Can I use an SSR with a motor - What about 220v? 1

[Douggg]

I'd like to know if I can use SSRs with a motor? If the motor is 220v do I use one on each leg?

Here's my application. I'm installing new pumps for my swiming pool. Filter and booster. After pricing the cost of a timer switch, (around $400) I got the idea of using SSRs.

My pumps are 220v and draw 8 amps when running. I have several SSR rated at 20a 30-280vac I was thinking of using a pair, one on each leg of the AC. (I’m in the US.) Or is there something wrong with my thinking?

Now for the timer. I have 2 empty spots on garden sprinkler system timer. Garden sprinkler solenoids are 24vac. My thinking is to use a diode (to make the AC DC), and add a cap (to smooth out the half wave).

See where I am going with this?

Here are my questions.

Can SSRs be used to control motors?
Or is there a problem with in-rush amperage?
Get too hot with motors?

Can they be used on 220v sinlge phase motors?
Do I use 1? Just one on one of the hot legs?
Or use 2? One on each of the hot legs?

Can 3-32vdc SSRs be controlled with 24vac? Do I need a diode and a cap? Or will the diode in the SSR work without the need for additional components?

Thanks for your advice.

 

[Skogsgurra]

Don't tinker with that if you have to ask those questions. You have to buy the right stuff and also have an electrician install it.

I might be an interesting excersise to try these things out on the lab bench. But let it stay there.

Gunnar Englund

http://www.gke.org

 

[itsmoked]

Besides I think you would have a problem with SSRs running a motor because of the starting current as you've considered.

You don't need to break both lines for a 240 load but I would if it had to do with a pool. Pools have a few subtle requirements for safety. Things like SSRs leak current even when they are off. Probably enough to kill someone. That's one reason you would want a contactor in lue of an SSR.

If you must proceed you could run the SSRs from your controller and then run contactors that break both legs to run the pumps.

Get the contactors working just the way you want them before going any further.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[OperaHouse]

Hope you are also including a GFI in this circuit too. You can likely find a complete system on ebay for less than you can obtain the pieces. I have an old hot tub and have found a great selection of replacement parts there for cheap. If you are set on this project, use a motor start contactor.

 

[Mike2468]

Since the number of on/off cycles are probably low, a 2 pole mechanical relay might be the best option. And, yes, there are very strict rules about GFIs and lock-out devices.

The slight leakage from an SSR would typically not be a problem since the pump's electronics "should be" completely isolated from the water. By the time that you are replacing the pumps, I do not know what condition that other electronics are in. Therefore, don't use an SSR. Use a simple timer and a simple mechanical contactor.

Gunner is correct. For a one-time installation, it is best to use hardware and an electrician who specifically are recommended and insured for a pool application.

 

[Douggg]

Thank you for your replies and sugestions. I have been looking for a qualified electrician who is familiar with SSRs and have been unsucessful. Do you know of anyone?

In all of your replies, no one addressed any of my questions.

Can SSRs be used to control motors?
Or is there a problem with in-rush amperage?
Get too hot with motors?

Can they be used on 220v sinlge phase motors?
Do I use 1? Just one on one of the hot legs?
Or use 2? One on each of the hot legs?

Can 3-32vdc SSRs be controlled with 24vac? Do I need a diode and a cap? Or will the diode in the SSR work without the need for additional components?

 

[Skogsgurra]

I will answer all your questions. But you should still leave this to an electrician.

Yes, SSRs can be used to control many types of motors.
Yes, there is a problem with in-rush current when starting most motors. This must be considered when choosing SSR.
"Get too hot with motors"" Do not really understand. It is like asking "Will this stick break?"

Yes, they can be used with 220 V (and the more common 230 V) motors.
Yes, you can use one in the hot wire.
Yes, you can use two. Do you really have two hot wires?

No. That would not work very well. You will probably get some sort of synchronous rectification which will result in DC magnetization of the motor. That will make current increase beyound the capacity of the mains fuse.
You will be much better off with "a diode and a cap" (I assume that you are referring to a rectifier with a smoothing capacitor). Beware, the rectified 24 V AC will give you just above 32 V DC. Control voltages from a transformer usually varies about +/-10 percent. That makes the voltage quite close to what the SSR input is rated for. You may need to do something about that.

Gunnar Englund

http://www.gke.org

 

[itsmoked]

[red]"In all of your replies, no one addressed any of my questions."[/red]

Are you serious?
I thought we did quite well...

SEE ###

Can SSRs be used to control motors?
### Yes with restrictions. Normally it's not done.

Or is there a problem with in-rush amperage?
### As I already stated YES there CAN be a problem with "starting current" (It is not called in-rush.)

Get too hot with motors?
### They only get too hot if the are incorrectly heat-sinked or over currented or both. The starting current flat blows them out if they aren't carefully sized and selected.

Can they be used on 220v single phase motors?
### To repeat; yes! (with design restrictions)

Do I use 1? Just one on one of the hot legs?
### (If you are insane enough to use one, on a motor, in pool service and we all recommend you don't) You only use one SSR to control any SSR controlled load.

Or use 2? One on each of the hot legs?
### See previous.

Can 3-32vdc SSRs be controlled with 24vac? Do I need a diode and a cap?
### Yes you need a diode and a cap

Or will the diode in the SSR work without the need for additional components?
### There is no diode in the input of most SSRs.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[machmech]

Hello Douggg

Really good emphasis on SAFETY here from all, Sure hope your real careful. loved ones can not be replaced!


Best wishes, Chuck

 

[OperaHouse]

Your first question was about operating a solid state relay from a 24V AC source. I suggest that you use a full wave rectifier, four diodes in a bridge, along with a small cap of 47-100uF on the input. The reason for this is that the timer may have a triac solid state output. A small resistor (470 ohm) may be needed to insure proper turn on/off by providing a suitable load on the circuit. If it is known to be a mechanical switch or relay you can use a single diode half wave rectification with the cap.

 

[Douggg]

Thank you to all who replied, very much appreciated. I've learned some things from your replies.

From your replies I have decided not to use an SSR to contoll the pumps. Instaed I will use a relays. I have a 1hp motor - 8a , so I’m thinking the ice cube type will work just fine. I have a couple and they are rated a 1.5 hp at 220v.

Contactors are used for higher amperage, right? Isn’t a contactor just a realy with replacable contacts? (The higher amperage reduces the life of the contacts.) Or does a contactor have additional circuitry?

As for pool/water safety, I would use the same GFI with an SSR as I would with a relay, right?

To prevent arcing/pitting of relay contacts, I was taugt to place a 47 microfarad/600v cap across the relay contacts. (Xc=1(2 pi f c) With a 47 micfrd cap Xc is very high value at 60hz. Is there any reason one would not want to do this? In the relay circuits I have seen it is uncommon to see a cap across the relay contacts. Can you tell me why?

Thank again. Your reples are very much appreciated.

 

[Skogsgurra]

It is very satisfying that you chose to choose a relay/contactor. It will make things safer.

Why then, spoil it all by connecting a 47 uF capacitor parallel to each contact? That, if anything, will destroy your contacts very rapidly. 47 microfarads is a very large capacitance and whenever the contacts are closed, these capacitors will discharge through the contacts and cause a very high momentary current.

I think you shall stop listening to that adviser. Not only will such a hefty capacitor ruin your contacts, it will also kill you if you think that the motor wires are safe because the contactor is off. At 220 (or 230) volts, a 47 uF capacitor will pass something between 60 and 70 milliamps. And that is not safe at all to the human body.

If you are concerned about electrical interference from the switching (which is why capacitors are placed parallel to contacts, then use something like 0.1 uF in series with 100 - 330 ohms). But that is nothing you need in an installation like the one you are describing.

See? There is way too much to think about. You shall absolutely contact an electrician for this job. And finding one that knows about SSRs, as you mentioned, is not at all difficult. How many did you really ask? (Yes, I know, you are not going to use SSRs).

Gunnar Englund

http://www.gke.org

 

[Douggg]

I have asked 15 licensed electricians, none know how to use a SSR.

I just reclaulated the cap value, I am in error.

Looks like it should be aaround a 1 pfd, NOT a 47 mfd. A 1 pfd has an Xc of 2.6 gigohms. Wouldn't that prevent arcing across the relay contacts?

Thanks

 

[Skogsgurra]

No. That is the same capacitance that you get from half an inch of a hook-up wire. As I said, leave this to an electrician. He hasn't got his head full of irrelevant facts.

Gunnar Englund

http://www.gke.org

 

[itsmoked]

Do not use any capacitor! None of the other billion pool pumps add that useless failure point. Why should you? It also provides a dangerous leakage avenue that should not be included. Bad idea!

You can use any relay you want that categorically states on its case 1HP 220(230)(240)as this means the relay can handle the starting current and can handle the stopping arc.(WITHOUT CAPS!)

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Douggg]

Gunnar,

Care to explain? As I said I have contacted 15 licensed electricians, none have used capacitors or SSRs. Where would I find one who is knowledgeable?

Can you tell me why some relays have caps across the contacts? Isn't it to prevent arcing?

With all do respect I'm posting so I can learn.

Can you tell me why at 60Hz you claim a 1 pfd cap is the same as a hook-up wire? I take that to mean a short? If one claculates it, its in the megohm range.

http://www.sengpielaudio.com/calculator-RC.htm

From what you are saying the formula for deturmining Xc is wrong. Pleae explain. Thank you.

 

[Skogsgurra]

No. I do not care to "educate" you. I have said what there is to say. Only someone that is eagerly trying to misunderstand would understand that the hook up wire should be used as a shorting link.

You have overtaxed my and still some engineers patience on a very early stage. This site is for discussing engineering problems and your hobby project is not such a problem. A discussion needs some basic knowledge to be meaningful. You don't have that knowledge.

BTW. The calculator in your link gives correct results. I see no problem there.

Gunnar Englund

http://www.gke.org

 

[itsmoked]

The electricians who deny knowning about capacitors across 'your' contacts do so because it isn't done. Skip the capacitor idea entirely... expunge it from your mind! piffft!!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Douggg]

This has nothing to do with the pool pump motors nor have I asked any electricians about the caps.

I have seen cap across relay contacts. My college electronics instructor was the one who taught us about the use of them while demonstrating capacitive reactance – Xc

Interesting that there are so many web sites that have the formula, here’s another and the resutling calculation is the same.

http://www.nepsi.com/capacitivereactance.htm

I’d like to know why you disagree.

 

[itsmoked]

Caps across relay contacts are to reduce radio frequency interference(RFI). So you might want to do that if you have a relay buried inside some electronics that are particularly susceptible to RFI. Otherwise for a pump or other such 'brute force' machine it isn't done. Furthermore it usually isn't just a cap, it's a snubber - a resistor in series with a cap.

Google "snubber".

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com