Today's question is more of a discussion, really.

I have a small single phase 220-240v AC fan motor - 0.45A, 40W output 900RPM capacitor start (poor picture of plate below). It is destined for an amateur wind-tunnel, and low, fairly reliable speeds of the order of 60RPM are required.

That's probably only achievable with a VFD? However, cost is a major issue. Cheapest item I can find is this sort of thing from ebay: https://www.ebay.co.uk/itm/Drive-In...120632&hash=item3fcb362bfa:g:R7EAAOSwCwxdL~dl

I can't find any data on this VFD beyond the poor ebay description. I have found reviews of similar items stating that the item was OK, but the manual was of no use. I see that the output is specified as 'Terminals U V W', but have no idea how to connect that to a single-phase motor.

Has anyone here any experience of using this type of kit, or any idea where I might get a generic manual from?

 

I am guessing that ebay ad is for a 3ph motor, it just says '1ph hot' which sort of points to a 1ph supply to a small 3 phase motor.!!?
Looks like a really heavy built motor for a 40w !
Quite a low RPM for a fan motor.
If this is a PSC motor where both windings are equal, you may be able to use a simple triac fan controller, may be worth a try as they are not much to buy or build.
Max.

 

You would need a single phase VFD inverter, not a 3-phase.

That is a 6-pole motor and would be commanded at 3 Hz in order to run at 60 RPM. That is nearly DC from the standpoint of the current in the windings, so the VFD current limit would need to be set low enough not to overheat the motor at that frequency. The capacitor may need to be changed to account for the lower frequency operation too. Some tuning of the VFD parameters will be required.

 

Most 1ph VFD's are not very successful, they tend to drop out of run easily on low rpms/loads hence the prevalence of many 1ph users opting for 3ph VFD/motor with 1ph supply.
This is why I suggest the triac idea, they are usually succesful upto about 1/2HP.
Max.

 

Hmm.... maybe I was reading things a bit hopefully? I see that there ARE such things as VFDs for single-phase motors - https://www.invertekdrives.com/variable-frequency-drives/optidrive-e3-single-phase for instance, but without further info I can't tell about that cheap ebay item.

The fan motor will be 6-pole 900 RPM for a 50hz AC supply. It's almost certainly PSC - but I really wanted some good slow speed control, and a triac will give a kind of PWM control, won't it? Jerky and noisy, and hard to set to a specific speed.

Having said that, all the books tell me that if I want precise speed control I want a 3-phase motor, so I suspect that I am looking for something that doesn't exist. There are, however, interesting articles talking about the U2008B phase control chip - here is a video which looks to be doing what I want https://www.youtube.com/watch?v=GyoswaSMnXw
Shame I can't find any good circuits, though...

 

Do you need variable speed, or could you use just one fixed speed?

 

- here is a video which looks to be doing what I want https://www.youtube.com/watch?v=GyoswaSMnXw
Shame I can't find any good circuits, though...

Look up U200B this is what that circuit is based on, many circuit examples out there.
It is a Triac based controller
Max.

 

Do you need variable speed, or could you use just one fixed speed?

Alas, variable, and able to adjust:
a) quite precisely
b) with good repetition

A wind tunnel application needs quite precise airspeed control, and with different objects in the test section we need a range of powers. I find that smoke flow visualisation works most effectively so long as the airflow is smooth and quite slow - of the order of a foot a second. Hence the need for very slow fan rotation...

 

If the motor were mounted in a relatively small diameter exit tube located a longish distance downstream of the chamber, then it could pull air thru the chamber entrance, thru a filter/flow straightener, past the test object, then thru a filter/straightener and into a reducer section to the exit tube. The motor could run as normal off the mains and the velocity in the chamber adjusted with baffles or orifice in the exit tube.

 

If the motor were mounted in a relatively small diameter exit tube located a longish distance downstream of the chamber, then it could pull air thru the chamber entrance, thru a filter/flow straightener, past the test object, then thru a filter/straightener and into a reducer section to the exit tube. The motor could run as normal off the mains and the velocity in the chamber adjusted with baffles or orifice in the exit tube.

The motor will indeed be mounted a long way from the test section, after a diffusion stage. It is surprising how sensitive flow conditions are in the working section of a wind tunnel. You need to correct, amongst other things, for the thickening of the boundary layer as the air passes down the test section, which results in a linear pressure variation which tends to drag the test object backwards (Google 'wind tunnel buoyancy' for more explanation). The use of baffles or an orifice in the diffusion stage would disturb the boundary states drastically, both upstream and downstream, and would require extensive flow recalibration each time a change was made. If an orifice were not completely concentric it would almost certainly result in a horizontal or vertical flow gradient in the test chamber, while a baffle would propagate a pressure pulse back up the diffuser. I would not like to predict what the boundary layer would do in any of those circumstances!

Interestingly you may sometimes have a static orifice in a closed-circuit tunnel, for better temperature control of the working medium. You will not usually find this illustrated in simple diagrams, it will be positioned very carefully, and will not be designed to impact flow control in any way...

 

Chasing u2008s has certainly been fruitful - this circuit certainly looks easy and cheap https://www.electronics-lab.com/project/ac-motor-speed-controller-using-u2008b/

But it is odd that there is no specification of line and neutral - does the U2008 not worry about that?

 

But it is odd that there is no specification of line and neutral - does the U2008 not worry about that?

What do you mean exactly?

 

What do you mean exactly?

I tend to think of single phase A.C. as comprising one wire with a voltage sine wave on it while the other is nominally flat - similar to earth and positive potential in a D.C. circuit. For some applications, such as a resistive light bulb, which is which hardly matters, but I thought that a complex IC might require connecting Line and Neutral to specific pins...

 

The two conductors of an AC supply are identical, there is no difference in the AC sine wave of each. You perhaps think of no evidence of signal on, say a neutral because it is earth grounded.?
IOW the two conductors have the same signal in each and current direction reverses every cycle.

 

A pure DC variable motor would be smooth and still be capable of electrical variance.
Possibly a balanced squirrel cage blower mounted using dampening material on quality pillow block bearings having no slop
and directly driven using spider coupling.

“Big whirls have little whirls,
That feed on their velocity;
And little whirls have lesser whirls,
And so on to viscosity.”

― Lewis Fry Richardson

 

Possibly a balanced squirrel cage blower mounted using dampening material on quality pillow block bearings having no slop
and directly driven using spider coupling.

Yes, I have seen this used on a number of university machines. And I know what i would like to do if cost was no object. Problem here is that we have a low spend requirement, and we already have a motor/fan combo. I suppose what I am really asking is, how do I make such a single phase AC motor rotate slowly, under good control, and at low cost... ?

 

Problem here is that we have a low spend requirement, and we already have a motor/fan combo.

Unless you need to ramp the air flow, couldn't you use different diameter or blade count fans?

I'm not sure about one of your earlier comments -

If an orifice were not completely concentric it would almost certainly result in a horizontal or vertical flow gradient in the test chamber, while a baffle would propagate a pressure pulse back up the diffuser. I would not like to predict what the boundary layer would do in any of those circumstances!

Since wind tunnels usually suck/pull, not blow, the air through them.

 

Probably if you were to do it all over, you would use a VFD that had single phase input and it would drive a 3 phase motor. This setup was used in a purchased mixer. It has the best chance of success.

 

I suppose what I am really asking is, how do I make such a single phase AC motor rotate slowly, under good control, and at low cost... ?

I think it will be very difficult, if not impossible, to get reliable control of a single-phase AC motor down as low as 1 rev per sec (60rpm), regardless of cost. Why not ditch that motor and use a more easily controlled geared DC motor or stepper motor instead?

 

Not a job for a stepper, they produce their max torque when holding--they are not good at providing torque to move loads.