Crydom SSRs - tips?
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From: Ecnerwal on 9 Apr 2010 18:12 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
From: Jasen Betts on 10 Apr 2010 09:02 On 2010-04-09, Ecnerwal <MyNameForward(a)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. Bye, --- news://freenews.netfront.net/ - complaints: news(a)netfront.net ---
From: Paul E. Schoen on 13 Apr 2010 14:20 "Jasen Betts" <jasen(a)xnet.co.nz> wrote in message news:hppsu3$dek$1(a)reversiblemaps.ath.cx... > On 2010-04-09, Ecnerwal <MyNameForward(a)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. 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
From: Ecnerwal on 13 Apr 2010 17:20 In article <qB2xn.80132$ao7.13085(a)newsfe21.iad>, "Paul E. Schoen" <paul(a)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 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
From: Ecnerwal on 13 Apr 2010 17:26
In article <MyNameForward-295C88.17205213042010(a)news.eternal-september.org>, Ecnerwal <MyNameForward(a)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. -- Cats, coffee, chocolate...vices to live by |