sound tone. If not its a measurement glitch.

There is a pitch change at the Inverter, It gets quieter.

 

[cct314]

Interesting.

Since it does this on more than one unit I am discinclined to think that it is an identical fault in both units.

Is the power supply feeding the inverter buckling under load and/or is it free of ripple from the modulated current draw of the inverter? With the Fluke the amount of superimposed AC should be easy to read on the DC input.

I would hope that the input of the inverter would have plenty of bulk capacitance to remove ripple, but this is not always so - particularly if the voltage is assumed to be a low-impedance source like a battery. I've also seen some power supplies' regulators do odd things in the presence of "strange" loads, particularly those that are not steady.

I don't know about this particular inverter, but many of the older units are self-excited - that is, they use a feedback winding on the transformer to drive the output transformers. If they see a load that they do not like (highly capacitive, "hard start" like a motor - neither of which apply in your case, except that light bulbs do have low "cold" resistance which can cause problems in some cases) - or if the input voltage source is particularly "flimsy" (e.g. long leads with high resistance, voltage sag) they can often do strange things: I had an old Tripp-Lite 60 Hz self-excited inverter that would do exactly what you describe under such conditions! On that Tripp Lite I added 4700uF of capacitance on the input 12 volt leads, very close to the unit, that made a dramatic improvement in its behavior.

I'm presuming that you have already checked this, but I didn't see it mentioned in the thread.

 

[737NUT]

Here are the test results at the DC input to the Inverter,
No Load
30.63Vdc 175mVac

Load on
30.45Vdc 254mVac

These are used to drive old radios in vehicles either boats or land They have some large caps in them and large transistors Very heavy for their size.