about DC motor control

[Chert]

CGwynne started a highly appreciated thread and made this statement:

"  Any 172x150x38mm DC fan will be a better substitute for the AC fans included stock. DC fans inherently allow you to vary and control the speed. AC fans really are only for a single fixed speed, a dimmer is a bad thing to do to an AC motor (notice the humming?), and a VARIAC is not that great of a solution either often with 0 RPM's until above 50% power and a non-linear progression from there.  "

Can anyone elaborate on this?  I am working on using PWM to control the DC motor of the drum rotation.  I have found that DC SSR is more expensive than the AC SSR I use for the PWM control of the fan.  My first efforts will involve a mosfet or array of mosfet, but at 24 V 2A, heat dissipation appears to possibly be an issue.  So I am acutely interested in how to control DC motors.

[rvtech]

Seems like this should work. You'll need the DC fan and an ample power supply.http://www.amazon.com/DROK-Controller-Regulator-Modulator-Indicator/dp/B00DVGGWC0/ref=pd_sim_469_7?ie=UTF8&refRID=11DAXT8VHHXQZPM7E4HP

[CGwynne]

2A seems like a lot for a DC fan motor?

I'm looking into swapping my AC fan out for a DC fan but as I also want to setup a cyclone I may not be using any of the stock fan equipment provided by Mr Li. Nevertheless am researching all the DC related exhausting.

I beleive Rvtech is correct that you just need a power supply and controller.

DC fan's can be 12, 24 or 48vdc (have even seen 36). And they will all work but it seems there are more speed controllers available for 24vdc fans so that would be the easiest to work with. Make sure you get one that is rated for a similar cfm. My 230VAC fan from Mr. Li is rated at 205 CFM. I'm still trying to lock down what the 120VAC fans are rated at.

I'm looking at possibly controlling the fan through a Phidget 1065 (through Artisan) but in discussion with Brian from Phidget he pointed out that the 1065 is not for brushless fans such as the Sinwan I mentioned in my reference post. (thanks for the acknowledgement). But perhaps that would work for you depending on the fan?

Here's the Phidget 1065.
Make sure to contact them first and tell them which fan you want to drive to make sure it works.
http://www.phidgets.com/products.php?product_id=1065

I should add that I am not an electrical engineer, I have a basic understanding from years of building PC's and can even solder if forced, but getting into more detail of SSR's for PWM control I'd have to look up

[dickcoffee]

I think it may be a good time to remind people of what happened to my Variac.  I got my Huky and Variac just about a year ago.  I was very happy with it and did about 75 roasts.  It sat over the winter as I had a major construction project going on in my home (converted a small bathroom that was at the end of my kitchen to a coffee nook).  When I started up roasting this spring, the variac was acting very erratic, like reving a  car engine and then letting up and then it started to trip the gfci connection.  Knowing nothing about electicity, I complained to the company where I bought it.  When I told the guy what I was using it for, he said, "well, that's your problem.  You can't use a Variac for that purpose." I haven't seen anyone else complain about a Variac failure.  I don't know why I'm the only one.  But, apparently, it's true that it's not a good use.  Since then I've been using a cheap dimmer which, while not as specific as a Variac does work.  Or, lately, I've gone back to just using the system as it was designed with the valve in the J tube.

[SusanJoM]

, "well, that's your problem.  You can't use a Variac for that purpose." I haven't seen anyone else complain about a Variac failure.  I don't know why I'm the only one.  But, apparently, it's true that it's not a good use.

Can someone explain to me why this is thought/said to be the case?  I thought this was very much exactly what Variacs were good for.  Mine hasn't had enough use to crap out yet, so I can't say that it won't, but I would like to understand the "why".

[CGwynne]

Caveat: I'm not enough of an electrical engineer to know if this is all correct but here's my understanding...Google is my champion

Variacs are designed to provide variable AC, hence the name. Anything that could benefit from varying the voltage of alternating current will work. Such as slowly bringing fragile tube amplifiers up to voltage instead of a switch which provides full power instantly. Or heating coils in a heater or heat gun that generate their heat via voltage resistance. Less voltage= less heating.

The problem for us is not the variac, it's the AC fan motor. AC fans are built for a fixed speed. We use variacs because they are better than standard light dimmers. Dimmers use triacs which feed an AC motor a signal that can damage the motor (hence the humming you can hear sometimes). Variacs work differently with a coil around the core and a wiper that contacts the coil at different voltage points, so it's a smoother way to "dim" an AC motor, but it's still not what an AC motor is designed for. A dimmer is pretty bad, a variac is better, but not ideal. Electrically speaking the proper way to vary the speed on an AC motor would not be to vary the voltage but to vary the cycle frequency (50/60hz) but devices to do that are far more expensive than variacs.

AC motors have a stall speed, a voltage which below, they will not spin. Nevertheless there is still current moving across the motor coil windings. I don't know enough about electrical engineering to know exactly what's happening but I do understand that the resistance to the load is bad for the motor when you are working near these stall speed voltages. The motor will overheat, and in our case where we dump exhaust heat from a coffee roaster right onto the fan hub enclosing this motor, we're making it worse.

If you don't mind the fan failing earlier than expected, then it's not a big deal, these are not expensive parts after all and we are not doing heavy duty or multiple back to back loads so it doesn't really matter. But it would not be done this way in an environment where reliability was more important. A better damper or a fan built with variable speed in mind would be used and DC is the solution for that. If you end up spending enough money to vary the fan speed in your Huky setup and that amount of money is similar to going with a DC variable speed fan setup instead, then take a look at that. My variac was almost as expensive as swapping everything out for DC, but I couldn't get the parts fast enough and at the time didn't want to wait, but I'll make the switch eventually.

As for dickcoffee's failed variac, perhaps it was a low VA variac and the resistance coming back from the fan when at low voltage (below stall speed) eventually proved to much for the variac? But that sounds strange, the Huky fans are not very powerful electrically and a variac is a pretty beefy thing.

A winding in the variac could burn out, or the wiper that contacts the windings could be damaged. Something happened while it was stored and not used. That it revved like a car engine suggests an issue where the wiper contacts the windings at different voltages. That sounds more plausible. But I don't buy that it was damaged by how dickcoffee used it. If a failed fan manufacturer said "your problem" ok yes, but not the variac company.

[dickcoffee]

Thanks for that.  I reckon I'll try again.  I've spent a lot more than the price of a variac on this hobby.  My fans still work fine by the way. 

[SusanJoM]

Excellent information, thank you.

What I have gathered from this is for a hobbyist like myself, the Variac is probably a good enough solution to wanting to control the fan.
For someone whose equipment stability is more critical, investing in a DC set up would probably be a better  solution since downtime and replacing parts could be more than just annoying.

Yes?

This bit (AC motors have a stall speed, a voltage which below, they will not spin. Nevertheless there is still current moving across the motor coil windings. I don't know enough about electrical engineering to know exactly what's happening but I do understand that the resistance to the load is bad for the motor when you are working near these stall speed voltages. The motor will overheat, and in our case where we dump exhaust heat from a coffee roaster right onto the fan hub enclosing this motor, we're making it worse. must be why I have read that it is not good to leave your Variac on 0.  That when you want the fan to be OFF it is important to turn the Variac OFF. 

[dickcoffee]

I bought a new Variac today.  The same on Hank has. 

[Chert]

CGwynne,

If the DC fan that fits in the bowl is a good choice for your cyclone, choose that one.  That way if someone develops a board for DC fan and drum motor control designed specifically for the HUKY, (via TC4 and artisan, perhaps) then that could also be used to control your cyclone.

Flint

[Kfir]

I was told that the best option to control AC motors/fans is a frequency inverter.

Kfir.

[CGwynne]

@Kfir, yes that's what I've come to understand as well. They are also known as a Variable Frequency drive.

https://en.wikipedia.org/wiki/Variable-frequency_drive

But they are more expensive than Variacs. And as SusanJoM, for a hobbyist, the Variac, at it's price point is good enough. When I was trying to source my Variac here in Bangkok, it turned out that getting an exact fit DC fan and controller was roughly the same price as the Variac but I would have had to wait for the parts and I needed to get some roasts out so went with the Variac at hand.

[Agrajag]

Bringing up an oldish thread - I'm looking at replacing the fan with a DC fan (instead of using a variac as above) however am not entirely sure of the options. DC fans that have the right airflow (and proper PWM) seem to be fairly expensive and I'm not really sure what the best options are for controlling them either. Can someone who has replaced the stock fan with a DC fan let us know exactly which fan you used and how you control it?

[Agrajag]

Replying to myself... after some more research I found this a 172mm 24V DC fan which was fairly cheap on ebay: PM240-24D-1751B-2TP. It can do 250CFM which is enough.

According to the pictures and datasheets it looks like it has 4 wires, one marked as CTRL which I would assume works similarly to 4 wire computer fans, so it's better than just plain DC voltage varying. The datasheets aren't very useful as far as how to actually control it though. I'm hoping to use an arduino and do pwm to that, will have to figure out what it needs. If anyone has any pointers on that part, let me know.

[Agrajag]

OK! The fan I ordered above finally arrived today, I hooked it up to my arduino and in no time at all had artisan controlling the fan. I reckon this is much better than using a variac, and cheaper too. The fan was $25 on ebay and you can get chinese knock off arduinos for about $10.

If you want to do the same, you want to get a fan that:

* Is 172mm, these seem to be standard and the same stand will fit in the holes (160mm apart)
* Has the appropriate CFM (the supplied one is 230CFM)
* 24VDC or less. Can be more but power supplies might be harder to find
* Has 4 wires. A 2 or 3 wire fan can work if you have another way to control the speed, but this is far easier with a 4 wire fan - one of the wires is a control wire, you send a simple PWM signal on that instead of varying the voltage. This way it's easy to use an external power supply for the fan and keep the arduino parts simple.

So you hook up the control wire directly to your arduino, and the arduino plugs into your PC running artisan. You can setup artisan to have a fan slider and set it to run a program. That program can just write to the arduino over serial. Your arduino code reads the input and then sends the appropriate PWM signal to the control wire pin.

I'm happy to expand on any of the above if anyone has any questions.

Video of it in action


Edit: youtube tags don't appear to be working on this forum? Not sure if it's just me. Link is https://www.youtube.com/watch?v=LJpfwPpzDi8