Seems a lot of “discussion” on running a dynamotor on 28Vdc. Aircraft systems are referred to as 28 volt systems, however should one take a look at the required charging voltage for a “24VDC” battery, as used in the aircraft in which most all dynamotor power was used, a Lead Acid battery, the nominal charging system voltage is around 27.5 v. Seems to me that chasing a 28 volt supply with little transformers, switchers, whatever is a futile chase. Get a power supply that is “robust” IE capable of at least 40 amps and is regulated. Aircraft Generators in most of the aircraft were around 200 amp units. Not much “droop” in voltage when one keys the transmitter. Of course the supply wire must be rated for the current, and corrected for the length of wire.
If one does not consider supply wire resistance, as well as return resistance, then expect non optimal performance. Aircraft normally use the airframe for ground return, which presents little measurable resistance, as long as installation is correct. In amateur use, which in most cases is not all original installation mountings, etc, there is always a ground resistance problem, at least in what I have observed.
A dynamotor “overhauled” ten years ago, depending on the grease utilized as well as brush/commutator condition will not provide optimal performance.
Running dynamotors is nice and nostalgic, however, better power supplies are readily available, and they don’t make all the noise.
Just my opinion.
Ran my first mobile 6 meter am rig on a dynamotor under the hood of a ’58 Ford. One didn’t transmit much without the engine running. But, that was what was readily available to a kid without a lot of “expendable” income.
R
Have you considered a super capacitor bank and appropriate pre-charge circuit. They are great to flatten inrush currents.
Mark, besides the wire losses, that 0.5 ohm resistor should also be shunted when you reach running speed. At the additional 3.5A under load, that’s 1.75V drop. Also make sure all your connections are tight, any connectors both crimped and soldered. Check for both AC and DC losses at every point in the path to the DM from the wall socket. You may be surprised at the many points of loss.
Howie WB2AWQ
Sent from Mail for Windows
You have a lot of losses along the way. Increasing the wire thickness will help a bit.
A 28-0-28 full wave rectification should produce 39.6V ( less the diode drop) at full input power. The variac may be contributing to the sag as well.
A regulated supply would be the optimal solution.
My 50A is a switch mode power supply that was designed for a military radio rack , so it has RFI filters.
EP is the 590V it should be with 28V at the supply end.
On Saturday, February 5, 2022, 10:36:29 AM EST, Mark K3MSB <[email protected]> wrote:
Your comments are much appreciated. I will attempt to answer all questions in a single response.
Dynamotor rebuild: Yes, I rebuilt it when I put the dynamotor into service about 10 years ago.
All caps were replaced in the BC-456 modulator.
This is what I'm using now, and it's not working out well. I measure 26.8V power off.
Power On Key Up: 23.8V Power On Key Down: 23.5V. This gives me an Ep of only 480V.
The above voltages are measured directly on the battery terminals.
Wire Sizes: This could be an issue and I realized it early on. I'm using 18 AWG doubled up.
That means each connection uses two wires in parallel.
Not optimal, but a single 18 AWG wire wouldn't do.
Changing to a larger wire size isn't a problem (I just have to get some).
The dynamotor is spec'd at 5.8 A @ 28VDC so I figured paralleled 18 gauge wire should work.
28-0-28V 50A. Using a full wave bridge rectifier. Positive lead goes to 0.5 ohm resistor for current inrush.
That's connected to a 20,000 uF filter capacitor. Using a 680 ohm 2W bleeder resistor.
I would prefer to use a 1 ohm, but didn't have one of sufficient wattage.
Using a IM-11 VTVM for all the following measurements (unless otherwise indicated)
I have panel meters for voltage and current in circuit -- voltages agree with VTVM readings.
With the Panel Meter set for 28 VDC:
Voltages / current measured at the filter capacitor terminals:
Key Down: 24V 7.5 A. 510V Ep 30W output
Voltages measured at the bridge rectifier output:
Voltages measured at the transformer output (using DMM)
If I increase the voltage to 30 VDC measured at the filter capacitor output:
Key Down: 25V 7.5A Ep 550V (which is what it should be) 40W output
Current spike on startup: Yup it is !! As expected.
No. Set it for 28VDC key up. 32 VDC would put 16 VDC on each of the 12.6 V rated tube filaments which will significantly shorten their service life
Robert Downs
I'm working on a new 28V power supply for my SCR-274N system as well as other projects.
I'm using a 28-0-28 V 50A Transformer with a bridge rectifier (so I'm not using the CT). I'm using a 220V 16A Variac to set the primary voltage.
When the setting set for 28V key up, I'm getting about a 4 to 5 volt drop key down.
I didn't expect that much voltage drop as the transformer is pretty stout for an SCR-274N.
Can I run the system on 30 to 32V key up without damaging the dynamotor?
This would be a short term fix as I suspect I'll need a VR circuit.
73 Mark K3MSB
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