Adding Variable Speed Control to AC Motor?
N.
I have an exhaust fan with a 115V, 60Hz, single phase motor, 1725 RPM,
1/2 HP, 8.8 full load amps. It is also a hazardous location type
motor, enclosed.
-Is it possible to convert this motor to variable speed?
-Are there economically priced variable speed controls one can buy to
make the conversion?
-Conversely, anyone have a schematic on how to construct one
economically?
Thanks a bunch.
Cheers,
Neal
John Popelish
Variable frequency/speed drives are available for 3 phase motors in
this size range for a couple hundred dollars. Unfortunately, they are
not directly usable with single phase motors. Are you willing ot
consider replacing your motor with a higher efficiency 3 phase model?
Here is an example of the kind of drive I am thinking of:
http://store2.automationdirect.com/
Click on Drives low on the right side. Check out the L100 series of
drives.
--
John Popelish
Bob Wilson
lovega...@yahoo.com says...
The motor you describe is a standard so-called "squirrel cage" induction
motor. These types do not repond well to attempts at speed control.
Bob.
N.
> Variable frequency/speed drives are available for 3 phase motors in
> this size range for a couple hundred dollars. Unfortunately, they are
> not directly usable with single phase motors. Are you willing ot
> consider replacing your motor with a higher efficiency 3 phase model?
Hi ya John,
Actually, I don't yet have the motor and fan, but I plan on purchasing
it.
I only have single phase service, so three phase solutions won't
apply.
> Here is an example of the kind of drive I am thinking of:
I checked the site but didn't find anything applicable (might of
missed it though).
The fan + motor aren't worth a $200 dolloar solution. If I could
cobble a solution together for $40, I'd be all eyes.
How do houshold items like 110VAC hand drills implement the variable
speed?
Apples and oranges?
Thanks.
Cheers,
Neal
roma
<ff510eaf.02010...@posting.google.com>:
go to page 27 >http://members.shaw.ca/roma
Sir Charles W. Shults III
For most AC speed controls that use motors driven by the phase, you need a
variable frequency drive. However, a single phase drive is really much
simpler than a multiphase one. It is little more than a sine wave
oscillator and a power driver. Still a challenge to build for most
hobbyists, but not impossible.
Cheers!
Chip Shults
New web page coming soon- old page expires 12-Jan-2002
http://sites.netscape.com/aichip
John Popelish
A variable speed drive rectifies the single phase power ot DC, and
generates its own 3 phase power from that, at different voltage and
frequency combinations that allow different speeds. It is actually
possible to run many single phase motors (like capacitor start and
capacitor run types) from this 3 phase source, (though a transformer
may be needed and the capacitor eliminated).
Induction motors are the most common types for fan use, because they
have no brushes and run quietly for a long time. Electric drills
usually use either series wound (universal) motors or permanent magnet
magnet DC motors, both having carbon brushes.
Sorry, but I don't foresee a 40 dollar motor and speed control
solution for this application that I could recommend.
--
John Popelish
Tony Williams
N. <lovega...@yahoo.com> wrote:
> I have an exhaust fan with a 115V, 60Hz, single phase motor, 1725 RPM,
> 1/2 HP, 8.8 full load amps. It is also a hazardous location type
> motor, enclosed.
> -Is it possible to convert this motor to variable speed?
> -Are there economically priced variable speed controls one can buy to
> make the conversion?
We had an argument about simple voltage control
of fan motors here on sed about a year ago. The
short answer is Yes, provided you use the proper
type of motor, designed for fan loads.
It will be a squirrel cage motor made with a
high rotor-resistance and of the capacitor-run
type (not capacitor-start with centrifugal switch).
The high rotor-resistance allows the motor to run
at high slip without excessive rotor current. But
this produces low torque at low speed.... which is
ok, because the fan-load itself is conveniently
non-linear, requiring a drastically reduced torque
at low speeds (a square-ish type law).
Speed control by simple voltage control is quite
common in Europe in the Air-Conditioning and
Ventilation applications. They can be SCR types
or even switched taps on an autotransformer.
To overcome stiction at switch-on, a commercial
controller will generally run up the fan on full
voltage and then back-off to the set (lower) voltage.
Just make sure you buy the proper induction motor.
--
Tony Williams.
ber...@ami.com.au
I had in my workshop a 1/3Hp motor on my drill press. Bought a brand
new 1 Hp capacity VFD from a friend (who bulk purchased 20). Bought a
1 Hp 3phase motor and now I have a variable speed drill press. the VFD
is single plase in and 3 phase out to the motor. I consider it a wise
investment.
aeolus
ber...@ami.com.au
I have a single phase induction motor fan for which I built a triac
speed controller and it works fine. BUT it is only suitable for fans.
The circuit components would certainly cost less than $A40 but whether
the parts are still available I do not know as I built it some 15
years ago. Still alternative parts might still be available.
The motor does not really approve of the steep transient on the
triac turn on and I guess the supply authority is not impressed
either. I think with a choke in series with the motor this could be
improved but I am to lazy to wind such an inductor.
aeolus
Rich Grise
dimmer. But, unless you intend for this to be a training project,
you'd be much better off to just get a 3-speed or variable-speed
exhaust fan off the shelf. I moved into my compartment a couple of
years ago, and one of the things I noted on my walk-through was that
the stove exhaust was trashed. So they replaced it. I have no idea
what it cost, but there is such a thing. It's got continuous speed
control. Also, I have a 3-speed window fan, where the switch just
has four wires, and there's off, low, med, high. You might look
into adapting one of these options.
--
Cheers!
Rich
"We have met the enemy, and he is us!"
- Pogo Possum, ca. 1950's
JOHN L STEWART
uses all 6 poles. Medium uses 4 poles & low uses 2.
Both lower speeds simply use the "slip" characteristic of the induction
motor & the fan loading to get the speeds other then full. They also
reflect a terrible power factor to the utility. John L Stewart
Sam Goldwasser
> Your 3 speed fan uses a 6 pole induction motor. The highest speed
> uses all 6 poles. Medium uses 4 poles & low uses 2.
Huh?
--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html
JOHN L STEWART
another
wound separately used for starting. After the motor reaches running speed,
the start winding is automatically unhooked. If you were to use some sort
of
hookup meant to vary the motor speed, the start winding could be
activated.
The result is lots of heat & eventual motor failure.
If the motor speed control were of the solid state variety, then the
result
would be a controlled voltage containing lots of harmonics. The harmonics
don't do any useful work in the motor, but are dissipated in the form of
heat.
That also causes overheating & eventual destruction. It also reflects lots
of harmonics back to the utility which displeases them greatly.
Bottom line is don't do it!!! John L Stewart PEng
Rich Grise
can get a controller really cheap, but you'd have to scrap
the motor and start over to use it. It might be more economical
to just cut into the ducts and divert some of the air flow,
if what you want to do is reduce the airflow, such that your
wife's hair doesn't get sucked up into the stove exhaust
filter.
It MIGHT - repeat MIGHT - be possible to come up with a
speed controller for the motor you call out, but either
we'd need you to provide much more information about the
motor, or you'd have to climb up in the duct, take it
down, and have somebody look at it, in which case, it'd
be cheaper to just install a new variable-speed motor,
and put your old one on consignment.
Good Luck!
--
Mark Daniels
<s...@saul.cis.upenn.edu> writes
>JOHN L STEWART <jh.st...@sympatico.ca> writes:
>
>> Your 3 speed fan uses a 6 pole induction motor. The highest speed
>> uses all 6 poles. Medium uses 4 poles & low uses 2.
>
>Huh?
I'll second that "Huh?"
Maybe he means 6 poles gives lowest speed, etc.
>
>--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
> Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
>+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
> | Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html
--
Mark Daniels
ber...@ami.com.au
Are you always back to front or are you sometimes upside down?
aeolus
ber...@ami.com.au
Since this is a FAN then you could use my fan speed controller which
has been in operation for some 15 years without incident. the only
problem is that in the intermediate speed range the motor is noisier
because of the steep waveform it receives. This could be overcome by
the addition of a smoothing choke but I am too lazy to wind one up as
the noise is not serious.
On 3 Jan 2002 17:06:27 -0800, lovega...@yahoo.com (N.) wrote:
aeolus
JOHN L STEWART
way the motor can deliver full torque to the fan. Using 2 or 4
poles reduces the torque available so the fan slows down.
BTW, the synchronous speed for a single phase, 6 pole motor
on a 60 HZ supply is 1200 RPM, so the motor always runs
slower then that. John L Stewart
JOHN L STEWART
There will be a quiz after this!!! Cheers John
JOHN L STEWART
from the ground up to have more "slip". If that is the case it
is possible the motor speed can be varied a little more,
by lowering the applied voltage. There is still a practical limit
set by the starting circuit & heating. The characteristic of the
motor that is adjusted to get the desired operation is the
resistance of the copper bars on the rotor.
These types may show up in applications where a high starting
torque is required
BTW, the synchronous speed for a single phase 4 pole motor on
a 60 HZ supply is 1800 RPM, so will always run slower then that.
The "slip" between the rotating field at 1800 RPM & the rotor
running at a lower speed is what gets the electrical energy
into the rotor. If there were no slip in the induction motor,
then no energy would be transferred.
Your Turn!! John L Stewart
JOHN L STEWART
to confuse, but I'm sure it does. The first line of my 8.54 AM post
should read as follows-
Most induction motors of that size have 2 windings, one for starting
& another wound separately used for running.
That's what happens when you do things in a hurry. I need an editor.
Any volunteers???
JOHN L STEWART wrote:
Most induction motors of that size have 2 windings, one for starting &
another
H. Dziardziel
>> >
>> > I checked the site but didn't find anything applicable (might of
>> > missed it though).
>> > The fan + motor aren't worth a $200 dolloar solution. If I could
>> > cobble a solution together for $40, I'd be all eyes.
>> >
>> > How do houshold items like 110VAC hand drills implement the variable
>> > speed?
>> > Apples and oranges?
>
As long as it does not use a centrifugal starting switch (but it
probably does for a motor that type) I would consider trying an
old small shop welding transformer both as a plain inductor
(choke) and step down transformer by using its secondary winding
and taps. A microwave oven transformer may even work depending
on the fan load.
Small power tools use ac/dc commutor motors which are a different
breed and work well with simple electronics speed control.
.
N.
Thanks for hitting this issue from various angles. It appears as if
the solution is more complex then I had imagined it might be. The
motor is a fully enclosed hazaedous location motor, expensive, and is
best served by not risking a reduction of motor life.
Thing is, the motor uses 8amps. One benefit to converting to variable,
would perhaps have been a lower amperage draw. I wanted to reduce the
CFM exhaust of the fan. Maybe the rough solution is mechanical rather
than electrical, an adjustable vent of some sort at the fan intake.
Aeolus, if you have a link to a schematic of your circuit, I'd be
interesting in taking a peek.
Thanks to everyone.
Cheers,
Neal
JOHN L STEWART
"N." wrote:
Me too. John L Stewart
ber...@ami.com.au
However if you send me a direct email with your postal address I will
try to hunt down the circuit in my monstrous filing system and send it
to you throught the postal system. I have often done this before, but
I rarely get thanks "kiss me foot" or even an acknowledgement of
receipt. Still one can hope!
aeolus
ber...@ami.com.au
Don't you believe in the formula:
Synchronous speed = 120 * frequency / number of poles ?
As a Chartered Professional Electrical Engineer, I always found it
valid, but you may be right, only I won't believe it till I experience
it personally. Don't think you can take objection to my policy.
On Mon, 07 Jan 2002 19:16:51 -0500, JOHN L STEWART
<jh.st...@sympatico.ca> wrote:
aeolus
JOHN L STEWART
here it is. Bottom line is there is no easy way to control their speed.
ber...@ami.com.au wrote:
> Hello John L Stewart,
>
> Don't you believe in the formula:
>
> Synchronous speed = 120 * frequency / number of poles ?
Yes, that formula is the same as we use here, so for single phase
motors that gets us synchronous speeds of
3600 RPM for a 2 pole, 60 HZ motor
1800 RPM for a 4 pole, 60 HZ motor &
1200 RPM for a 6 pole, 60 HZ motor.
However all induction motors have something called "slip".
That is the difference between the synchronous speed & the
actual speed of the rotor. For example a 6 pole, 60HZ single phase
motor might show 1140 RPM on the name plate.
Since there is a difference, a rotating magnetic field is induced into
the rotor by the stationary field poles. In fact there are two (2) rotating
magnetic fields traveling in opposite directions in any single phase motor.
That's why any single phase motor needs some kind of starting help.
At rest the rotor doesn't know which way to go.
The induced field in the rotor is what transfers the power needed to drive
the external mechanical load & the resistance & magnetic losses in the rotor.
In order to get some speed variation control in a single phase induction
motor the rotor can be designed so that it has a somewhat higher
resistance then that normally used. With that comes more slip & losses,
but for light load applications it is tolerable.
The window fan Rich Grise mentioned earlier (see below) is an example
of an induction motor designed for limited load applications where some
speed variation is desirable. I have several here in the form of a
"Box Fan". They all have a 6 pole motor. The speed variation is
achieved by connecting either one, 2 or 3 sets of pole pairs to the
supply. In this case starting is achieved by "shaded poles", another
subject in itself.
With only one set of pole pairs connected the developed torque is at
a minimum so that a slow speed is produced. Connecting the 2nd & 3rd
pole pairs results in more torque so that the fan speed increases.
With all poles connected the speed comes up to the name plate 1140RPM.
There is no easy way to vary the speed of a single phase induction motor.
There are many controllers for 3 phase motors but that doesn't help here.
They can cost 1/3 to 1/2 that of the controlled motor. A common way to
do it is through a full wave, 3 phase bridge supplying DC to a Solid State
Pulse Width Modulation system. A variable frequency output
controls the motor speed.
Around 1960 I built a 100 watt, 120 volt supply to drive a single phase
synchronous motor. It used 20 of the 6AQ5, supplied by plus & minus
300 volt DC supplies. It used constant current feedback so that as the
frequency varied, so did the applied voltage, that being one of the requirements
of any of the variable speed schemes. I still have the working end of it!!
Hope this helps those trying to figure out what to do with their motors.
John L Stewart
> >> >> Rich
John Woodgate
<jh.st...@sympatico.ca> wrote (in <3C3D03D7...@sympatico.ca>)
about 'Adding Variable Speed Control to AC Motor?', on Wed, 9 Jan 2002:
>It used 20 of the 6AQ5, supplied by plus & minus
>300 volt DC supplies.
Why did you use so many low-power tubes instead of two or maybe four big
ones?
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
After swimming across the Hellespont, I felt like a Hero.
Reg Edwards
> Why did you use so many low-power tubes
> instead of two or maybe four big ones?
Probably for the same reason that when 1KW radio transmitters were needed in
quantity by the military in WW2 it was appropriate and more reliable to use
in the RF power amplifier 20 readily available 807's in series-parallel
rather than one whopper which nobody would ever have heard of if one went
soft and glowed blue in the middle of battle.
Other valves used for anything and everything were 6SN7, 6SL7 (double
triodes) and EF50's. There were far more EF50's (VR91's) manufactured (high
Gm HF pentodes) than any other type of valve in the whole of the history of
radio, radar and TV.
---
Reg
JOHN L STEWART
lots of drive. I worked at U of Toronto, Physics in the School
of Graduate Studies, first as a technician & eventually got
Professional Engineer status. There was a large quantity of
cast off electronic equipment & parts, so the 6AQ5's made sense
at the time. If I was doing it now I would follow a different path.
The amp in question is based on the Futterman but with
improvements so that the output runs in the pentode mode
& is balanced. You can see an example of the original
Futterman in
Audio Anthology Volume 3, page 10-
A Transformerless 25 Watt Amplifier for Conventional Loudspeakers
Dickie & Macovski
This one is a Series Push Pull Amp (Futterman?).
I built one in about 1955. It successfully drove a 16 ohm loudspeaker.
The test results of my version are long gone.
The output stage is asymmetrical so the driving circuitry should have
been bootstrapped.
It wasn't. The designer fixed the problem with 40 db of feedback!!
Cheers John L Stewart
John Woodgate
about 'Adding Variable Speed Control to AC Motor?', on Thu, 10 Jan 2002:
>Like many back at that time I didn't have too much money but
>lots of drive. I worked at U of Toronto, Physics in the School
>of Graduate Studies, first as a technician & eventually got
>Professional Engineer status. There was a large quantity of
>cast off electronic equipment & parts, so the 6AQ5's made sense
>at the time. If I was doing it now I would follow a different path.
Ah! An economic reason. Makes perfect sense.
John Woodgate
<g4fgq...@btinternet.com> wrote (in <a1k5p9$r9atk$1@ID-
124126.news.dfncis.de>) about 'Adding Variable Speed Control to AC
Motor?', on Thu, 10 Jan 2002:
>
>> Why did you use so many low-power tubes
>> instead of two or maybe four big ones?
>===============================
>
>Probably
Well, possibly.....
>for the same reason that when 1KW radio transmitters were needed in
>quantity by the military in WW2 it was appropriate and more reliable to use
>in the RF power amplifier 20 readily available 807's in series-parallel
>rather than one whopper which nobody would ever have heard of if one went
>soft and glowed blue in the middle of battle.
I agree with that; workhorse 807's would be much better, although I
would expect 20 to give more than 1 kW.
>
>Other valves used for anything and everything were 6SN7, 6SL7 (double
>triodes) and EF50's. There were far more EF50's (VR91's) manufactured (high
>Gm HF pentodes) than any other type of valve in the whole of the history of
>radio, radar and TV.
Indeed, although SP41s and SP61s together probably ran the EF50 close.
My point was that the 6AQ5 is a B7G version of the 6V6 and runs
**extremely** hot. I would have thought that something bigger and cooler
would be better.
Mark Daniels
<jh.st...@sympatico.ca> writes
There is actually a very easy way to vary the speed of a single phase
induction motor. Several years ago I designed a very simple controller
for doing just that. The big problem is the start and run capacitor as
it is sized for operation at a specific voltage and frequency.
Disconnecting it after the motor has started is the simplest way of
eliminating the problem. In my application it caused juddering of the
motor whilst running, which was due to the non-sinusoidal input
waveform, so disconnecting the cap killed two birds with one stone.
The waveform used was the classic three level square wave, timed to
eleiminate all triple n harmonics. The timing is 2:1 mark-space ratio.
i.e. the positive pulse occurs for two master clock pulses, the dead-
time takes one clock, the negative pulse, two clocks and the dead-time
again one clock.
Varying the frequency and voltage such that the V/f ratio is always
constant will give quite a range of speed control. Obviously, this
should not be done by changing the mark-space ratio given above, if
harmonic elimination is required. Varying the d.c. into the inverter is
the simplest way.
I am not going to claim that this method is perfect, but it works very
well and we had no problems with motors overheating beyond that expected
with low speed operation. Oversize the motor, accept a limited speed
control range and you can get away with a hell of a lot.
--
Mark Daniels