220Vac Mains Sensor

[Cyberfreak]

Hi All

I need to measure the AC voltage and current with a Micro-controller. And by doing that, calculating the load of the device under measurement.

I have one idea of measuring the current, is to use a Current transformer with a 1:200 ratio to bring down the current and voltage to a safe level. By connecting up a "burden" resistor in parallel on the secondary, the voltage across will represent the current on the primary. But, what I am not certain off, is that voltage should be still AC? Must it go through a Half-wave or Full wave rectifier to be interfaced with a micro with an 10bit ADC build-in? 3.3V DC micro that is 5V tolerant.

What will I use for the Voltage measuring side? Also a type of step-down transformer to be isolated from the mains?

Please help.
Cyberfreak

 

[Nigel Goodwin]

Check EPE magazine, they have published two projects for what you want (both use hall effect current transformers), the first was years ago (Sept 96) and used the 16C84, the second was last year (Feb 2005) - the new one uses the 16F877, and is dual channel.

I built the original one back in 96, and it still works well!.

 

[Cyberfreak]

220Vac motor control with a micro

Hi Nigel

Thank you for your info. It is exactly what I require. I will purchase the back issue.

I also require to control the speed of the AC motor with the same micro. Please find attached an image from the U208B datasheet to control the speed of such motor with a potensiometer. How would I interface this with the micro? Replacing that potensiometer with a digital pot? The potensiometer is still on the AC side?

Any views on this...

Cyberfreak

 

[Hero999]

An AC motor?

This won't work with a real AC motor (a squirrel cage type), but it'll work with a universal motor (one with carbon brushes), what sort of motor is it?

 

[Cyberfreak]

220Vac Danrol Motor control from a Micro

Hi Hero999

I finally found the correct AC motor I must use. It is a motorised drum motor from Danrol. See **broken link removed** , it is used for a conveyer belt and I need to control the startup speed and stop speed of the conveyer with a micro. If I can't use the U208B, what can I use to control?

Also find attached the Danrol Wiring Diagram.

Please help...
Cyberfreak

 

[Hero999]

Wow, I'm still none the wiser, I'm still guessing it's a squirrel cage type, the 6uF capacitor is a good clue since it's probably used for power factor correction. I don't know if your simple phase control circuit will work. I have a feeling you need a more complecated variable frequency drive, give your circuit a try by all means but I think it might overheat. I'm sorry I can't give you an alternative circuit as I'm no expert.

 

[oldtimer]

The cap. is probably for phase shift.

 

[Cyberfreak]

Digital Pot for AC Motor

Hi again

While I'm waiting for the U208B samples, to build and test that part, I need to figure out what to do with the variable control side of the motor. To control the speed of the motor with the micro. The pot in the U208B diagram is used on the AC side. How will I do this? There are many Digital pots from Maxim. I'm not an expert, but my feeling is it will not do the job, because who knows what current goes through that pot. Maybe I'm wrong. Can someone please give me some ideas?

Cyberfreak

 

[Hero999]

You don't need a resistor, you just need a vairiable voltage source negitive with respect to neutral on pin 6. I wouldn't recommend this circuit anyway since it isn't isolated from the mains, I've designed an isolated voltage controlled phase controller but it isn't finished, I'll post it if you like but I'm sure Nigel Goodwin over there can suggest a design for this involving a PIC.

 

[Nigel Goodwin]

The speed controller mentioned looks to be ONLY for universal AC/DC motors, the motor in question does not appear to be of that type and can't be controlled that way!.

 

[Cyberfreak]

Hi Nigel

Any idea how to control this motorized drum AC motor from Danrol then.

**broken link removed**

I also found this app note, but are still busy reading through it. Looks promising for isolation point of view, but still not sure it will work on this motor. Unfortunately I can't get more info on this motor. I have one in my possession for testing.

Any more views....

Cyberfreak

 

[Nigel Goodwin]

Hi Nigel

Any idea how to control this motorized drum AC motor from Danrol then.

**broken link removed**

I also found this app note, but are still busy reading through it. Looks promising for isolation point of view, but still not sure it will work on this motor. Unfortunately I can't get more info on this motor. I have one in my possession for testing.

That's just another phase control circuit, like it says for universal motors.

Have you tried emailing the manufacturer?, presumably they must supply more details than seem to be on the website (which doesn't even seem to tell you what sort of motors they are?).

 

[Hero999]

The speed controller mentioned looks to be ONLY for universal AC/DC motors, the motor in question does not appear to be of that type and can't be controlled that way!.

I agree, it won't work, you need a variable frequency drive, a simple phase controller will overheat!

 

[Hero999]

On second thoughts, a variable frequency drive won't work since it's a single phase motor with a phase shift capacitor which will only work at the mains frequency.

https://www.epanorama.net/links/motorcontrol.html#ac

Induction motors are practically fixed speed devices. The there is practically only two methods to change the rotation sped of AC induction motor: use frequency converter or use motor with sperate winding for different speeds. In some applciations motors with dual speed winging are used. The applications where accurate speed control is needed, you need s frequency converter. A frequency converter can run a three phase AC motor at very wide speed range quite well (the performance of motor is usually reduced outside it's optimal operation speed).

There are variable frequency drives that allow induction motors to run on different speeds. But on those applications mechanical load and the speed range must be considered, because on those applications motors can get very hot very fast. The problem is that a 60 Hz (or 50 Hz) motor does not have enough iron in it to allow efficient 25 Hz operation. The motor will run hot due to not having enough inductive reactance at the reduced frequency. Dropping down to 10 Hz would make it even worse. A motor designed for variable frequency drive has more iron. Also, it might use a different iron/steel alloy to allow efficient operation at higher frequencies (say 400 Hz). With a light mechanical load and a good motor combined with a good vraible frequency drive controller, it's sometimes possible to get a reasonable speed range using a variable frequency inverter. A good vriable frequency drive device controls both frequency and voltage. The better ones even take into account that at very low speeds the resistance of the coils cannot be negelected. In VFD (variable frequency drive) system the incoming single phase power is rectified and filtered, and three-phase power is generated from the DC rail using three half-bridges. You get to set the frequency over a range so you can vary the speed of your motor, plus a nice digital display etc. It's a bit harder on the motor insulation than just running it from the line, but well-designed motors should be okay. The reason why VFD is hader for motor insulation is that the unductance in the wiring to the motor allows spikes and ringing at the motor itself. The waveforms that go from VFD to motor are typically quite far from ideal sinewave.

Frequency converter does not work with AC induction motors that are run from single phase power source, because the operation of the needed motor phase conversion capacitor is very frequency sensitive (works as expeted only at normal mains frequency).

Pehaps using a bridge rectifier an capacitor to convert the mains to DC, then modified sinewave inverter and reducing the duty cycle a low frequencies might work but then there's sitll the problem with the phase shift capacitor not working at the lower frequencies, unless you start off fast then slow it down.

Somehow I don't think this is possible, not without a lot of messing around anyway.

 

[Cyberfreak]

Thank you Hero999/Nigel/oldtimer

I have emailed the manufacturer to obtain more information on this motor. I will give sum feedback when I know.

Regards
Cyberfreak