Crydom SSRs - tips?

E

Ecnerwal

Guest
My well pipe comes into the building about 50 feet from where the
electrical service comes in and the well power goes out. Rather than run
100 feet of wire that actually carries current to get to the pressure
switch & back, I've grabbed a somewhat oversized Crydom "brick" SSR to
power the pump by the direct route, and will use the pressure switch &
100 feet of wire just to turn the Crydom (an AC input model) on.

I see that heat-sinking is rather important for aggressive uses of these
- I'll be down the lower end of that, having gotten a 75 amp unit for a
6.4 amp full-load / 25 amp locked-rotor pump motor. Couldn't see any
downside to overkill in the specs, and the high-amp units have better
junction to case heat flow.

Any gotcha's not obvious from the spec sheet I should be on the lookout
for? These certainly seem like very handy parts. I could see making more
use of them.

--
Cats, coffee, chocolate...vices to live by
 
On 2010-04-09, Ecnerwal <MyNameForward@ReplaceWithMyVices.Com.invalid> wrote:
My well pipe comes into the building about 50 feet from where the
electrical service comes in and the well power goes out. Rather than run
100 feet of wire that actually carries current to get to the pressure
switch & back, I've grabbed a somewhat oversized Crydom "brick" SSR to
power the pump by the direct route, and will use the pressure switch &
100 feet of wire just to turn the Crydom (an AC input model) on.

I see that heat-sinking is rather important for aggressive uses of these
- I'll be down the lower end of that, having gotten a 75 amp unit for a
6.4 amp full-load / 25 amp locked-rotor pump motor. Couldn't see any
downside to overkill in the specs, and the high-amp units have better
junction to case heat flow.

Any gotcha's not obvious from the spec sheet I should be on the lookout
for? These certainly seem like very handy parts. I could see making more
use of them.
Click to expand...
Your pump motor is an inductive load that may require special treatment.
check the datasheet.

Bye,

--- news://freenews.netfront.net/ - complaints: news@netfront.net ---
 
"Jasen Betts" <jasen@xnet.co.nz> wrote in message
news:hppsu3$dek$1@reversiblemaps.ath.cx...
On 2010-04-09, Ecnerwal <MyNameForward@ReplaceWithMyVices.Com.invalid
wrote:
My well pipe comes into the building about 50 feet from where the
electrical service comes in and the well power goes out. Rather than run
100 feet of wire that actually carries current to get to the pressure
switch & back, I've grabbed a somewhat oversized Crydom "brick" SSR to
power the pump by the direct route, and will use the pressure switch &
100 feet of wire just to turn the Crydom (an AC input model) on.

I see that heat-sinking is rather important for aggressive uses of these
- I'll be down the lower end of that, having gotten a 75 amp unit for a
6.4 amp full-load / 25 amp locked-rotor pump motor. Couldn't see any
downside to overkill in the specs, and the high-amp units have better
junction to case heat flow.

Any gotcha's not obvious from the spec sheet I should be on the lookout
for? These certainly seem like very handy parts. I could see making more
use of them.

Your pump motor is an inductive load that may require special treatment.
check the datasheet.
Click to expand...
Here is a datasheet from the Crydom website for peak-firing SSRs designed
for high inductive loads. But they are all DC controlled.
http://www.crydom.com/en/Products/Catalog/p_s24.pdf

The complete catalog shows many SSRs with random (or phase-controllable)
turn-on (with a -10 suffix).
http://www.crydom.com/en/Tech/crydom_us.pdf

I have used these with a custom-made SCR firing circuit that provides
adjustable initial phase angle to minimize the inrush caused by the DC
offset which occurs with zero crossing turn-on. It can be done with a 555
timer and a zero-crossing detector.

Here is detailed article on DC offset. Probably way more than you need to
know:
http://www.basler.com/downloads/CTerrors.pdf

Here is a somewhat weird website but it has good information on motor
startup inrush currents:
http://yxx2007.spaces.live.com/

A rather involved discussion about inrush in a big pump motor:
http://www.eng-tips.com/viewthread.cfm?qid=29146

More about big motors:
http://www.alstraindustries.com/Troubleshooting%20Induction%20motors.pdf

Probably "too much information". I thought I could find a simple reference
that explained this phenomenon. What you can do is run an LTSpice simulation
where you apply a sine wave to an inductor, and observe the current. It will
lag the voltage by 90 degrees, so if you apply a sine wave that starts at 90
degrees, the current will be a nice sine wave that starts at zero (or
course), but its amplitude peaks will be constant. However, if you start at
the zero crossing (0 degrees), the current waveform will have an initial
peak of about twice the normal and then a succession of diminishing peaks
until it stabilizes.

Another effect is remanent magnetism that will occur if there is an unequal
number of positive and negative half-cycles, which is essentially a DC
component. Depending on the phase applied next time, there could be a very
high peak surge because the laminations are already magnetized in that
direction, and saturation causes current similar to a locked rotor
condition.

Sorry if this is too much detail. If your SSR can handle the locked rotor
current and as long as it is protected by thermal overload and a time delay
circuit breaker or fuse, even a zero cross SSR should be OK.

Paul
 
In article <qB2xn.80132$ao7.13085@newsfe21.iad>,
"Paul E. Schoen" <paul@pstech-inc.com> wrote:

Sorry if this is too much detail. If your SSR can handle the locked rotor
current and as long as it is protected by thermal overload and a time delay
circuit breaker or fuse, even a zero cross SSR should be OK.

Paul
Click to expand...
Too much information is better than too little.

What I could glean from the data sheets (I have ones for the AC input
versions - this particular model is obsolete, making it inexpensive
through sources of supply other than where I found links to datasheets
at http://octopart.com/a4875-crydom-6963 ) seemed mostly to be promoting
random turn on specifically for things like phase control. When using it
simply as a relay, I could not find anything to indicate that ZTR was a
problem, but they are really not too detailed, either, and assume you
already know what the assumptions are, I think.

I do have a ZTO model, but given I went both over-amp and over-voltage
(75-480 for a 6/25-230 load) I suspect it will be fine. If not I'll buy
something different when I kill this one. There is a circuit breaker,
sized for the pump, feeding it, of course.

Without going to the bother of simulating it, I have some doubts about
behavior at turn-on, given both that a motor is not purely inductive,
and also that current following voltage 90 degrees does apply to a
steady state pure inductance & sinewave, but almost certainly does not
apply when switching one on in the discontinuity of the first half-cycle
or so. No voltage, no stored magnetic field, no current...current rises
with voltage, but as magnetic energy is stored, current begins to lag.
If turned on at peak voltage, there has to be an inrush in time with the
initial voltage, until field is built up, so handwaving a 90 degree lag
isn't going to cut it for that discontinuous condition. There will be an
inrush current to build up the field in either case.

Now, when it comes to turn off, there certainly will be current flowing
at zero volts, and then there will be the inductive spike in voltage
when the system is shut off. Much of that will probably be eaten by the
lightning suppression at the well head.

Starting the motor always takes more current (inrush) than running it
does - that's simply a matter of paying for the inertia needed to start
it from rest (nearly the equivalent of locked rotor) to the rated speed
in a few cycles.

None of this is controlled with the mechanical relay normally used for
this type of thing, so I can't imagine that it will be any worse for the
pump motor.

--
Cats, coffee, chocolate...vices to live by
 
In article
<MyNameForward-295C88.17205213042010@news.eternal-september.org>,
Ecnerwal <MyNameForward@ReplaceWithMyVices.Com.invalid> wrote:

Without going to the bother of simulating it
Click to expand...
This is primarily the bother of getting simulation set up on my computer
- something I've had in the copious spare time project list for a long
time. Copious is ironic in that description.

Otherwise I'd be quite happy to settle it - but it might be easier to do
with with an oscilloscope and a motor, around here - but then I'd need
one of those DC powered random turn-on SSRs to see how it varied in real
life.

--
Cats, coffee, chocolate...vices to live by
 
"Ecnerwal" <MyNameForward@ReplaceWithMyVices.Com.invalid> wrote in message
news:MyNameForward-613D64.17264813042010@news.eternal-september.org...
In article
MyNameForward-295C88.17205213042010@news.eternal-september.org>,
Ecnerwal <MyNameForward@ReplaceWithMyVices.Com.invalid> wrote:

Without going to the bother of simulating it

This is primarily the bother of getting simulation set up on my computer
- something I've had in the copious spare time project list for a long
time. Copious is ironic in that description.

Otherwise I'd be quite happy to settle it - but it might be easier to do
with with an oscilloscope and a motor, around here - but then I'd need
one of those DC powered random turn-on SSRs to see how it varied in real
life.
Click to expand...
You are correct that a motor is not purely inductive, and in fact if it is
capacitor start the current may actually lead the voltage for a while and
then it becomes a dynamic load which may be mostly resistive as it is doing
work. The turn-off of an inductive load with a SSR actually occurs when the
current reaches zero, but a motor load acts as a generator as it coasts to a
stop. And I think the starting capacitor may kick back in when the
centrifugal switch closes and it may act as a dynamic brake to some extent.
You may want to add an R-C snubber across the motor and/or the SSR. Some
SSRs have overvoltage protection which actually turns them back on to avoid
catastrophic breakdown.

You probably won't have any problems using your oversized SSR. But there is
a huge body of knowledge about various types of motors and their
characteristics and the best ways to control them. I am partial to 3 phase
induction motors and I have played around with variable frequency PWM motor
drives, and I even rewound an old 1/2 HP single phase pump motor to run on
three phase at about 8-12 VAC from a SLA battery.

Paul
 
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