Automated variac to control cooler motor

[alwaink]

Hi,

I have a cooler powered by a small motor. The cooler motor is adjustable 0-60V (the higher voltage, the colder the cooler gets). Since a soft start is preferred, I use a typical Variac to slowly turn the voltage up in the lab.

However, I'd like to set up a simple input/output system, where the input is the thermocouple temp reading of the environment. Based on this temperature, I would like the output voltage to adjust 0-60V accordingly.

My quesetion is if it's possible to have the variac automated, without having to manually turn the dial? I know you have to change the tap manually in a transformer, and turn the dial in a Variac due to number of coil turns. I just don't know if I can vary the voltage output in some way automatically.

Appreciate your feedback. Thanks.

 

[itsmoked]

Probably be better if you could cobble up a PWM(ON/OFF) system. I also don't see how your motor is going to do anything at 2V.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

What powers/currents are involved? Is this a 1k+ or just one-off project? A variac seems to be the worst solution ever when it comes to cost. There are so many alternatives.

Also, is this a DC motor or a universal motor. It surely cannot be an AC motor of any kind.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[sreid]

If it's a universal motor, then possibly

http://www.crydom.com/products/productFamily.aspx?id=97

 

[alwaink]

The motor takes up to about 60VAC and 3.5A single phase. This non-manually adjustable voltage is a preferred method for future end users to run the cooler without spending a lot of time paying attention to the ambient temp and adjusting the cooler voltage themselves.

It could potentially become a $1k+ projrct...I really don't know what's available out there to achieve this. Hopefully not too expensive.

 

[jraef]

First off, you need to identify the type of motor more accurately as pointed out above. There are lots and lots of different types of motors, some AC, some DC, some hybrid, some can be speed controlled some cannot, of those that can, some can be controlled with voltage only, some cannot. Anything else we conject here is pretty much meaningless without knowing what you have to begin with.

Secondly, you cannot simply feed a thermocouple into a variac or variac-like device to control output. As the temperature goes up, the speed goes up, then when it works and the device gets cooler, the speed goes down. It will never stay at any stable level, it will always be "hunting" up and down, eventually switcging from fully on to fully off. You need in between is what is called a "feedback loop controller" of some sort, typically a PID or at least a PI loop controller. If you don't know what that is, you have a lot to learn, Google is a great place to start.

The cheap and dirty explanation is that they take a setpoint (which you enter) of whatever temperature you want to maintain, monitor the thermocouple and then determine a process signal that would go to your variable controller (of whatever kind it needs) that powers the motor. The controller determines the proper Proportional (P) response to the signal change, Integrates (I) the rate of change in the response and Derives (D) the predictive response time.

 

[Skogsgurra]

That voltage is somewhat non-standard. Isn't there anything more standard available? (I know there is) Is this a motor you just happened to have lying around? In that case, it would be a far better approach to find a motor that can be easily controlled with a triac (power is just right for that).

A complete triac solution with temperature control and trigger circuits can be produced at a very low price. That is what is inside most bath-room ventilators and those particular boards can be easily produced at ten dollars or less (How I know? I designed a lot of them).

The variac solution is just wrong.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[alwaink]

I didn't know the type of motor would be important in this case. I just wanted the voltage supply to the motor to be controlled without much manual operation. I don't know what type of motor this cooler has, I'll have to check with the manufacturer.

I was thinking of a simple board with a microcontroller that can take the thermocouple input reading, and with some programming, output to some interface that can adjust the voltage to the motor.

 

[Skogsgurra]

A variac plus motor to adjust it will cost around 100 times more than a simple triac. The circuit board and the processor will be the same cost in both cases.

Get my point?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[alwaink]

Ok, thanks. I will look into the triac option. This sounds like a cheap way of doing it.

 

[Marke]

The triac option will be an issue if it is an AC motor at 60 Volts only. It would be preferable if the motor was able to run off mains voltage. This would reduce the cost of the controller. See if there are 230 volt motors available for the cooler.

Best regards,

Mark Empson

http://www.lmphotonics.com