[Amps] Crouzet SSR - FAQs

Jim Thomson jim.thom at telus.net
Tue Feb 11 02:27:58 EST 2014

Message: 8
Date: Mon, 10 Feb 2014 19:46:26 -0600
From: "Paul Kraemer" <elespe at lisco.com>
To: <amps at contesting.com>
Subject: [Amps] Crouzet SSR - FAQs

There is a wealth of information here worthy of reading which may further 
understanding of solid state relays, the application, what they can and 
can't do and the myth of zero turn on related to inductive loads


Paul K0UYA 

##  Scroll down part way in the url...and  u will see that the AC SSRs   all use ZERO CURRENT shut off. 

They also state that SCR  SSRs are better for high inrush and inductive loads......VS the resistive load  SSR’s. 

I looked at P+B  70A  SSRs  years ago..with one in each leg of the 240 line.  The heat sink requirements  were huge..
like 1 square foot  PER 70A  SSR device.  And that was using .25”  thick AL plate. 

P+B did have one good design example however....like a typ industrial application.   The example they used was a 150 HP
electric motor, running on 600V, 3 phase power.  The SSRs  could be series for more V....and paralleled for more I.  It all works,

I can see the worse case scenario would be the case where the amp is running 1.5 kw out RTTY – FM..and the SSRs are then
shut off.  The back EMF  would be just wicked.  Transorbs are built into some SSRs.    Transorbs and movs can also be
wired across the output of each SSR  device. 

The peak current every 8.3 msecs in normal operation is wicked.... like 4-6  times what you measure with a clamp on ammeter.
This is with a C input filter.  Most SSRs I have seen still have a small leakage current on their outputs.  

In the end, I gave up on em....and used heavy duty contactors instead.  135A CCS,  3 pole contactors, NIB, are still only 
$50.00  each at the surplus place in the usa.   They come with 220 vac coils....so the coils can be run on anything from
208-240 vac  easily..with no drop resistor required.   They also come with 3 x sets of AUX contacts.   You can do the
same with 20-50A  3 pole contactors. 

We paralleled all 3 x poles with 1 inch x 1 inch  Angle AL  to make 1 big 400A  spst contactor.   Ditto with a 2nd contactor.
1 assy used in each leg of the 240 V line.  A 3rd assy used for the step start.    For a laugh, I bought several  3 pole, 300A
CCS contactors. These used 120 vac for the coil.... but 1st went to a FWB..with no cap.   The raw output is 108 vdc, which
is what is actually applied to the solenoid coil. 

SSRs have their place...... but with the big heat sink requirements in some cases,  I really see no real practical advantage...except 
a SSR can be shut off faster....which will  eliminate follow on current.  A HV fuse in series with a 50 ohm glitch resistor in series with 
the B+ is ample for most cases.  A 2nd HV fuse can be wired in series between the plate xfmr sec and FWB.   If you are real hardcore,
you can use an adjustable  plate overcurrent  setup.....to open off the coil to the contactor  in the 240 vac primary.   

Jim   VE7RF 

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