[ARC5] SCR-274N Dynamotor Question

Mon Feb 7 12:43:39 EST 2022

I will reiterate what I said early on.  A wet or gelled battery will do the same thing better and cheaper. The biggest advantage is the lack of a huge surge attendant with charging a big bank of DLCs or electrolytic.  The floating battery provides a low impedance to the load, and more accurately represents the environment thein which equipment was designed to operate.  Scott W7SVJSent from my Verizon, Samsung Galaxy smartphone
-------- Original message --------From: Doran Platt <jeepp at comcast.net> Date: 2/5/22  7:20 PM  (GMT-07:00) To: ARC-5 Mail List <arc5 at mailman.qth.net> Cc: ARC-5 List <arc5 at mailman.qth.net> Subject: Re: [ARC5] SCR-274N Dynamotor Question 

   Have you considered a super capacitor bank and appropriate pre-charge circuit. They are great to flatten inrush currents.

    On 02/05/2022 1:36 PM howard holden <holden7471 at msn.com> wrote:

    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

     From: J Mcvey via ARC5Sent: Saturday, February 5, 2022 9:32 AMTo: Mark K3MSBCc: ARC-5 ListSubject: 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> 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] On Behalf Of Mark K3MSBSent: Friday, February 4, 2022 19:38To: ARC5Subject: [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 

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