Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!wuarchive!uunet!mcsun!ukc!slxsys!stevem
From: stevem@specialix.co.uk (Steven Murray)
Newsgroups: sci.electronics
Subject: Re: solid state relays
Message-ID: <1991May15.132910.1396@specialix.co.uk>
Date: 15 May 91 13:29:10 GMT
References: <12500@qisoff.phx.mcd.mot.com> <1991May14.135524.25014@wsrcc.com>
Organization: Specialix International, London
Lines: 25

wolfgang@wsrcc.com (Wolfgang S. Rupprecht) writes:

>hbg6@citek.mcdphx.mot.com writes:
>> I am building a controller for the evaporative cooler on my house. The
>> blower motor has two speeds, selected by providing power to either
>> the HIGH terminal, or the LOW terminal.

>> I really want to use solid state relays to avoid the cost and 
>> complexity of contactors and the associated drivers.

>You will also want to use a good snubber network and a zero crossing
>detector (for turn on).  This will reduce RF bursts that will lead to
>radio interference and also an early destruction of the SCR's.

Expanding on this a bit, driving motors with SCR's or solid state relays,
especially anything more than 'one motor, one relay' can be painful.  You
can tend to get problems with inductive spikes and back EMF - zero crossing
switch-on is fine, but that doesn't control the switch off.  Snubbers are
great, VDR's aren't such a bad idea either.  Aim to protect the SSR's with
VDR's rather than stop interference from the load.
-- 
Steven Murray
uunet!slxsys!stevem  stevem@specialix.co.uk
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