AC fan speed control by periodic on/off switching

[Don Lancaster]

This will instantly destroy any fan based on a conventional inductin
motor. Only motors with brushes can be controlled by a lamp dimmer.
However, the lack of speed feedback will cause major problems.


--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[MooseFET]

This will instantly destroy any fan based on a conventional inductin
motor.

No, it takes them a while to get up to ignition temperature. They
smoke quite a lot before that actually fail. Also, they tend to
either run near full speed or drop to a very slow speed. They just
won't go a about 1/3rd normal speed.

Only motors with brushes can be controlled by a lamp dimmer.

Successfully, yes.

However, the lack of speed feedback will cause major problems.

For a fan that isn't really the case. The loading rises smoothly with
increasing RPM. At any give voltage it will settle to a speed.

 

[MooseFET]

Not instantly, but the asymmetric output will not be good over a period.
Eventually I expect the motor to die.

I wonder about putting a capacitor in series. This can be a dangerous
idea, however. You don't want to make a tuned circuit that somehow
rings up to a large voltage.

There is another problem with the light dimmer idea. They are not
designed to work with inductive loads. When the current in the motor
stops, the mains voltage won't be at the zero crossing. There will be
a large dv/dt right after the current stops that would likely
retrigger the triac.

 

[joseph2k]

There is something that I have wanted to try out for quite a while.
It is a variation on dropping cycles that I think would lead to lower
losses in the motor. I will explain it and others can shoot it down
if I am wrong.

To decrease the operating speed of an induction motor, it is best to
lower both its voltage and frequency. A device like a triac can be
fired to allow an alternation through or not.

The triac it fired on the positive alternation.
The negative alternation is skipped
The positive alternation is skipped
The triac is fired on the negative alternation
The positive alternation is skipped
The negative alternation is skipped
... repeat

This lowers the frequency to 1/3rd that of the mains frequency.

The RMS voltage on the motor would be 1/sqrt(3) of the normal line
voltage. However, the frequency content will be a bit funny. If I
did my math right, only 0.4 of that will be at F/3.

That is doable as well, but the controls needed for say 10% steps like OP
indicated make that approach very complex.

 

[MooseFET]

MooseFET wrote: [.....]

The triac it fired on the positive alternation.
The negative alternation is skipped
The positive alternation is skipped
The triac is fired on the negative alternation
The positive alternation is skipped
The negative alternation is skipped
... repeat

This lowers the frequency to 1/3rd that of the mains frequency.

The RMS voltage on the motor would be 1/sqrt(3) of the normal line
voltage. However, the frequency content will be a bit funny. If I
did my math right, only 0.4 of that will be at F/3.

That is doable as well, but the controls needed for say 10% steps like OP
indicated make that approach very complex.

I think it is well within the scope of what a small microcontroller
could do. For extra credit, lets see if we can figure out how to get
a measure of the speed of the motor.

We have a complex waveform on the motor. If we measure the voltage
and current, we should be able to figure out an impedance curve for
the motor. The impedance will be very high near the synchonous speed.

 

[Rich Grise]

OK. Since Google Groups doesn't do binaries I'll email it to you
tomorrow and I'll also post a copy to abse for anyone else who might
be interested.

I'm interested.

I'm curious how you'd do it. I voted for integral-cycle, but
pseudo-bangbang with zero-crossing MOCs sounds interesting as well.

Then again, is there that much difference between the two? ;-)

Thanks,
Rich

 

[Rich Grise]

Oh, OK. Never mind my other post where I asked you to post the schem.

Cheers!
Rich

 

[ectoplasm]

Yes. A better solution than mine if you don't need to count
individual cycles, so I won't waste my time designing and posting
the circuit I had in mind.

Actually, the schematic I found using the double 555 (http://
casemods.pointofnoreturn.org/pwm/circuit2.html) only sets the absolute
duty cycle. I.e. if you change the frequency, the duty cycle in
seconds would stay the same, and the % duty cycle is changed. You'd
need to adjust it again.

So mr. Fields... if your solution can handle this better, if you could
post the schematic anyway? Thanks!

 

[John Fields]

Actually, the schematic I found using the double 555 (http://
casemods.pointofnoreturn.org/pwm/circuit2.html) only sets the absolute
duty cycle. I.e. if you change the frequency, the duty cycle in
seconds would stay the same, and the % duty cycle is changed. You'd
need to adjust it again.

---
Which is exactly what you want.

Lets say that you have a fan running with a couple of 555's in
series so that the output into the TRIAC driver looks like this:

___________________ __
___| |___________________|

|<-------300------->|<-------300------->|
|
|<-----------------600----------------->|


where each positive pulse is 300 60Hz cycles wide and the period of
the waveform is 600 60Hz cycles.

Now lets say that the fan is running too fast and that you want to
slow it down. What you would do would be to either decrease the
width of the positive portion of the waveform (decreasing the number
of cycles into the fan) without disturbing the width of the low
portion:

___________ __
___| |___________________|

|<---150--->|<-------300------->|
|
|<-------------450------------->|



or increase the width of the low portion of the waveform while
leaving the high portion alone:

___________ __
___| |___________________|

|<---300--->|<-------600------->|
|
|<-------------900------------->|



Either way would cause the fan to stay on for less time than it was
on, which is what you'd want to cause happen if the fan was running
too fast.

 

[Michael A. Terrell]

Actually, the schematic I found using the double 555 (http://
casemods.pointofnoreturn.org/pwm/circuit2.html) only sets the absolute
duty cycle. I.e. if you change the frequency, the duty cycle in
seconds would stay the same, and the % duty cycle is changed. You'd
need to adjust it again.

So Mr. Fields... if your solution can handle this better, if you could
post the schematic anyway? Thanks!

You need to use <> around a URL, not () See below:


Actually, the schematic I found using the double 555
<http://casemods.pointofnoreturn.org/pwm/circuit2.html> only sets the
absolute duty cycle. I.e. if you change the frequency, the duty cycle in
seconds would stay the same, and the % duty cycle is changed. You'd need
to adjust it again.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[John Fields]

---
Which is exactly what you want.

Lets say that you have a fan running with a couple of 555's in
series so that the output into the TRIAC driver looks like this:

___________________ __
___| |___________________|

|<-------300------->|<-------300------->|
|
|<-----------------600----------------->|


where each positive pulse is 300 60Hz cycles wide and the period of
the waveform is 600 60Hz cycles.

Now lets say that the fan is running too fast and that you want to
slow it down. What you would do would be to either decrease the
width of the positive portion of the waveform (decreasing the number
of cycles into the fan) without disturbing the width of the low
portion:

___________ __
___| |___________________|

|<---150--->|<-------300------->|
|
|<-------------450------------->|



or increase the width of the low portion of the waveform while
leaving the high portion alone:

___________ __
___| |___________________|

|<---300--->|<-------600------->|
|
|<-------------900------------->|



Either way would cause the fan to stay on for less time than it was
on, which is what you'd want to cause happen if the fan was running
too fast.

---
Aaarrghhhh!!!

I must be getting senile.

You can do it with a single 555 astable:
..
..VCC>--+---------------------------------+
.. | |
.. [Ra] +---------+ |
.. | 7 |_ | 8 |
.. +----------------O|D Vcc|-----+
.. | 6 | _| 4 |
.. +----[Rb]----+----|TH R|O----+
.. | | 2 |__ | 3 |
.. +--[DIODE>]--+---O|TR OUT|-----|--->OUT
.. | | GND | |
.. [C] +----+----+ [0.1µF]
.. | | 1 |
..GND>---------------+---------+----------+
..

RaC determines how long the output will remain high and RbC
determines how long it will be low.

You could also use a pot (about 1 megohm) to make adjustment easy,
and then C would be about 10µF to give you a period of about 10
seconds. The 1k is in there so you don't toast the chip if you
crank the pot all the way up to Vcc.

..
..VCC>--+---------------------------------+
.. | |
.. | +---------+ |
.. | 7 |_ | 8 |
.. [POT]<--+---[1K]---O|D Vcc|-----+
.. | |A 6 | _| 4 |
.. | [DIODE] +----|TH R|O----+
.. | | | 2 |__ | 3 |
.. +-----+------+---O|TR OUT|-----|--->OUT
.. |+ | GND | |
.. [C] +----+----+ [0.1µF]
.. | | 1 |
..GND>---------------+---------+----------+
..


Here's an LTSPICE circuit list you can use to run a simulation:

Version 4
SHEET 1 880 748
WIRE 208 160 -16 160
WIRE 784 160 432 160
WIRE 208 224 144 224
WIRE 512 224 432 224
WIRE 624 224 592 224
WIRE 656 224 624 224
WIRE 784 224 784 160
WIRE 784 224 736 224
WIRE 208 288 64 288
WIRE 480 288 432 288
WIRE 512 288 480 288
WIRE 624 288 624 224
WIRE 624 288 592 288
WIRE 208 352 176 352
WIRE 64 368 64 288
WIRE 480 384 480 288
WIRE 512 384 480 384
WIRE 624 384 624 288
WIRE 624 384 576 384
WIRE 144 448 144 224
WIRE 480 448 480 384
WIRE 480 448 144 448
WIRE 176 480 176 352
WIRE 784 480 784 224
WIRE 784 480 176 480
WIRE 784 496 784 480
WIRE 480 512 480 448
WIRE -16 592 -16 160
WIRE 64 592 64 448
WIRE 64 592 -16 592
WIRE 480 592 480 576
WIRE 480 592 64 592
WIRE 784 592 784 576
WIRE 784 592 480 592
WIRE -16 640 -16 592
FLAG -16 640 0
SYMBOL Misc\\NE555 320 256 R0
SYMATTR InstName U1
SYMBOL voltage 784 480 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 5
SYMBOL res 608 272 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 58 VTop 0
SYMATTR InstName R1
SYMATTR Value 900k
SYMBOL res 752 208 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R2
SYMATTR Value 100k
SYMBOL res 48 352 R0
SYMATTR InstName R3
SYMATTR Value 1000
SYMBOL cap 464 512 R0
SYMATTR InstName C1
SYMATTR Value 12e-6
SYMBOL diode 576 368 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName D1
SYMATTR Value 1N4148
SYMBOL res 608 208 R90
WINDOW 0 12 110 VBottom 0
WINDOW 3 -39 59 VTop 0
SYMATTR InstName R4
SYMATTR Value 1000
TEXT 16 616 Left 0 !.tran 20 uic

 

[ectoplasm]

Thanks for the schematic. I plan to test it on a test board; I think
it's useful. As for your explanation in your last post where you said
that frequency doesn't need to be altered: you are right, as a speed
regulation the pulsewidth method suffices. Though I would like to be
in control of frequency too (I like to think of the fan spinning up &
slowing down as a sound likened to sea waves, when it has a long
period of, say >= 8 seconds). For the method you aim at, a period of 1
second or less would work, no? (C=1µF) It would then probably go
unnoticable that it's switching on/off to attain low speeds.

 

[ectoplasm]

I'm using the MOC3041 triac driver. I have some questions regarding
its use, though. This is from the data sheet: (applies to MOC3041,
3042, 3043) (paste in Notepad please)
..
.. Rin 1 _________ 6 360
.. VCC>---^^^^---| |---^^^^--+----+---------<HOT
.. | | | |
.. | | | |
.. 2| MOC3041 |5 | >
.. ----------| |-o TRIAC > 39
.. | | G/ | |
.. | | / | | 240Vac
.. 3| |4 / | |
.. o-| |---+- | ___
.. |_________| | | ___ 0.01
.. > | |
.. 330 > | |
.. | | |
.. -----+-------LOAD---<NEUTRAL
..

"Typical circuit for use when hot line switching is required. [...]
Rin is calculated so that IF [IR LED current] is equal to the rated
IFT of the part [...] 15 mA for the MOC3041. The 39 ohm resistor and
0.01 µF capacitor are for snubbing of the triac and may or may not be
necessary depending upon the particular triac and load used."

And:

"* For highly inductive loads (power factor < 0.5), change this value
to 360 ohms." [talking about the 39 ohm resistor? There is no '*' in
the schematic.]

I got some questions about this; hope someone can help me out:

Q: I am using 110Vac. Do I need to change any components? Will the 360
ohm resistor become 180 ohm? And how about the 330 ohm at pin 4 / the
gate?

Q: What is 'snubbing' of the triac? Do I need to change these values
(39 ohm, 0.01 µF) for a standing fan for home use of 60 watts? What if
this resistor / capacitor is not there, what's bad about it?

Q: I use the 556 (dual 555)'s output to Rin. The 556's VCC is 12V so I
assume that's what's on its output pin. With the IR LED at 1.8V and 15
mA, I use Rin at 680 ohms in a test setup (works fine). Is my Rin
value correct? The data sheet doesn't mention the IR LED typical
voltage.

The triac I am using is the BT137 (max 600V, 8A).

Thanks,
E.

 

[ectoplasm]

Direct link to the MOC3041 data sheet (pdf):
<http://www.web-ee.com/Electronic-Projects/data/moc3041.pdf>
(or Google...)

 

[John Fields]

Direct link to the MOC3041 data sheet (pdf):
<http://www.web-ee.com/Electronic-Projects/data/moc3041.pdf>
(or Google...)

 

[John Fields]

I'm using the MOC3041 triac driver. I have some questions regarding
its use, though. This is from the data sheet: (applies to MOC3041,
3042, 3043) (paste in Notepad please)
.
. Rin 1 _________ 6 360
. VCC>---^^^^---| |---^^^^--+----+---------<HOT
. | | | |
. | | | |
. 2| MOC3041 |5 | >
. ----------| |-o TRIAC > 39
. | | G/ | |
. | | / | | 240Vac
. 3| |4 / | |
. o-| |---+- | ___
. |_________| | | ___ 0.01
. > | |
. 330 > | |
. | | |
. -----+-------LOAD---<NEUTRAL
.

"Typical circuit for use when hot line switching is required. [...]
Rin is calculated so that IF [IR LED current] is equal to the rated
IFT of the part [...] 15 mA for the MOC3041. The 39 ohm resistor and
0.01 µF capacitor are for snubbing of the triac and may or may not be
necessary depending upon the particular triac and load used."

And:

"* For highly inductive loads (power factor < 0.5), change this value
to 360 ohms." [talking about the 39 ohm resistor? There is no '*' in
the schematic.]

---
Use this data sheet:

http://www.fairchildsemi.com/ds/MO/MOC3041-M.pdf
---

I got some questions about this; hope someone can help me out:

Q: I am using 110Vac. Do I need to change any components? Will the 360
ohm resistor become 180 ohm? And how about the 330 ohm at pin 4 / the
gate?

---
The Fairchild data sheet has the answers to those questions
---

Q: What is 'snubbing' of the triac?
---
http://en.wikipedia.org/wiki/Snubber
---

Do I need to change these values
(39 ohm, 0.01 µF) for a standing fan for home use of 60 watts? What if
this resistor / capacitor is not there, what's bad about it?

---
You may never be able to turn the fan off.
---

Q: I use the 556 (dual 555)'s output to Rin. The 556's VCC is 12V so I
assume that's what's on its output pin. With the IR LED at 1.8V and 15
mA, I use Rin at 680 ohms in a test setup (works fine). Is my Rin
value correct? The data sheet doesn't mention the IR LED typical
voltage.

---
The Fairchild data sheet gives 1.25Vf with If = 30mA, and National's
LM555 data sheet gives a voltage drop of about 1.5V at that current,
so:

Vcc -(Vled + (Vcc - Vout))
R = ----------------------------
Iled

12V - (1.25V + (12V - 10.5V))
= ------------------------------
0.03A


= 308 ~ 300 ohms, (the closest standard 5% value)

and the resistor will be dissipating:


P = I²R = 0.03A² * 300R =0.27 watts, so you'd want to use a 1/2
watt resistor.

For 15mA If it would be about twice that resistance (but 1/2 the
wattage), so 680 ohms might be a little high. Personally, (even
though the MOC3041 is guaranteed to trigger at 15mA worst case) I'd
go with 30mA of current just to be really, really sure.
---

 

[John Fields]

Thanks for the schematic. I plan to test it on a test board; I think
it's useful. As for your explanation in your last post where you said
that frequency doesn't need to be altered: you are right, as a speed
regulation the pulsewidth method suffices. Though I would like to be
in control of frequency too (I like to think of the fan spinning up &
slowing down as a sound likened to sea waves, when it has a long
period of, say >= 8 seconds). For the method you aim at, a period of 1
second or less would work, no? (C=1µF) It would then probably go
unnoticable that it's switching on/off to attain low speeds.

---
I don't know anything about your fan, so I can't say...

BTW, there are a couple of errors in the schematic which didn't get
caught and fixed:

1. The 40103 should be a 74HC40103 and,

2. R1 needs to be 4700 ohms.

 

[ehsjr]

I'm using the MOC3041 triac driver. I have some questions regarding
its use, though. This is from the data sheet: (applies to MOC3041,
3042, 3043) (paste in Notepad please)
.
. Rin 1 _________ 6 360
. VCC>---^^^^---| |---^^^^--+----+---------<HOT
. | | | |
. | | | |
. 2| MOC3041 |5 | >
. ----------| |-o TRIAC > 39
. | | G/ | |
. | | / | | 240Vac
. 3| |4 / | |
. o-| |---+- | ___
. |_________| | | ___ 0.01
. > | |
. 330 > | |
. | | |
. -----+-------LOAD---<NEUTRAL
.

"Typical circuit for use when hot line switching is required. [...]
Rin is calculated so that IF [IR LED current] is equal to the rated
IFT of the part [...] 15 mA for the MOC3041. The 39 ohm resistor and
0.01 µF capacitor are for snubbing of the triac and may or may not be
necessary depending upon the particular triac and load used."

And:

"* For highly inductive loads (power factor < 0.5), change this value
to 360 ohms." [talking about the 39 ohm resistor? There is no '*' in
the schematic.]

I got some questions about this; hope someone can help me out:

Q: I am using 110Vac. Do I need to change any components? Will the 360
ohm resistor become 180 ohm? And how about the 330 ohm at pin 4 / the
gate?

Q: What is 'snubbing' of the triac? Do I need to change these values
(39 ohm, 0.01 µF) for a standing fan for home use of 60 watts? What if
this resistor / capacitor is not there, what's bad about it?

Q: I use the 556 (dual 555)'s output to Rin. The 556's VCC is 12V so I
assume that's what's on its output pin. With the IR LED at 1.8V and 15
mA, I use Rin at 680 ohms in a test setup (works fine). Is my Rin
value correct? The data sheet doesn't mention the IR LED typical
voltage.

The triac I am using is the BT137 (max 600V, 8A).

Thanks,
E.

You can decrease the 360 to 180, the 330 is fine.
I'd use 100 & .01 for the snubber, but it's not
critical. See the url below for some snubber info.
http://www.epanorama.net/circuits/semiconductor_relays.html#snubber

The IR led in the MOC3041 probably has a vf closer to
1 volt than 1.8, but the 680 is fine.

Ed

 

[Marra]

I have done this a few times using different methods.

If using AC remember that you have to balance positive and negative
phases otherwise the motor might overheat.

Phase control works well.

Swicthing one full mains cycle then missing one full one works too.

www.ckp-railways.talktalk.net/pcbcad21.htm

 

[ectoplasm]

Thanks for the great help, John! The Fairchild datasheet really
helped.
And also thanks to ehsjr.

And now that we're talking about power, I think of one thing: do I
need to add a fuse in series with the BT137 triac? Since the triac can
handle up to 8A and probably has no protection for over-current, I'd
have to use an 8A fuse, right?