[ARC5] SCR-274N Dynamotor Question

Sat Feb 5 13:36:21 EST 2022

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<https://go.microsoft.com/fwlink/?LinkId=550986> for Windows

From: J Mcvey via ARC5<mailto:arc5 at mailman.qth.net>
Sent: Saturday, February 5, 2022 9:32 AM
To: Mark K3MSB<mailto:mark.k3msb at gmail.com>
Cc: ARC-5 List<mailto:arc5 at mailman.qth.net>
Subject: Re: [ARC5] SCR-274N Dynamotor Question

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 <mark.k3msb at gmail.com> wrote:

GM Gents

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.

Float Power Supply:
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.

New Power Supply:

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 up:  28V    4A
Key Down:  24V  7.5 A.   510V Ep   30W output

Voltages measured at the bridge rectifier output:
Key up:  29 V DC
Key Down:  26 VDC

Voltages measured at the transformer output (using DMM)
Key up:  28.34 VAC
Key Down:  27.41 VAC

If I increase the voltage to 30 VDC measured at the filter capacitor output:
Key up:  30V  4A
Key Down:  25V  7.5A    Ep 550V (which is what it should be)  40W output

Current spike on startup: Yup it is !!   As expected.

73 Mark K3MSB

On Sat, Feb 5, 2022 at 2:29 AM Robert Downs <wa5cab at cs.com<mailto:wa5cab at cs.com>> wrote:

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

From: arc5-bounces at mailman.qth.net<mailto:arc5-bounces at mailman.qth.net> [mailto:arc5-bounces at mailman.qth.net<mailto:arc5-bounces at mailman.qth.net>] On Behalf Of Mark K3MSB
Sent: Friday, February 4, 2022 19:38
To: ARC5
Subject: [ARC5] SCR-274N Dynamotor Question

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
______________________________________________________________
ARC5 mailing list
Home: http://mailman.qth.net/mailman/listinfo/arc5<https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fmailman.qth.net%2Fmailman%2Flistinfo%2Farc5&data=04%7C01%7C%7C02b2dccd4cc44c83c0c008d9e8cd716d%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C637796791360541169%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=TGaSeKrhetzpIDXqCqNrfQIaTTV87jGBRLsxt4gmJhE%3D&reserved=0>
Help: http://mailman.qth.net/mmfaq.htm<https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fmailman.qth.net%2Fmmfaq.htm&data=04%7C01%7C%7C02b2dccd4cc44c83c0c008d9e8cd716d%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C637796791360541169%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=VUUK3V3dgTLPtUD9qRFufdccqLrSqzJ3%2BWGNltLYmDE%3D&reserved=0>
Post: mailto:ARC5 at mailman.qth.net<mailto:ARC5 at mailman.qth.net>

This list hosted by: http://www.qsl.net<https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.qsl.net%2F&data=04%7C01%7C%7C02b2dccd4cc44c83c0c008d9e8cd716d%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C637796791360541169%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=JuA2zaFCofgrXysiV08jXnJ4TT%2Bm8i2YoN6AdOVx5Y4%3D&reserved=0>
Please help support this email list: https://www.qsl.net/donate.html<https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.qsl.net%2Fdonate.html&data=04%7C01%7C%7C02b2dccd4cc44c83c0c008d9e8cd716d%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C637796791360541169%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=I7C5vRKF9IVHy6r68Cj9Zz7uhZslbSH6BrBkGTpujXU%3D&reserved=0>

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/arc5/attachments/20220205/a88c12a9/attachment-0001.html>