[Amps] Crouzet SSR - FAQs
jim.thom at telus.net
Tue Feb 11 02:27:58 EST 2014
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
## 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.
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