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Remotely Controlled Variac

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Arjames090686

New Member
Hi Guys,

I am new to this site and the world of electronics! I am currently in my second year of a ONC in Electrical/Electronic Engineering and start my project this year. I have kindly been given the below scenario by one of my colleagues in work and was wondering if anyone had any suggestions about how i would go about this.

I have been doing research into how Variacs work etc and also have been given a lot og Forest M Mimms notebooks which show various OP Amps and Comparators etc which is proving helpful but a few hints and tips would be great to set me on the right track.if i have not provided enough information then feel free to ask and i will endeavour to find the answers for your questions.

Spec below:


As i mentioned, i am learning and
Objective: To remotely control the output voltage from a variac. Once the required voltage has been sent the variac must track that voltage without further intervention from the host PC / PLC. The voltage can be changed by the host. The variac will then track this new voltage.

Technical Details:
Input Voltage 230VAC 50Hz
Voltage range: 10VAC to 264VAC
Resolution: 1V
Response: 3V to 5V per second.
Accuracy: ±3V around required voltage.

The controller must be powered without the need for additional bench top power supplies.
The prototype can have an analogue signal to demonstrate the control principle if it makes it easier.
The variac will be controlled from a PLC or PCI digital IO card. Both these devices will have relay output (zero volt contacts)
Up to 24 inputs and 24 outputs are available (the minimum used the better).

Custom PC / PLC software can be written by the customer to provide the desired signal’s. You will need to advise early in the development, how many I/O lines are to be used and later what the signals required are.

Wish list extras:
Display showing the variac output voltage VAC.

Cost Target: A Digital variable AC supply is approx £3000, but has a lot of extra functionality (See AC Power Supplies (AC Power Source): Compact AC Power Supply (CV/CF) PCR-M Series | KIKUSUI ELECTRONICS CORP. -Products-).

Delivery date: Project submission date.
 

Arjames090686

New Member
hi all,

right, i have been poked in the direction of using a potential divider to drop the voltage from 240vac to something like 10vac. i believe an R1 value of 10ohm and an R2 value of 1ohm, is this correct?

would it be wise to convert this into dc current? if so, would i be right in saying a bridge rectififier would be required?

My first thoughts are to drop the mains to something workable (i.e low voltage). i can then maybe use this low voltage and build some kind of op-amp/comparator circuit which would drive a servo motor. The idea is to maintain the constant 240vac output from the variac and after doing some research i think i need to use a reference voltage which would then dictate whether the motor turns clockwise or anticlockwise???

As mentioned above, i am a complete beginner and until a month ago i didn even understand how a potential divider worked so please bare with me if all of the above is nonsense!!!

any advice would be greatly appreciated.
 

BrownOut

Banned
I can't understand how your project is supposed to accomplish it's task, or what voltage dividers and resistors are supposed to do. I can't offer any advise, because I totally don't understand your issues.
 

Arjames090686

New Member
hi,

sorry, like i said i am a complete beginner.

I asked one of my colleagues in design for a project to do for college. they currently have a variac that is controlled via a motor fixed to the wiper arm. if the voltage output voltage drops below say 230vac, the motor is activated in a clockwise direction. if the voltage goes above 250vac, the motor activates in an anticlockwise direction.

the idea is for me to work on asystem where they can have a constant set voltage. so if the mains input goes up and down, the motor continuosly adjusts the variac to maintain 240vac for example.

im sorry if this is not very clear. i bet you hate answering these type of questions
 

BrownOut

Banned
I don't do alot of this type of thing, however... I suggest you connect a 20:1 transformer, or thereabouts. That way, you can try to sense a safer 12V, which will also make things easier to work with. Then, you can rectify and filter your 12V, and use a couple comparators to sense when it's higher/lower than tolerance. Drive your DC motor from the output of your comparators, with appropriate amplification and buffering.

I'm no expert, but that's the way I would do it.
 
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Arjames090686

New Member
thanks for the assistance.

even though i didn get to the answer you gave, it is good to see that i was starting to get somewhere along the right track with the comparators etc.

i have spent weeks just researching different types of motors, different op-amps etc and i think i am starting to get the electronics bug.

hopefully one day it will all click and make sense lol
 

BrownOut

Banned
If you would like to try to draw up an idea, I could tell you if you and I are on the same track or not.
 

Arjames090686

New Member
i am going to spend this weekend putting everything down on paper and try and spec some king of initial concept. As it is only an ONC project, the main part of the project is the project planning, logbook etc but i really want to get the project working as otherwise there would be no sense of satisfaction!!!
 

BrownOut

Banned
Do yourself a favor, and keep it simple. Don't try to make the tolerance too tight. The tighter you try to make it, the more complicated the system will have to be, as well as less stable. Use a simple, loose hysteresis scheme. If you don't know what hysteresis is, ask your professor. If he doesn't know, demand to see his credentials.
 
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foretek

New Member
MCU to control variac

Hi, I think you can build your project based on this following system. first, get the spec of the motor, and the circuit driving the motor. Then use a MCU to control the motor drving circuit. Use MCU A/D to sense the setup point(pot position), and the actual AC voltage, compare them to get the error, then use the error to drive the motor drving circuit in order to match actual AC voltage with setup point.
----------------------------
hi,

sorry, like i said i am a complete beginner.

I asked one of my colleagues in design for a project to do for college. they currently have a variac that is controlled via a motor fixed to the wiper arm. if the voltage output voltage drops below say 230vac, the motor is activated in a clockwise direction. if the voltage goes above 250vac, the motor activates in an anticlockwise direction.

the idea is for me to work on asystem where they can have a constant set voltage. so if the mains input goes up and down, the motor continuosly adjusts the variac to maintain 240vac for example.

im sorry if this is not very clear. i bet you hate answering these type of questions
 
Last edited:

Arjames090686

New Member
foretek, thanks for your suggestions. having spent several weeks researching i am now going along the track of measuring the output from the variac and then feeding this output voltage back into a comparator circuit via a potential divider. as you have mentioned the comparator would work on the basis of comparing the actual output voltage against a reference voltage that would be set initially and this would generate an error signal. this error signal would drive a low voltage dc motor that would be connected to the shaft of the variac, making the adjustments required.

the above is just an idea and is probably flawed in several ways but this is all part of the fun of doing a project isnt it!!!

to be honest i have really struggled to get this far but i am now trying to find how i would set reference voltages etc and what type of motor i would want to use, servo, stepper etc
 

DJDAudio

New Member
I know this is old, but a great topic it is.

Many years Ago I had the same problem, I wanted to make a Simple 'crude' regulated Variac but found no good plans out there.

So I setup a Pitman gear-head motor along side the Variac, the output shaft I put a rubber wheel, this 'rode' along the wiper arm disk, this made for a great gear reduction. ( direct drive needs allot of torque ) and High precision.

So I picked an output voltage I wanted to keep. Lets say 12V so I used a second smaller power supply that I could set for 12V out, now if the output of the Variac was 12V I would just put both legs of the motor one on each power supply the motor would see 0 Potential and would not move, if the voltage went down by lets say 3V the motor would see a 3V potential and then would move, with some simple Diode logic, it could know what direction to rotate.

The cool part was a large voltage differential would move the motor very quickly and then as it approached the set voltage it would slow down and come to a smooth halt.

It was a pure Analog Regulated high current power supply.

When I finish the project I have now, I may sit down and rebuild this project, I forgot how much fun it was to watch work.

Oh, and by adjusting the small power supply you could adjust the voltage regulation set point.
 
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