[ARC5] SCR-274N Dynamotor Question

[J Mcvey]

 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    4AKey Down:  24V  7.5 A.   510V Ep   30W output

Voltages measured at the bridge rectifier output:Key up:  29 V DCKey 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  4AKey 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 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 

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