Monitoring AC motors

Randy

I have several three speed 120 volt motors in FCUs (heating and/or cooling units) that I would like to monitor with a Stamp. I would like to know which speed they are on and if they are indeed running. A selecter switch determines what leads are feed 120 volts. I figure I could simply check for voltage on the three lines to determine what if any speed was selected, allthough I do have some questions about how to do that safely and not have gliches due to an AC wave form. The mystery to me is how to verify that they are running and not open or frozen.

First on the list is of course it must be inexpensive. Depending on the outcome of this experiment I could be looking at several hundred motors.

I don't know if I could monitor current cheaply but if so there are a number of issues that might this impractical.

While they are all similar there are at least ten different models of motors with slight speed differences and current draw. They are impedance protected so if they are stuck they will not pop a breaker but simply get warm, I don't know what the current draw is if frozen and I would guess it varies with the selected speed.·The load·each motor·drives is not the same as the blower wheels vary in size not to mention dirty filters and duct length.

Air flow will also not be consistant and many units have two motors into a common duct.

An optical·disk on the shaft would be labor intensive - not cheap.

Anyway so much for why I can't come up with a solution. Any ideas how to do this?

Randy
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stamptrol

Randy,

In the consulting business, there's a rule to keep in mind when promising the customer a solution:

- you can have it FAST
- you can have it CHEAP
- you can have it GOOD

.......pick any two!

You have some constraints no matter which way you choose to solve the current monitoring problem. Some options are:

1. monitor the selector switch position to know what speed the motor is supposed to be on. Detect that the fan is turning or not ( use Hall sensor and magnet ). This assumes of course that if the selector asks for speed 2, the fan actually gets to that speed.
2. put an isolated current sensor (with DC output voltage ) in the common lead of the motor. Each speed will have a different current draw. Get the stamp to monitor the current and select which speed is on at the moment.
3. generate a fan speed signal opticaly or with Hall sensor. Let stamp figure out how fast the fan is turning.

Being impedance protected, I take it that these are small motors so there's probably not a lot of room to add dollars to the price. Each option will require some extra hardware; each option will require a stamp and some programming; and testing.

Can you give us a feel for how much investment you can make in the project?

Cheers,

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Tom Sisk

http://www.siskconsult.com
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John R.

I would add that you might want to be looking at an SX chip for cost reasons over the STAMP.

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John R.

8 + 8 = 10

Randy

Thanks Tom and John

Cheap, cheap and cheap! The boss man is bugged by a dozen units and will spend maybe five hundred dollars to see what is going on with them. In the same breath he said if he likes the results he might want to do three hundred plus of them. That will never happen as I know the funds would not get approved for even a thousand dollars. I need to apply the post holiday stress question and see if he is serious.

My previous Stamp based solutions have been limited to a single Stamp. While I though of the cost and quantity of the other components I didn't give thought to the number of Stamps and the cost of them. I will look at SX if this is going to expand to numerous units. The programing scares me as I have trouble enough with PBASIC.

That said I still wish to pursue how to do this. I am wondering about using a vibration sensor on the fan housing to detect operation. All I need is a simple yes it is running or no it is not. By monitoring the selecter switch I will know what speed was selected, beyond that I only care that the motor has not failed. I have never used a vibration sensor before and don't have any idea if it would work. Thoughts?

Randy

Daphyl

One word of caution...

If it's anything like a evaporator or condenser fan you're going to monitor, those babies can really draw some current at startup.· Make sure your hardware and software are able to handle it.

David

stamptrol

Randy,

OK, I understand your constraints! Barnyard style it is!

Go ahead and monitor the switch position ( with opto isolation or dedicated low voltage contacts).

To see if the motor is running, is there some way to glue a small Hall effect sensor on the stator iron of the motor? The normal flux leakage should affect the Hall sensor and let you feed its signal into an A/D ( like LTC1298) where the stamp can see that the motor is running.

Or, if you can't get at the motor, glue the Hall sensor's sensitive face to the head of a nail ( like a 3" board nail). Wrap a few ( maybe 5 or 6) turns of the motor's common lead around the shaft of the nail. Any motor current will affect the sensor.

Because most Hall devices can sense both N and S fields, you may need a precision rectifier circuit ( one op-amp, one small signal diode) on the DC signal fed into the stamp.

Other than your time, this should get the parts price down to 20 bucks or so!

Let us know how it works out.

Regards,

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Tom Sisk

http://www.siskconsult.com
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PhilTheMotorman

Randy,

I'm not able to give any solutions on the·high tech·side of your questions, but I know the low tech.··The normal method to monitor if a blower is operating is with a pressure differential switch or a sail switch.· If you monitor the voltage at each speed control lead of the motors and use the switches I have mentioned you should satisfy the requirements you have listed.· If you are using an alarm circuit, make sure you use both the voltage to the motor (on)·and the pressure or sail switch (off) and make it a manual reset.· Your motors are impedence protected, but have an in-line/automatic reset/·thermal protector.· These motors will overload, trip out, cool down, reset, and cycle like this·untill the overload finally fatigues and opens permanently.· You may not detect a developing problem if your alarm is not a manual reset.

I am no engineer, just guy who works on motors and blowers every day.

Hope this helps,

Phil

Randy

Thanks to all of you for the replies and ideas. I am getting a good feel for what to do and of course I have thought of some other restrictions. Mostly how to get this info to a host pc. Many of these FCUs are a hundred plus feet and two to four stories away. I will need to do some multiplexing at the least to keep cabeling simple. There will probably be no alarm just a periodic monitoring.

Anyway first I need to see if they are serious about wanting me to pursue this in light of costs. I will be out of the country untill February so I will have some time to tie my brain in knots while simplifying the vision of how to do this. At this point even if they axe the project I want to figure out how to make this work so if nothing else I will put together at least one from my pocket just to know I can. It is no wonder I have money! But I got lots of toys.

It will be a month or two but I will post again when I have a working solution or most likely a question regarding communicating the data to a pc.

Randy

Mike Green

In terms of sensing whether there's power applied to a winding, a simple solution is to use an optoisolator and series resistor (to drop the AC to 1.7V at 10ma). The output of the optoisolator is just a transistor and could connect to a Stamp or directly to a twisted pair with a 20ma current source at the other end. You'll get 60Hz current pulses at the other end which you can detect with a Stamp. If you need to multiplex all these sensors, you should consider having a Stamp at the motor end of things. It could be powered locally (from the motor supply) or fed power over the cable from beyond. You could use RS422/RS485 drivers for the serial connection for reliability over that kind of distance. Basically, the Stamp would send some kind of unique character (like "!") at the beginning of the message, then a series of bytes with a bit for each sensor followed by the one's complement of each byte (for checking) and maybe even a checksum at the end. These would be send continuously. The monitoring Stamp or PC would discard any messages that failed to check properly. If too many errors occur or no messages are received within some time period, the monitoring Stamp or PC could signal a communications failure.