[sci.electronics] solid state relays

hbg6@citek.mcdphx.mot.com (05/09/91)

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. The current
spec. for the relays I have is in range but I have three concerns:

1 The SSR can easily handle the run current of the motor but is
the starting current likely to kill it?

2 Is the normal failure mode for a SSR open or shorted?

3 What would happen if both the HIGH and LOW terminals on the
motor are powered at the same time.

Bottom line; if the SSR fails closed and power is applied to
both terminals am I going to burn up a $200 blower motor?

Thanks,
John (everything over TTL is high voltage) Schuch

.....................................................................
John Schuch - Motorola Inc., Computer Systems Division (602)438-3008
All opinions expressed are mine and not Motorolas,        their loss.
.....................................................................

youngqd@jacobs.CS.ORST.EDU (Dean Youngquist) (05/12/91)

In article <12500@qisoff.phx.mcd.mot.com> 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.

>text deleted

>Bottom line; if the SSR fails closed and power is applied to
>both terminals am I going to burn up a $200 blower motor?
>
The SSR could fail either open or shorted.  I would suggest some old-
fashioned slow-blow fuses in series with them power source before it splits
into the high/low windings.  If both windings were somehow powered
simultainiously the fuse would blow.  Make sure the fuse is not rated too
high over normal maximam current.

Dean Youngquist
youngqd@jacobs.cs.orst.edu

wolfgang@wsrcc.com (Wolfgang S. Rupprecht) (05/14/91)

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. The current
> spec. for the relays I have is in range but I have three concerns:

> 1 The SSR can easily handle the run current of the motor but is
> the starting current likely to kill it?

Yes.  Of course.  The more you beat on a device the faster it will
wear out.  While the SCR's (as any semiconductor) can tolerate a bit
of extra surge current, it does stress them.  Do this too often and
you break them.

> 2 Is the normal failure mode for a SSR open or shorted?

Both.  This is what fuses are made for.

> 3 What would happen if both the HIGH and LOW terminals on the
> motor are powered at the same time.

Lots of current would be drawn.  Smoke would be sure to follow.
Use a simple interlock between the two SCR's, and USE A FUSE.

> Bottom line; if the SSR fails closed and power is applied to
> both terminals am I going to burn up a $200 blower motor?

Not if you use a fuse.  ;-)

Oh one more thing.  You should really think about what the voltage on
the low speed SCR will look like when the high speed winding is
energized.  Hint: You will probably see quite a bit more than the
input voltage across the low-speed SCR.  Rate this SCR appropriately.

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.

-wolfgang
-- 
Wolfgang Rupprecht    wolfgang@wsrcc.com (or) uunet!wsrcc!wolfgang
Snail Mail Address:   Box 6524, Alexandria, VA 22306-0524

whit@milton.u.washington.edu (John Whitmore) (05/15/91)

In article <1991May14.135524.25014@wsrcc.com> wolfgang@wsrcc.com (Wolfgang S. Rupprecht) writes:
>hbg6@citek.mcdphx.mot.com writes:

>> I really want to use solid state relays to avoid the cost and 
>> complexity of contactors and the associated drivers. The current
>> spec. for the relays I have is in range but I have three concerns:
>
>> 1 The SSR can easily handle the run current of the motor but is
>> the starting current likely to kill it?

>Yes.  Of course.

	Nope, I don't agree.  Unlike bipolar transistors, SCRs have
no 'hot-spot' failure mode (they're like MOSFETs in that regard),
so they are EXTREMELY tolerant of surges.  Motorola gives their
12A SCRs a 300A surge rating; your motor is unlikely to kill
'em.  Lightning strikes, however...

>> Bottom line; if the SSR fails closed and power is applied to
>> both terminals am I going to burn up a $200 blower motor?

>Not if you use a fuse.  ;-)

	Well said.

	John Whitmore

stevem@specialix.co.uk (Steven Murray) (05/15/91)

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
I am speaking, but  | If these are your opinions, then we are in agreement!!
not for my employer.| Flames, spelling errors, complaints > /dev/null

winans@sirius.mcs.anl.gov (John Winans) (05/16/91)

In article <1991May15.132910.1396@specialix.co.uk> stevem@specialix.co.uk (Steven Murray) writes:

[stuff deleted]

>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.

Excuse my ignorance please... but... uhmmm.....  what is a VDR?

--
! John Winans                     Advanced Computing Research Facility  !
! winans@mcs.anl.gov              Argonne National Laboratory, Illinois !
!                                                                       !
!"The large print giveth, and the small print taketh away"-- Tom Waits  !

jbertoia@medar.com (Jeffrey A. Bertoia) (05/18/91)

In article <1991May15.132910.1396@specialix.co.uk> stevem@specialix.co.uk (Steven Murray) writes:
>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.
>
Good move!

>>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.

Expanding yet further and refuting some.

In the following text I use SCR/SSR relatively interchangably.

The zero crossing detector for turn on will definately reduce the RFI generated
but will have little effect on the overall life of the SCR/SSR as long as it
has been properly sized for the application.  Note: In sizing you must consider
not only the steady state but also the surge ratings.

Medar's business is resistance welding controls which generally involves phase
controlling a very large transformer.  More often than not we swicth SCR's on
at times other than the voltage zero-crossing.  We rarely 'lose' SCRs due to
current or voltage transients.  A far more common failure mode is improper
cooling.

Now a snubber, will help extend the life of an SCR in it's off state (i.e. it
will help protect the SCR for overvoltage transients in it's non-conducting
mode).  It will also assist in turn-on if the gate drive circuit is
inadequate,  however if I remember, the original post regarded SSRs which
have an integral drive circuit which is properly sized.  The problem with
snubbers in these applications is that the off state over voltage
protection is marginal due to the capacitors inability to store the energy.

An additional consideration in using a snubber is that 'some' voltage will
be present on the load side of the snubber at _all_ times do to the leakage
through the snubber.  This may or may not be tolerable.

If overvoltage is the main concern a MOV (Metal Oxide Varistor) will do the
job nicely.  A MOV will 'break-over' at a specified voltage quencing the
spike.  Note: Size the MOV correctly otherwise things WILL get exciting...
Lots of noise and smoke will follow.

As I remember the original post... There was also a question about what if
the SSR fails on.  One of the frequent failure modes in SCRs is to fail on,
but the is not necessarily a problem.  The circuit can be properly fused to
stop any potential problems.  Most motors have sigificant thermal mass and
can tolerate short over current conditions.  For the very cautious a small
protection circuit using (small) interlocking relays should be no problem
to design thus preventing both SSRs from coming on simultaniously.  This
exercise is left to the reader.

jeff
-- 
Jeffrey A. Bertoia				Medar, Inc.
...!uunet!medar!jbertoia			38700 Grand River Ave.
jbertoia@medar.com				Farmington Hills, MI 48335

jeffd@csd4.csd.uwm.edu (Jeffrey Alan Ding) (05/31/91)

In article <4814@cocoa46.UUCP> reichert@motcid.UUCP (Chuck KD9JQ) writes:
>Tried E-Mail...but no luck
>
>John....What you want is a "Zero Crossing" Solid State Relay that operates
>off +5VDC.  These will switch the 240VAC or 120VAC line when the Voltage
>when the line voltage is zero, therefore very little current is switched!
>
>		Chuck Reichert  KD9JQ


Does anybody make a Solid State Relay that DOES NOT have the Zero Crossing
circuit inside?  If so I would be interested who and where I can get them.

Please replay E-mail.  Thanks.

jeffd@csd4.csd.uwm.edu