Question on AC Motor Conrtol

MarcL

I am a computer programmer and am learning electronics and microcontrollers (or at least trying to).· I seem to be having trouble not with microcontroller programming, but with what electronic equiptment I need to build a project.· Can anyone here shed some light on what I need to buy and point me in the right direction?· Here is what I am trying to do:

I have a simple machine that runs on 120 ac current and an adjustable user timer so the user can run the machine for 1-10 minutes depending on what he/she wants to do.· Within this machine, there are water pumps, air pumps, and three motors all running on the same AC voltage.· After reading a microcontroller programming book, I would like to automate this system for the user.·

Part #1) I would like to implement a microcontroller so I can tell the machine to when to power motor A, when to run the water pump, when to run the air pump, and how long each should run for.·

Part #2)· I would like to implement a tiny touchscreen lcd so the user can select options from there.·


I am just not sure where to begin and what to buy for this project.· Can anybody help?·


Marc

Chris Savage

How much current do each of the 120VAC devices draw?· Being new to electronics makes it difficult to recommend options for controlling such devices, since high-voltage devices require a great deal of safety and attention to details.

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Chris Savage
Parallax Tech Support
csavage@parallax.com

MarcL

The water pump, air pump, and cooling fans draw a very small current.· Less than 4 watts each.· There is a small motor that is 2 amps and the large motor is 5.5 amps.· Total power consumption for the entire machine is less than 900 wats with everything going.· I appreciate the attention to safety as I want this project to be done correctly.· I am anxiously awaiting·your response so I know what parts to purchase!

LoopyByteloose

Relays may be more reliable for the higher amperage AC motors.
If you don't want speed control, it would be much simpler too.

There are solid state relays that can handle such power, but they are tempermental with inductive loads.· It is best to have the relay shut down bottom sides of the AC for safety.· The solid state relays usually are single poled.

The simplest approach is Parallax's Opt-22 Relay board as you can mix and match plug-in relays with input isolation. In the case of failure, you can just remove and replace quickly. {I looked at the Parallax Industrial secton and I didn't see anything for 5AMPs at 120VAC - so you may have to shop elsewhere for heavier relays to be connected off the board.}

There are several· relay systems out there, but you have to create your own interface to the BasicStamp. So, it really depends on how much you want to build and how much you want to program.· Visiting a retail outlent in person will give you more clarity that shopping on the Internet.·It is easier to visualize size and the completed system.

Parallax also has a complete unit for industrial control with isolation [noparse][[/noparse]the Stamp PLC]. If you are considering use and reuse of such an item on several projects, it may be worth the investment.

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The touch screen is more of a challenge. You will have to locate a product that can do both Input and Output. If it has serial interface, it won't place much demand on the Stamp, but the cost may be more than your machine control. If it has parrallel interface, you may have to use two Stamps.

Parallax has a very little [noparse][[/noparse]2x8] LCD with 4 buttons. This may be your best choice. Don't let the small size fool you as there is 'hidden display memory' that you can jump or scroll too.

Touch Screens are very demanding of programing resources and knowledge. Are you considering reverse engineering a Palm replacement touch screen or using a full-sized one?

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G. Herzog in Taiwan

Post Edited (Kramer) : 10/2/2005 11:56:44 AM GMT

MarcL

Thanks for the input.· I do not need any motor speed control, just an on/off mechanism for the motors.· So you are saying I need a solid state relay and the OPT-22 relay board would be a good start too?· Are there any other parts I would need?


I'm not so much worried about the touch screen as that would be an add-on to the completed machine.· The 2X4" lcd you reccomended would probably do the job well.· I would eventually like to use a nice color 5 X 7 screen, but that can be done after part 1.·

I think you are right on going to a store versus the internet, but I don't have any stores close to me.·



·

Chris Savage

If you use Solid-State Relays, besure they can handle the current.· Many of the more common ones are around 3A, but your one pump runs 5.5A, so you should probably get a 10A SSR and make sure you fuse everything right where the power comes in, before any control circuitry.

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Chris Savage
Parallax Tech Support
csavage@parallax.com

LoopyByteloose

Hi MarcL,
While I really do like the idea of solid state relays, what I was saying is to use the mechanical ones because I am not sure how well the solid-state relay will handle a large inductive load and the mechanical relay can be a double pole switch which provides complete shut-down in the off position.

Also, I have read that they may fail in an ON position which is not really optimal safety design --- you might get a runaway pump or machine.

The OPT-22 relay board and related controller board will be very helpful to eliminate a lot of electronic engineering and DIY construction. And you will have something that you can reconfigure for other things as you knowledge increases and your projects change.

Anything with 120VAC must be fused. You have both potential electrocution and fire hazards, so take fusing seriously. And put a ground to the case.

Parallax does have a Touch screen, but it is $300USD and I really sensed you are just beginning. That is quite an investment. The real advantage of the 2X8 LCD is that it uses the same pins for both input and output - so you have more pins available for other uses [noparse][[/noparse]It uses 7 of your 24 available pins]. You could do the same think with the other parallex LCD displays, but you would have to build your keypad.

You can download Parallax Texts for free. Take a look at 'What is a Microcontroller' and the Industrial Control text. Both will help a lot.

You managed to get me going and I build a little 4 relay board today in about 3 hours. The relays are controlled by the Stamp and use only 5 volts, but they can switch up to 0.5 amps at 120VAC or 1.2 amps at 24VDC.

As you see, with a little know-how - you can DIY many things. Some of us are more software oriented and some of us are more hardware oriented. What you buy really depends on where your pleasure most lies - in the building or the programing.

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G. Herzog in Taiwan

japer

hello
dont forget the power of 3's, startup on motors can be as high as
3 time the motor running and if the motors are in a dusty enviroment
were drag can be a problem this can be an issue, as can tempature extremes.

japer